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parasitology
10.7.97
dr lok rides again

yes, virginia, 
there is yet another handout today - Myiasis.

we were discussing the 
Diptera, the order of true flies. We talked about the suborder 
nematocera, which includes the delicate small biting flies, mosquitoes, 
biting gnats, noseeums, and so forth, sand flies, etc, which are 
annoyances and vectors of some important veterinary pathogens.

next 
suborder is brachycera, the short-horned flies. the only family we'll 
discuss is the family Tabanidae, including horseflies  and deerflies
The 
horseflies are large, heavy bodied adult flies - 2.5 cm in lenght as 
adults, often dark blue to black, with patterning or dappling of the 
wings. they have large compound eyes and often the eyes are brightly 
colored or multicolored - eg, the deerfly may have bright green eyes - 
hence "greenhead". these flies have piercing, scissorlike stylets for 
mouthparts and a sponging structure called a labellum. they are blood 
feeders. 
life cycle of tabanids is semi-aquatic to terrestrial. not 
found in moving water, but rather semiliquid to terrestrial habitats. 
some larvae are predacious on smaller invertebrates eg insect larvae, 
snails. they will find protective habitat to form pupal stage on dry land 
usually. adult females take blood meal to initiate egg development. 


importance of these tabanids is - they are large bloodsuckers, capable of 
ingesting 0.4 ml of blood at once, and hundreds of them may feed on one 
horse or cow during the day. so these flies can cause significant blood 
loss, or at least severe annoyance. can see interrupted grazing and 
decreased production, wt loss. the structure of the mouthparts is  - two 
scissorlike stylet mandibles and maxillae. the stylets are encased in the 
labium, and there is a soft spongy structure called the labellum kind of 
around it to soak up blood. this bite really hurts. it is painful and 
very irritating. so the host will try shooing the flies away as fast as 
possible. so one thing that characterizes feeding behavior of these flies 
is interrupted feeding. to get one blood meal, will probably have to feed 
off of several hosts. the labellum, therefore, can become highly 
contaminated with viruses, bacteria, bloodborne protozoans, etc. 
therefore it can transmit adherent infectious particles by simple 
contamination. we call this "mechanical transmission" ***important to 
remember that*** there are many pathogens known to be spread this way. in 
africa, trypanosoma evansi, which causes sura in horses, is transmitted 
exclusively this way. some other trypanosomes require intermediate host 
but this one doesn't. we also know that the viral agent of equine 
infectious anemia is transmitted by tabanid flies. 

that's all for them. 

next is the group of "muscoid" flies. houseflies are musca domestica, so 
these flies in the suborder cyclorrapha are flies that look like 
houseflies, or "muscoid" flies. they may vary in size and color but have 
similar overal configuration. includes family chloropidae of eyegnats (?) 
and family muscidae which includes houseflies, horn flies, face flies, 
stable flies, and tse tse flies. thick cuticle, often black/grey 
markings, wings held at angle over body, body covered with stout spines 
or hairs. life cycle --> eggs are laid, larvae hatch, larvae feed 
actively and finally pupate. need moist or semiliquid medium, decaying 
organic material such as fecal pats in pastures or so forth. egg stages 
are elongate, look like rice grains to me. larvae themselves are what we 
call "maggots." they are wormlike in appearance, have a set of mouthhooks 
that can tear into the food substrate. they have "spiracles" - openings 
to respiratory tract - which can be used to speciate them. pupation is 
the process of the last stage of development. modifies cuticle, inflates 
with hemolymph pressure, darkens it - forms oval shaped puparium or pupal 
case. many species pupate. pupae are pretty big - about same size as fly.


within family muscidae is a range of feeding types and substrates. one 
group of obligate blood feeders and one nonbloodfeeding.

