Transcript Biology of Malaria Vectors and Parasite-Vector Relationships Dawn Wesson Tulane Department of

Biology of Malaria Vectors and
Parasite-Vector Relationships
Dawn Wesson
Tulane Department of
Tropical Medicine
1
Malaria Vector Biology



Anopheline Life Cycle – habitat
preferences, types of habitat, unpolluted
water
Effect of human activities on habitat
creation – agriculture, irrigation, etc.
Biology of Malaria Vectors – General and
Specific
2
Family Culicidae
> 3500 species
3 subfamilies:
Anophelinae - Anopheles, Bironella and Chagasia,
~ 500 species
Toxorhynchitinae - Toxorhynchites, 70+ species
(all non-bloodfeeding)
Culicinae - Aedes, Culex, Haemagogus, Mansonia,
and all other genera, > 3000 species
Anophelinae
Toxorhynchitinae
time
Culicinae
3
Anopheles mosquito life cycle
4
eggs
5
Anopheline
Culicine
Adult
6
Genus Anopheles
6 subgenera:
Cellia - >230 species, most important Old
World malaria vectors (Africa and Asia)
Anopheles - >180 sp., were the most important
malaria vectors in Europe and N. America
Nyssorhynchus - >40 sp., most important New
World malaria vectors
Kertezia - >10 sp., NW, bromeliads
Lophopodomyia – 6 sp., NW tropics
Stethomyia – 5 sp., NW tropics
7
Anopheles Habitat Preferences



Effects of human activities
Major malaria vectors tend to be colonizing
species in temporary habitats free of
established predators
They have evolved with humans to take
advantage of these environments
8
LARVAL HABITAT - An. albimanus in Cuba
WHO/TDR/Service, 1992
9
LARVAL HABITAT An. bellator in Brazil
from bromeliades
WHO/TDR/Service, 1992
10
LARVAL HABITAT - An. pseudopunctipennis in Mexico
WHO/TDR/Service, 1992
11
LARVAL HABITAT - An. stephensi from
water tanks on rooftops in Dubai
WHO/TDR/Service, 1992
12
WHO/TDR/Lindsay, 1991
LARVAL HABITAT Irrigation ditches
provide Anopheles
breeding sites in the
Gambia
WHO/TDR/Olliaro, 1988
LARVAL HABITAT Standing water
created by road
building in Benin
13
LARVAL HABITAT - Rice fields and irrigated areas provide
Anopheles breeding sites in Viet Nam and the Gambia
WHO/TDR/Lindsay, 1991
WHO/TDR/Martel, 1994
14
Water storage
pots, breeding site
of An. gambiae
and other
mosquitoes in
Nigeria
WHO/TDR/Ragavoodoo, 1992
Roof water breeding
site of An. arabiensis
in Mauritius
WHO/TDR/Service, 1992
15
Biology of Anopheles gambiae
Anopheles
gambiae
WHO/TDR/HOLT Studios, 1992
16
Anopheles gambiae




Major malaria vector in sub-Saharan Africa
Typical anopheline life cycle, but extreme
preference for living around and feeding
on humans
Preferred oviposition sites – small
temporary pools in full sunlight
Seasonal abundance correlates with
rainfall
17
Anopheles gambiae – life cycle




Other sites – irrigated areas (rice fields);
drying streams in dry season; habitats
created by humans
Eggs laid on water or damp soil; hatch 48
hr. – 2 weeks
Larvae can crawl across damp soil from
drying pool to another with water
Larval development - <week with sufficient
temperature and food
18
Anopheles gambiae – life cycle





Larvae are filter feeders on surface film
– algae and bacteria
Pupation in full sunlight – can be
induced in laboratory with light
Pupal development in 24 hr. – 3 days;
temperature dependent
Adult emergence at night
Both sexes need 24 hr. to reach sexual
maturity – male terminalia (genitalia)
rotate 180.
19
Mosquito Emerging
from Pupal Exuvia
20
Anopheles gambiae – adult behavior



