FLY STRIKE!!
by Susan Wiley
There is nothing so horrifying, so repugnant, so disgusting, as a breeder’s first encounter with fly strike. I had been raising English Angoras for six years in central Illinois, and had never encountered the problem, until I moved to southern Michigan. I was in the middle of shearing an English Angora buck. I had finished the top side and gently flipped him over to start work on the underside, when I gasped in horror. His crotch area was a wiggling mass of maggots!!!!!! I screamed and raced to the bathroom with him. What could I kill the maggots with? First I dipped him in alcohol… the maggots didn’t like it, but it didn’t kill them. Next I dipped his hind end in bleach… the same result. At this point he started kicking, and maggots were flying through the air, climbing up the bathroom walls and all over the tub and sink and me. It was an absolute nightmare.
Well, we both survived the ordeal. But since then, I have had more encounters with fly strike, and have become a reluctant expert in this area. I learned how to treat the rabbits, but I never had a clear answer to my many questions: “Why me?” “What fly is causing this problem and how do I keep it from happening?” “Are my fly strike problems the same as friends on the West Coast are experiencing?”
I tried looking in the rabbit literature on fly strike, but found little useful information. Not until my husband, a Professor at the University of Michigan, mentioned to me that he was having lunch with a colleague from Michigan State University (Dr. Rich Merritt) an expert on (among other things) various biting and flesh eating flies, did I get on the right trail. Dr. Merritt kindly loaned us quite a few studies on fly strike in foxes, voles and humans. The following is a brief synopsis of what I have gleaned from that literature and from my own experiences.
WHAT IS FLY STRIKE?
The scientific name for fly strike is myiasis, “the condition resulting from the invasion of tissues or organs of man or animals by dipterous larvae.” Diptera (the insect Order of two-winged flies) is comprised of over 120,000 species. Of these, several different families of flies (and a number of species in each) are known to attack animals. Each species has a different mode of attack, a different life style, and a different range of occurrence. The problem was clearly a lot more complicated than I ever suspected. There are blood-sucking maggots, flies that lay eggs under the skin of human and animal babies, flies that attack healthy tissue, flies that are attracted to necrotic tissue and excrement. What kind(s) are we dealing with in the rabbit barn?
The Cuterebrids – botflies
One family of flies known to attack rabbits is the Cuterebridae, which includes the genus Cuterebra. This is the fly commonly known as the botfly, causing a condition in rabbits (and other animals) known as “warbles.” The genus is strictly North American in its’ distribution and contains a number of species that parasitize rodents and lagomorphs. It is a large fly, about 20 mm or more in length. The abdomen is usually shining black or blue, but may be fuzzy or reddish. The larvae occur singly, living in cysts under the skin, which open to the outside. The preferred location for deposition is the neck and shoulder region of the rabbit. This fly does not usually cause much damage, because there is usually only one. However, its presence can predispose the rabbit to bacterial infection or parasitism by another kind of fly that is more serious.
The Sarcophagids – fleshflies
Another family of flies that may attack rabbits is the Sarcophagidae. Flies in this family are medium-sized and typically found around the carcasses of dead animals. Some breed in carrion, others in excrement, others parasitize invertebrates, or breed in decaying vegetable matter. The genus Wohlfahrtia , commonly called the grey fleshfly, belongs to this family, and is a common parasite of mice, foxes, mink, rabbits, dogs, cats and even man. This fly is gray with a black abdomen, or gray abdomen with black spots in longitudinal series. In the species Wohlfahrtia magnifica the female deposits larvae into sores, cuts, wounds and body openings such as nose, eyes and ears. The larvae burrow into tissues and grow rapidly. They molt in three days, and in another 3-4 days they are ready to crawl out of the wound and pupate. The larvae are extremely hardy, and specimens can survive for considerable periods of time in alcohol, pure hydrochloric acid, turpentine, etc.
