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CHAPTER 1
INCIDENCE OF PARASITIC INFECTION
INTRODUCTION
The first thorough description of the relation between a
digenetic
trematode and its snail host resulted from the investigations
into the life
cycle of a liver fluke, Fasciola hepaticaL. and its development
in Lymnaea
truncatula (Mull.) by Thomas (1883). Ever since numerous
biologist have
been attracted by the simingly inexhaustible variety of life
cycles in which
these parasites utilizes molluscan hosts. Though many life
cycles remain
unknown but the importance of the sub-class Pulmonata of the
class
Gastropoda in the evolution of the modern digenean is clearly
evident. The
digeneans are characterized by a complex life cycle in which
usually at
least one of the hosts is a mollusc. The work was initiated
early in 20th
century to undertake or investigate larval trematode infection
in the
freshwater snails. During the fall of 1913 the study of the
larval trematode
infestation in the freshwater snails was undertaken by William
(1914) at
the suggestion of Prof. Henry B. Word as an attempt to open up
this
underdeveloped field. The snails studied, which were obtained
from
several sources, yielded a surprisingly large number of species
of cercaria,
belonging to a wide variety of trematode groups. Ancient period
of study
the grouping of cercariae is done following classification of
Luhe (1909).
Some of the digeneans that develop in pulmonates are well known
for their
medical or veterinary importance, which is largely due to the
outstanding
success of their snail hosts in a variety of freshwater
habitats.
Schistosomiasis and Fascioliasisremains urgentproblems, partly
because
the snail hosts continue to be abundant, despite attempts to
control their
population by chemical and non-chemical (biological) means.
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Fortunately, much of the extensive and rapidly increasing
literature
dealing with relation between pulmonates and the Digenea is
gathered into
reviews. Recent contributions cover the intermediate host of
Schistosomiasis andFasciolasis (Jordan and Webbe, 1969; Berrei
1970;
Kendall, 1970) and wider fields (Ulmer, 1971; Brooks, 1969).
While
Wright (1971) and Erasmus (1972) describe the biology of
trematodes with
special reference to experimental studies and most of them
involving
pulmonate snails.Trematode parasites are essentially a component
of the
fauna associated with aquatic environment and are generally
overlooked
until epidemic disease makes the association inescapable
(Erasmus 1972).
Free living digeneans larvae may be exceedingly abundant in
water, an
infected snail may discharge clouds of cercariae and in the
words of Basch
(1975) snails in their natural habitats may be subjected to a
barrage of
miracidia derived from numerous species of trematode parasitic
in local
vertebrates.
In the life histories of Digenea employ either two or three
hosts, a
few cases are known in which a single species of snail may
serves both as
first and second intermediate host. The egg - laying generation
lives in
vertebrate and their eggs must reach water or dump soil where
the
miracidium already developed insidethe egg, before leaving the
host may
remain alive. No miracidium can withstand desiccation.
In order to proceed further life cycle of the parasite, it is
necessary
for the miracidium, with perhaps a single exception, to get into
the body
of a mollusc, such as an aquatic or terrestrial snail or a
bivalve. Entry is
passive, the egg is eaten by the mollusc if. This is the usual
method of
entry for miracidia of these trematodes which produce small
eggs.
Examples are the Heterophyidae, Opisthorchidae, Brychylaemidae
and
Plagiorchidae.It is also well known fact that both terrestrial
and aquatic
snails are avid eaters of feces and that they gather around the
dung of
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vertebrates to feed. The entry into the molluscan host by most
miracidia
of medium or large size is brought about by their active
penetration
through soft mucous surface tentacle, head-foot organ, the
mantle or
possibly of the lining of the respiratory chamber into which
they have
been observed to disappear.
The act of penetration into the molluscan host tissue may
require
only a few seconds or possibly as much as a minute. Penetration
is
accomplished by the joint action of muscular movements, activity
of cilia
located on the epidermal plates and the lysis of the host cells,
or perhaps of
the mucoproteins constituting the intercellular cements by the
secretion
discharged from the penetration glands. These secretions
probably
containhyaluronidase which has been found by Levin et al.(1948)
in
cercaria possessing penetration glands. The actual process of
penetration is
probably the same whether the miracidia attack a soft external
surface of
the host or the epithelial lining of the digestive tract by
those emerging
from ingested eggs.
In the field of parasitology, in particular d igenean parasitic
infection
to the snail hosts, starts from swallowing of trematode‘s eggs
or
penetration and enterance of miracidial larval form into the
snail body.
After the enterance of the miracidia there starts the incubation
period and
this lasts till the emergence or the start of release or
discharge of cercaria
from the snail body called as period of prepatency. The start of
cercarial
release is known as begin of patency period and this lasts till
it is free of
infection. This start of cercarial release till complete
shedding of the larvae
is called as period of patency. These two periods i.e.
prepatency and
patency are different from parasite to parasite and snail host.
This may be
depending on the infection rate i.e. number of miracidial
infestation to the
snail.
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Whether the small miracidia which are ingested while the egg
comes
into contact with suitable molluscan hosts appears to be largely
a matter of
chance. The hazard involved in making connections with suitable
mollusc
are truly enormous, a risk in which is compensated bythe worm
and large
output of cercariae released by the sporocysts or rediae which
develop in
the molluscan host resulting from the successful establishment
of a
miracidium.