Male mosquito swarming behavior –
females fly into swarm to mate (not well
documented in wild An. gambiae but does
occur in lab colonies).
Male activity increases at sundown.
Changes in antennae (plumes folded up
during day – open to detect female flight
sound; Johnston's organ)
Males attracted to females and mate in
flight – females probably mate only once
(?) – store sperm in spermathecae
21
Anopheles gambiae – host seeking



Mated An. gambiae females seek blood at
night (after sundown) - ~90% of
bloodmeals taken from sleeping human
hosts and they usually rest on the inside
walls of the house to digest the meal
Egg development takes about 48 hrs
during warm season – longer in cooler
weather
Oviposition occurs at night – usually the
2nd night after a bloodmeal
22
Anopheles gambiae – host seeking


The female then searches for another bloodmeal
- during warm season, a female is capable of
ovipositing every other night
This behavior has implications for the timing of
host seeking by An. gambiae females – early
evening blood-seeking females are probably
feeding for the first time (they have not laid
eggs yet – nulliparous), while older (parous)
females tend to seek blood later at night (they
have to oviposit first)
23
Anopheles gambiae – host seeking


Extrinsic incubation period (minimum) of
Plasmodium falciparum in the mosquito is 8-10
days – so under ideal conditions, the female
would take 5-6 bloodmeals in the process of
acquiring parasites and living long enough to
transmit them (about 2 weeks)
In real life…environmental factors will usually
affect time line – temperature, rainfall, wind will
interfere with the ability to oviposit and bloodfeed at will. Most field collected An. gambiae
females with P. falciparum sporozoites in their
salivary glands have taken 3-4 blood meals
24
Physiology of
Gonotrophic
Cycle
• 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, which
gradually develops and then fades
• Mature eggs induce preovipostion behavior, leading to oviposition
25
Other factors influencing host seeking…







Host defensive behavior
Mosquito age – older mosquitoes more likely to seek
blood even when gravid
Larval nutrition – if poor, blood may go to support adult
metabolism
Mating status – unmated less likely to host seek
Nutritional status of male with which female mated –
poor nutrition in male results in more host seeking
Mosquito species – some, such as An. gambiae, host
seek every 24 hrs. until replete (even if gravid!)
All of these factors potentially contribute to multiple
bloodmeals per gonotrophic cycle, increasing the
potential for malaria transmission
26
Malaria Parasite-Vector
Relationships

Malaria Transmission Cycle

Parasite Infection Specificity

Mosquito Immune Defenses
27
midgut infected with oocysts
salivary glands
gametocytes
macrogametocyte
microgametocyte
salivary
glands
zygote
sporozoites
oocyst with
sporozoites
oocyst
sporozoites
ookinete
cross section of oocyst
Plasmodium Development in Anopheles
Alimentary Canal
29
Alimentary Canal
Within the alimentary canal, the malaria parasite
encounters various structural and physiological/
biochemical characteristics that can influence its survival
 The noncellular (chitinous) peritrophic membrane (PM)
can be an effective physical barrier, preventing midgut
infection
 Vector specificity for malaria pathogens may be linked to
the rate of PM formation versus the rate of ookinete
production in bloodmeal
 Adult mosquitoes secrete PM1, while larvae secret PM2
 PM1 secretion is triggered by dramatic extension of the
midgut epithelium during ingestion of a bloodmeal

30
Alimentary Canal
After ingestion, the gametocytes go through a complete
sexual cycle in the midgut lumen and develop into
motile ookintes (~16-24 hrs)
 Invasion of gut epithelilal cells occurs about 30 hrs after
bloodmeal
 In P. gallinaceum / Ae. aegypti , Plasmodium secretes a
chitinase in order to penetrate the PM (inhibiting
chitinase blocks transmission). Trypsin, secreted by the
mosquito, activates parasite chitinase.
 This system may vary in different mosquitoes – PM
formation in An. stephensi variably detected

