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History:  78 yo male patient admitted for a chronic ulcerative post-traumatic lesion on his left ankle.  The patient suffered an accident riding his motorcycle 20 days ago; his left ankle was trapped under the vehicle for at least one hour until he was helped.  The motorcycle accident 20 days earlier resulted in a painless non-suppurative lesion for which local herbs, and oral cephalexin was used for 2 days.  Eight days before admission here he presented to a local hospital where had a debridement and was begun on clindamycin and cephalexin but the ulcer continued to progress and the patient had a feeling of something moving inside the wound.

Epidemiology:  Born in in the highlands but has lived in Satipo, in the central rainforest, for most of his life.  Still works in a lumber-mill.

Physical Examination:  BP: 180/90; afebrile; ulcerative lesion in the left ankle with irregular borders, the center is erythematous and suppurative [see Image A].

Laboratory Examination:  Hematocrit 40%, WBC 15700 (no bands, 71% neutrophils, 3% eos), fasting glucose: 136.  X-ray of the extremity was normal without signs of osteomyelitis.

Discussion:  After removing most of the necrotic tissue, approximately 11 live maggots were removed [see Image B].  On further questioning the patient related that over 100 similar larvae had been extracted from the wound during the initial debridement.  The maggots are fourth stage Cochliomyia hominivorax [see Image C].  The distinctive features of Cochliomyia are: smooth external aspect (opposite to hairy maggots), pigmented (other species are unpigmented) dorsal tracheal trunks, and two features difficult to observe in the present image – open peritreme (at the tail) and posterior spiracles not in cavity.

Myiasis is defined as the infestation of humans or other vertebrates with larvae of Diptera (or true flies), with the larvae feeding on the host’s living or necrotic tissue.  Cochliomyia hominivorax is an obligate parasite and an extremely destructive species whose larvae only feed on viable tissues in live hosts.  The female fly lays a batch of 10-500 eggs at the edge of any type of skin wound, no matter how small the wound is.  This includes tick bites and navals of newborn animals or humans.  In addition eggs may be laid on mucous membranes including vagina, orbit, mouth or nose [Image D shows extraction of a nasal maggot via an oral incision in another patient].  The highly invasive larvae rapidly produce deep lesions that may be 5 cm or more deep.  The spines may be used as anchors, making removal difficult.  The odor of the lesion attracts other gravid females so that wounds may contain hundreds or thousands of larvae.  When mature, the larvae fall from the host to pupate in the soil and develop into adult flies.  The cycle takes about 3 weeks but may be several months in cooler climates such as in the Andes.  The original description of human infestation with screwworm was in 1858 in prisoners on Devil’s Island in French Guiana.  Coquerel described a fatal outcome in 3 of the 6 men (hominivorax means “man-eater”) involved and screwworm was said to be responsible for hundreds of deaths over the years there.

Cattle are the preferred host for larvae but all warm-blooded animals, including humans, seem at risk.  Screwworm is a devastating agricultural disease and remains one of the major causes of morbidity and mortality amongst livestock in tropical and sub-tropical regions.  Once an animal becomes infested with screwworm, death is generally inevitable unless the wound is cured.  Large programs in the 1970s and 1980s using sterile male flies have eradicated new world screwworm from the United States.  Similar programs are ongoing in Mexico and Central America.  The current southern range extends to Uruguay and northern Chile.  Any case of human Cochliomyia hominivorax myiasis imported into the USA is regarded as a public health emergency by the US Department of Agriculture.  A case several years ago in Alabama resulted in a large-scale quarantine of an entire neighborhood until application of appropriate pesticide could be effected.  A bioterrorism potential is apparent.  Chrysomya bezziana, the old world screwworm is widely distributed outside the Americas.

In travelers, furuncular myiasis due to Dermatobia hominis, the human botfly, is a much more common form of myiasis.  A larva penetrates intact skin.  The larva lives only in the subdermis, is most often single, and appears as a furuncle or boil characterized by a small punctum through which the larva obtains oxygen [see arrow, Image E; from Gorgas Teaching Files].  Occlusion of the respiratory punctum such as with paraffin or petroleum jelly often causes the pear-shaped larva [see Image F] to come out and is curative.  Professor Hugo Lumbreras, the founder of our Tropical Medicine Institute, used to teach of the traditional approach to myiasis in Perú, which involved the occlusion of the entrance of the wound by a solution made from basil leaves, the odor of which forced the larvae out of the wound.

At our institute we see about ten outpatients per year with screwworm infestation and over the last ten years and ten patients have required hospitalization.  Dermatobia hominis occurs in Perú but is generally not serious enough to warrant referral to an institution such as ours.