February 2016

A 58 year old male with a lateral canthus lesion

Stephanie Wright, MD, and Rocco LaSala, MD

Overview

In early September, a previously healthy 58 year old male presented to an outside ED with a two-week history of a slowly enlarging, painful nodule at his right lateral canthus. The skin overlying the nodule was erythematous and inflamed, and drained clear fluid from a small punctate opening. He denied any systemic symptoms or vision changes. He was diagnosed with a presumed inclusion cyst with secondary infection, prescribed erythromycin ointment, and scheduled for surgical excision.

Over the following week, the lesion worsened, with increasing spontaneous lancinating pains and hyperalgesia of the right side of his face. Upon the application of antibiotic ointment to the lesion, he extracted a large, moving object from the punctate opening, which he brought with him to scheduled appointment. On subsequent questioning, the patient revealed that he lived in a rural area, worked closely with cattle and had never traveled outside of the United States nor beyond the West Virginia state border in many years.

Gross Description

The object (Figure 1) measured 1.7 x 1.0 x 0.4 cm. The exterior surface was covered entirely by small, dark brown spines. The anterior end contained two small fanglike protrusions whereas the posterior end demonstrated paired spiracles.

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Figure 1 – Object extracted from right lateral canthus (images courtesy of John Strazanac, PhD).

Diagnosis

What is the most likely explanation for the patient’s lesion ?

Answer

Please select an answer above.

Discussion

Myiasis is defined as an infestation of tissues by fly larvae (i.e. maggots). Many flies of the order Diptera do not require a living host but rather lay their eggs on refuse or rotting meat. Their rapid and predictable life cycles are even useful in determining time of death in forensic entomology. However, such species can produce myiasis in a living host, typically within a necrotic wound. Calliphora spp., Lucilia spp., and Phormia regina have been known to cause facultative myiasis. Larvae of these species may feed only on necrotic tissue, and decontaminated strains of Lucilia sericata have been approved for use in medical wound debridement.

Other fly species are obligatory parasites of wounds and invade into healthy surrounding tissue. The old-world screwworm Chrysomya bezziana is found throughout the Middle East, the Indian subcontinent, Southeast Asia, and Indonesia. The new-world screwworm Cochliomyia hominivorax was once present as far north as the southern United States, but the population has been controlled by the release of sterile male flies. Its territory is now limited to Central and South America. Wohlfahrtia spp., known as fleshflies, can infest a living host through mucus membranes as well as in wounds. W. magnifica spans from the Mediterranean to China, while W. vigil is present across the northern United States, and throughout Central and Southern Europe and Russia.

Other species of flies are known to parasitize healthy tissue in hosts. In Subsaharan Africa, the Tumbu fly Cordylobia anthropophaga causes furuncular lesions. In Central and South America, Dermatobia hominis, the human botfly, is a well-known cause of myiasis in healthy travelers returning to the United States. Because these two species are the most common agents of human myiasis worldwide, it was reasonable to seek additional history pertaining to the case patient’s prior travel. Given that he had no prior travel to these regions and that the extracted larva bared little resemblance to D. hominis (Figure 2), a search for alternate agent of myiasis endemic to this region was initiated.

Several genera of botflies are known to veterinary medicine for parasitizing specific mammals: Hypoderma (cattle), Oestrus (sheep), Gasterophilus (horses), and Cuterebra (rodents/rabbits). Although any of these species may incidentally infest humans, the most common culprit is Cuterebra.

Cases of Cuterebra myiasis have been described throughout the United States with most reports originating in the Northeast and Mid-Atlantic states. The Cuterebra life cycle begins in late summer and early fall, when adult flies deposit their eggs in close proximity to the habitat of the intended host, such as soil and plants at the entrance of a rabbit or rodent burrow. Upon passing, the eggs attach to a host’s fur and heat from its body triggers hatching into first instar larvae. There, larvae can penetrate unbroken skin or, in some cases, may seek out a mucosal entry point such as the mouth or eyes. Following entry, the larvae will mature for several weeks either directly in situ or, depending on site of entry, following a migratory pathway to the dermis. Only a single larva dwells in the enlarging nodule, colloquially called a “warble”. When mature, it will exit through its former breathing hole, fall into the soil, and pupate until the following spring.

Inadvertent infestation by Cuterebra spp. in humans likely occurs following exposure to first instar larvae directly from the environment or indirectly through carriage on a pet. First instar larvae are ~1mm in length, so transfer of the parasite can easily go unnoticed. Penetration through unbroken skin or entry through small breaks in the skin may produce mild irritation but is more commonly sub-clinical. The most common sites of human infestation include the face, scalp, neck, and back, although ophthalmic, nasopharyngeal, tracheobronchial and genital areas have also been reported rarely.

Although humans are an unintentional host for Cuterebra spp., larvae will continue to develop over several weeks, eventually maturing into the third instar forms (as seen in Fig 1). However, the maturation cycle is often interrupted as a result of medical intervention. Extraction of larvae by surgical excision is the preferred method of removal, yet application of antibiotic (or other) ointments can occlude the breathing hole and, as in this case, may prompt premature migration or facilitate manual removal with forceps.

Species of Cuterebra can be difficult to distinguish phenotypically, particularly for immature instars. To assist in identification of this specimen, a 413 bp segment of the cytochrome oxidase II (COII) mitochondrial gene was amplified using primers to highly conserved regions. Amplicons underwent bidirectional Sanger sequencing and were compared to known COII sequences of botflies in NCBI GenBank. Alignment confirmed taxonomic placement within the Cuterebra genus (Fig 3), yet similarly high sequence identities for this single gene (99.9 % and 99.8% with C. fontinella and C. grisea, respectively) were insufficient for definitive taxonomic placement in either species. That this specimen represents a distinct species with underrepresented sequence deposits in NCBI also cannot be entirely excluded.

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Figure 2 – Four larvae of D. hominis, removed from a human host (CDC, DpDx Image).
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Figure 3 – Evolutionary relationships of taxa inferred using the UPGMA method. Evolutionary distances were computed using the Maximum Composite Likelihood method and are in the units of the number of base substitutions per site. The analysis involved 15 nucleotide sequences with a total of 413 positions in the final dataset. All positions containing gaps and missing data were eliminated. Evolutionary analyses were conducted in MEGA5.

The patient’s remaining canthal lesion was surgically resected and revealed no additional larva. Microscopically, the lesion consisted of granulation tissue with an eosinophilic infiltrate. The patient has since recovered uneventfully.

Resources

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