The species Wohlfahrtia vigil is common in the northern U.S. and the adult has a black abdomen with gray spots. The female lays larvae on the skin of an animal, and the larvae may wander quite far before penetrating the skin. They cause abscess-like lesions, and the larvae can be observed as it sticks its hind end out to breath while feeding (yes, they breath through their…) This fly usually only parasitizes babies with thin skin, but it can take advantage of openings made by botflies, for example. In one study of vole populations in the Vancouver area, parasitism by this species was strongly linked to the botfly occurrence and the author believed that W.vigil was able to parasitize adult animals only because of the botfly openings. Parasitism occurred June through October, with death of the vole almost certain. Larvae were found feeding on body tissues in the inguinal region (lower abdomen), along the hind legs and along the lower back. Death of the voles often appeared to be from hemorrhaging. The larvae penetrated abdominal cavities, and could complete their development on dead hosts (unlike the botfly).
The adult W. vigil is not attracted to fetid odors. In fact, Wohlfahrtia spp. adults are known to be attracted to flowers and sweet things. Gassner and James cited cases in which heavy infestations on mink and fox farms disappeared when pens were moved from fields planted with crops such as alfalfa and rye.
The Calliphorids – blowflies
The family Calliphoridae (blowflies) are often also called “bluebottle” and “greenbottle” flies because of their usually metallic blue, green, or copper color. For the most part these medium sized flies feed on decaying animal material, but some can also feed on live animals. For example, the species Callitroga americana, a bluish metallic fly, attaches its eggs (10 to 400) on dry tissue near a wound. The eggs hatch in 11-21 hours and penetrate the tissue in a head-down position. They are gregarious and produce pocket-like injuries. This fly is widespread over most of the U.S. and South America, and causes a lot of damage to cattle, sheep and goats. There are reports of 20% mortality in infested populations.
Another potentially dangerous Calliphorid is Callitroga macellaria , the Common Screwworm Fly. This fly has a green body with an orange head and is found throughout north and south America. Females may deposit a thousand eggs each, hatching in 4 hours under favorable conditions. The adults feed on anything from garbage to the nectar of flowers, and are usually found in the vicinity of carrion. It does not form the pocket injuries, though, and may usually be a secondary invader, present in the wool near injuries but not attacking live tissue.
But it is another blowfly that I suspect is the source of our problems here at Wiley Woolies. Phaenicia sericata, commonly called the greenbottle fly or the English sheep fly. It is a metallic green fly and distributed over most of the world. The common breeding medium is carrion, and can be reared from manure and garbage. It is, however, attracted to foul smelling sores and soiled wool, and is one of the principal sheep maggots of the British Isles, South Africa and New Zealand. It causes a serious form of myiasis. The young larvae can bore deeply into healthy tissue.
This summer I actually captured an adult female as she attempted to crawl through the wool of an already infested rabbit. Using a key supplied by Dr. Merritt my husband identified the beast as a greenbottle fly. I have suspected these metallic pests for some time. Ever since the day I set some wet, dyed angora outside to dry and watched a horde of green metallic flies settle on the wool and begin depositing what looked like small, live maggots. I had washed some soiled angora, dyed it blue with some Cushings dye, washed it, rinsed it and set it out on a lawn chair. The maggots traveled around in the wet wool, but eventually died as the wool dried out. I don’t know what attracted the flies, though it was probably the odor. The dye-bath had been acidified with vinegar, and there was still a strong odor after washing, but whether it was the vinegar, or some lingering smell from the dye, I don’t know.
The identification of this fly helped me to understand why we have such trouble with fly strike here in southern Michigan. This is sheep country. One of my close neighbors has over 3,000 head. I have a small flock myself. Farmers around here are occasionally troubled with what they call “screwworms,” and dead sheep are often left to the turkey vultures and maggots to consume. There is also a lot of road kill available. I am sure that Phaenicia sericata finds this region a good breeding ground. As for the part of central Illinois where I lived previously, there were few farm animals and fewer road kills. It was big corn and soybean country. And that might explain why I never encountered fly strike there.
TREATMENT OF FLY STRIKE
So here is a scenario: You are walking down a row of caged rabbits and you smell something unpleasant. You follow the odor to a certain cage, and there is a rabbit sulking in the back, in obvious pain. You reach in and pull the rabbit out, turn him over, and there it is: a soiled crotch area, with squirming maggots. What do you do?
