Toxoplasmosis and Transmission of Toxoplasma gondii

INTRODUCTION

Toxoplasmosis is a parasitic zoonotic infection found

throughout the world, commonly carried by cats and other

animals (Asad et. al., 2006), it is caused by a zoonotic obligate

intracellular protozoan parasite; Toxoplasma gondii, which has the

capacity to infect all warm-blooded animals. Generally, T. gondii

is among the most prevalent parasites in the global human

population, Generally, it is estimated that about one third of

the World’s population is infected with T. gondii (Tenter, et. al.,

2000; Dubey and Jones, 2008; Swai and, Schoonman, 2009). T.

gondii is an opportunistic parasitic infection in immune

compromised hosts (Ferreira and Borges, 2002).

Geographical Distribution

Prevalence of the infection varies widely, depending on social

and cultural habits, geographic factors, climate, and

transmission route. It has been reported that the prevalence

is higher in warm and humid areas (Studeničová, et. al., 2006)

and in areas where raw or undercooked meat is traditionally

eaten.

High prevalence of the infection have been reported among

pregnant women and women of childbearing age from different

foci in Latin America, parts of Eastern/Central Europe, the

Middle East, parts of south-east Asia and Africa (Pappas et. al.,

2009). However, the prevalence of T. gondii in pregnant women in

China was less than 10% (Gao, et. al., 2012). In Africa, overall

seroprevalence rate as high as 92.5% has been reported (Ayi,

et. al., 2009). Most pregnant women infected with T. gondii are

chronically infected while few acquire the infection during

pregnancy (Nowakowska et. al., 2006). Pregnant women with acute

infection during pregnancy are at risk of congenitally

transmitting the infection to the foetus. Felids play a

crucial role in the epidemiology of this parasitic disease

because they are the only definitive hosts, shedding and

excreting millions of infective oocysts in a short period of

time in their faeces (Dubey, and prowell, 2013).

In most adults it does not cause serious illness, however,

blindness and mental retardation can be caused in congenitally

infected children and severe diseases in those with

compromised immunity. A recent study indicated that infection

with T. gondii is associated with abdominal hernia (Alvarado-

Esquivel and Estrada-Martínez, 2011).

Congenital Toxoplasmosis

T. gondii infection acquired by pregnant women during gestation

and its transmission to the foetus continue to be the cause of

tragic yet preventable disease in the offspring (Remington et.

al., 2006). It has been estimated that 500–5000 infants each

year are born with congenital toxoplasmosis in the United

States (Roberts and Frenkel, 1990). Although the majority of

infants appear to be healthy at birth, significant long-term

sequelae may become obvious only months or years later. Most

pregnant women with acute acquired infection do not experience

obvious symptoms or signs (Remington, et. al., 2006; Boyer,

et. al., 2005). A minority may experience malaise, low-grade

fever, and lymphadenopathy. Rarely, pregnant women will

present with visual changes due to toxoplasmic chorioretinitis

(Garweg et. al., 2005). Congenital toxoplasmosis (CT) occurs

in infants following maternal transmission. It can result in

foetal death and abortion and in syndromes that include

neurologic and neurocognitive deficit One of the late sequelae

of congenital toxoplasmosis is chorioretinitis (Al-Azawi et al.,

2013). It could also cause mental retardation, blindness,

epilepsy, and death (Petersen, 2007).

Toxoplasmosis could be severe and life-threatening during

pregnancy, and to foetuses, and new born babies (Robert-

Gangneux et al., 2009).