Almost all known digeneans have early stages which are parasitic
in
mollusc (Gastropods, Scaphopods or lamellibranches) and the
adults are
internal parasites of vertebrates. A common life cycle comprises
a phase of
asexual multiplication in a primary (first intermediate)
molluscan host,
followed by a period encystment within a secondary intermediate
host and
finally development to maturity with sexual reproduction within
a
vertebrate (definitive) host. In most complex life cycles the
parasites
require a third intermediate host. Usually the first and second
intermediate
hosts are molluscs. Penetration into the first molluscan host is
achieved
either within water by a free swimmingmiracidium larvae or
parasites‘ egg
may ingested by the host and hatch in its gut. The established
miracidium
commonly develops into a mother sporocyst which gives rise in
one type
of life cycle to one or more generations of mother sporocysts or
in other
type of life cycle to one or more generations of rediae.
Sporocysts are sac like organisms which absorb nutrient
through
their surface, while rediae are equipped with a pharynx and gut
and can
ingest pieces of host tissue. Daughter sporocyst and rediae
produces
another stage, the cercaria, which in most life cycle has a
brief free
swimming existence, terminated by encystment as a
metacercaria.
Encystment may occur either on vegetation (as a Fasciola) or
within the
second intermediate host, which may be a different individual of
the
species which serves as a first intermediate host. In some life
cycles the
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second intermediate host is ingested by a definitive host. The
Schistosomes
are unusual in having cercaria that penetrates directly into the
definitive
host. An extensive range of information about the life cycle may
be
obtained from Wright (1971) Erasmus (1972) and Canning and
Wright
(1972).
Cort (1914) wrote his preliminary report ―Larval trematodes
from
North American Fresh Water Snails.‖ The snails examined by Cort
were
from different localities throughout the United States and from
various
ecological situations. During his survey he described fourteen
new species
of cercaria. Cort et al.,(1937)carriedout an extensive survey,
by
determining the incidence of 17 species trematodes in over
7000
specimens of Lymnaea emarginataangulata (Sow) from two lakes in
North
America.O‘Roke (1917) published a short paper on ―Larval
Trematodes
from Kansas Fresh water Snails.‖ His study was less pretentious
than that
of Cort‘s but added several new species in the field of
Helminthology.
Later on Faust (1919) presented his finding in ―The American
Naturalist‖,
which deals with a biological survey of cecariae. His probe
includes
examination of total number of host record was 72. The net
outcome of the
survey was 61 species of larval trematodes and there were eleven
species
of larval trematodes recorded from two or more hosts. He states
that ―The
molluscs most heavily infected are the ubiquitous species.
Planorbistrivolvis and Physagyrinathe Western species Lymnaea
proxima.
The effect of the exposure dosage of miracidia on the biology of
the
snail host and the subsequent development of the larval stages
of the
parasites is an interesting problem and in recent years number
of
researchers got attracted towards this area of
research.(Najarian,1961;Chu
et al.,1966; Pan, 1963; Pesigan et al., 1958; Schreiber and
Schubert (1949a
and 1949b); Hanson (1975) recently found that
Schistosomamansonicercarial embryos freed from sporocysts
and
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developed to swimming cercariae when cultivated in presence
of
Biomphalariaglabrata embryos (Bge) tissue culture. Resently,
Paul and
Joseph (1977)made invitro developmental studies
onSchistosomamansoni
cercariae.
The proportion of the total body weight of heavily infected
snails
that is contributed by larval trematodes was estimated to be
about 20% by
Hurst (1927) and Wesenberg Lund (1931) in the case of
Echionostomarevolutum (Froelich) in Physaoccidentalis
Tryonand
LeucochloridiumparadoxumCarus in Succinaeaputris(L). But the
data
given by Cheng (1971) indicated that this proportion can be
about 27% for
Physasayii (Tappan) infected with Echionostomarevolutum.
Numbers of experiments were made on the cercarial release
from
snail hosts. Krull (1941) reported that the snail
Pseudosuccineacollumellawhen exposed to a single miracidium
of
Fasciola hepatica the number of cercaria emerged ranged from 14
to 629
per snail host. Rothschild (1939, 1942)has indicated that as
many as a
million might emerge from a single host. Elon and George (1954)
made
observations on the number of daughter sporocysts and cercariae
produced
in Physagyrina after exposure to single and multiple
Ochetosomatides egg
exposures. They have demonstrated the pattern of cercarial
shedding for
the larvae of the Ochetosomatidesand the number of daughter
sporocysts
resulting from one or more egg exposures and the duration of the
cercarial
shedding period.
While studying population dynamics of the vector snail in the
field,
the question arise and can be answered by laboratory studies
under
controlled conditions (Sturrock and Sturrock, 1970). He observed
a
reduced growth rate in B. glabrata infected at the age of two
weeks. In
other experiments on the influence of S. mansoni on the growth
of B.
glabrata or B. pfeifferi a temporary accelerated growth rate was
found
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(Chernin, 1960; Pan 1963, 1965; Sturrock 1966 and Sturrock
and
Sturrock, 1970). In some cases infected snails become eventually
stunted
(Pan, 1963 and 1965). On the other hand ‗gigantism‘ has been
supposed
to occur in field-infected specimens of some other species of
gastropods
(Linke, 1934; Wesenberg-Lund, 1934; Rothschield, 1941, Boettger,
1953
and James, 1965).
Generally, a higher mortality rate (Chernin, 1960, Pan 1963
and
1965; Chu et al. 1966; Sturrock 1966 and 1967 and Sturrock and
Sturrock,
1970) and an increased susceptibility to bad conditions eg.
Desiccation
(Brumpt, 1941; Oliver et al., 1954) and high temperatures (Etges
and
Gresso, 1965) has been found in snails infected with
Schistosomaheamatobium. During the past quarter of the century
there has
been much interest on the lines of parasitic affects the
behavior, ecology
and evolution of their hosts (Price, 1980; Rollinson and
Anderson, 1985;
Dobson, 1988; Barnard and Behnke, 1990; Anderson and May 1991
and
Toft et al. 1991).
