New World Screwworm Myiasis

[Cochliomyia hominivorax] [Callitroga americana] [New World Screwworm] [Primary Screwworm]

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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.

Information on the unrelated Old World screwworm, Chrysomya bezziana, can be found on the Obligate Myiasis page.

Life Cycle

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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 [PDF – 2 pages] 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. For more information on the current outbreak, please see [New World Screwworm Outbreak in Central America].

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.

Additional clinical information on risk, diagnosis, treatment, and prevention can be found on the Clinical Overview of New World Screwworm page.

Cochcliomyia hominivorax third instar larvae are usually 6.5 – 17 mm long and muscidiform (tapering anteriorly and truncate posteriorly) in shape, with encircling bands of short 1- 2- and 3-pointed spines on each body segment. Salient morphologic features that aid in distinguishing them from other obligate myiasis-causing species and from related members of Calliphoridae that may cause facultative/incidental myiasis include dark tracheal trunks (Figure D) and the appearance of the posterior spiracular plates (Figure E).

Figure A. Two third instar C. hominivorax larvae presented in lateral view (top) and dorso-lateral view (bottom). Note the sharp, curved mouth hooks on the anterior end (white arrow) the dark tracheal trunks faintly visible through the body wall at the posterior end.

Figure B: A third instar C. hominivorax larva shown in ventral view.

Figure C: A close-up view of the anterior of the third instar larva, showing sharp, curved mouth hooks (black arrow). The anterior spiracles (white arrowheads) are also visible in this orientation.

Figure D: Dorsal-posterior view showing the tracheal trunks of C. hominivorax that originate from the spiracular plates (arrows) and extend into the body. These breathing tubes are pigmented dark enough to be visible through the body wall and extend across at least two body segments—a combination of features which is unique to C. hominivorax among the myiasis-associated fly larvae and thus very useful for identification.

Figure E: Posterior view of the anal segment of a third instar C. hominivorax larva showing spiracular plates. Note the three straight spiracular slits (SS) on each spiracular plate, and the dark peritreme (P) that incompletely surrounds each spiracular plate. Note that second instar larvae will only have two slits on each side, and the minute first instar larvae have two spiracular openings but lack a surrounding peritreme.

Figure F. Adult fly of C. hominivorax. Image courtesy of United States Department of Agriculture Animal Plant Health Inspection Service.

Confirmation of New World screwworm (NWS) myiasis depends on morphologic identification of larvae extracted from wounds. Several features aid in distinguishing Cochliomyia hominivorax from other myiasis-associated species, most importantly the morphology of the spiracular places and the darkly pigmented tracheal trunks that extend across multiple body segments. These features are illustrated and described in the image gallery and New World screwworm Bench Aid. [PDF – 3 pages] Note that these aids, and most identification keys, only pertain to the morphology of 3^rd instar larvae. The earlier stages are much smaller and lack some of the more obvious identifying traits of the 3^rd instar.

As for all myiasis cases, patient travel history and clinical presentation often help guide laboratory identification of recovered maggots.

Suspected New World screwworm infestations in humans should always be confirmed by reference laboratories experienced in the morphologic analysis of fly larvae. For human cases, DPDx offers confirmatory identification via telediagnosis or by physical specimen examination. Animal cases should be submitted to USDA/APHIS.

Prompt referral for species identification is critical both for clinical management and for decisions about initiating appropriate surveillance and control interventions in the area.

Other Resources

DPDx is an educational resource designed for health professionals and laboratory scientists. For an overview including prevention, control, and treatment visit www.cdc.gov/parasites/.