most important 
blood feeders would be tse tse group. tse tse transmit african 
trypanosomiasis in cattle and humans in equatorial africa. trypanosoma 
brucei causes a fatal dz in cattle and has rendered lots of land useless 
to livestock operations because the cattle can't survive around this 
parasite. tse tse also transmists the trypanosomes which cause sleeping 
sickness in humans. the stable fly is another blood sucking muscoid. 
you've probably seen them at the jersey shore. look a lot lik houseflies. 
similar grey/black markings, about the same size. but they have a long 
piercing/sucking stylet that the housefly doens't have. mouthparts - 
labium, long sucking tube, and on the bottom of that is a "labella" which 
is a disk shaped structure with denticles on it. this is what the fly 
uses to abrade the skin of the host. proboscis can flip up and down on a 
hinge type structure. common on beaches. very pestiferous in different 
livestock operations as well. have been incriminated as mechanical 
vectors of some trypanosomes although not totally proven. they have 
painful, irritating bite, interrupted feeding pattern, so this could 
certainly be possible. at least potential mechanical vectors. unlike 
housefly which breeds in manure, stable flies like to breed in moist, 
decaying plant material eg silage or so forth. that's the typical place 
for it to lay eggs. the biomass of these things in a site like that is 
amazing. a similar fly to stable fly is a fly called the horned fly, and 
this is about half the size of a housefly but otherwise looks like one. 
it feeds similarly to the stable fly. if you look at where the horned fly 
feeds, it is around the bases of livestock horns that they mostly feed. 
they cause annoyance, irritation, can interrupt grazing patterns causing 
decreased production or decreased wt gain. also acts as intermediate host 
for stephana filaria of cattle (microfilaria in skin). 

those are really 
all of the bloodsucking muscoid flies. 
the non blood sucking flies ---> 
the housefly is the best known one of these. if you've ever been in a big 
poultry house you might have seen a huge mass of flies all over the 
place. the population gets huge due to high concentration of fecal 
material, which is where larval stages hang out. houseflies have gotten 
more of a bad rap than they deserve. they have been incriminated as 
vectors of many things but without evidence. they do act as vectors of 
habronema in horses, though. feeding apparatus of nonbloodsucking muscoid 
fly has a large labella but no piercing structures. the labella is a 
large spongy thing at the end of the proboscis. probably more important 
in livestock is the face fly. these are close relatives of houseflies, 
usually seen outside, don't come inside much. they tend to congregate on 
the faces of large animals like cattle, horses, deer, etc. they feed on 
nasal d/c, ocular d/c, lacrimal secretions. they will also land on wounds 
and eat serous exudates. they are vectors of habronema as well. face 
flies are an introduced species from europe brought here in the 1950s and 
spread through the US - they are an annoyance, interrupt grazing 
patterns, decrease production. also mechanical vectors for pinkeye in 
cattle. they irritate the host so they move from host to host a lot.


glossing over the family calliphoridae and sarcophagidae right now. we'll 
get back to them later. they are cyclorraphan flies though.

another 
muscoid fly doesn't look like a fly at all. The sheep tick, melophagus 
ovinus, superficially appears ticklike. they have no wings at any point 
in their development. are members of the louse-fly family- hippoboscidae. 
they are flat, with thick leathery cuticle covered with spines. antennae 
recessed into pits. these flies are also bloodsucking flies. they take a 
blood meal on their host. the whole life cycle is on the host. no 
freeliving phases. this particular species lives on sheep and can cause 
unthriftiness, anemia, unattractive wool, self excoriation, etc. another 
interesting thing is that thefemales sort of nourish the larvae within 
her body, and deposit them just before they form the puparium. important 
ectoparasite of sheep.

hippoboscids are also important ectoparasites in 
wildlife species. if you ever see wild pigeons or birds you know they 
have a lot of these lousefly parasits. lipoptena cervi is a parasite of 
deer that is also a lousefly, and it does have wings.

aspects of control 
for order diptera: in general, sanitation procedures. reduce sites of 
development of nematocerans. proper stall construction, timely manure 
removal, removal/elimination of areas of silage, soft feed, etc. manure 
is breeding site for face fly andhouse fly so one approach to using 
chemical larvacides is to formulate them as feed additives that are 
passed in the feces unmetabolized, so they kill the larvae when they 
hatch in feces. you need to make a pesticide without effects on mammals. 
one commonly used insecticide is raybon, an organophosphate. there are 
also a lot of sprays, or third generation type insecticides, used - and 
some parasitic wasps are commercially available but not sure how well 
they work. for environmental control, outside, you need to think about it 
some more. feedthrough larvicides are good. oil/water based sprays onto 
animals work but are labor intensive. systemic organophosphate pourons 
are used too, but also very labor intensive. more often you see forced 
use methods - run through dips, or dustbags placed in the field that 
animals have to run under. also, plastic ear tags impregnated with 
pyrethrin are used to control face flies.

