Malaria VectorBiology

8/13/2019 Malaria VectorBiology

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Biology of Malaria Vectors andParasite-Vector Relationships

Dawn Wesson

Tulane Department ofTropical Medicine


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Malaria Vector Biology

Anopheline Life Cyclehabitatpreferences, types of habitat, unpolluted

water Effect of human activities on habitat

creationagriculture, irrigation, etc.

Biology of Malaria VectorsGeneral andSpecific


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Family Culicidae > 3500 species3 subfamilies:

Anophelinae - Anopheles, Bironel la and Chagasia,~ 500 species

Toxorhynchitinae -

Toxorh ynch ites, 70+ species

(allnon-bloodfeeding)

Culicinae- Aedes, Culex, Haemagogus , Manson ia,and all other genera, > 3000 species

Anophelinae

Toxorhynchitinae

Culicinaetime


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Anopheles mosquito life cycle


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eggs


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Anopheline Culicine

Adult


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Genus Anopheles6 subgenera:

Cell ia- >230 species, most important Old

World malaria vectors (Africa and Asia)

Anopheles- >180 sp., were the most importantmalaria vectors in Europe and N. AmericaNyssorhynchus->40 sp., most important New

World malaria vectors

Kertezia- >10 sp., NW, bromeliadsLophopodomy ia6 sp., NW tropics

Stethomyia5 sp., NW tropics


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AnophelesHabitat Preferences

Effects of human activities

Major malaria vectors tend to be colonizingspecies in temporary habitats free ofestablished predators

They have evolved with humans to take

advantage of these environments


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LARVAL HABITAT - An . albimanusin Cuba

WHO/TDR/Service, 1992


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LARVAL HABITAT -

An. bel lator in Brazil

from bromeliades



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LARVAL HABITAT - An. pseudopunct ipennisin Mexico



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LARVAL HABITAT - An . stephens ifrom

water tanks on rooftops in Dubai



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LARVAL HABITAT -

Standing water

created by road

building in Benin

WHO/TDR/Olliaro, 1988

LARVAL HABITAT -

Irrigation ditches

provide Anopheles

breeding sites in theGambia

WHO/TDR/Lindsay, 1991


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WHO/TDR/Lindsay, 1991

WHO/TDR/Martel, 1994

LARVAL HABITAT - Rice fields and irrigated areas provide

Anophelesbreeding sites in Viet Nam and the Gambia


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15WHO/TDR/Service, 1992

WHO/TDR/Ragavoodoo, 1992

Roof water breedingsite ofAn. arabiensis

in Mauritius

Water storage

pots, breeding site

ofAn. gambiaeand other

mosquitoes in

Nigeria


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WHO/TDR/HOLT Studios, 1992

Anopheles

gambiae

Biology ofAnopheles gambiae


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Anopheles gambiae

Major malaria vector in sub-Saharan Africa

Typical anopheline life cycle, but extremepreference for living around and feedingon humans

Preferred ovipositionsitessmalltemporary pools in full sunlight

Seasonal abundance correlates withrainfall


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Anopheles gambiaelife cycle

Other sitesirrigated areas (rice fields);drying streams in dry season; habitatscreated by humans

Eggslaid on water or damp soil; hatch 48hr.2 weeks

Larvaecan crawl across damp soil from

drying pool to another with water

Larval development -


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Anopheles gambiaelife cycle

Larvae are filter feeders on surface filmalgae and bacteria

Pupationin full sunlightcan be

induced in laboratory with light Pupal development in 24 hr.3 days;

temperature dependent

Adult emergenceat night Both sexes need 24 hr. to reach sexual

maturitymale terminalia (genitalia)

rotate 180.


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Mosquito Emerging

from Pupal Exuvia


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Anopheles gambiaeadult behavior

Male mosquito swarming behaviorfemales fly into swarm to mate (not welldocumented in wildAn. gambiaebut doesoccur in lab colonies).