31
Bloodmeal processing - steps
1.
2.
3.
4.
Removal of excess water from the
bloodmeal
Breakdown of vertebrate blood cells
(hemolysis)
Hydrolytic degradation of macromolecules
in the bloodmeal (digestion)
Absorption of small molecules into the
midgut epithelial cells and subsequently into
the hemocoel
32
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 (hemolytic
factors including small peptides and free fatty
acids)
33
34
Absorption of Bloodmeal Nutrients

Differences between insects that show continuous
digestion (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 about
absorption of other molecules like lipids, vitamins,
and minerals
35
Peritrophic Matrix (PM)



The peritrophic matrix is a layer of acellular
material separating ingested food from epithelial
cells
“peritrophic” comes from the Greek word peri for
around; 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 PM
is not -- it is a sheath of cheesy material of
amorphous appearance. The word matrix is more
suitable!
36
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
37
Structure of salivary glands



Structure varies among insect phyla
In mosquitoes, salivary glands of both sexes are
paired organs located in the thorax, and each
gland consists of 3 lobes connected to a main
salivary gland duct (male sg’s small)
Female sg’s have two identical lateral lobes and
one shorter medium lobe. Lateral lobes can be
divided according to proximal and distal regions
(different regions secrete different proteins)
38
Function of the salivary glands



Saliva contains enzymes that digest
sugars
Salivary gland secretions play a role in the
maintenance of feeding mouthparts saliva acts as a lubricant
In ticks, water in ingested blood is cycled
back through the sg’s where it is returned
to the host
39
Salivary Glands and Bloodfeeding
 Salivary
glands produce a saliva that facilitates
rapid and efficient feeding (hemagglutinin,
anticoagulant, antiplatelet activity, vasodilators)
 Parasites
can increase the probability of their
transmission by modifying arthropod salivary
activities
 Malaria
sporozoites infect the female-specific
salivary gland lobes (distal-lateral and medial)
40
Salivary Glands and Bloodfeeding -2
 Parasite
invasion causes cellular damage in the
glands – 4-5x reduction in apyrase activity
 The salivary apyrases of blood-feeding arthropods
are nucleotide hydrolysing enzymes and have
been implicated in the inhibition of host platelet
aggregation through the hydrolysis of extracellular
ADP.
 Sporozoite-infected mosquitoes take longer to
probe – more sporozoites released
 Also, more interrupted feedings – bite more
frequently before achieving successful bloodmeal
41
Immune responses of vectors


Arthropod immune responses are not like
vertebrate antigen-antibody reactions but
the internal defense mechanisms are still
specific and effective in destroying
pathogens and parasites.
Much of what we know comes from
immune studies of lepidopteran larvae.
42
Cuticular and gut barriers


The arthropods possess a rigid cuticle that
functions as a barrier to potential
pathogens. Microorganisms do not
penetrate 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?).
43
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 are
recognized 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 an
immune response but they fail to develop due to
physiological or biochemical incompatability
44
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)
45
Summary
• Anopheles gambiae is well adapted to take advantage
of temporary aquatic habitat associated with human
activities (farming, construction, etc.)
• Behaviors such as preferential feeding on humans and
resting in homes keep it closely associated with us.
• The association between Anopheles mosquito and
Plasmodium parasite is controlled by a series of physical,
physiological and biochemical interactions, which may
lead to a successful infection followed by transmission to
a new host.
46
Additional Reading for More Detail:
Biology of Anopheles mosquitoes – general
Medical Entomology for Students, 4th Edition – pp. 33-51
Biology of Anopheles gambiae mosquitoes
Biology of Disease Vectors, 1st Edition – pp. 75-77
Host seeking behavior in mosquitoes – general
Biology of Disease Vectors, 2nd Edition (BODV) – pp. 277-287
Midgut structure and Peritrophic Matrix
BODV – pp. 289-310
Bloodmeal Processing, Egg Development and Osmotic Regulation
BODV – pp. 329-362
Immune Response in Vectors
BODV – pp. 363-376
Salivary Glands and Saliva in Bloodfeeding Insects
BODV – pp. 377-386
47