Clinical types of myiasis:
(1) pseudomyiasis
(2) myiasis colonizing devitalized tissue
(3) myiasis with invasion of normal tissue
Pseudomyiasis:
(1) This involves the presence of maggots (1) in the mouth, external ear, anus, GI tract, urethra or vagina, without tissue invasion or (2) in clothing.
(2) The eggs may have been deposited in stool or filth in the patient's clothes or bed, and the larvae migrate to the person's body surface for warmth or when disturbed. Larvae identified within the GI tract may have been ingested in food.
Colonization of devitalized tissues only:
(1) Most blowfly larvae are facultative parasites that are deposited in devitalized tissues and which do not invade adjacent healthy tissue.
(2) There is usually a pre-existing wound associated with a history of diabetes, alcoholism, peripheral vascular disease, pressure ulcers, trauma or cancer.
Invasion of normal tissue:
(1) Most commonly associated with the screwworm fly or botfly.
(2) The larvae may be deposited (1) into normal tissue or (2) into a wound then extend into adjacent normal tissue.
(3) The larvae may be associated with a subcutaneous furuncle or burrow.
(4) This may be more likely to occur in the subtropics and tropics.
Laboratory evaluation of fly larvae:
(1) While an expert may be able to speciate larvae, an adult fly is usually more easily identified.
(2) Larvae should be harvested if present. Some larvae should be placed in fixative, while others should be reared to adulthood if possible.
(3) If the larvae are to be transported to a reference laboratory, then care should be taken not to send the larvae in airtight containers since they can suffocate. Attention should also be paid to temperature and dehydration.
(4) Identification of the stage and species can help determine (1) when the eggs were probably laid, and (2) if the species is a facultative or invasive parasite.
Laboratory evaluation of the patient:
(1) culture of affected wound
(2) blood culture
(3) blood ammonia (heavy infestations in animals may be associated with elevated blood ammonia)
