Causal Agent
Cochliomyia hominivorax, the New World screwworm (NWS), is a fly in the family Calliphoridae (blowflies, bottleflies). Other less common names for this species include the “primary screwworm” or the historic, deprecated binomial Callitroga americana.
Importantly, the larvae (maggots) of C. hominivorax are obligate parasites that infest and consume living flesh of warm-blooded hosts, unlike other calliphorid flies (including Cochliomyia macellaria, or the “secondary screwworm”) which causes facultative myiasis.
Adult female Cochliomyia hominivorax flies are attracted to odors produced by open wounds and mucous membranes of warm-blooded animals; even wounds as small as a tick bite may attract flies. The female fly then deposits a cluster of eggs, up to several hundred, on or near pre-existing wounds or on mucous membranes inside the nose, mouth, ears, eyes, and umbilicus of newborns . Eggs hatch into first-instar larvae which burrow into the tissue and feed on the living flesh, leading to extensive destruction of tissue . The larvae continue to molt and develop into second- and then third-instar larvae over a period of approximately 7 days, after which they drop from the host and pupate in upper layers of the soil The adult fly emerges from the pupa after 7 – 54 days depending on temperature and humidity . The female fly mates with a male only once and then may lay up to 3,000 eggs within her 10 – 30-day lifespan.
Hosts
Any warm-blooded animal is a potential host for Cochliomyia hominivorax. Infestations primarily occur on large hooved livestock such as cattle, sheep, and horses. Cases are less commonly reported from humans, domestic dogs, and various wild mammals (e.g., deer, rabbits, opossums, etc.,), and birds.
Geographic Distribution
The New World screwworm is endemic in South America, parts of southern Central America, and some Caribbean islands. The historic range of the New World screwworm once extended upward into the southern and western United States, until intensive eradication efforts utilizing the sterile male technique resulted in a modern endemic range restricted below the Darién gap of Panama. Historically, interstate transport of livestock led to seasonal outbreaks in many more northern states during summer months, but winter freezes restricted the year-round survivable range of New World screwworm in the United States to southern Florida, southern Texas and Puerto Rico.
Since 2023, a growing outbreak of New World screwworm has resulted in a resurgence of human and animal cases across Central America and Mexico, where the parasite had previously been eradicated.
Clinical Presentation
Cochliomyia hominivorax larvae are highly destructive and tear into the host’s tissue with sharp mouth hooks, causing wounds and sores to expand in size and depth. As the larvae develop and feed, affected areas become painful, swollen, and may bleed or produce a foul odor; larvae are typically visible in the wound oriented head-down with their posterior spiracles exposed. Infestation of the mucosa and subsequent destruction of oral, nasal, and ocular tissues may also occur.
Untreated infestations are potentially fatal. Bacterial superinfections may also occur in C. hominivorax-infested sites. If left unattended, facultative myiasis-associated species can eventually become attracted to the deteriorating wound site and may co-infest New World screwworm lesions. Some important risk factors for infestation among persons living in or traveling to endemic regions include proximity to livestock, exposed open wounds, and sleeping outdoors. A reduced ability to deter the fly and attend to wounds also makes infants and the very elderly or infirm more vulnerable to infestation.
Source: https://www.cdc.gov/dpdx/newworldscrewwormmyiasis/index.html