Male activity increases at sundown.Changes in antennae (plumes folded upduring dayopen to detect female flight

sound; Johnston's organ) Males attracted to females and mate in

flightfemales probably mate only once(?)store sperm in spermathecae


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Anopheles gambiaehost seeking

MatedAn. gambiaefemalesseek bloodatnight (after sundown) - ~90% ofbloodmeals taken from sleeping human

hosts and they usually rest on the insidewalls of the house to digest the meal

Egg development takes about 48 hrsduring warm seasonlonger in coolerweather

Oviposition occurs at nightusually the2nd night after a bloodmeal


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The female then searches for another bloodmeal- during warm season, a female is capable ofovipositing every other night

This behavior has implications for the timing ofhost seekingbyAn. gambiaefemalesearlyevening blood-seeking females are probablyfeeding for the first time (they have not laid

eggs yetnulliparous), while older (parous)females tend to seek blood later at night (theyhave to oviposit first)


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Extrinsic incubation period(minimum) of

Plasmodium falciparumin the mosquito is 8-10daysso under ideal conditions, the femalewould take 5-6 bloodmeals in the process of

acquiring parasites and living long enough totransmit them (about 2 weeks)

In real lifeenvironmental factors will usuallyaffect time linetemperature, rainfall, wind will

interfere with the ability to oviposit and blood-feed at will. Most field collectedAn. gambiaefemales with P. falciparumsporozoites in theirsalivary glands have taken 3-4 blood meals


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If, after locating host and ingesting blood, the blood meal is large,distention-induced host seeking inhibition is triggered

This tapers off as the blood is assimilated and excreted

Eggs mature producing oocyte-induced host-seeking inhibition, whichgradually develops and then fades

Mature eggs induce preovipostion behavior, leading to oviposition

Physiology ofGonotrophicCycle


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Other factors influencing host seeking

Host defensive behavior

Mosquito ageolder mosquitoes more likely to seekblood even when gravid

Larval nutritionif poor, blood may go to support adultmetabolism

Mating statusunmated less likely to host seek

Nutritional status of male with which female matedpoor nutrition in male results in more host seeking

Mosquito speciessome, such asAn. gambiae, hostseek every 24 hrs. until replete (even if gravid!)

All of these factors potentially contribute to multiplebloodmeals per gonotrophic cycle, increasing thepotential for malaria transmission


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Malaria Parasite-Vector

Relationships Malaria Transmission Cycle

Parasite Infection Specificity

Mosquito Immune Defenses


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ookinete

oocyst

gametocytes

macrogametocyte

microgametocyte

zygotesalivaryglands

oocyst with

sporozoites

sporozoites

salivary glands

sporozoites

midgut infected with oocysts

cross section of oocyst PlasmodiumDevelopment inAnopheles


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Alimentary Canal


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Alimentary Canal

Within the alimentary canal, the malaria parasiteencounters various structural and physiological/biochemical characteristics that can influence its survival

The noncellular (chitinous) peritrophic membrane (PM)can be an effective physical barrier, preventing midgutinfection

Vector specificity for malaria pathogens may be linked tothe rate of PM formation versus the rate of ookineteproduction in bloodmeal

Adult mosquitoes secrete PM1, while larvae secret PM2

PM1 secretion is triggered by dramatic extension of themidgut epithelium during ingestion of a bloodmeal


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Alimentary Canal

After ingestion, the gametocytes go through a completesexual cycle in the midgut lumen and develop intomotile ookintes (~16-24 hrs)

Invasion of gut epithelilal cells occurs about 30 hrs afterbloodmeal

In P. gallinaceum/Ae. aegypti , Plasmodium secretes achitinase in order to penetrate the PM (inhibitingchitinase blocks transmission). Trypsin, secreted by themosquito, activates parasite chitinase.

This system may vary in different mosquitoesPMformation inAn. stephensivariably detected


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Bloodmeal processing - steps

1. Removal of excess water from thebloodmeal

2. Breakdown of vertebrate blood cells(hemolysis)

3. Hydrolytic degradation of macromoleculesin the bloodmeal (digestion)

4. Absorption of small molecules into themidgut epithelial cells and subsequently intothe hemocoel


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Hemolysis of Bloodmeal

Hemolysis breaks down cells to release proteins

and other nutrients, making them accessible to

the digestive enzymes

Hemolysis may be achieved mechanically

(cibarial armature) or biochemically (hemolyticfactors including small peptides and free fatty

acids)


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Absorption of Bloodmeal Nutrients

Differences between insects that show continuousdigestion (eg, tsetse flies -- absorption occurs

through specialized cells) vs those that show batch

digestion (eg, mosquitoes -- same cells that secrete

enzymes also carry out absorption)

Processes range from simple diffusion (eg,

absorption of sugar into the hemolymph) to active

transport (amino acids); little is known aboutabsorption of other molecules like lipids, vitamins,

and minerals


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Peritrophic Matrix (PM)

The peritrophic matrix is a layer of acellularmaterial separating ingested food from epithelial

cells

peritrophic comes from the Greek word peri foraround; trophic is the Greek word for food. The

PM surrounds the food bolus.