Toxoplasmosis in Immuno-compromised people

Among the immuno-competent people, toxoplasmosis is usually

asymptomatic, subclinical or benign, and can be classified as

congenital, acquired or ocular (Oyibo et al., 2009). It could

result in the chronic persistence of cysts within host

tissues; the cysts normally lie dormant, probably for life

(Malla, et. al., 2005). However, it can be severe and life-

threatening to immune-compromised patients (Robert-Gangneux et

al., 2009), causing severe encephalitis through acute infection

or reactivation of latent infection (Innes, 2010; Hang et al.,

2007). It may precursor spontaneously resolved symptoms such

as fever, malaise, and lymphadenopathy, indicating symptomless

latent infection (Montoya and Liesenfield, 2004). Immune

compromised patients such as in HIV infections, subjects are

at risk of developing acute toxoplasmosis due to reactivation

of the organism if their CD4+ T-cell count decreases below 200

cells/μL (Martinez, et. al., 2002; Jayawardena, et. al.,

2008). Since the pandemic of HIV infection has spread

throughout the world, toxoplasmosis has been implicated as one

of the most important opportunistic infections in HIV/AIDS

patients (Nissapatorn, et. al., 2002; Montoya and Remington,

2000; Walker and Zunt, 2005). Moreover, in up to 10% of HIV

infected immune competent individuals, it causes cervical

lymphadenopathy or ocular disease (Walker and Zunt, 2005).

Ocular Toxoplasmosis

This occurs when the parasite enters the eyes. If parasites

reach an eye and they yield a focus of inflammation, the

lesion is progressed to retinitis and involves the choroid

secondarily. Immune responses of the host appear to induce

conversion of the parasitic forms, from tachyzoites to

bradyzoites and their encystment. The cyst may remain inactive

in the scar or nearby for a long time. However, when the cyst

ruptures with release of organisms into the surrounding

retina, retinitis may be reactivated. The reactivation of

retinitis is known to develop at the border of old scars and

is attributed to the rupture of tissue cysts which are located

within old lesions. Sometimes, however, new lesions are found

at locations distant from old scars. Some studies have

suggested a possible route of infection from the brain to the

eye through the optic nerve; however, now ocular infection is

most likely mediated via the bloodstream (Park and Nam, 2013)

Immunity

Both cellular and humoral immune responses are associated with

toxoplasmosis. Acute infections produce IgA and IgM. The

occurrence of IgG in individuals indicates prior infection

with T. gondii. IgE antibodies severe infection (Otubanjo, 2013).

Diagnosis

Diagnosis is usually achieved by the following methods

Serology: Serological tests involves detecting specific

antibodies like IgM and IgG through ELISA.,

The Sabin-Feldman dye test

Tissue cysts may be observed in stained biopsy specimens.

Diagnosis of congenital infections can be achieved by

detecting T. gondii

Ploymerase chain reaction (PCR): DNA in amniotic fluid, cerebrospinal fluids, blood samples etc. can be detected using PCR

Treatment

Treatment is with combination of sulfadiazine and pyrimethamine.

TRANSMISSION OF T. gondii

T. gondii exists in 3 forms, all of which are possible to infect

hosts as a form of zoonosis. These forms are:

1. Tachyzoites; which can infect almost all nucleated cells

through a process of active invasion,

2. Tissue cysts (containing bradyzoites); which are formed

primarily in the brain and skeletal muscles during the chronic

phase of infection, and

3. Oocysts which are produced during the sexual cycle that

takes place in the intestine of acutely infected felines

(Tenter, et. al., 2000)

The routes of infection are therefore:

By ingestion of tissue cysts from undercooked or raw meat

(primarily pork and lamb) (Dubey, 2004; Dubey, 2010;

Dehkordi et. al., 2013) .

Direct contact with cats and owning of pets. This will

increase the proximity of humans to the cats (which

excrete oocysts with their faeces) and pets like dogs

have been found to be transport hosts of the parasite.

Consumption of water or food contaminated by oocysts

excreted in the faeces of infected cats (Montoya and

Liesenfeld, 2004).

Recently, T. gondii has been reported in many marine

mammals, suggesting the possibility that the

contamination of seawater with T. gondii may be more common

than realized (Dubey et al., 2003b).

Congenital infection; this may occur following maternal

infection during pregnancy.

Oocysts in soil can be spread mechanically by flies,

cockroaches, dung beetles, and earthworms.