moving on to today's lecture, 
Myiasis.
note: we're still talking about the muscoid flies 
definition of 
myiasis: the infestation of organs and tissues of living vertebrate 
animals by larval flies (diptera). 

basically you have larval flies 
acting as endoparasites. they are often highly specialized but not 
always. The first way to classify this is by degree of association 
between larvae and host.

relationship can be obligatory, or facultative. 
fly larvae may HAVE to live in host, or simply use the host if it is 
available or by accident. 

within those groups, we can also classify the 
different forms and flies we see being involved by the location in the 
host, or by mode of invasion. sometimes the infection is enteric, 
confined to GI tract. can be urogenital. can be traumatic - must be 
preexisting wound in host for ovipositing fly. cutaneous - hatched larvae 
actively penetrate host skin. finally, can see a grotesque example of 
nasopharyngeal myiasis. 

specifics:

probably the most prominent family 
of myiasis of facultative kind is the family calliphoridae, or blowflies. 
calliphora is particularly important as cause of accidental or 
facultative enteric myiasis. normal place for egg deposition is rotting 
plant material or carrion. if an animal eats that, larvae may grow in GI 
tract and be passed out in a living state. so you may see dogs/cats 
passing these larvae. these are just cases where the animal ate some 
material that was contaminated with the egss of these flies. 

some flies 
are attracted to urogenital orifice to oviposit, and larvae migrate in - 
Fannia spp seen in man, or anal glands of dogs. this is also facultative 
- larvae usually freeliving.

another example of facultative myiasis is 
cutaneous myiasis caused by larvae of greenbottle fly in sheep and cows 
etc. phaenicia serricata is the real name. this occurs accidentally when 
animals get feces on their haircoats. flies deposit eggs on feces, so if 
skin is contaminated, flies will lay eggs here. esp common on soiled 
hindquarters of sheep. larvae hatch out, and feed on the soiling material 
and also on the skin of the host. in areas where there is a lot of fly 
pressure this can be a problem on sheep farms. this is also called 'fly 
strike' in sheep, b/c the female flies come in and forcibly strike the 
skin, lay eggs really fast, and leave. thick wooled sheep are more 
susceptible to fly strike. similar syndrome occurs in dogs, esp older 
ones that sit around and have soiled haircoats. in sheep operations they 
modify farming practices to reduce blowfly strike - shaving wool around 
butts "crutching", also Mule's operation - tightening up folds of skin 
which are easily soiled in hindquarters. (see page 2 of handout). the 
blowflies which cause fly strike are just as happy to be growing in 
carrion or feces, freeliving. so you can try to make sheep less 
attractive, but you can't really get rid of them entirely. 

ok. leaving 
facultative myiasis. 
obligatory myiasis: flies never seen in freeliving 
situation - obligatory endoparasites.

- gasterophilis spp, the botfly of 
horses, the enteric parasite. "horse bots"
morphologically, adults of 
these flies are easy to recognize - kinda look like honeybees - kinda 
fuzzy, yellowish hair, dark clear wings. larval stages don't really look 
like maggots per se. they are stouter, fatter, more grublike. life cycle: 
female fly will deposit her eggs usually on the forequarters of the horse 
- legs, or face. some will oviposit on grass of pasture. anyway, eggs are 
eaten, and stimulated to hatch by warmth and moisture in horse mouth. 
they hatch in the mouth and first stage larvae migrate along oral mucosa 
- may see them in peridontal ulcers. they make their way down into the gi 
tract by migrating along the oral mucosa and the esophagus - not just 
swallowed. oh, by the way, the ovipositing occurs during the fall. the 
larval phases in the horse occur during the winter. so the larvae get 
into stomach, and develop to maturity - large clusters of bot larvae in 
the stomach. towards the next spring, the bot larvae release the mucosa 
and are passed in feces, where they pupate on the ground. adults emerge 
and fly around that summer. life cycle takes a whole year nd provides 
parasite with protection from harsh winter weather. 
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