The snail counts per unit of time method measures the density of
the
snail population (Oliver and Scheidermans 1956) in the marked
area only,
not the total population. These authers investigated infection
rate of
parasites by shedding and crushing method. A detailed
description of the
various types can be found in Cheng‘s report (1973). Some are
classified
according to the position and number of body suckers. Some
are
categorized according to the shape and relative size of their
tails, while
some cercariae are categorized morphologically by specialized
body
structure like the Xiphidiocercarie, the stylet bearing
cercariae. The various
species have one characteristic in common; they all have an
anterior
marginof the oral sucker.
Molluscs provide an environment that the parasite exploits
to
achieve considerable growth and reproduction. Intimate contact
between
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parasite and host is required and undoubtedly the interaction
extend to a
molecular dialogue, which likely triggers development in the
parasite and
may or may not the full force of the snail internal defence.
One of the most characteristic features of trematode-mollusc
interaction is their specificity. Despite the fact that aquatic
environment are
usually inhibited by several snails species, each host species
is normally
found infected with specific larval trematodes and never with
others,
despite the array of trematode miracidia present in the
environment. The
use of the restricted group of host by the parasite is a
phenomenon known
as ―host specificity‖. Trematodes show high specificity in their
use of
molluscan intermediate hosts more than in their definite
vertebrate host.
Only certain species combinations are compatible (i.e. the
parasite
recognizes, penetrates and develops within the snail). Most
digenean
species can develop successfully in only a single snail family,
genus or
even species (Adema and Loker, 1977).
The occurrence of multiple infections in nature may provide
a
possible mechanism for intermediate host switching the process
by which a
trematode transfers to a new host. The miracidial penetration of
the
―wrong‖ host would normally result in elimination of the
incompatible
parasite, but if the invading parasite survives by overcoming
the snails
internal defence system, the infection may persist.
Joosse and Van Elk (1986); while working on experimentally
parasitized trematode host snailLymnaea stagnalis by the
miracidia of
Trichobilharziaocellata, determined the period of prepatency and
patency.
Similar type of observations were made by Pan (1963 and 1965)
and
Sturrock and Sturrock (1970) studied the process of egg-laying
in
experimentally infected and non-infected planorbid snail,
Biomphalariaglabrata, during early prepatency, prepatency and
patency
period of trematode infection.
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Transmission of digenetic trematodes from the snail to the
next
host in the life cycle depends largely on the proportion of
snails that
release cercaria as well as the number of cercariae released
from each
snail (Anderson and May 1991). These factors may be
quantified
respectively, in terms of the prevalence and intensity of patent
infection
(Margolis et al., 1982, Bush et al., 1997). In field population
both these
factors tend to increase with increasing snail size. Among
freshwater
snails, for example, a positive correlation between snail size
and the
prevalence of infection was reported in 73% of the field studies
reviewed
by Sorenson and Minchella (2001); size may also be positively
correlated
with intensity of infection (Smith, 1984). For both mammalian
and avian
schistosomes, the prevalence and intensity of infection tend to
be highest
among the largest snails (Sturrock, 1973; Kulesa et al., 1982;
Loker,
1983, Woolhouse 1989 and Niemann and Lewis, 1990).
Large snails are older on an average, than small snails, within
a
given population (Minchella et al. 1985). This age variation
translates into
differential exposure to and duration of infection among snails
of differing
sizes because larger older snails may have been exposed to more
miracidia
or may have been infected for a longer period and thus have
patent highly
productive infections. Snails size itself (regardless of age)
might in fact
affect trematode development; if for example, a relative large
snail
provides more space, greater energetic resources or both for
production of
cercariae.
In recent year‘s wealth of information gathered on larval stages
of
digenetic trematodes from freshwater snails, described
throughout the
world. As not by Robert and Janovy (2000), the global prevalence
of
several animal parasites has not changed in 50 years. Of
particular interest
is Schistosomiasis where although the worldwide distribution of
people
infected may have changed due to eradication programs, as in
Japan the
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number of people at risk are (200 million) infected and dying
(20,000/year)
from Schistosome infection have not diminished (Oliveria et al.,
2004).
Parasitic Platyhelminthes are important economically and
socially and
deserve the attention they receive. They deserve the attention
not because
of they are important economically or medically, but because
they are
fascinating animals that provide challenges to understand
associations of
animals and model systems in any field of biology.
According to Kerney (1999), the pond snail, L.stagnalis is a
Holaretic freshwater snail and a common host for many trematode
parasite
species (Loy and Haas 2001). One or more species of cercariae,
sometimes
more than ten, may be found in freshwater and terrestrial
gastropod (Ito,
1980). The freshwater snail Paludomuspetrosus is one of the
freshwater
snail found in the mountains of Southern Thailand. This species
is
common in areas where, Paragonimuswestermanione of the animal
lung
fluke that lives in carnivores. Duangduen et al., (2003)
reported four types
of larval trematodes from the freshwater snailP.petrosus,
Ashrafi et al.,
(2004) studied 4830 different snails from Quiland Province,
Iran, from the
point of larval stages of Fasciola, in the snail Lymnaea
gedrosiana. Only
seven (0.35%) specimens were found infected with larval stages
of
Fasciola species. Hamann (2006) described specimens of
Glypthelminsvitellinophilum from the anuran
Lysapsuslimellus,
Cipangopaludinalecythis an edible snail as a host for a new
Furcocercous
cercaria from Manipur (Gambhir et al., 2008). Very recently a
faunistic
survey has been made by Sharif et al., (2010) to isolate
cercariae from
Lymnaeid snails in Central areas of Mazandaran, Iran. Sami and
Ghaleb
(2011) investigated larval stages of digenetic trematodes in
the
prosobranch gastropod snails from freshwater bodies in
Palestine.Most
freshwater snails can become intermediate hosts for trematode
cercariae
which may be transmitted to people and animals.