Humans may also acquire toxoplasmosis by petting dogs

that have rolled over in infected cat faeces (Frenkel et

al., 1995; Lindsay et al., 1997). T. gondii infection in dogs

are important because the infection can cause serious

illness in dogs. It is important to note that dogs can

also be transport hosts for T. gondii oocysts and dog meat

is consumed by humans in several countries (Liu, et. al.,

2012).

Through organ transplant; Organ transplant recipients can

develop toxoplasmosis due to transmission of the parasite

with the transplanted organ from a Toxoplasma-seropositive

donor to a Toxoplasma-seronegative recipient. Heart

transplantation is the most common type of organ

transplantation procedure when this occurs, as cysts form

in the cardiac muscles (Martina et al. 2011; Derouin and

Pelloux 2012). However, toxoplasmosis is an uncommon

outcome from organ transplantation as only 5% of human

pathogenic parasites have reportedly caused significant

illness in transplant recipients (Barsoum 2006). It is

also possible that parasite transmission could occur as

the result of blood transfusion or haematopoietic stem

cell transplantation. The chance of either of these

occurring is very low and could only occur if the donor

was recently infected with T. gondii and so had tachyzoites

present in their blood and bone marrow (Derouin and

Pelloux 2012).

There is also pseudo-vertical transmission through the

milk, and sexual transmission through the sperm (Dubey,

2010; House, et. al., 2011; Dass, et. al., 2011).

LIFE CYCLE OF T. gondii

The organism undergoes both schizogonic and sexual cycles

typical of coccidians. It consists of two cycles which are the

intra-intestinal (enteric) cycle and Extra-Intestinal (exo-

enteric) cycle.

The Intra-intestinal cycle is characterized by a direct,

monoxenous life cycle in the intestine of the natural feline

definitive host, the domestic cat, and other feline host of

the genera; Felis and Lynx. This involves several schizogonic

cycle followed by gametogony (sexual reproduction. Sexual

reproduction occurs only in the cat.

The Extra-intestinal cycle is a heteroxenous cycle, involving

other mammalian hosts as intermediate hosts and also, the

feline host with strictly asexual reproduction. It involves

oocysts, sporozoites and tissue cysts bradyzoites.

Infection of the feline definitive host occurs when a cat

consumes an intermediate host (such as a mouse or bird)

infected with tissue cysts. Upon ingestion of a tissue cyst by

a susceptible cat, the walls of the cyst are digested by

proteolytic enzymes and bradyzoites are released. The

bradyzoites undergo asexual reproduction followed by sexual

reproduction in intestinal epithelial cells to produce

microgametocytes and macrogametocytes. The microgametocytes

fertilise the macrogametocytes, leading to the production of

zygotes. The zygotes differentiate into unsporulated oocysts

and are shed in the faeces of the definitive host (Ortega

2007; Jones and Dubey 2010).

After a prepatent period of up to 10 days following primary

infection with tissue cysts, a cat may shed more than 100

million oocysts into the environment over a 2-3 week period

(Tenter et al. 2000).

Figure 1: Life cycle of Toxoplasma gondii. (Source: Black and

Boothroyd (2000))

Factors influencing Transmission of T. gondii

There are several factors that ensure successful transmission

of T. gondii and they could be classified into parasite, host

and environmental factors and they are listed below:

Parasite factors

Toxoplasma gondii is considered as one of the most successful

parasites in the world.

This success is first illustrated by the worldwide

distribution of the parasite, from arctic to hot desert

areas, including isolated islands and in cities (Dubey,

2010).

The parasite also has the ability to affect host’s

behaviour. Infected rats are less fearful of the scent of

cats, thus making them readily available for predation by

the feline hosts. Infected humans are also prone to

behaviour changes such as hallucinations and reckless

behaviour (Otubanjo, 2013).

The parasite is able to infect, or be present in, the

highest number of host species: any warm-blooded animal

may act as an intermediate host, and oocysts may be

transported by invertebrates such as filtrating mussels

and oysters (Dubey, 2010; Miller, et.al., 2008).

