A “boring” worm? New World screwworm neuromyiasis in a patient with Darier disease

Introduction

Cochliomyia hominivorax, also called Coquerel or New World screwworm (NWS), is an obligate parasitic fly primarily endemic to parts of South America and the Caribbean Islands. Human infestation by NWS is rare in the United States but can potentially be fatal. Individuals with open wounds are at risk, especially when traveling to endemic areas. Here, we present the case of a 15-year-old girl with Darier disease with chronic scalp lesions who developed an intense focal headache after a hiking trip to Peru, culminating in the discovery of a scalp ulcer harboring 45 NWS larvae.

Case report

A 15-year-old girl developed an intense focal occipital headache the day after she returned from a 3-week hiking trip to Peru. She had a 7-year history of Darier disease that was only partially responsive to isotretinoin, along with recurrent staphylococcal infections on her scalp. Examination showed a 4.5 mm “punched-out” ulcer of the scalp posterior to the vertex without surrounding erythema or purulence. Undulating movement was noted at the ulcer base (Fig 1). Occlusion of the lesion with bacitracin ointment encouraged the egress of 45 cream-colored 4-7 mm larvae (Fig 2). Larvae were sent for identification and recognized to be Cochliomyia, with cylindrical bodies with a reddish tinge, prominent rings of spines, 12 body segments, and mouth hooks in the anterior end (Fig 3). Histologic examination revealed a thick, chitinous cuticle with widely-spaced spines on the abdominal surface, beneath which were layers of striated muscle and internal organs (Fig 4). Magnetic resonance imaging showed no evidence of intracranial extension. Ivermectin 400 mcg was administered once and full-thickness excision of the regional scalp skin, subcutaneous tissue and periosteum was performed to assure full larval clearance. The intense headache dramatically improved within hours of NSW removal and the patient recovered well without long-term sequelae.

Fig 1.

Scalp ulcer of the scalp containing Cochliomyia hominivorax larvae.

Fig 2.

Cream-colored 6 mm screwworms. The narrowed end corresponds to the 2 mouth hooks. The blunt, wide end contains the breathing spiracles.

Fig 3.

Microscopic whole-body appearance of the NWS larva. The larva has a cylindrical body with a reddish tinge, prominent rings of spines, and 12 body segments. The mouth hooks can be visualized in the anterior end (bottom right). Bar = 1 mm. NWS, New World screwworm.

Fig 4.

Routine histologic appearance of a hematoxylin and eosin-stained cross-section of C. hominivorax. The 2 hook-like mouthparts bore aggressively into tissue, causing tissue destruction.

Discussion

Cochliomyia hominivorax is the only primary screwworm in the Cochliomyia genus causing myiasis in living tissue.¹ In the Calliphoridae family which includes Cochliomyia, Chrysomya bezziana (Old World screwworm) is another obligate parasitic species.² Whereas the NWS is primarily endemic to areas of South America (Venezuela, Colombia, Ecuador, Brazil, Paraguay, and Uruguay) and the Caribbean Islands (Cuba, Dominican Republic, Jamaica, and Trinidad, and Tobago), Old World screwworm is endemic to areas of Africa and Southeast Asia.² C. hominivorax and C. macellaria overlap in geographical distribution, but the former can also be found in areas of Southern Canada and the United States.² NWS larvae feed on living tissue (thus the name hominivorax or “man-eater”) and primarily infest cattle and other livestock, causing substantial economic loss for farmers.² In contrast, C. macellaria feed on carrion.² NWS can be distinguished microscopically and sometimes visibly by their distinctive pigmented tracheal trunks in the final third of the body, which are not found in C. macellaria larvae.¹ The more common post-travel myiasis infestation is the human botfly (Dermatobia hominis), which uses a vector insect to deposit a single egg on a human host, in contrast to the 100-400 eggs inoculated by the NWS fly.²

The United States and Mexico eradicated the NWS in 1982 using a male fly sterilization technique.³ Since that time, rare cases of human infestations have been reported in travelers returning from endemic regions,^4,5 leading to the risk of carrying NWS to new regions.³ Open wounds, such as the oozing scalp lesions of our adolescent with Darier disease, are the main risk factor for human NWS infestation.⁶ Within 12-24 hours after the female NWS fly deposits eggs into human wounds, eggs hatch and larvae begin to burrow (“screw”) head-first into tissues, anchoring their bodies with ringed spines (resembling a screw) as they ingest their pathway with 2 large mouth hooks (Fig 2).² Patients typically describe intense focal progressive pain as the larvae destroy the underlying tissues.⁷ Pruritus leads to scratching, which facilitates larval burrowing into tissue.⁷ Examination often reveals ulcer(s) with characteristic cream-colored segmented larvae.⁸

NWS infestation can be deadly, especially when involving the scalp. Larvae may burrow through the skull, dura, and into the brain, leading to an associated 8% mortality.³ Magnetic resonance imaging may gauge the extent of tissue destruction. Manual extraction is commonly used, but surgical exploration to assure complete clearance is key.⁹ Oral ivermectin has been administered to paralyze larvae,¹⁰ but its use has not been validated as a single agent. Topical or systemic antibiotics can be administered to decrease the risk of secondary bacterial infection.

Our report highlights a rare case of NWS neuromyiasis in the United States. Given the potentially fatal complications of NWS infestation and risk of imported myiasis in our “global village,” patients with risk factors such as chronic cutaneous wounds traveling to endemic areas should be warned about the signs of myiasis and counseled on preventative measures against parasitic infections. Moreover, physicians should remain vigilant about the possibility of NWS infestation in the United States, despite its eradication. For patients at risk because of travel to endemic areas and scalp wounds, skin examination should be performed, as early detection is crucial to prevent long-term complications and/or mortality.

Conflicts of interest

None disclosed.