How do mosquitoes find their host

How Does Mosquito Detect Human Being PRESENTED BY ANTU BHATTACHARJEE

BARASAT GOVT. COLLEGEWEST BENGAL STATE UNIVERSITY

MSC. 1ST SEM Reg no.-002559 of 2013

ROLL : Bgc/zool NO: 14105Year-2014

INTRODUCTION Mosquitoes are small, long-legged, two-

winged insect have been around for more than 30 million

years. Mosquitoes that act as disease vectors rely

upon olfactory cues to detect humans for blood feeding.

Female mosquitoes feed on vertebtrate(eg. human,cow,bird) blood for egg

laying.

PURPOSEMosquitoes transmit pathogens that cause

diseases such as dengue, yellow fever, filariasis and most significantly human malaria.

The dual receptor(designated as cpA(Gr 1,2,3) of maxillary palp of mosquito of CO2 and skin-odorants as a key target that could be useful to disrupt host-seeking behavior and thus aid in the control of disease transmission.

HOST FINDING PROCEDURE:Female mosquitoes are attracted to us by

smelling the carbon dioxide(CO2) we exhale, being capable of tracking us down even from a distance.

But once they get close to us, they often steer away toward exposed areas such as ankles and feet, being drawn there by skin odors.

Olfactory Parts and Tissues of MosquitoThe antenna,maxillary palp and the labellum(tip of

proboscis) of the mosquito are the three sides of mosquito

sensing.Sensory organs called sensilla cover these tissues and house Odoront Sensory Neurons (ORNs).

The graph clearly shows that a mosquitoes ability to find a host is greatly reduced upon the removal of antennae or maxillary palp or proboscis

Odorant-binding Protein (OBP)OBP genes encode small ,globular,hydrophilic

proteins(14-20 KDa and 11KDa respectively) that are excreted in significant amount out of the cell into the peri-receptor space.

OBPs in the mosquito’s antennae and mouth parts function in a way to concentrate odorant molecules and help carry them to the insect’s actual olfactory receptors, leading to odorant detection.

NOTE: Mosquitoes have higher concentrations of these scent-detecting substances in their sensory organs at night.That’s why mosquitoes detect human scent better at night

Sensory Neuron Membrane proteins (SNMPs)

SNMPs was first described as a pheromone binding protein in the antenna.

They are membrane receptors ,and are directly related to CD36 family fatty acid transporter receptors which have been shown to bind a number of ligands in various biochemical pathways .

Twelve to fourteen CD36 like genes have been identified in the genome of Ae. aegypti and An. gambiae.

The mosquito antennae which is the principle adult olfactory appendage,remains the least understood is so far as the molecular basis of olfactory physiology.

There is a growing appreciation of antennal expression patters of several genes that encode signal transduction components

Olfactory Properties of Mosquito Antennae

Olfactory Properties of Mosquito Labellum

The proboscis is a long slender oragn housing the feedind stylets of the adult mosquito, harbouring numerous chemosensory sensilla at its bulbous tip known as labellum.

Three morphologically and functionally different labellar sensilla have been identified.TYPE –I(T1) and TYPE-II(T2) sensilla are found on the external surface of the labellam while the TYPE-III(T3) is present on the oral surface.All three types are sensitive to taste stimuli and several odorant stimuli including butylamin and several aliphatic carboxylic acids.

Fig: The labellum of Anopheles gambiae exhibits olfactory responses.Schematic dorsal view of mosquito head and brain,anterior(A),posterior(P),dorsal(D) and ventral axis,as indicated.(A) neurons of the labellum project to the posteromedial region of the Antennal Lobes(AL) in the brain.B a subset of T2 sensilla on the labellum respond to odorant stimuli in single sensillum(SSR) experiments.Coincidely the T2 sensilla express OR6 and OR7.SOG: Sub Esophageal Ganglion.OL:Optic Lobe

Olfactory Properties of The Maxillary Palp

The maxillary palp of mosquitoes consists of five segments.

The maxillary palp is considerably less complex ,harbouring a single type of chemosensory sensillum,the Capitate peg.

these structures contain three ORNs that form stereotyped triads within each capitates peg sensillum.

The capitates peg(Cp) A and B neurons renpond with high sensitivity to Co2 and 1-octen-3-ol,respectively.

CpC neuron might be involved in the detection of the mosquito pheromones.

Fig: Fig: The labellum of Anopheles gambiae exhibits olfactory responses.Schematic dorsal view of mosquito head and brain,anterior(A),posterior(P),dorsal(D) and ventral axis,as indicated.(A)Palpal ORNs project to a posteromedial region of the antennal lobes(AL) in the brain.(B) Capitate pegs(Cp) are located on the ventral side of the 2nd,3rd and 4th segment of the maxillary palp.Each neuron in the Cp(CpA,CpB,CpC) respond to different odorant stimuli and expresses a different odorant stimuli and expresses a different set of odorant receptors(Or) or gustatory receptors(Grs).SOG:Suboesophageal ganglion,OL:optic lobe.

Odorant receptors(ORs)

Traditionally classified as G-protein coupled receptors(GPCRs),insect ORs,Aa/AgORs belong to a novel membrane protein family.In addition to their characterstic lack of primary sequence conservation, insect OR proteins are recognised as functionally obligate heterodimers composed of conventional and highly divergent OR and a highly conserved non-conventional co-receptor called OR7 in mosquito.