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The present freshwater pulmonate snail Lymnaea acuminata is
inhabitant of varieties of aquatic habitats VIZ freshwater
ponds, pools,
ditches and rivers is proned to get exposed varieties of natural
and artificial
stress conditions.Through rain water runoff the water at their
habitat gets
contaminated by varieties of contaminants and there can be
chances of
trematode infection in the water due to human and other animal‘s
excreta
entering through rain water runoff. With this insight the
present chapter
was framed to cover various aspects as under:
This first chapter deals with the incidences of larval trematode
infection
to the freshwater pulmonate snail Lymnaea acuminata. The topic
includes
following subtopics as under.
i. Frequency of larval trematode parasitic infection in the
snail Lymnaea
acuminata during two years of study period.
ii.Determination of period of patency (cercarial release period)
in
naturally infected snails and larval release.
iii. Different types of larval trematode parasites in L.
acuminata.
iv. Number of cercariae released during period of patency.
v. Frequency of particular type of parasitic infection during
two years
study period in L. acuminata.
vi.Effect of infection on Survival of snail during patency
period
vii.Host size dependence larval trematode infection and
cercarial release
during patency period
viii.Effect of larval trematode parasitic infection on shell
size and weight
of the snail during patency period.
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MATERIAL AND METHODS
The freshwater snail species of Lymnaea acuminata were
procured
from different water bodies in and around the city Aurangabad
during two
years of study (from Jan. 2009 to Dec. 2010). Immediately snails
were
washed with tap water in the laboratory in order to remove mud
particles
and algal material present on the shell. Normal sized intact
healthy snails
(22 ± 1mm shell length) were sorted out and maintained in 100
ml
dechlorinated tap water. Dechlorination of tap water was done at
laboratory
conditions.Water in the trough getting exposed to open air at
least 24 hours
prior to use. Next day visual observations were made for
parasitic infection.
i. Frequency of larval trematode parasitic infection in the
snail
Lymnaea acuminata
In order to find out frequency of parasitic infection during
every month
for total two years study period, release of cercaria in the
snail water in the
beaker, indicative of natural infection to the snail.
Fortnight collection of snail was made and randomly sorted
out
normal sized, intact and healthy snails showing creeping
movements (70-80
numbers) were maintained individually in separate 250 ml
capacity beakers.
Observations were made for continuous subsequent three days to
check
normal and trematode infection to the snails. Number of infected
snails was
counted, and infection rate in percent was calculated for every
month
during two years of study period (Jan.2009 to Dec. 2010).
ii. Determination of patency period in naturally infected
Lymnaea
acuminata
After getting sorted out naturally infected snails, after
completion of
prepatency or incubation period, there starts the release of
cercaria, this is
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called as begin of patency period. Cercarial release is with
slow rate at the
beginning of patency for two days is called initial phase of
patency period.
Then starts peak phase of patency period, during which there is
intense
release of cercariae for three days and once again there is
reduced rate of
cercarial release, this lasts for two more days and called as
post phase of
patency. In this way patency period of Lymnaea acuminata is of
seven
days, which starts from beginning of larval release to complete
stop of
cercarial emergence or released by the snail.
iii. Different type of larval trematode parasites found in snail
L.
acuminata
Different larval trematodes emerged during patency were
collected
separately and got centrifuged. The larvae settled at the bottom
of the tube
were transferred to cavity block. First of all the larvae got
preserved in 4%
formalin. These preserved larvae then got processed for
morphological
study. After getting washed with distilled water, got stained
with either
heamatoxylinor neutral redand passed through increasing grades
of alcohol
for dehydration. Parasites cleared in xylol and mounted over the
micro
slide under DPX medium. Observations were made under
compound
microscope for their identification. The trematode cercarial
larval
characters such as position and number of suckers, shape and
size of the
tail and morphologically by specialized body structures were
taken into
account for their identification. While identifying the cercaria
a systematic
key reference by Frandsen and Christensen (1984) was followed.
These
different parasitic pathogens were photographed.
iv. Number of cercariae released during period of patency
Naturally infected medium sized snails got sorted out and were
maintained
individually in separate beaker for cercarial release. 4-5
infected snails
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continued for cercarial release separately. After every 24
hrs.snails were
separated and transferred in new beaker with freshwater. The
cercariae in
infected snailwater were killed with addition of formalin (2-3
drops). One
ml cercarial water taken in cavity block. With the help of
capillary tube
dropper, cercariae were sorted and counted. The same procedure
was
continued for total 7 days period of patency i.e. initial phase
of patency,
peak phase of patency and post phase of patency. An average
with
standard deviation of cercariae released during every day was
calculated
for seven day of patency period.
v. Frequency of particular type of larval trematode infection in
the
snail Lymnaea acuminata
Naturally infected snails got sorted out and maintained
individually
in separate beakers of 250 ml capacity. 30- 40 infected snails
were
identified and kept under observation for different type of
larval trematode
pathogen identification. The cercariae released in the snail
water was
collected and after killing the cercaria with addition of 2-3
drops of 10%
formalin were observed under binocular microscope for their
identification. After identification of particular type of
cercaria, percentage
of snails got infected with particular type of trematode larvae
was
calculated.
vi. Effect of infection on Survival of snail during patency
period
The survival of both infected and non-infected snail also
studied by
keeping batches of infected and non-infected normal sized snails
in
separate trough with sufficient dechlorinated tap water and ad
libitum as
food material with hydrophytes collected from snail‘s habitat
provided,
during patency period. The water changed after every 24 hrs. in
both
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troughs. Changes in survival number of snails in both the trough
was
noted down daily at normal room temperature 27±20C.
vii. Host size dependence larval trematode infection and
cercarial
release during patency period
In order to determine number of cercariae released by different
sized
snail host of L. acuminata were collected from Kham River,
Aurangabad.
