FLIES
There are many types of flies that are important in causing human disease. We will discuss some here but note that this is not inclusive of all flies that can be clinically important. Some are ectoparasites. Others are important vectors of parasites, bacterial pathogens, and viral pathogens.
Musca domestica (common house fly)
Musca domestica (adult) and larvae removed from the lung of a patient with pulmonary myiasis (see video below)
The common house fly is perhaps the most familiar of the flies. It is a mechanical vector of bacterial, viral, and parasitic diseases. The common house fly is also known to cause myiasis, a parasitic infestation by the larvae of the fly. Generally the eggs are laid in exposed or compromised body tissue. The eggs hatch releasing numerous larval stage flies or maggots. These maggots will feed on damaged tissue and eventually develop into mature flies. Maggots can be introduced into unusual host sites such as the lungs of patients that have been incapacitated for an extended length of time (elderly patients that are bed ridden are at risk).
Tsetse Fly (Glossina Species)
The Tsetse Fly is an important vector of Trypanosoma brucei which is the causative agent of African Sleeping Sickness. The genus name is Glossina and there are multiple species present in the genus that will transmit the parasite.
The Sandfly (genus Lutzomyia and Phlebotomus)
The name Sandflies include a large variety of flies and is generally used to describe any number of species or genus of biting blood sucking flies found mostly in sandy areas. In the world of parasitology the most important of the Sandflies transmitting parasites are those of the genus Lutzomyia and Phlebotomus. Sandflies are aggressive feeders and transmit Leishmaniasis as well as various viral diseases (Toscana Virus, Sandfly Fever Sicilian Virus, Sandfly Fever Cyprus Virus, and Sandfly Naples Fever Virus).
Botfly (Dermatobia hominis)
Botflies are called a variety of names (warble flies – gadflies – and heel flies). They belong to a family of flies known as the Oestridea. The human bot fly is Dermatobia hominid. There are other bot flies that will infect the skin of other mammals. The human bot fly has many suitable hosts available such as cattle and dogs which will be in close proximity to humans. But suitable hosts can extend to cats, monkeys, sheep, and many other mammals.
The human bot fly is described as a large fly with a dense coating of hair. It is approximately the size of and has the appearance of a bumblebee with a yellow face, blue metallic abdomen, and orange legs. Though the bot fly is not known to transmit disease, it will however cause myiasis in the subcutaneous layers of the skin of the host. This parasitism in some cases can also occur within the gut. The life cycle generally requires a blood sucking arthropod vector to act as a transporter of the fly eggs to the skin surface of an unsuspecting human or other mammalian host. These mechanical vectors include mosquitos and ticks which are captured in flight to have the eggs deposited on them. Once the eggs are on the skin surface, the warmth of the skin will induce the hatching/release of larvae. The larvae will then make their way into the human tissue by either burrowing or entry through a compromised area of skin. Maturation of the larvae then occurs within 30 days in the skin or may take longer. During this maturation there is the formation of pustules, skin irritation, and the uncomfortable sensation of movement under the skin. The larvae survive in the skin of the host by breathing through spiracles flush with the skin. Secondary infections are possible but as mentioned before the fly is not known to be a vector for any diseases. The maturation of the larvae in the skin to the third instar larval stage generally takes about 30 days whereupon the larvae will crawl out and drop to the soil where it will pupate. Once the larvae emerge from the pupae stage as adults their prime objective is to find a suitable mate. And the cycle continues with the egg laden female depositing its eggs on the transporting vector to eventually have larvae be introduced into another host.
Geographically, the human bot fly can be found in Central and South America.
Life Cycle of the Botfly
Removal of the larvae from the feeding site is the best treatment and there are various methods that can be used. Surgical removal of the larvae is a simple yet invasive way of retrieving the larvae that can result in secondary infections if the surgical site is not cared for. Another method is the blockage of access to air which will coax the larvae into exiting the area. This is best done by blocking the spiracles of the larvae so that the larvae can not breathe. This can be done by covering the air access site with petroleum jelly, or beeswax, or any substance of similar consistency. Not being able to breathe, the larvae will migrate out of the lesion for easy removal.