Millions of infective oocysts can be shed and excreted

in a short period of time in the faeces of the cat (which

is the only definitive host) (Dubey, and prowell, 2013).

The ability of oocysts to survive for long periods.

Oocysts are known to survive on fruits and vegetables for

long periods (Kniel et al., 2002). Also, Sporulated oocysts

may survive during several years and may disperse through

water movements, soil movements and microfauna

High Infectivity of the parasite. Ingesting a single

sporulated oocyst may be sufficient to infect an

Intermediate Host and begin the asexual reproduction

phase (Dubey, 2010). This classical life cycle thus

relies on a prey predator relationship and on

environmental contamination, like other parasites

(Deplazes, et. al., 2004).

Host Factors

The presence of proteolytic enzymes in the gut of the cat

that helps to digest the walls of the cysts to release

the tissue cyst bradyzoites.

Predatory behaviour of cats, that is, cats could become

infected when they feed on rats whose tissues contain

tissue cysts (bradyzoites).

Socio-cultural practices of humans, that is, the practice

of eating raw or poorly cooked meat. It has been reported

that the prevalence is higher in areas where raw or

undercooked meat is traditionally eaten.

Owning pets like cats and dogs.

Maintaining poor personal hygiene, such as the habit of

not washing hands after handling pets.

The physiological state of the host. Studies have shown

that people most at risk of developing clinical symptoms

include immune-compromised individuals, pregnant women

who acquire (or have a reactivation of) the infection

during gestation, foetuses that are congenitally infected

and individuals who have previously been infected in utero

(Tenter et al. 2000; Jones et al. 2003; Montoya and

Liesenfeld 2004; ESR 2010).

Environmental Factors

The following conditions have been seen to affect the length

of time required for sporulation to occur,

Temperature and

High farm density.

Poor environmental sanitary conditions.

Temperature: It has been reported that the prevalence is

higher in warm and humid areas (Studeničová, et. al., 2006) and

in areas where raw or undercooked meat is traditionally eaten.

Studies have shown that lower temperatures slow the

sporulation rate (Lindsay et al. 2002; Jones et al. 2003; Hill et

al. 2007). A study conducted by Lindsay (2002) demonstrated

that unsporulated oocysts can survive in the environment at

4°C and retain their ability to sporulate for at least 3

months. Sporulated oocysts are more resilient than

unsporulated oocysts. Once sporulated, oocysts maintain

infectivity in moist soils for up to 18 months and in water

and seawater for several years at 4°C (Lindsay and Dubey

2009). The duration of infectivity, however, decreases with

increasing temperatures. Infectivity is maintained for at

least 200 days in the temperature range of 10–25°C, for 1

month at 35°C, for 1 day at 45°C and sporulated oocysts become

non-infective after 1 minute at 60°C (Dubey 1998a).

Unsporulated oocysts die within 24 hours when stored at 37°C,

whereas sporulated oocysts can survive for over a month at

35°C and 9 days at 40°C (Lindsay et. al. 2002). Constant freezing

at -21°C kills unsporulated and sporulated oocysts within 1

and 28 days, respectively (ESR 2010). T. gondii tissue cysts

remain viable in infected meat stored at refrigeration

temperatures of 4°C for up to 19 days. Cooking infected meat

to internal temperatures of 67°C or higher inactivates the

tissues cysts (Dubey 2004). Freezing meat at -10°C for 3 days

or -20°C for 2 days or treatment with gamma irradiation at a

dose of 75 krad is also sufficient to kill tissue cysts (El-

Nawawi et al. 2008). Tachyzoites that may be found in the milk

of intermediate hosts are inactivated by pasteurisation

(Tenter et al. 2000). Studies have shown that farming areas

and weather conditions such as cool and wet winters are

associated with increased T. gondii seroprevalence in cats. As

cat infection determines the environmental contamination by

oocysts, climate and landscape characteristics should be taken

into account to improve the risk analysis and prevention of T.

gondii (Afonso, et. al., 2013).