Immediately after getting to laboratory, were washed in order to
remove
mud particles and algal material attached if any. Different
sized animals
such as 5 ± 1mm, 10 ± 2mm,15 ± 1mm and 20 ± 1mm shell length
were
sorted out and maintained in 100 ml dechlorinated water taken in
250 ml
capacity glass beakers. Separately observations were made in
order to
detect naturally infected snails. 5-6 specimens of different
sizes selected
were maintained individually for release of cercaria. Naturally
infected
snails start release of cercaria under laboratory condition.
After identifying
cercarial release by each sized snails continued for cercarial
count for total
period of patency. Counting of cercaria was done as usual
method
mentioned earlier. Total number of cercaria released by each
sized animals
was calculated. Average count of cercaria by5 ± 1mm, 10 ± 2mm,
15 ±
1mm and 20 ± 1mm sized snails was calculated for total 7 days
period of
patency.
viii. Effect of larval trematode parasitic infection on shell
size and
weight of the snail during patency period
Non- infected snails when collected randomly along with
infected
one are small in size (18±1mm shell length) compared to
parasitized ones
(20±1mm shell length). Continuous observations were made on
measurement of shell length and weight of animals in both
infected and
non-infected snails during different phases of patency. Shell
length in mm
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was measured with the help of calibrated scale and weight by one
pan
electronic balance in terms of mg ± S.D.
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OBSERVATIONS AND RESULTS
i.Frequency of larval trematode parasitic infection in the
snail
Lymnaea acuminata
70-80 normal sized healthy snails measuring 20-22 mm shell
length were sorted and maintained individually in 250 ml beaker,
during
every month of the two years study period. The frequency of
trematode
larval infection observed in percent is depicted in the table 1.
No infection
was observed during summer months i.e. from February to May
during
both the years of study. Snails start getting invaded by
trematode larval
pathogens from June onwards. Heavily infection was observed in
the
month of September and October, 2009 and 2010. Maximum infection
50
to 60 % was found in the month of September respectively in 2009
and
2010.
ii.Period of patency in the snail Lymnaea acuminata
Naturally infected snail specimens were identified in the
laboratory
and cercarial release period was observed and mentioned in the
form of
observation table 2. The total period of patency was observed
and lasts for
seven days‘ time period. Depending upon the number of cercariae
released
by theinfected snails, the total period of patency can be
divided into three
phases-
A. Initial phase of patency:
Since the cercarial release starts with emergence of few
cercaria
during first two days.
B. Peak phase of patency:
From the 3rd
to 5th
day of cercarial release, is the peak phase of
patency because during these days there is enormous number of
cercarial
release by the snail L. acuminata. (See table 2)
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21
C. Post phase of patency:
During 6th
and 7th
day of patency, the frequency of cercarial release
decreases compared to peak phase of patency. Generally in the
present
study, after 7th
day of cercarial release, there is no further release of
cercaria observed.
iii.Different types of trematode larval pathogens (cercariae)
found in
the snail L. acuminata
Various type of larval trematode pathogens got invaded in
the
naturally infected L. acuminata is shown in the form of
observation table
4. In all some six types of cercaria have been identified during
study
period. Of the total parasitized snails 43.25 % snails were
found invaded
by, cercaria of Fasciola hepatica and least number of infected
snails 2.33
% were found invaded by the cercaria of
Diplostomumhepaticum.
iv. Number of cercariae released during period of patency
During patency period large number of cercariae released from
the
infected snail. The cercariae released from the snail were kept
separately
without snail host to check their viability. The cercaria could
not survive
more than 24 hrs. in laboratory conditions and die in the
beaker. The
mortality rate was found to be higher among infected snail than
the normal
snail. Again the behavior of the snail observed, the infected
snail was slow
as compare to non-infected one, which shows active movement.
The
freshly released cercariae get used for their identification
after killing by
formalin.(Table 3)
v. Frequency of cercaria released during period of patency:
From the start of period of patency to the 7th
day, number of cercaria
released is given in the observation table 3. From the table it
is observed
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22
that from first day of patency, to completion of peak phase of
patency there
is continues increase in the cercarial release (1009 ± 25 to
2400 ± 30)
observed. The data included in the table is average of three
infected snails
along with standard deviation. Again on 6th
and 7th
day there is decrease in
the number of cercarial emergence from the parasitized snails.
Maximum
numbers of cercariae (2400 ± 30) were released on fourth day of
patency
i.e. during peak phase of patency period. Least number of
cercariae were
released (800± 12) on last day of patency.
vi. Effect of infection on Survival of snail during patency
period
Gradual decrease in the percentage of snail‘s survival form
both
infected and normal group showed in table 6. Mortality rate was
high in
infected snails than non-infected group. At beginning both
groups show
same survival percentage, but in infected group during post
phase of
patency the mortality rate got increases and survival rate
decreases upto
65.62 % at the end of 7th
day. Whereas the normal snails group shows
83.54% at the end of 7th
day of patency.
vii. Snail host size dependence cercarial release during period
of
patency in Lymnaea acuminata
The number of cercaria released by naturally infected,
different
sized snails of L. acuminata is summarized in the observation
table 5.
Total number of cercaria released by small sized (5±1mm shell
length)
snail is 2300 ± 27 during total period of patency. Medium sized
snails (10
± 1mm and 15 ± 2 mm shell length) released respectively 5100 ±
39 and
7400 ± 35 cercaria. Maximum numbers of cercaria were released by
the
snail size having 20 ± 2 mm shell length released 9700 ± 53
cercariae.
Number of cercaria invaded in snail is directly proportional to
the size of
snail hostLymnaea acuminata.
