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parasitology
10.6.97

dr lok

The order Diptera, continued.
last hour we had just begun to introduce the phylum ARTHROPODA and talked 
about what constitutes that group: classes crustacea - crabs, crawfish; 
centipedes; millipeds; insecta - insects aka hexapoda; arachnida - 
spiders, scorpions, ticks, mites. 

general physiology and morphology was discussed. 
different develompmental patterns were discussed - types of 
metamorphoses: complete or incomplete types used by insects; also diff 
kinds used by arachnidae.

finally we discussed the behavioral and structural adaptations required 
for an arthropod to be a good vector. We talked about evolution of 
mouthparts, need to have correct skin piercing equipment, and about food 
preferences, need to feed on animal and not plant to be important as 
vector. 

a few general comments about controlling arthropod vectors and pests:
emphasis is on *not* eradication of pest populations, but on IPM: 
integrated pest management - maintaining the population of the pest below 
some economic/disease transmission threshold. There is some threshold 
short of outright eradication where this occurs. Eg, crop protection - 
there is a threshold below which it doesn't pay to spray. In medicine, we 
think of a threshold below which disease cannot result or transmission of 
pathogens cannot result. there is a finite population threshold where 
this occurs. IPM also stresses a multiplicity of approaches to control: 
not one single modality. In fact, it seeks to minimize the use of 
chemical toxicants in pest control. There are many other methods besides 
pesticides and drugs to control pest populations. Eg, physical or 
chemical barriers - keeping animals away from pest population -> screen 
windows, insect repellents. Environmental manipulations are also 
important in breeding populations. site reductions --> reductions of 
specific sites needed for arthropod breeding. Modified farming 
practice--> flies live in cow manure and bird manure, so manure 
management will control pest population.

be that as it may, chemical pesticides are part of the armament for pest 
and vector control. geneology of different kinds of chemical pesticides:

- First generation of chemical pesticides was derived around turn of the 
century when some plants were discovered to have pesticide property. eg, 
plants evolved defense mechanisms. Rotenone (from roots of plants) and 
pyrethrum (from plants of genus chrysanthemum) - naturally occuring 
substances in plants. These compounds were extracted and concentrated and 
were/are effective pesticides. Rotenone is still used on young puppies 
and kittens - less vertebrate toxicity

gross chemical poisons like arsenicals were also used at this time.

-Second generation of chemical pesticides showed up around end of WWII - 
spin off of development of nerve gases and so forth. These drugs are 
synthetic organic molecules: chlorinated HCos, organophosphates, etc - 
have neurotoxic effects on insects. these all have a level of selectivity 
b/w vertebrate and insect neurons, but it is incomplete. there is a toxic 
threshold with all of these beyond which you have unintended toxic 
effects on nontarget species. DDT is a chlorinated hydrocarbon. malathion 
is an organophosphate (controls mosquitoes). Sevin is a carbamate used by 
home gardeners. Pyrethrins are synthetic analogs to the first generation 
pesticide pyrethrum. these are probably least offensive from a standpoint 
of mammalian toxicity (pyrethrins, that is.)

- third generation of chemical pesticides include a lot of different 
insect/arthropod hormone mimics. these chemicals mimic naturally occuring 
reproductive hormones. they are applied strategically at times when they 
will disrupt development of the arthropod pest. this also includes 
metabolic inhibitors, eg luferenon which inhibits chitin, and pheromones 
- chemical communicators. these pheromones are used mainly as 
attractants. 

we know that insects and other arthropods have their own populations of 
parasites and pathogens. There are nematode parasites which live in 
arthropods, and these may be biological control agents for the 
arthropods. there are also minute wasps which lay eggs in other species 
of insects, killing their hosts. "parasitoids". 

questions about different classes of insecticides?
no.

starting off now going through the class hexapoda, the insecta. 