High farm density: Research has shown that areas with high

farm density are high risk places for the prevalence of T. gondii

because farms are favourable areas for the presence of

domestic cats (Turner and Bateson, 2000).

Poor Environmental Sanitary Condition: Environmental

condition such as contamination of the environment with

faeces of cats containing oocysts increase the chance of

infection.

REFERENCES

Afonso, E., Germain, E., Poulle, M., Ruette, S., Devillard,S., Say, L., Villena, I., Aubert, D., and Gilot-Fromont, E.(2013). Environmental Determinants of Spatial and TemporalVariations in the Transmission of Toxoplasma gondii in itsDefinitive Hosts. International Journal for Parasitology: Parasites andWildlife 2:278–285

Al-Azawi, A.K.A., Al-Rawe, I.H.A., and Al- Bayati, R.Y.J.(2013). Seroprevalance of Toxoplasmic chorioretinitis inBaghdad Province. International Journal of Science and Nature 4(1),68-71.

Alvarado-Esquivel, C., and Estrada-Martínez, S. (2011).Toxoplasma gondii Infection and Abdominal Hernia: Evidence of aNew Association. Parasit Vectors 4:112

Asad, Y., Abuodeh, M.D., Basem, S., and Hattar, M.D. (2006).Concurrent Malaria and Toxoplasmosis: A Case Report. JRMS;13(1): 48-50.

Ayi, I., Edu, A., Apea-Kubi, K., Boamah, D., Bosompem, K., andEdoh, D. (2009). Sero-epidemiology of Toxoplasmosis amongstPregnant Women in the Greater Accra Region of Ghana. Gh MedJ, 43:107-114

Berenreiterova, M., Flegr, J., Kubena, A.A.,and Nemec, P.(2011). The Distribution of Toxoplasma gondii Cysts in theBrain of a Mouse with Latent Toxoplasmosis: Implicationsfor the Behavioral Manipulation Hypothesis. PLoS One 6:e28925.

Black, M.W., and Boothroyd, J.C. (2000). Life Cycle ofToxoplasma gondii. Microbiol. Mol. Biol. Rev. 2000, 64(3):607.

Boyer, K.M., Holfels, E., Roizen, and N., (2005). Risk Factorsfor Toxoplasma gondii Infection in Mothers of Infants with

Congenital Toxoplasmosis: Implications for Pre-natalManagement and Screening. Am J Obstet Gynecol. 192:564–71.

Dass, S.A.H., Vasudevan, A., Dutta, D., Soh, L.J.T., Salposky,R.M, and Vyas, A. (2011). Protozoan Parasite Toxoplasma gondiiManipulates Mate Choice in Rats by Enhancing Attractivenessof Males. PLoS ONE 6: 27229.

Deplazes, P., Hegglin, D., Gloor, S., and Romig, T. (2004).Wilderness in the City: The Urbanization of Echinococcusmultilocularis. Trends parasitol 20: 77-84.

Dubey JP (1998a) Toxoplasma gondii Oocyst Survival Under Defined Temperatures. Journal of Parasitology 84(4):862–865.

Dubey, J.P., Zarnke., R., Thomas, N.J., Wong, S.K., Van Bonn,W., Davis, J.W., Ewing, R., Mense, M., Kwok, O.C.H.,Beckmen, K.B., Romand, S., and Thulliez, P., (2003b).Toxoplasma gondii, Neospora caninum, Sarcocystis neurona, andSarcocystis canis-like Infections in Marine Mammals. Vet. Parasitol.116, 275–296.

Dubey, J.P. (2010). Toxoplasmosis of animals and humans, 2ndedition. Boca Raton: CRC Press. 313 p.

