Flies

There are over 80,000 species of flies and mosquitoes and these as a group vector more diseases than any other order. Characteristics of the order include one pair of wings, halters (knoblike reduced second wings), and complete metamorphosis. The adults vector diseases mechanically and biologically, while the larvae often invade tissue of higher animals. Members of this order have a wide range of breeding habitats including plants, flesh, excrement, and standing water.

Myiasis

Myiasis is the condition where fly maggots infest organs and tissues of living humans or other animals. There are several types depending which part of the body is infested and how it occurred. The more common types include the following: enteric (both intestinal and gastric), urinary, nasal, auricular (ear), ophthalmic (eyes) and dermal/cutaneous. When wounds are involved it is referred to as traumatic. Accidental myiasis occurs due to ingestion of maggots by consuming infested food. This can also occur when the maggots invade an anal opening. Semi-obligate myiasis occurs when the maggots invade dead tissue and progress into live tissue that surrounds a wound. Obligate myiasis is when the maggots only infest living tissue.

Flesh Flies and Blow Flies

The flies most frequently associated with traumatic myiasis are the flesh flies and blow flies. Blow flies are often found in houses and have a characteristic green, copper or blue metallic coloration. The larvae feed on dead animals, excrement, and occasionally living flesh.

The flesh fly is found near carrion, excrement, and decaying organic matter. It has a characteristic checkerboard pattern to the abdomen and has 3 dark stripes on the thorax. These flies can get quite large. Instead of laying eggs, flesh flies give birth to live maggots, which are either deposited or dropped mid-air. I was once standing in a motel room in Costa Rica and a flesh fly deposited a larva on my arm. Within less than a minute the larvae began to attempt to bore into my skin.

In the United States most cases of traumatic myiasis are more or less confined to rotting tissues. Typically these flies deposit their larvae or eggs (hit) that hatch and feed chiefly on the decaying tissue associated with wounds. This feeding activity rarely harms the person or animal, and in some cases can actually be beneficial. It was found during World War I that wounded troops not found immediately but left in the battlefield for a few days developed less of certain types of deep-seated infections than those soldiers taken directly to the hospital. The wounds of the soldiers left in the field were infested with fly maggots, which secreted a material named alantoin, a natural antibiotic. The introduction of blow fly maggots into wounds is still practiced worldwide by many primitive people and the use of sterilized maggots for this purpose is now receiving some attention by physicians in the United States.

Under certain conditions this type of infestation can be dangerous, if not deadly, to animals. Of course, this is partially due to the inability of many animals to clean infested wounds. For example, we once had a rabbit that accidentally dipped its dewlap (a large flap of skin under the chin) into its drinking water every time it drank. As a result some of this tissue became excessively damp and began to rot. A blowfly made a hit of several dozen eggs and by the time we discovered it, the maggots were feeding on the entire underside of the still living animal. Of course the animal had to be destroyed.

One example of semi-obligatory myiasis is the screwworm fly, Cochliomyia hominovorax, which is attracted to the wounds/sores of cattle and other animals. The eggs are laid in batches of 200 to 500 in 2 to 10 day-old wounds and hatch after 14 to 18 hours. The larvae initially feed of rotting tissue but quickly bore into healthy flesh and feed for 3 to 5 days. Once fully developed the larvae drop to the ground to pupate and after 7 days emerge as adults. Wounds infested with these flies become extensive, attracting other flies and even more of the screwworm flies. In some cases the infested animal becomes weakened, may not feed or drink and, if not treated, may die. This pest is no longer found in the United States because of a successful sterile male release program. This blowfly is still a major pest of cattle, ranging from Mexico to South America.

In many areas of the world human myiasis from this fly is fairly common. In 1935, prior to its eradication in the United States, over a hundred cases of human myiasis occurred in Texas alone. In one documented case, a female fly deposited her eggs up the nostril of a man who had a cold. Apparently, the fly was attracted to his nasal discharge. The first symptoms were those of a severe cold. As the larvae cut away through the various tissues of the head, the victim became slightly delirious and complained of intense misery and discomfort in the nose and head. When the larvae finally cut through the soft pallet, his speech was impaired. Despite attempts to remove the larvae the patient, after a short recovery, had a relapse as the Eustachian tubes were invaded. The tissue damage was extensive and the head and face showed the characteristic swelling of screwworm myiasis. During the autopsy over one hundred larvae were removed from the man’s brain.

House Fly

The house fly, Musca domestica, is one of the most well known insect pests in the world. This fly has 4 longitudinal stripes on the thorax (Figure 87). The eggs are laid in batches of 100-150, with a female capable of producing over 1,000 in her lifetime. The eggs hatch in about 8-12 hours and the larvae are worm-like with no visible head area. The larvae or maggots go through 3 successive molts that, under optimum conditions, take 2 to 5 days. With many flies the outer skin of the last instar larva hardens prior to pupation and forms a capsule like structure within which is found the pupae. The puparium is dark and will take 3 to 4 days to develop into the adult fly. Under ideal conditions the house fly can complete one life cycle in as little as 7 days. With this number of eggs and speed of development these insects have a tremendous reproductive capacity. Someone once figured out that if one female house fly in April laid all her eggs and all her offspring survived and reproduced similarly, by August there would be 191,0100,000,000,000,000,000 flies or enough to cover the earth by 43 feet. This obviously doesn’t happen. The limiting factor is the availability of food. Of course, in the United States we have health codes that, when followed, greatly reduce the breeding sites of these pests.

The adult lives for about 30 days and prefers to oviposite on decomposing manure or vegetable matter. Normally most fly infestations around the home are of local origin; however, these flies are capable of flying up to 20 miles if need be.

The house fly occasionally causes enteric myiasis. One interesting case resulted when an elderly lady went to the doctor complaining of intestinal and stomach cramping. After considerable consultation, a stool sample revealed living house fly maggots and pupae. Apparently she had consumed some food that was infested with maggots. These maggots passed through the digestive system and were basically impervious to digestive enzymes and other digestive processes. Normally under these conditions maggots in the digestive system may survive but do not complete their life cycles. Rectal myiasis has also resulted when house fly adults lay their eggs around the anus. Upon hatching the maggot migrate up into the rectum causing considerable discomfort. (So, wipe well!)

The most important medical implication of the house fly is its potential to vector disease causing organisms. The house fly freely enters the home, restaurants and other places where human food is available; it just as freely inhabits situations where human and animal excrement is available. It feeds on human food and excrement! Because the fly can only feed on liquids it regurgitates digestive enzymes from its stomach to dissolve solids. In doing so it may also regurgitate drops of excrement. The fly is also structurally adapted for picking up pathogens. Its mouthparts are provided with many fine hairs and ridges that readily collect germs and filth. The tarsi are a complex structure of fine hairs and sticky pads that enhances it potential for vectoring diseases. Studies have indicated that a single house fly can carry as many as 6 million bacteria on its body. Flies have been known to be contaminated with more than 100 species of pathogenic organisms, including the causative agents of amoebic and bacterial dysentery, typhoid fever, cholera, salmonella, anthrax, leprosy, yaws, trachoma, polio, and infectious hepatitis. In addition they have been demonstrated to carry the eggs of certain pathogenic worms including pinworm, tapeworms and hookworms.

Lesser House Fly

The lesser house fly, Fannia canicularis, commonly inhabits houses as well but is found all over, not just primarily in the kitchen, as is the house fly. The larvae are quite small and have spines covering the body and are usually found in excrement and decomposing organic matter. These flies can be readily distinguished from the house fly by their flight pattern of hovering or flying in circles in garages and breezeways. They can vector many of the same diseases as the house fly.