-
23
viii. Effect of larval trematode infection on shell size and
weight of the
snail during patency period
Effect of trematode parasitism on shell length and total body
weight
of Lymnaea acuminata observed during patency period and is
summarized
in the observation table 7. There is a slight variation in the
shell size of
non-infected and naturally infected snails. The control snails
weight
remain constant (18±1mm) while the infected snail shows
gradually
increase in shell length (20±1mm to 23±1mm) and body weight
(230±2 mg
to 237±2 mg) respectively.
Following different types of cercariae were found invaded in
the
snail L. acuminata-
1. Cercarial form of Fasciola hepatica.
2. Cercarial form of Plagiorchissp.
3. Cercarial form of Echinostome sp.
4. Cercarial form of Pseudo echinoparyphium sp.
5. Cercarial form of Trichobilharziaocellata.
6. Cercarial form of Diplostomumhepaticum.
The distinguishing features of these cercariae are as
follows:
1. Fasciola hepatica – (Linnaeus 1758)
a. The cercaria has a heart shaped body and bears a long
tail.
b. Mouth is surrounded by oral sucker.
c. Most of the adult organs are present.
d. Rudiments of reproductive organs are also present.
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24
2. Plagiorchis sp.-(Müller in 1780)
a. The cercaria displays a total length of approximately
450µm.
b. The swimming behavior of cercariae is with snaking,
crawling
movements.
c. Mouth is surrounded by oral sucker, ventral sucker also
present.
d. The tail is short, not longer than the body length.
3. Echinostome sp. –(Rudolphi 1809)
a. A good feature of cercariae is of its tail with finger like
pulled out end.
b. The total length of cercaria is approximately 780 µm.
c. The intestinal branches reach the end of the body.
d. The mature rediae of the parasite contain up to 30 cercariae
in various
stages of development.
4. Trichobilharziaocellata – (La Valette 1855)
a. The furcocercaria is with its body approximately 800 µm, very
large
and easy to distinguish with naked eye.
b. Good features for recognition are bifurcated tail hence
called as
furcocercus, is considerably longer than the body length.
c. The bifurcated parts being half as long as tail stem.
d. The pharynx and oral sucker fuse together in to head
organ.
e. In the process of movement they take up a typical position
towards
lights, hence photo positive, in which the tail is held at about
900
to the
body and the cercaria with the acetabulum attached
underneath.
-
25
5. Diplostomumhepaticum – (Retzius 1786)
a. It is also furcocercariae, with body length approximately 400
µm.
b. It is identified easily by its typical swimming position. The
cercariae
hangs motionless with their bodies bent in the water and let
themselves
be carried along by the current, interrupted to only by a short
and fast
swimming movements.
c. It is having a strong arming of oral sucker and
acetabulum.
6. Pseudoechinoparyphium sp. –
a. It is a representative of Echinostomatidae family. Total
length of the
cercaria measures approximately 1.2 mm; is the largest found
echinostome cercaria to date.
b. The most striking feature of cercaria is strongly developed
intestinal
system, which appears to be subdivided.
c. The tail exhibits no fin edge.
-
26
Table 1
Frequency of larval trematode infection in the snail Lymnaea
acuminata during two subsequent years, from January 2009 to
December 2010.
Month and
year
Frequency of
infection in %
Month and
year
Frequency of
infection in %
January 2009 05 January 2010 07
February 02 February Nil
March Nil March Nil
April Nil April Nil
May Nil May Nil
June 09 June 11
July 30 July 15
August 35 August 25
September 50 September 60
October 49 October 45
November 20 November 39
December
2009 10
December
2010 13
-
27
Table 2
Different phases of patency in the freshwater snail, Lymnaea
acuminata.
Period of
patency
in days
Phases of Patency
Each Phase of
patency in
Days
1
2
Initial phase of
patency 2
3
4
5
Peak phase of
patency 3
6
7
Post phase of
patency 2
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28
Table 3
Cercarial release during patency period in Lymnaea acuminata
Period of patency in days
Number of cercaria
released
Mean ± S.D.
1st 1009 ± 25
2nd 1400 ± 15
3rd 2000 ±27
4th 2400 ± 30
5th 2200 ± 20
6th 1600 ±15
7th 800 ±12
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29
Table 4
Incidence of different trematode larval infection to the
snailLymnaea acuminataduring infection period.
Sr.Number Cercaria of following
trematode
Infected snails in
%
1 Fasciola hepatica 43.25
2 Plagiorchisvespectilionis 20.84
3 Echinostome sp. 15.62
4 Pseudoechinoparyphium
sp. 10.41
5 Trichobilharziaocellata 07.55
6 Diplostomumhepaticum 02.33
Total 100.00
-
30
Table5
Snail host size dependence cercaria release during period of
patency in Lymnaea acuminata.
Snail size in mm
Number of Cercariae released during patency period in days Total
± S.D.
1 2 3 4 5 6 7
5±1 200±1 200±2 400±3 600±4 500±5 300±5 100±7 2300±27
10±1 300±2 500±3 700±2 1100±9 1300±7 700±8 500±5 5100±39
15±2 800±3 700±2 900±5 2000±7 1400±7 1000±8 600±6 7400±35
20±2 900±4 1100±6 2400±8 2200±9 1600±11 1000±9 500±6 9700±53
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31
Table 6
Effect of parasitic infection on mortality of host snail,
Lymnaea
acuminata.
Patency
period in
days
Survivalist of non-infected
snails in %
Survivalist of Infected snail
in %
1 98.21 97.23
2 96.43 95.51
3 96.35 94.32
4 94.16 92.34
5 93.67 88.11
6 89.52 78.70
7 83.54 65.62
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32
Table 7
Effect of trematode parasitism on shell length and total body
weight of
Lymnaea acuminata observed during patency period.