Order DIPTERA. diptera is a subdivision of the class insecta/hexapoda. 
greek scholars know diptera means "two wings". recall the generalized 
insect. We said each segment of the thorax had a pair of wings. but in 
diptera, the hind pair of wings is vestigial, hardly there at all. only 
one real pair of wings. there are large compound eyes. all dipterans 
"true files" undergo COMPLETE metamorphosis. complete metamorphoses 
involves the pupal stage. most spp lay eggs, but some nourish immature 
stages in female body and deposit mature larva or pupa on ground. 
mouthparts may be sucking kind or piercing kind, so they do have vector 
potential. the immature stages inhabit many habitats: some aquatic, some 
terrestrial, some are highly specialized and immature stages live in 
living vertebrate tissue. how are true flies organized? order diptera has 
three suborders: nematocera, brachycera, cyclorrhapha. don't worry about 
those names right now. the division is based mainly on antenna morphology 
in adult fly. 
nematocera have long, multi-segmented antennae - "long horned fly".
nematocera include mosquitoes, biting gnats. these are small, delicate 
little flies. also nosee'ems, black flies. we can also differentiate 
based on immature stages. nematoceran larval bodies are divided into 
three distinct segments - head, thorax, and abdomen. pupal stages are 
"exorate" (?) you can see developing structures of adult on exterior 
surface of pupa. adult stages are small, delicate flies which leave pupa 
through t or y shaped opening.

brachycera - these diptera have "short horns" - the antennae are 
segmented, but much shorter. these include horseflies, deerflies 
(greenheads) (these are biting flies). their larval stage is cryptically 
subdivided - wormlike in appearance. feeds with opposing mandibles. Pupae 
are exorate. adults in this group are large, robust flies.

cyclorrhapha - blunt antennal segments, form a club shape. usually has 
terminal sensory hair or "arista" on it. lots of 
chemosensory/mechanosensory apparatus on it. kinda looks like a feather 
to me. like arista records :). typical life history of these flies - 
which include the housefly = is that they hatch from egg and go through a 
series of wormlike larval stages, which have feeding apparatus which is a 
series of biting hooks which work in parallel with eachother. these 
larvae are often called "maggots". toward end of larval period, a hard 
shell, ovoid shaped, forms around developing pupae. it is NOT exorate. 
exterior is smooth and hard. when it's done, a circular cap pops off and 
adult emerges from the "puparium" or pupal case, which was formed from 
the last, unshed cuticle of the final larval stage. 

can also look at wing venation - but this is beyond the scope of this 
class. one thing you will see in lab in a few weeks is that in general as 
you go through suborders you see a decrease in number and complexity of 
veins in wings. nematocera most complex, cyclorrhapha least complex.

[now entering today's handout...]


suborder nematocera,the long horned flies:

contains four families of importance

-culicidae - the mosquitoes: tiny, delicate flies, long slender legs, 
complex venation on wings, wings covered with scales. long piercing 
proboscis. adult female mosquito has long, hollow, piercing stylet 
mouthpart to penetrate capillaries under the skin of the host. life 
history of mosquitoes: metamorphosis is complete. larval and pupal stages 
are all aquatic, to some degree. they need standing water, or can grow on 
margins of slowmoving watersheds. don't like moving water. will use tin 
cans, discarded tires, etc. eggs are laid on water surface or just above 
it where they are liable to be flooded by rain. in adult female, blood 
feeding is required to initiate egg development. so only adult females 
eat blood. the blood provides material to make yolk. act of distending 
abdomen with blood starts the neuroendocrine cascade giving rise to 
ovarian follicular development. 

mosquitoes and disease: first, mosquitoes are very annoying. they CAN 
exsanguinate horses and cattle if there are A LOT of them but they don't 
really get a chance to do that these days. more important is their role 
as vector. They are classic vectors of protozoal plasmodium spp, which 
cause malaria.
The sexual stages of plasmodium occur in the invertebrate host, which is 
the definitive host. the human who gets malaria is the intermediate host. 

mosquitoes also vector filarial nematodes, including those causing human 
elephantiasis, and canine heartworm. finally, in public health we'll talk 
about mosquitoes as viral vectors. st. louis encephalitis, yellow fever 
virus, dengue virus, equine encephalitis virus. so this is also very 
important. 