Dubey, J.P., and Jones, J.L. (2008). Toxoplasma gondii Infectionin Humans and Animals in the United States. Int J Parasitol.38:1257-1278

Dubey, J.P., and Prowell, M., (2013). Ante-mortem Diagnosis,Diarrhoea, Oocyst Shedding, Treatment, Isolation andGenetic Typing of Toxoplasma gondii Associated with ClinicalToxoplasmosis in a Naturally Infected Cat. J. Parasitol.99(1):158-160).

El-Nawawi, F.A., Tawfik, M.A., and Shaapan, R.M. (2008)Methods for Inactivation of Toxoplasma gondii cysts in Meat andTissues of Experimentally Infected Sheep. Foodborne Pathogensand Disease 5(5):687–690

ESR (2010) Toxoplasma gondii. Ministry for Primary Industries, New Zealand.

Ferreira, S.M., and Borges, S.A.(2002). Some Aspects ofProtozoan Infections in Immune Compromised Patients – Areview. BioLine Int System, 97(4):443–457.

Gao, X.J., Zhao, Z.J., He, Z.H., Wang, T., Yang, T.B., Chen, X.G., Shen, J.L., Wang, Y., Lv, F.L., Hide, G., and Lun, Z.R. (2012). Toxoplasma gondii Infection in Pregnant Women in China. Parasitology, 139:139-147

Gilot-Fromont, E., Lélu, M., Dardé, M.L., Richomme, C.,Aubert, .D., Afonso, E., Mercier, A., Gotteland, C., andVillena, I. (2012). The Life Cycle of Toxoplasma gondii in theNatural Environment.

Hill, D.E., Sreekumar, C., Jones, J., and Dubey, J.P. (2007) Toxoplasma gondii. Ch 12 In: Simjee S (ed) Foodborne diseases.Humana Press, Totowa, p. 337–353

House, P.K., Vyas, A., and Sapolsky, R. (2011). Predator CatOdours Activate Sexual Arousal Pathways in Brains ofToxoplasma gondii Infected Rats. PLoS ONE 6: 23277.

Innes, E.A. (2010). A brief history and overview of Toxoplasmagondii. Zoon Pub Heal. 57, 1–7.

Jayawardena, S., Singh, S., Burzyantseva, O., and Clarke, H.(2008): Cerebral Toxoplasmosis in Adult patients with HIVinfection. Clin Med J Resid Hosp Physician 44(7):17–24.

Jones, J., Lopez, A., and Wilson, M. (2003). Congenital Toxoplasmosis. American Family Physician 67(10):2131–2138.

Lindsay, D.S., and Dubey, J.P. (2009). Long-term Survival of Toxoplasma gondii Sporulated Oocysts in Seawater. Journal of Parasitology 95(4):1019–1020

Lindsay, D.S., Blagburn, B.L., and Dubey, J.P. (2002) Survivalof Non-sporulated Toxoplasma gondii Oocysts Under RefrigeratorConditions. Veterinary Parasitology 103(4):309–313.

Malla, N., Sengupta, C., Dubey, M.L., Sud, A., and Dutta, U:(2005). Antigenaemia and Antibody Response to Toxoplasmagondii in Human Immuno deficiency Virus Infected Patients. BrJ Biomed Sci 28:104–109.

Martinez, E., Mago, H., Rocha, R., and Pacheco, M. (2002):Epidemiological findings and Prevalence of Toxoplasma gondiiantibodies in HIV-positive patients in a Venezuelanhospital. Valencia Int Conf AIDS . 7–12:14.

Miller, M.A., Miller, W.A., Conrad, P.A., James, E.R., Melli,A.C., Leutenegger, C.M., Dabritz, H.A., Packham, A.E.,Paradies, D., Harris, M., Ames, J., Jessup, D.A.,Worcester, K., and Grigg, M.E. (2008). Type X Toxoplasmagondii in a Wild Mussel and Terrestrial Carnivores fromCoastal California: New Linkages between TerrestrialMammals, Runoff and Toxoplamosis of sea otters. Int. j. parasitol58: 928-937.