Phases of Patency
Period
Body shell length
(mm) Body weight (mg)
Control 18±1mm 225±2
Initial phase 20±1mm 230±2
Peak phase 22±1mm 245±3
Post phase 23±1mm 237±2
-
33
-
34
-
35
-
36
DISCUSSION
Many aquatic snails act as an intermediate host for larvae
of
trematode parasites. Some part of the life cycle is being spent
within the
body of snails as primary host. The adult form causes a number
of
dreadful diseases to man and his domesticated animals. In Japan
the
numbers of people at risk (600 million), infected (200 million)
and dying
(20,000 per year) from Schistosome infections (Oliveira et al.
2004).
From economical point of view worldwide losses due to
Fasciolasis are
conservatively estimated as some US $ 3.2 billion per annum
(Piedrafita
et al. 2004). Parasitic Platyhelminthes are important
economically and
socially and deserve much more attention. They deserve the
attention not
simply because they are fascinating animals that provide
challenges to
understanding associations of animals,but as model systems for
study of
any field of biology.
At the habitat of Lymnaeid snails which act as an intermediate
host,
free swimming miracidia of Fasciola hepatica approach toward
these
intermediate hosts by increasing their rate of change of
direction, when
they enter the areas of snails. A sort of chemosensory stimulus
is being
released into the water through mucus secretion from L.
acuminata
(Neuhans, 1941). These trematode larval pathogens possess a
remarkable
ability to discriminate among snail‘s species while approaching
them. This
has been reported by investigators working with miracidia of
liver fluke, F.
hepatica (Thomas 1883 and Neuhans, 1941 and 1953). Also it has
been
experimentally proved by Chipev (1993), by varying the number
of
introduced miracidia of F. hepatica in the vicinity of non-host
and target
snails, concluded that there must be a species - specific chemo
orientation
is involved at the time of infestation to primary intermediate
snail host.
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37
One of the most characteristic features of trematode-mollusc
interactions is their specificity. Despite their fact that
aquatic environments
are usually inhabitated by several snail species, each host
species is
normally found infected with specific larval trematode and never
with
others, despite the array of trematode miracidia present in the
environment.
This use of a restricted group of hosts by the parasite is a
phenomenon,
known as ―host specificity‖. In the present investigation the
intermediate
snail host L.acuminata, most of the time got invaded by only one
type of
cercaria.
The occurrence of multiple infections in nature may provide
a
possible mechanism for intermediate host switching, the process
by which
a trematode transfers to a new host. The miracideal penetration
of the
wrong host would normally result in elimination of the
incompatible
parasite, but if the invading parasite survives by overcoming
the snail
internal defence system, the infection may persist. The same
possibility
cannot be ruled out for present incidences of occurrence of two
types of
cercariae found invaded in the body of naturally infected snail
L.
acuminata by Plagiorchis and Echinostomum. Such type of
double
infection may interfere with snail internal defence system,one
factor that
might facilitate hosts shifts, in this way, there is presence of
other digenean
species infecting the same snail host. A suppressive effect from
one
parasitic infection may allow other parasites to eventually
adapt to a new
host.
During two years of study from the point of incidences of
cercarial
infestation in naturally infected snail L. acuminata, found
heavily invaded
by larval trematodes during post monsoon period i.e. during
September
and October 2009 and 2010. Choubisa (2008) while screening
freshwater
gastropod snail from the point of larval trematode infection in
the tribal
region of Southern Rajasthan, reported that the most favourable
season for
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38
cercarial infection was found during late rainy or prewinter
season. During
this season more than 95 % matured snail species found release
of different
kinds of cercaria like the present intermediate snail host
Lymnaea from
Aurangabad.
Total length of the cercarial shedding period is called as
period of
patency. The shedding period begins with a ―flushing – out of
larvae from
the naturally infected snail body. In L. acuminata cercarial
release starts
with moderate number of cercariae on first two days with
gradual
increasing order up to 5th
day, which is peak phase of cercarial shedding.
Similar pattern of larval shedding has been observed by Elon and
George
(1954) while working on experimentally infested positive
snail
Physagyrina in the laboratory. Usually a moderately large number
was
released on the first day followed by more or less steady
increase in
number the next 2 or 3 days, culminating or attaining peak
initial peak
number on 4th
or 5th
day. In the present investigation ―flushing – out‖
period of the number of cercarial release for further two to
three days
declined to establish a lowest level of count is referred as
post phase of
patency.
L. acuminata is found distributed both lentic and lotic type of
water
body and is having a surface dwelling habit. May be because of
this fact it
is found invaded by diversified species of trematode cercarial
larvae has
been reported by Choubisa (2008) in various genera of the
family
Lymnaeidae. Many workers have reported the diversity of the
snails of
lentic habitats and their pathogenic cercarial fauna from
different
geographical areas. He also found that the larval digenean
infection in
surface dwelling snails from the lentic environment especially
in the
perennial ponds or reservoirs was higher than those of bottom
dweller
species in the same habitat. The present snail L. acuminata may
be due to
surface dwelling habit shows maximum diversity of trematode
cercarial
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39
larvae. In the life cycle of the digenetic trematodes the radial
stage is a
very important step, it could represent a form of resistance to
unfavorable
environmental conditions, raising generations of daughter redia
during
intramolluscan development. Location and structure of rediae
of
Echinostomaperaensei collected from intermediate host
Lymnaea
collumella (Pinheiro et al. 2004) are similar to the present
finding on redia
present in Lymnaea acuminata. The mature rediae collected from
L.
acuminata are colorless and are located in the peripheral region
of the
digestive gland.