The first line of attack is the embroidery scissors, which I use for shearing. Cut away all soiled wool from the infected area. Cutting each maggot in half with the scissors is the quickest way to dispatch them. (Remember to drop a few of them into a vial of alcohol for later identification!). Then shear the entire rabbit. If you have caught the infestation early enough, this may be all you need to do. However, if the rabbit’s tissues have been invaded, there may be extensive damage. Skin, muscle, nerves, blood vessels may have been consumed. Some breeders have reported hindquarter paralysis following a case of fly strike. Nerves or even the spinal column can be damaged by the maggots. Many myasis-causing species secrete ammonia and other substances to facilitate cell death and decomposition; these might have toxic effects on the rabbits as well. In some cases, the wound may be foul, like an open bedsore in a human. First wash the rabbit with a mild shampoo and then rinse. Then I pour on hydrogen peroxide, which helps oxidize the filth and necrotic tissue. Then I may wash and rinse again, followed by a nice long soak in hot water with Epsom Salts, to promote healing. After the bath, the wound may be covered with an antibiotic ointment. Never put a damp rabbit outside where it may attract new flies. There is some indication in the scientific literature that flies may be attracted by wool hydrolysis. Wait until the bunny is thoroughly dry.
In severe cases, the flies may have burrowed so deeply into the rabbit that this cleaning method is not sufficient. Screwworm sprays, designed for sheep, work very well at killing the maggots and preventing reinfestation. However, these sprays can be dangerous for the rabbit and for you. Read the ingredients and warnings, and carefully consider before using. Some of these toxic sprays may persist for long periods of time.
PREVENTION OF FLY STRIKE
The best cure, of course, is prevention. The first line of defense is to keep the flies out of the rabbitry. Screening can help a lot, but it is not chap nor is it fail-safe. Flies can sneak through when you open a door, or find a crevice to crawl through, and one gravid female can do a lot of damage. German Angoras seem particularly susceptible to fly strike, probably because of their increased wool density and their lethargic habits. A rabbit in hot, humid conditions with a heavy wool coat may not keep itself clean, and thus attract flies. That is why a 3 month shearing schedule during the hot, humid “fly months” is so important in preventing fly strike. The hot, humid months are also when bacteria and coccidiosis are more prevalent, leading to more poopy bottoms and even diarrhea, a definite fly attractor! Keeping rabbit bottoms clean is absolutely necessary during the “fly months,” and it requires daily checking. Each rabbit should be removed from its cage, flipped over, and the crotch area checked for any feces buildup. Dry feces don’t attract flies – they need moisture. On the other hand, plain old wet wool can attract them. The fly larvae can kill a rabbit in a day or two, so daily checking is a must, if you have a problem in your area.
A MODEST STUDY PROPOSAL
Had enough of this? I’ve only discussed a few of the many kinds of flies that could potentially cause fly strike. Clearly we would all be better off if we knew who our (fly) enemies were! We could attack the problem knowledgeably. For example, fly paper hanging in the rabbitry may catch a lot of the lowly housefly, but if your problem is the Greenbottle Fly, fly paper isn’t going to do much good. These flies zoom in on their target, they don’t buzz around and lazily land on fly paper. And a stinky fly trap isn’t going to do much good if your problem is the Gray Fleshfly… you would have better luck with a sweet trap. And trapping a particular adult fly in your barn doesn’t necessarily imply that you have identified the fly that is causing you problems with your rabbits.
We need more information from all over the country. Here is what I am suggesting. When you encounter a case of fly strike, please put some of the maggots into a test tube, vial or jar of alcohol (ethyl is preferred and available at many drugstores, but plain old rubbing alcohol will do), and write down as much information as possible: date, time, weather conditions, and where found on the rabbit, how many observed, the type of damage you observed, etc., etc. Very carefully seal the container with an appropriate lid and then tape it down with mailing or duct tape. Pack it so that it will survive in the mail! You can send this material to me at the address below. I will get the larvae identified and compile records of what we are encountering and where, and will report back to everyone as our information base grows.
Mail to:
Susan Wiley
18490 Bethel Church Road
Manchester, MI 48158
Reprinted from IAGARB News, Winter 2000
