Myiasis: A Primer
“Myiasis is the infestation of live vertebrate animals with dipterous larvae, which at least during a certain period of time feed on the host’s
dead or living tissue, liquid body substances or undigested food” (Zumpt 1965: xi). Several classifications have been established to further
describe such parasitism. These categories include (1) obligatory parasites comprised of dipteran species whose larvae normally develop
within or on living tissues of vertebrates. This classification is further divided to include larvae living in: (a) nasopharyngeal
(i.e., relating to or affecting nose and pharynx) cavities of herbivorous mammals, (b) dermic or subdermic tissues of certain mammals,
(c) cutaneous and subcutaneous tissues of some avian species, and (d) the alimentary canal of horses and other mammals. The second
category includes (2) facultative parasites comprised of dipteran larval developing in decomposing organic materials or the necrotic
tissues of living animals (Guimarães & Papavero, 1999).
Dermatobia hominis larvae can burrow directly into the area where first contact is made with the host or crawl and then burrow
into subcutaneous tissue. The larva stays in the area of entry where it develops within a painful subdermal tumor. The most common
infection site in humans tends to be uncovered areas, but they can also enter the eyes and nostrils. Penetration of the dermal tissue
occurs within 5 to 10 minutes (Neiva & Gomes, 1917).
Upon reaching the subcutaneous tissues, the larva develops within a tumor open to the outside air to facilitate respiration (Guimarães
& Papavero, 1999). The larval period is somewhat poorly understood in regards to the length of individual instars but is thought to vary
due to temperature (Neiva & Gomes, 1917). However, the overall time interval for larval development is thought to be around 46-47 days.
During this period, larvae eat necrotic tissues associated with the entry wound (Guimarães & Papavero, 1999).
Mature larvae then exit the host and fall to the ground. The larva will then try to penetrate the substrate for a suitable place to pupate,
however, if the ground is hard they will pupate above ground but will not emerge as adults (Neiva, 1910). Neiva and Gomes (1917)
found that under experimental conditions mature larvae develop into pupae in about 24 hours. The pupal period ranges from 23-29 days
after which the adult emerges through the operculum of the pupal case (Ribiero & Oliviera, 1987; Neiva & Gomez, 1917).
Adults do not feed and must therefore mate, reproduce, and lay their eggs (females) on the reserves acquired while in the host organism.
The adult life span ranges between 2-19 days. Under experimental conditions, Ribiero et al. (1993) found females to live between
3-11 days and males 2-11 days. Futhermore, they found females to die immediately following the oviposition of 1-438 eggs with males
dying on the 4th day after emergence.
Myiasis by Dermatobia hominis is recorded in native fauna including: monkeys, jaguars, pumas, agoutis, grisons, armadillos,
toucans and ant birds (Guimarães & Papavero, 1999).
Reports of parasitization of humans by D. hominis are also not uncommon. In fact, The Washington Post published an article
in the November 11th 2003 Health Section which detailed an account of such parasitism in a Bethesda, MD resident having just returned
from a visit to Costa Rica. The woman first noticed two red, oozing sores on her leg. She also reported having felt movement within
the areas of the sores and experienced stabbing pain as well. One physician intern suggested tick bites and expressed concern regarding
lyme disease. Another physician put her on an antibiotic to treat a putative case of staph infection or Lyme disease. Copious amounts
of fluid oozed from the sores, and the patient had to change her pajamas and bed clothes daily. The woman was referred to a dermatologist
who instructed her to quit taking the antibiotics, assured her that she had an allergic reaction to some bites, and that the malady would go
away in two weeks. Several weeks had passed, and the patient was feeling more pain and movement within the now ½ inch, blue-green tumors.
Another dermatologist who was equally perplexed took a biopsy and sent it to a pathologist who reported no unusual results.
Ultimately, the woman diagnosed her own condition after a visit to the library where she stumbled across “The Travel Doctor: Your
Guide to Staying Healthy While You Travel” by Mark Wise. She found text describing her wounds (e.g., shooting pains and movement)
and heeded the book’s advice to suffocate the maggot using peanut butter. Responding from the lack of air, the larva burrowed partially
to the surface of her skin. The next day she applied duct tape and upon pulling it off discovered a white piece of tissue coming off with
the tape. Unable to remove the entire larva, she called her dermatologist’s receptionist and told her that she knew the diagnosis. The
patient asked her doctor to look up myiasis in her medical journal, the doctor agreed with her self-diagnosis, and the doctor proceeded
to surgically remove two larvae; one of which was approximately 1 inch long by 1/4 inch thick and the second even larger. Both the patient and
doctor were very disgusted by the appearance of the removed larvae. The patient followed up by showing her first dermatologist what
had been discovered and e-mailed Mark Wise to tell him that his book helped solve a mystery that had boggled four different doctors.
However, not all flies associated with myiasis are deleterious or otherwise a nuisance to their vertebrate hosts. In fact, some species
of Calliphoridae (Blowflies) have been observed on infected wounds since the mid-1500’s and once their advantageous interaction
was elucidated used by physicians. Ambroise Paré (1509-1590), the chief surgeon to Charles IX and Henri III, discovered in 1557
during the battle of St. Quentin that maggots often infested pus-forming wounds but was not privy to the benefits of their presence
(Sherman et al., 2000). Likewise, in 1829, Baron Dominic Larrey the chief surgeon to Napoleon found that these infestations prevented
infection and hastened healing (Sherman et al., 2000). Physicians used such flies to cure very serious infections such as temporal
mastoiditis and perineal gangrene where traditional surgical and antibiotic treatments were ineffective. Today, these beneficial
maggots serve as an adjunct to modern medicine rather than solely as a last resort. They are used to treat several maladies including
the foot wounds of diabetic patients. Sherman et al. (2000) document other conditions in which these beneficial fly larvae are
employed to remove dead tissue and speed healing.
Illustrations & Photos | Distribution | Vectors | Eggs | Myiasis | Literature
Content by Jonathan M. Eibl and Norman E. Woodley. Last Updated: August 31, 2004.