Randomly collected snail species of L. acuminata, size
ranging
from 5-22 mm shell length were found naturally infected by
various types
of larval trematodes after getting crushed the animals. It has
been observed
that there is a specific relationship between size of the snail
and infestation
by pathogens of larval trematodes during intense infestation
period i.e. in
the months of September and October of two subsequent years of
the
study. The numbers of cercariae shed from the naturally infected
snails
were quantified in the laboratory. Small sized infected snails
released less
number of cercariae compared with moderately sized (20 mm shell
length)
infected snails during their total period of patency. Age of the
mollusc is
probably a controlling factor for many species of miracidia.
Among the
blood flukes, Schostosomatidae, Spirorchidae and in
Paragonimuskellikotti
(Troglotrematidae), also in Clinostomummarginatum
(Clinostomatidae)
many experiments have shown that young snails from a few days to
a few
weeks are most readily penetrated by miracidia. Miracidia of
some species
rarely or never penetrates snails over 2-3 months of age. But
this limitation
does not appear to exist in certain other species. In the
present
investigation it has been observed and confirmed that number of
larval
trematodes emerged is directly proportional to the size of the
snail host.
-
40
The analysis of prevalence in relation to maturity and age
produces a
more complicated picture for two reasons - First, high
prevalence of
trematode infections in large sized host have to be attributed
to host
maturity, because the trematode larvae exclusively invaded
the
reproductive organs of the adult individuals. The gonadal tissue
was
completely replaced by sporocysts or rediae, which corresponds
with
Ankel‘s (1962) findings. Second, the trematode prevalence in
mature
individual increased with size suggested by Probst and Kube,
(1999).
Ballabeni (1995) hypothecated experimentally parasite induced
gigantism
in hermaphrodite mollusc Lymnaea peregra infected with the
trematode
Deplostomumphoxini. The present pond snail L. acuminata infected
in
nature also show slight difference in their shell size when
observed during
period of patency. The parasitically infected snail suffers
total reproductive
inhibition caused by parasites like the prediction derived by
Minchella
(1985) and Ballabeni (1995). Due to parasitic infection by
larval trematode
there is increased host mortality rate, may be because of this
fact there is
no gigantism resulted, though there is slightly increased in
shell length of
infected snails than normal non-infected host snails. It is well
known that
trematode infection influence the growth rate of molluscs.
Chernin (1960)
and Pan (1963 and1965) reported that experimental infections
with
Schistosomamansoni accelerated the growth rate of
Australorbisglabratus.
The effect of infection can however be influenced by the age of
the
host. Thus significant in shell size and body weight occur in
Lymnaea
stagnalisonly if the trematode infection is established at an
early stage
(Joosse, 1964). Away from the results of Pan (1965) gigantism is
observed
in the present investigation on naturally infected L.
acuminatasnail
compared to non-infected normal host snails.
Whitlock et al. (1977) made a comparison of two laboratory
method
of maintaining field collected Lymnaea tomentosa for the
production of F.
-
41
hepaticametacercariae. It has been observed that the number
of
metacercariae recovered per snail dissected increase with in
initial shell
length up to 8 mm. Naturally infected various sized of snails of
L.
acuminata showed increased number of cercaria with increase in
size of
snails.
Many workers have reported the diversity of the snails of
lentic
habitats and their pathogenic cercarial fauna from different
geographical
areas (Pandy and Agrawal, 1978; Choubisa and Sharma, 1983).
The
digenean trematode Diplostomumspathaceum is well known parasite
in
fishes where it occurs as metecercercaria in the eye lenses of
the host and
often causes parasitic cataract (Sheriff et al.1980). While
screening the
present freshwater snail L.acuminata from the point of larval
trematode
infections, it has been observed that the cercarial stage of
Diplostomumwere found invaded in the body of snails collected
from
Salim Ali Lake Aurangabad. Fish host may be the definitive host
of this
trematode larval pathogen because many fishes are the
inhabitants of this
lake as associate animals.
Lake water inhabitants of snail L. acuminata in the present
study
found heavily infected with cercarial larval trematodes compared
with
snail species collected from lotic water dwellers.
Wealth of information is available on morphology and various
types
of larval trematode stages such as furcocercous cercaria were
described by
Azim (1935), Xiphidiocercaria by Azim (1936) EL- Gindy and
Hanna
(1963) and Sakla and Khalifa (1981) and Pleurolophocercous
cercariae
were described by Khalifa et al. (1977) and Fahmy et al. (1986)
in Egypt.
Recently Yousif and his co-workers (2010) described morphology
of new
eleven types of cercariae from prosobranch snail,
Melanoidestuberculatain
Egypt. Of these cercariae they have describe two new types of
cercariae
which were released from M. tuberculatus. Based on variations
in
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42
superficial morphological characteristic features some ten
different types
of cercarial forms have been noticed in the naturally infected
present
intermediate host snail L. acuminata. Similar type of
observations were
made by Cort (1914), while working on larval trematodes, some 14
new
species of cercaria have been collected from freshwater snails
from
different localities throughout the United States and from
various
ecological situations.
The emergence of two types of cercariae from single snail host
is
referred as a case of double infection. Such type of incidence
occurred
rarely in present study. Two types of furcocercariae have been
found
infested in L. acuminata at one or two occasions of naturally
infected
snails may be due to identical host- specificity of two
different digenetic
trematode parasites. Two types of furcocercariae namely
Trichobilharzia
and Diplostoma have been found infesting different snail
specimens of L.
acuminata, collected from same locality the city Aurangabad
(M.S.).
These two furcocercous cercaria never found infesting together
in the same
animal during two years of study period. Similar type of
infection have
been also observed by Mukharjee (1966); Jain (1970);Pande and
Agrawal
(1978) and Choubisa (1986b).
The present study provides to estimate larval trematode
parasites
among lymnaeids snails and their zonotic importance in animal or
human
health.