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parasit
10/8/97

continuing with Myiasis, and then moving on 
to fleas. Hoo boy.

[started by repeating a buncha stuff from yesterday, 
which I wrote down YESTERDAY and am not writing down again.]

[remember, 
first we talked about nonobligatory myiasis in sheep and old dogs. then 
we talked about obligatory myiasis with horse bots]

horse bots: light 
infestations not a serious clinical problem. heavy infestations as seen 
on the slide will occlude GI tract, and will compete with host for 
nutrients. will irritate gastric mucusa and intestines when passing out. 
will cause gastritis.

treatment/control: oral insecticide drugs: old 
standby is dichlorvos, an organophosphate, widely used in houses - no 
pest strips, etc. very potent organophosphate - called "Task". also 
trichlorphan is used but narrow safety margin. Ivermectin is now the 
mainstay for controlling this 

often dosing with ivermectin in fall to 
kill a variety of intestinal roundworm parasites, which will also kill 
early migrating stage of gastrophelus as it migrates through oral mucosa. 


another form of myiasis is also obligatory and in livestock, and is 
called "traumatic myiasis" - here, fly larvae require preexisting wound 
to gain entry into the host. Screw worm fly - small, green/blue/metallic, 
bright orange compound eyes. the female will be attracted to serous 
exudate around a wound and also to exposed soft tissues like placentas. 
they lay eggs there. eggs will hatch out in that wound site, and you'll 
have a mass of larvae feeding on that tissue. they have big darkly 
pigmented tracheal trunks leading back from the two large spiracular 
plates. the larvae can do extensive tissue damage - looks like they ate 
most of this cow's ear from a small tick bite where they started!

control: minimize likelihood of wounds by removing any barbed wire, sharp 
things, etc. cover wounds left by vets eg incisions etc. 

we talked a 
bit about biological control - it works, here. screw worm flies have been 
eradicated by much of the continental US. remember, adult female insects 
have a spermatheca to store sperm from a single mating. so, they've been 
mass rearing these flies, irradiating the male pupae to sterilize them, 
and releasing them. they copulate without releasing sperm. females never 
mate again. this has been really successful in controlling these flies. 
they are not found in the USA anymore.

cutaneous myiasis - hypoderma 
genus. cattle grubs, warbles. adult hypoderma bovis looks a bit honeybee 
like. has fuzzy orangish hair. has very stout fat grublike mature larvae. 


adult females lay eggs on hair of host around hocks, heels. therefore 
they are sometimes called "heel flies." these flies come in and strike 
rapidly and fly away, to avoid the host defenses. this behavior results 
in some sterotypical behavior in the host. cattle avoid striking flies by 
"gadding" or running at full speed across the field, with tail up in the 
air. so we call them "gadflies" also. the larvae can actively penetrate 
the skin -no wound needed. they migrate up through legs and into the 
viscera and go up to the dorsal aspect of the animal. they form an 
intradermal pocket in the skin on the back of the cow, and become third 
instar stage (or whatever). it sits with the caudally located spiracle up 
against an opening to the outside. we call the pocket in which it grows a 
"warble". it's pretty big, 1-2 cm in length. toward the end of the cycle, 
they back out, fall to ground, pupate on the ground. this goes on during 
fall (ovipositioning), winter (in host), and spring (mature in warble) 
and falling out in late spring/early summer. you can see scars on the 
cows left from the warbles. if you see a warble on the cow you should be 
careful trying to rupture it and get out the larva. you do not want to 
rupture the larva in situ and break the larva - can get allergic 
reaction. better to pull larva out by gentle traction. heavily 
parasitized cattle can get secondary infections in the warbles, eg screw 
worm infestations! hides from these animals have reduced market value. 
gadding behavior interrupts normal grazing patterns, too. to control 
these suckers, there are some different systemic organophosphates - 
raybon, phenthion, etc. don't need to know names. just know there is a 
class of systemic organophosphate insecticides that can be applied 
topically, will penetrate outer tissues and will kill the larvae in situ. 
ivermectin is also a good agent to use esp for early migrating larval 
stages. 

