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Thursday, September 4, 2014

Wound myiasis (Warning: GRAPHIC PHOTOS!)


As you may know - myiasis is the infestation of a living animal's tissues by fly larvae, or maggots. These infestations occur all over the world, but are most common in tropical climates. The larvae can feed on the host's living or dead tissue, bodily fluids, or even ingested food. In animal husbandry, myiasis leads to great economic losses, since it reduces milk production and hide quality. In humans, maggots are known for causing highly unpleasant - and sometimes dangerous - wound infestations. Poverty, bad living conditions and a lack of hygiene are predisposing factors. Inadequate medical and nursery care of the elderly, psychiatric patients, alcoholics, and other helpless patients, especially those with the inability to discourage flies from depositing eggs or larvae, also makes humans prone to wound infestation (on this blog, I've described the cases of an alcoholic from Brazil with a huge maggot-ridden cancer wound and a boy with cerebral palsy whose tracheostomy became infested with fly larvae, probably as a result of neglect). Poor eyesight may make it harder to detect myiasis. Human natural disasters may be another predisposing factor for wound infestation by fly larvae.

Wound myiasis is most often initiated when flies lay eggs in necrotic, hemorrhaging, or pus-filled lesions. Wounds with alkaline discharges (pH 7.1 to 7.5) have been reported to be especially attractive to blow flies. The presence of necrosis is also an important factor. In human cases, there is usually only one offending species in the lesion, although mixed infestation can occur, reaching rates of 3% in one series.

Cochliomyia hominivorax, the New World screw-worm fly, or screw-worm for short, is a singularly unpleasant species of parasitic fly, well known for the way in which its larvae eat the living tissue of warm-blooded animals. While the maggots of many fly species eat dead flesh, and may occasionally infest an old and putrid wound, screw-worm maggots are unusual because they attack healthy tissue. C. hominivorax is found in the New World tropics. There are five species of Cochliomyia but only one species of screw-worm fly in the genus; there is also a single Old World species in a different genus (Chrysomya bezziana). 
 
Both New World and Old World screw-worms cause multiple infestations, with 100 to 500 eggs. The females often lay eggs on or near a wound. After hatching, the larvae immediately begin feeding, causing an extensive destruction of tissue and a foul-smelling bloody discharge. Tissues around the lesion become swollen, and pockets may be eaten out beneath the skin. C. hominivorax and C. bezziana myiases are typically very painful. Tissue invasion and local destruction caused by the larvae when leaving the necrotic tissue - more mature larvae are often more invasive - lead to significant local pain and secondary bacterial infection. Other symptoms include fever, chills, bleeding, and fistula formation. Cavernous lesions are formed, so it is difficult to extract the larvae in a single session, and this delay makes the situation more dangerous. An infested person may actually die from tissue destruction.

Scary, right? And here are some photos. Please don't lose your lunch.


Myiasis due to C. hominivorax in a B lymphoma patient. The photo shows a huge ulcer filled with larvae.



 And here is a close-up shot of the ulcer, along with the maggots in all their wriggly glory. Ugh.


 (Images from: Francesconi and Lupi 2012)

C. hominivorax myiasis in a basal cell carcinoma case. Note how large the larvae are.

Lying weak and bedridden, with stinking fluid seeping from your wounds and maggots slowly eating you alive, sounds like something from a Stephen King novel - but sadly, it does happen in real life.


Literature:

Francesconi F, Lupi O. (2012) Myiasis. Clin Microbiol Rev. 25(1): 79-105.


 

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