Montoya, J.G., and Remington, J.S. (2000): Toxoplasma gondii. InPrinciples and Practice of

Infectious diseases. 5th ed. Edited by Mandell, G.E.,Beneth, J.E., Dolin, R.Odon: Churchill Livingstone;2000:2858–2888.

Montoya, J.G., and Liesenfeld, O. (2004). Toxoplasmosis.Lancet, 363:1965–1976.

Nissapatorn, V., Kamarulzaman, A., Init, I., Tan, L.H.,Rohela, M., Norliza, A., Chan, L.L.,

Latt, H.M., Anuar, A.K., and Quek, K.F. (2002):Seroepidemiology of Toxoplasmosis among HIV- infectedPatients and Healthy Blood Donors. Med J Malaysia, 57(3):304–310.

Nowakowska, D., Stray-Pedersen, B., Spiewak, E., Sobala, W.,Małafiej, E., and Wilczyński, J. (2006). Prevalence andEstimated Incidence of Toxoplasma infection Among PregnantWomen in Poland: a decreasing trend in the youngerpopulation. Clin Microbiol Infect 12:913-917

Otubanjo, O., (2013). Elements of Parasitology, Panaf Publishers,Inc., Nigeria, Pp. 169-171

Oyibo, W.A., Oladosu, O.O., Agomo, C.O., Ojuromi, O.T.,Anunobi, C.C., and Soyebi, K. (2009). CongenitalToxoplasmosis: A Review of its Pathology, Immune Response

and Current Treatment Options. Sierra Leone J Biomed Res 1 (1),9-20,

Pappas, G., Roussos, N., Falagas, and M. E.(2009).Toxoplasmosis Snapshots: Global Status of Toxoplasma gondiiSeroprevalence and Implications for Pregnancy andCongenital Toxoplasmosis. Int J Parasitol 39: 1385-94

Park, Y.H., and Nam, H.W. (2013). Clinical Features andTreatment of Ocular Toxoplasmosis. Korean J Parasitol. 51(4):393–399.

Petersen, E. (2007). Toxoplasmosis. Semin Fetal Neonatal Med, 12,214–223.

Qing-Xin, L., Shuai, W., Li-Qun, W., Jun, X., Wen-Jue, G.,Guo-Fang, L., Bin, Z., Hai-Bin, Z., and Li-Hua, G. (2014).Seroprevalence of Toxoplasma gondii Infection in Dogs and Catsin Zhenjiang City, Eastern China. Asian Pac J. Trop Biomed 4(9):725-728

Remington, J.S., McLeod, R., Thuilliez, P., and Desmonts, G.,(2006). Toxoplasmosis. In: Remington, J.S., Klein, J.O.,Wilson, C.B., and Baker, C., (eds.). Infectious Diseases ofthe Foetus and New-born Infant. 6th ed. Philadelphia:Elsevier Saunders, 947–1091.

Robert-Gangneux, F., Year, H., D'Herve, D., Guiguen, C.(2009). Congenital Toxoplasmosis after a Preconceptional orPericonceptional Maternal Infection. Pediatr Infect. Dis. J. 28,660-661

Studeničová, C., Benčaiová, G., and Holková., R. (2006).Seroprevalence of Toxoplasma gondii Antibodies in a healthypopulation from Slovakia. Eur J Intern Med . 17:470–473.

Swai, E.S., and Schoonman, L. (2009). Seroprevalence ofToxoplasma gondii Infection Amongst Residents of TangaDistrict in North-East Tanzania. Tanz J Hth Res, 11(4):205–209.

Tenter, A.M., Heckeroth, A.R., and Weiss, L.M. (2000).Toxoplasma gondii: From Animals to Humans. Int. j. parasitol. 30:1217-1258.

Turner, D.C., and Bateson, P.B., (2000). The Domestic Cat. TheBiology of its Behaviour.

Cambridge University Press, Cambridge.

Walker, M., and Zunt, J.R. (2005): Parasitic central nervoussystem infections in Immunocompromised hosts. Clin Inf Dis40:1005–1015.