Peritrophic membrane was termed >100 years

ago but membrane implies lipid bilayer. The PMis not -- it is a sheath of cheesy material of

amorphous appearance. The word matrix is more

suitable!


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Other important points -- PM

The signal that activates PM secretion is the

physical distention of the midgut epithelium; eg,

ingestion of partial bm does not trigger PM formation

Mosquitoes, blackflies, and sandflies secrete

different type of PM during larval life

PM is permeable to digestive enzymes

Possible barrier to pathogen infection


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Structure of salivary glands

Structure varies among insect phyla

In mosquitoes, salivary glands of both sexes are

paired organs located in the thorax, and eachgland consists of 3 lobes connected to a main

salivary gland duct (male sgs small)

Female sgs have two identical lateral lobes and

one shorter medium lobe. Lateral lobes can bedivided according to proximal and distal regions

(different regions secrete different proteins)


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Function of the salivary glands

Saliva contains enzymes that digest

sugars

Salivary gland secretions play a role in themaintenance of feeding mouthparts -

saliva acts as a lubricant

In ticks, water in ingested blood is cycledback through the sgs where it is returned

to the host


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Salivary Glands and Bloodfeeding

Salivary glands produce a saliva that facilitatesrapid and efficient feeding (hemagglutinin,anticoagulant, antiplatelet activity, vasodilators)

Parasites can increase the probability of theirtransmission by modifying arthropod salivaryactivities

Malaria sporozoites infect the female-specificsalivary gland lobes (distal-lateral and medial)


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Salivary Glands and Bloodfeeding -2

Parasite invasion causes cellular damage in the

glands4-5x reduction in apyrase activity

The salivary apyrases of blood-feeding arthropodsare nucleotide hydrolysing enzymes and have

been implicated in the inhibition of host plateletaggregation through the hydrolysis of extracellularADP.

Sporozoite-infected mosquitoes take longer to

probemore sporozoites releasedAlso, more interrupted feedingsbite more

frequently before achieving successful bloodmeal


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Immune responses of vectors

Arthropod immune responses are not like

vertebrate antigen-antibody reactions but

the internal defense mechanisms are stillspecific and effective in destroying

pathogens and parasites.

Much of what we know comes fromimmune studies of lepidopteran larvae.


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Cuticular and gut barriers

The arthropods possess a rigid cuticle that

functions as a barrier to potential

pathogens. Microorganisms do notpenetrate the exoskeleton unless there is

a wound.

Many potential pathogens are ingested.Some are passed on through the feces or

through regurgitation. Some are walled off

by the peritrophic matrix (barrier?).


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Possible outcomes following exposure

of an arthropod to a parasite

susceptible arthropod: the parasite receives

appropriate stimuli from the biochemical

environment and develops successfully

resistant arthropod: some or all of the parasites arerecognized as foreign by the cellular/humoral

components in the hemolymph, and the arthropod

immune response sequesters and destroys parasite

refractory arthropod: the parasites do not elicit animmune response but they fail to develop due to

physiological or biochemical incompatability


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Cellular immunity in insects

Phagocytosis. In mosquitoes,

phagocytosis activity is a function of the

numbers of hemocytes present Encapsulation. The main defense

mechanism of insects against invaders

too large to be phagocytosed is

encapsulation. Phenol oxidase enzymes

are involved in melanotic encapsulation of

parasites (worms and malaria parasites)


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Summary

Anopheles gambiae is well adapted to take advantageof temporary aquatic habitat associated with humanactivities (farming, construction, etc.)

Behaviors such as preferential feeding on humans andresting in homes keep it closely associated with us.

The association betweenAnopheles mosquito andPlasmodium parasite is controlled by a series of physical,physiological and biochemical interactions, which maylead to a successful infection followed by transmission toa new host.


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Additional Reading for More Detail:

Biology ofAnopheles mosquitoesgeneralMedical Entomology for Students, 4thEditionpp. 33-51

Biology ofAnopheles gambiae mosquitoesBiology of Disease Vectors, 1stEditionpp. 75-77

Host seeking behavior in mosquitoesgeneralBiology of Disease Vectors, 2ndEdition (BODV)pp. 277-287

Midgut structure and Peritrophic MatrixBODVpp. 289-310

Bloodmeal Processing, Egg Development and Osmotic RegulationBODVpp. 329-362

Immune Response in VectorsBODVpp. 363-376

Salivary Glands and Saliva in Bloodfeeding InsectsBODVpp. 377-386

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Malaria VectorBiology