The stealthy dreadful threat to mankind – ‘Screwworms’.

Screwworms, specifically the New World Screwworm (NWS – Cochliomyia hominivorax), are flying larvae those feed on living flesh and can burrow deep into wounds or orifices. It is a common threat to livestock, while not as common in humans, although they often infest humans nowadays. These screwworm infestations can be painful and potentially life-threatening if left untreated.

The screwworm flies are attracted to and lay their eggs on and in open wounds and mucous membranes. The name screwworm refers to the feeding behaviour of the larvae (maggots) as they burrow (screw) into healthy tissue. NWS larvae cause extensive damage by tearing into the host’s tissue with sharp mouth hooks. The wound can become larger and deeper as more larvae hatch and feed on living tissue. As a result, NWS can cause serious damage to the animal or person it infects. Bacterial superinfection can also occur because of the NWS infestation. The buggers can also squirm into the belly buttons of newborn mammals.

New World screwworm infestations begin when a female fly lays eggs on a wound or orifice of a live, warm-blooded animal. The odour of a wound or an opening such as the nasal or eye openings, umbilicus of a newborn, or genitalia attracts female flies. Wounds as small as a tick bite may attract a female fly to feed. One female can lay 200 – 300 eggs at a time and may lay up to 3,000 eggs during her 10- to 30-day lifespan. The Eggs then hatch into larvae that burrow into the wound to feed on the living flesh. After about 7 days of feeding, larvae drop to the ground, burrow into the soil, and pupate. The adult screwworm fly emerges from the soil after 7 – 54 days depending on temperature and humidity. Female flies mate only once in their lifespan.

The notoriety of the species gained widespread attention, when Mr. Donald Trump led United States, Government have imposed a ban on the import of Mexican beef cattle for 15 days due to the detection of screwworm in shipments. The US restricted Mexican cattle shipments in late November following the detection of the pest, but lifted the ban in February after protocols were put in place to evaluate the animals prior to entry into the country. But there has been an “unacceptable northward advancement” of the screwworm, the US Department of Agriculture said in a statement on Sunday.

“The last time this devastating pest invaded the US it took our livestock industry 30 years to recover.  This can never happen again”, the United States Agriculture Secretary Brooke Rollins said on her ‘X-Handle’.

It is mention worthy, Mexico’s Health Ministry issued an epidemiological warning this month after the first human case of screwworm myiasis, or parasitic infestation, was confirmed on April 17 in a 77 year’s old woman living in the southern state of Chiapas. A detailed insight has revealed that, NWS is endemic in the American tropics and subtropics which includes the countries of South America, Cuba, Haiti, and the Dominican Republic. There have been recent cases in Central American countries for the first time in many years.

People at higher risk include:

• Those living in rural areas in regions or countries where NWS is endemic, and where livestock are raised.
• People who frequently work with livestock.
• Anyone with open sores or wounds, including from a recent surgery, as the flies will lay eggs on open sores.
• Vulnerable populations, including people who are immunocompromised, those at extremes of age, and people experiencing malnutrition.

Testing:

Clinicians can diagnose NWS by finding fly larvae (maggots) in tissue. Identification of the genus or species involves comparing certain morphological structures on the larvae, including the anterior and posterior spiracles, mouthparts, cephalopharyngeal skeleton, and cuticular spines. Travel history can also be helpful for ascertaining the genus.

Prevention:

• The best way to prevent New World Screwworm (NWS) is to avoid exposure.

• Patients with NWS symptoms, with a history of recent travel to regions where NWS is endemic, should be isolated.

• Patients should avoid spending time around livestock or where livestock are housed while in rural areas in NWS endemic regions.

• Patients must clean and cover wounds, wear loose-fitting, long-sleeved shirts and pants and socks, and use Environmental Protection Agency (EPA)-registered insect repellents.

• Patients should avoid sleeping outdoors and protect sleeping quarters with screens or bed nets, especially where people have wounds or active nasal or ocular discharges. Screening of hospital windows and doors is essential.

Treatment:

To treat NWS, is to physically remove the larvae from the infested tissue. All suspected NWS larvae should be collected and placed into a leak-proof container with 70% ethanol. The volume should be sufficient to fully submerge larvae, which will kill and preserve them for confirmatory diagnostic examination. If 70% (or greater) ethanol is unavailable, 70% (or greater) isopropanol or 5 – 10% formalin are acceptable alternatives, although not preferred. There are anecdotal reports of using ivermectin in off-label treatment of NWS infestations in humans. However, there are no controlled, double-blind studies measuring the impact of ivermectin use on myasis.

Indian Context:

Wound myiasis in the Indian subcontinent is most commonly caused by old world screw-worm (Chrysomya bezziana). A 4-year-old malnourished girl presented with full thickness rectal prolapse following acute diarrhea with a large wound and screwworm myiasis of the rectum. Turpentine oil was applied to immobilise the maggots followed by manual extraction. Prolapse was successfully treated by manual reduction followed by strapping of the buttocks. Child was thriving well and gained 2 kg weight in follow up after two weeks.

What remains to understand is; what steps does Government of India warrant to protect the countrymen from this dreadful myiasis? The issue needs a detailed retrospection of the understanding on the subject; an impeccable road map from understanding-orientation-prevention of this threat to mankind in future ahead.

Team Maverick