end family culicidae

next family within suborder nematocera is the simuliidae - the so called 
"black flies". note: for purposes of learning this, it is more important 
to know organization and for testing, it's ok to know the common names 
instead of the latin or greek names. so. the black flies have long 
antennae also, but otherwise look different from mosquitoes. wings clear, 
devoid of scales. mouthpart more short, scissorlike. lacerates host skin 
so flie can lap up pool of blood. not vessel feeder like mosquito - adult 
female black flies are pool feeders. blood feeding stimulates egg 
development. black flies are tiny, 1-2 mm long. humpbacked appearance. 
remember immature moquitoes grow in standing water. black fly larvae 
develop in fastmoving creeks, rivulets, rivers, streams. larval/pupal 
stages are attached to rocks and vegetation - in white water areas that 
are highly oxygenated. pupal stages attach via crude silken cocoon. when 
adult fly finishes growing in pupa, it emerges and floats to surface in 
an airbubble and flies away. re all the nematocera - there are up to 
7,8,9 larval stages, each punctuated by a molt, between hatching and 
pupal stage. larvae feed on particulates - they are filter feeders - eat 
plankton, etc. many blackflies in poconos, etc. adult females take blood 
meal on host. are source of annoyance, for one thing. also they inject 
saliva into bite site, which has a lot of pharmacologically active stuff 
in it to inhibit clotting. often highly antigenic or allergenic, so 
multiple bites can precipitate a hyperallergenic response to the bites. 
probably more important is the role as vectors of onchocerca spp, the 
subcutaneous filariae in humans that cause "river blindness". also they 
are vectors of protozoa of birds called leukocytoon. 

next family in suborder nematocera is the family ceratopogonidae which 
includes the genus culicoides. this family includes biting gnats and 
nosee'ems. these are very very small flies, can get through conventional 
window screens. they feed similarly to black flies - pool feeders, have 
very very painful bite. active during dawn, will feel them biting you at 
dusk when camping. life cycle can be aquatic OR semiterrestrial. found in 
mud, most soil, decaying plant litter. might be find around margins of a 
stockpond containing pasture runoff. also seen in cracks or crevices of a 
drying stream bed, or in outflow from a dairy barn - fecal/milk 
contamination, mud overlain with layer of water. typical habitat for 
larval ceratopogonids. Adults most active at dawn and dusk (hey, like 
rabbits!). also they subject the host to some localized allergic 
responses in skin - chronic dermatitis may result from many flies feeding 
on one host, most obvious/noteworthy in horses, which get "sweet itch" or 
"queensland itch" in summer months, and which occurs along ventral 
midline of horse. generalized dermatitis along ventral midline which is 
preferred feeding site. goes away in the winter.
medical significance: (aside from the dermatitis) they act as vectors of 
onchocerca cervicalis L3 in horses and other onchocerca spp (so hard to 
know what is causing the dermatitis - the sweet itch goes away in winter, 
though, and onchocercal dermatitis stays all year.also act as vectors of 
arboviruses - agent of bluetongue in sheep. may also possibly vector 
leucocytozoon protozoa.

next family (still in suborder nematocera): psychodidae - phlebotomines, 
sand flies. they look a bit like noseeums, but adult female holds her 
wings up at about 60 angle from body when feeding. life cycle is strictly 
terrestrial. they develop in high humidity protected sites which have 
lots of organic debris. typical site is in rodent burrows. the nestling 
rodents make good hosts for adult fly. also some relatives of sand flies 
can grow in dirty restroom drains and so forth but those aren't blood 
feeders. adult female sand flies are blood feeders. they transmit the 
agent of leishmaniasis - leishmania spp. (protozoa). causes severe 
disfiguring disease. also some arboviruses and some bacterial pathogens.

quick recap:
suborder nematocera. four families: culicidae- aquatic life history, 
standing water, blood feeding females, malaria vector, hw vector, EE 
virus vector.
blackflies: aquatic lives, fastmoving not standing water, females blood 
feeders, vector onchocerca, leukocytozoon. 
noseeums: semiterrestrial - mud, moist soil. vicious biters. chronic 
dermatitis in horses. vectors of arboviruses (blue tongue) and onchocerca 
in horses
sandflies: vectors of leishmania, terrestrial life cycle. 
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