CO2 Receptor A distinct set of AgOrs and AgGrs are expressed in the triad of ORNs in the

maxillary palp of An. gambiae(Jones et al 2007,Lu et al 2007).The CpA neuron co-expresses three CO2 receptors,AgGr22,AgGr23, AgGr24.And the other two ORNs express AgOr7 along with AgOr8 or AgOr28,respectively(Jones et al 2007,Lu et al 2007).Several other AgOrs could be detected from palpal cDNAs,albeit only after 35 cycles of PCR amplification(Latron and Biessmann 2008).Three AaGrs, AaGr1, AaGr2 and AaGr3,are closed homologs of AgGr22,AgGr23 and AgGr24 respectively and are presumably expressed in the Ae. Aegypti equivalent of the CpA neuron.

olfactory signal transduction pathways.

Here, a 7 transmembrane odorant receptor protein (OR) lying within the ORN dendrite interacts directly with odorants or alternatively (?) in the context of odorant-binding protein (OBP) complexes. In both cases, subsequent interactions with heterotrimeric G-protein complexes (G/G′) activate downstream effector enzymes adenyl cyclase (AC) and phospholipase C (PLC). This leads to the synthesis of the second messengers cyclic AMP (cAMP), diacylglycerol (DAG) and inositol 1,4,5 triphosphate (IP3) that regulate several cation (Na+, Ca++) channels that carry the transduction current. In addition, OBPS are hypothesized (?) to interact with odorant degrading enzymes (ODEs) to clear unbound ligand from the extracellular space. Signaling is terminated (bulbed lines) by decoupling of OR/G-protein complexes by arrestins (A) and G-protein coupled receptor kinases (GRKs).

ROLE OF CO2 IN HOST-SEEKINGAtmospheric levels of CO2 were estimated at

0.03-0.04%), with an increase of 0.06% to 0.10% due to plant respiration in a tropical forest.

Schmidt & Nielsen (1975) estimate that 275 ml/min of CO2 is given off by a human subject, resulting in a concentration of between 0.01% and 1.0% in air coming off the subject.

mosquitoes may respond to a change in CO2 concentration rather than a certain threshold level.

Experiment: An example of the

selected host-seeking behavior of 60 female mosquitoes monitored for a short period (75 min) by automated-device. CO2 was delivered for 2 sec at 15 min, 30 min, and 45 min (arrow). Note that female mosquitoes show "touch-down" behavior (blue) in response to the change of CO2

concentration (red).

BEHAVIOURAL RESPONSETake off:When mosquitoes were exposed to an

airstream to which 0.2% carbon dioxide was added, the rate of take-off was greatly increased for a period of about two minutes before falling off again to a low level.

Sustained flight: In a tunnel,an airstream uniformly permeated with carbon dioxide at a concentration of 0.2 or 0.05% was flowing, Mayer & James (1969) found that Ae. aegypti failed to progress more than a short distance up the tunnel. It is presumed that the mosquitoes settled again soon after take-off.

ROLE OF LACTIC ACID IN HOST-SEEKINGAcree et al. (1968) identified lactic acid as a mosquito

attractant L isomer of lactic acid was 5 times as attractive as the D

isomer.Lactic acid is produced as a result of vertebrate muscle

metabolism and is only produced in the L(+) isomer .Smith et al found that lactic acid evaporating from

human hands was within the range of 23-133µg/h.an enzymatic decomposition of lactic acid eliminates the

attractive effect of human skin residues for A. aegypti.This implies that all components which contribute to the

attractiveness of skin odour are only effective when lactic acid is present.

FEMALE MOSQUITOES WILL BE ATTRACTED TO A SOURCE OF HEAT Mosquitoes landed three times as

often on a clothed robot when the robot's "skin" temperature was 98°F (36.7°C) than one with "skin" temperature of 50-65°F (10.0-18.3°C).

Fig:Mosquito TRPA1 is a candidate thermo-sensing protein for host-seeking behavior. (A-L) AsTRPA1 (+) cells (green) are present along the whole proboscis and in association with sensory neurons (magenta: HRP-positive neuron, D and J). Note that some sensilla contain AsTRPA1 (+) signals (A, F, and H). All bars: 5 μm. (M-O) TRPA1-activating agent (AITC) disturbs the selected host-seeking behavior. Female mosquitoes were exposed to 1% (v/v) AITC at ZT16-19. Note that volatile AITC(allyl isothiocyanate ) reduces the selected host-seeking behavior compared to controls.

Experiment Combining heat and odor

Air was pumped in on one side of the wind tunnel and the mosquitoes were released on the other, downwind, side. Clean air was used as control. The treatments consisted of a heat source at the air inlet, and a combination of heat and foot odor.

The presence of a heat source near the air inlet did not affect the flying path, but more mosquitoes landed near the inlet When the air contained foot odor.

The results show that the mosquitoes use both odor and heat to find their host and that either of these two stimuli alone is not enough.

CONCLUSION With this information,it should be possible to

identify and formulate chemicals that will act as attractants, repellents and other agents targeted against the olfactory basis of mosquito behaviours. Increased awareness of the importance of these approaches will be helpful in the fight against insect-borne diseases.

ACKNOWLEDGEMENT Sincere thanks to Dr. Debajyoty Chakraborty, the Head of

the Dept. of Zoology, Barasat Govt. College for valuable suggestions and providing infrastructure facilities.

Thanks to Dr. Tuhin Saha, Dr. Madhumita Manna, Dr. Debjani Sarkar, Dr. Sanjoy Poddar,Dr.Tanaya Dey, Dr. Srikanta Guria, for providing encouragements and various help during the entire period of study.

Lastly, the co-operation received from the classmates is also acknowledged.