A case of a 54-year-old woman who presented with a breast mass is reported. Histologically, a chronic granulomatous inflammatory response was observed. The response was associated with an organism diagnosed as a fly larva, Dermatobia hominis (human botfly). The incidence of myiasis, infestation by fly larvae, presenting as a long-standing breast mass and mimicking a neoplasm is extremely rare, especially in the United States.

Myiasis is the infestation of the body by the larvae of Diptera, which extract nutrients from the host's tissues, fluids, or food.1–7  Dermatobia hominis (human botfly) is an obligate Diptera that needs to feed on a host to complete its development. Temperate and tropical climates are most favorable for Diptera; they are indigenous to Central and South America and have been reported in the United States. The larvae are usually seen in the skin and subcutaneous tissue, but they have been observed in almost every part of the human body, and early in the infestation they usually evoke little cellular reaction. An abscess or numerous neutrophils and eosinophils may be detected adjacent to the organism, especially if the larva remains in the body and dies. Rarely, a granulomatous response has been reported around the larva.

A 54-year-old-Hispanic woman from El Salvador presented with a breast mass. Two years before surgery, the patient had been assaulted with a blow to the right breast, which resulted in a laceration and development of a mass. Pain and discomfort in that region up to the time of surgery had been noted. The mass was tender and had been stable in size for several months. The patient also complained of intermittent pain on the right and left sides of the chest that would wax and wane, both with and without exercise.

Physical examination revealed a palpable, firm, mobile, occasionally tender nodule just deep to a scar in the upper outer quadrant of the right breast directly below the axilla. A mammogram revealed an oval, well-circumscribed, 2-cm, benign-appearing mass that contained calcifications (Figure 1). The differential diagnosis included a partially calcified hematoma or fat necrosis within a lymph node. There were scattered calcifications within the right breast thought possibly to be related to the prior breast trauma. No suspicious clusters of microcalcifications or other masses were seen.

Results of routine laboratory studies, including serum chemistries, urinalysis, and complete blood cell count, were noncontributory, and there was no evidence of eosinophilia. When the mass was removed, a small amount of material exuded from the core, suggestive of the contents of a sebaceous cyst. The patient was discharged to her home and subsequently lost to follow-up.

After surgical extirpation, the surface of the mass was marked with india ink, fixed in 10% neutral-buffered formalin, embedded in paraffin, then sectioned and stained with hematoxylin-eosin.

Gross examination of the breast showed a portion of yellow-tan fibroadipose tissue measuring 2 cm in maximum dimension. Histologic examination of the breast lesion revealed a cystic structure with an adjacent inflammatory granulomatous response characterized by numerous lymphocytes, histiocytes, including multinucleated giant cells, and desmoplasia. An organism thought to be a parasite was noted, but the exact species was unknown. Consultation with the Armed Forces Institute of Pathology stated that this was the larva of D hominis (human botfly) and represented myiasis. The structures that were readily identified included a thick exoskeleton with spines, breathing tubes, striated muscle, and posterior spiracles (Figure 2). The larva was degenerated, suggesting that it had been dead for a long time. Focally, spines were centered within foreign body giant cells (Figure 3).

Dermatobia hominis is the most important fly to produce myiasis. Dermatobia hominis is a common parasitic infestation seen in Central and South America, and cases are now encountered outside the endemic regions.1–3 

Clinically, myiasis occurs primarily on the exposed skin involving the face, scalp, and extremities. However, cases of invasion of other body sites, including the brain, have been reported.1 The larvae enter the skin into the subcutaneous tissue, with production of an itchy papule. As the larvae feed and grow, a furuncular erythematous nodule with a patent punctum (warble) is produced. Movement of the larval spiracle (respiratory apparatus) may be seen at the punctum. The exudate around the punctum may be serous, serosanguineous, or seropurulent. An intense inflammatory reaction may be seen in the surrounding tissue during a later stage of the infestation.1,5 The larvae may also invoke a granulomatous inflammatory cell response if present for a longer period. Symptoms may include mild pruritus, periodic stinging, or intense cutaneous pain, which may lead to agitation and insomnia.

The adult human botfly is about the size of a bumblebee (1.5–1.8 cm). It has a yellow face, metallic blue abdomen, and orange legs. The adult botfly does not feed, so the mouth parts are vestigial. In the life cycle of D hominis, the female attaches to a vector onto which she deposits her eggs. The vector is usually the female Psorophora mosquito,1,2 but the gamut may include a tick, any other biting arthropods, or even a plant.3 After the mosquito has landed on a warm-blooded subject, the larvae, attracted to the warmth of the blood, hatch from the eggs and penetrate the skin through the bite of the vector through the opening of a hair follicle, a skin fold, or another site. The larvae have been seen on uncovered extremities but have been identified in practically every location on the skin surface. The larvae may survive on the skin for up to 3 weeks. Other hosts include birds, dogs, and cattle.3 

After the larvae have penetrated the skin, they feed and grow on the skin exudate. There may be 2 or 3 moltings. The larvae grow from a cylindrical shape to a mature, stage III larvae, measuring 1.8 to 2.4 cm in length, and have the appearance of an inverted flask. They have mouth hooks and 2 to 6 rows of dark hooklike processes that keep the them from being easily withdrawn. The spiracles of the larvae are flush with the skin surface so respiration may occur. Complete larval development may take approximately 1 to 3 months. The larvae extrude themselves from the skin and must go to the ground to pupate. Pupation may take 2 to 3 weeks, and then the adult fly emerges.

It is important to distinguish the larvae of D hominis from other conditions that can produce similar lesions. Other organisms that may have a similar histologic appearance include ticks and Tunga penetrans. Ticks have legs and a mouth and do not burrow deeply into the skin. They have spines and a cuticle less than 50 μm in thickness. Tunga penetrans is a flea that almost always involves the feet, has legs, does not have a spine, and has a cuticle greater than 20-μm thick. The larvae of D hominis do not have legs, have large and sparsely distributed spines, and have cuticles greater than 50-μm thick. As the larvae of D hominis are usually symptomatic, they are promptly removed from the human host. This case is unusual as the larvae died but remained in its host for such a long period of time. The degeneration of the larva with the adjacent chronic granulomatous inflammatory cell response was consistent with the clinical history of the mass being present in the patient for many months. Treatment for D hominis consists of evacuation of the larvae and systemic antibiotics for any possible secondary infection. The punctum of the larvae may be occluded by material such as petroleum jelly or pork fat, which prevents the organisms from breathing. This hypoxia stimulates the larvae to emerge from the patient. Also, agents noxious to the organism, including lidocaine hydrochloride, may be injected under the cutaneous mass, followed by extraction. Larvae may then be removed by lateral pressure. Surgical excision is usually unnecessary while larvae are alive but may be performed with a scalpel and forceps when the larvae are dead or decayed.4,6 The larvae are removed surgically by making a cruciate incision, avoiding the central hole, because this may result in larval damage, portions of which are then left in the region. After removal of the organism with probe and forceps, the organism should be checked to ensure that it is complete and intact.6 

The larvae of D hominis do not normally infest humans. Humans are infected with D hominis when they enter the environment of the natural host and come in contact with vegetation and detritus that has eggs containing infective larvae that are ready to hatch. The best preventive measure is avoidance of areas endemic with D hominis.8 

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