there's a relative of hypoderma thot occurs in cats, rodents, 
etc. cuterebra usually lives in rodent or lagomorph. the larvae are 
pretty big. unlike hypoderma the larvae tend to develop at the site of 
initial penetration. they dont' migrate all over the host. the eggs are 
laid around entrances to rodent burrows by females, and they hatch when 
the host passes by, and the larvae grab onto host and penetrate skin. 
dogs and cats who sniff around rodent and rabbit burrows can pick up 
these larvae. you see a subcutaneous nodule with a fistula to the 
outside, containing the grublike larva. in kittens, which are abnormal or 
aberrant hosts for cuterebra, sometimes there is migration to pharyngeal 
area or CNS and this can be fatal. 

dermatobia hominis: human myiasis. 
similar to cuterebra. sstays in local cysts. central and south american 
jungles have these hanging around.

last example of obligatory myiasis - 
nasopharyngeal myiasis mainly in sheep - nasal bots: oestrus ovis is the 
fly. the adult deposits larvae in or around nostrils. they forcibly 
strike at the facial area. larvae crawl up to nasal sinuses and grow in 
there. the next spring they back out and pupate in the soil. it's 
probably uncomfortable for the sheep, who will snort and shake heads and 
stuff. sometimes you see the larvae coming out. heavily infested sheep 
may paw the ground and ambulate abnormally. these nasal maggots may be 
controlled with intranasal dichlorvos or late summer/early fall 
ivermectin to get the migratory stages. 

ok. moving on to today's topic 
- the fleas.

order siphonaptera of the class insecta of the phylum 
arthropoda.
ignore the ad for Program on the flea slides.
fleas are 
wingless laterally compressed ectoparasites. this makes them well adapted 
to move around between hairs. their legs are modified for jumping. 
blood 
loss, annoyance, tissue damage, disease vectors. 

fleas look only like 
fleas. very highly specialized. body has head, thorax and abdomen. no 
wings. piercing/sucking mouthparts. antennae recessed into pits. small 
inconspicuous eyes. on head in some species is the "genal comb" (sp?) 
looks like whiskers. this comb and its number of teeth is used to 
speciate fleas. on thorax are three pairs of legs. basal segment of each 
leg (coxa) is highly developed with big muscles. first thoracic segment 
may have a pronotal comb - some have it and some dont - species specific. 
on abdomen are reproductive organs, sensory hairs caudally (pygidium), 
spermatheca in females (also species specific). 

internal structure of 
fleas is much like the generalized insect. note that between the foregut 
and the midgut are some chitinized spines extending into the lumen of the 
gut, radially - this is an important structure. these spines of the 
proventriculus are important to the mechanism by which yersinia pestis is 
transmitted.

life cycle: fleas use complete metamorphosis. they pass 
from egg to larval stages (larvae have chewing mouthparts)(larvae live 
OFF the host, not on the host)(can't even develop at host body 
temperature). generally larval stages are found somewhere in close area 
inhabited by host. fleas tend to parasitize nesting mammals - larvae 
often found in host bedding. cat's favorite spot on the carpet, or 
whatever. will spin silken case and form pupa inside. pupa is quiescent 
stage. the immature phase of life cycle is temperature dependent - faster 
when warmer. once adult flea forms inside pupa, you have to get it 
out...it won't do anything til a host approaches. it will wait until it 
senses a host. it can remain quiescent for up to two months. flea control 
has been really problematic b/c people do not want to treat the house. 
they only want to treat the pet. eggs hatch in about 2-4 days. larvae 
take about 7-14 days to reach pupal stage. pupae can last 1 week - 2 mos.


slides of flea dirt, eggs, larvae, etc.

for many years, people thought 
dog and cat fleas would readily jump on and off the host. now, we know 
that adult fleas can only survive for very short times (hours to days) 
off the host, and they don't really leave the host unless they start 
having crowding problems. the perception that fleas are jumping on and 
off is usually caused because as you enter an infested area, the fleas 
will hatch in unison and attack the host as it approaches. 




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