What Veterinarians Need to Know About Screwworms

Known as the primary screwworm, Cochliomyia hominivorax causes myiasis and its geographical range extends through Central and South America. Adult flies lay eggs in fresh wounds and hatch quickly, usually within 24 hours, fully developing within a week.¹ The larvae feed on living tissue and may be fatal if left untreated.¹ Females mate once in their lifetime and environmental control is typically through the release of sterile male flies. This can effectively break the lifecycle and limit further infestations.^1-3

Current concerns in the United States regarding the primary screwworm center around infested cattle in Mexico.³ The close proximity to the United States has put many on alert; increased monitoring of livestock, wildlife, and pets is recommended. The most at-risk include those closest to the border or those moving between Mexico and the United States. 

This article will address some of the most common questions general practitioners might have about this parasite of increasing concern.

What causes infestations and which hosts are vulnerable?

Most vertebrate hosts, including humans, are susceptible to screwworm infestations. Any fresh wound (e.g., castration, ear or tail docking, newborn navel, branding, shearing cuts, tick bites, cuts, abrasions) or mucus membrane (e.g., nostril, ears, mouth) can serve as a potential site for egg laying by a female fly.^1-4 

How does it harm the host? 

Once eggs hatch, larvae feed on living tissue (obligate myiasis), including bone. They feed as they develop through their larval stages. Within 3 days of infestation, larvae (maggots) should be visible. They will start small and gradually increase in size to a third instar larva (FIGURE 1). Wounds will enlarge and get worse, have a foul odor, and larvae (maggots) may be visible. In many instances, the development of necrotic tissue associated with these wounds and screwworm infestations will attract other flies, and larvae from additional fly species will begin to feed on the surrounding dead tissue (facultative myiasis).^1-4 

Figure 1. Cochliomyia hominivorax, primary screwworm, third instar larva (10x magnification).

How are screwworm infestations diagnosed? 

Screwworm larvae are diagnosed based on distinguishing morphological characteristics. One of the most notable characteristics are the dark tracheal trunks (FIGURE 2) extending from spiracular plates (FIGURE 3), which allow the larvae to breathe while consuming tissue.² 

Figure 2. Cochliomyia hominivorax, primary screwworm, dark tracheal trunks of third instar larva (40x magnification).

Figure 3. Cochliomyia hominivorax, primary screwworm, spiracular plates of third instar larva (80x magnification).

Look for larvae (maggots) and smelly wounds that are increasing in size and not healing. Animals may be painful, not eating, and/or lethargic.^3,4 

Is there a treatment for screwworm infestations? 

Environmentally, the release of sterile males to mate with female flies and break the lifecycle is the best approach to overall management.^1-4 On a host, removal of larvae (maggots) and wound care is essential. Make sure to use gloves while cleaning wounds and handling larvae. Larvae should be preserved in 70% ethyl alcohol or isopropanol.^3,4 The use of insecticides or off-label ivermectin have been reported, but efficacy has not been proven.³ Off-label use of nitenpyram,⁵ spinosad,⁶ lotilaner,⁷ afoxolaner,⁸ and sarolaner,⁹ has shown efficacy at expelling larvae within 2 to 6 hours after treatment and larval death within 24 hours post-treatment. Prevention or treatment of secondary bacterial infections may also be necessary. 

Can a host survive infestation? 

This will depend on the severity of infestation and speed of intervention. 

How can infestations be prevented in companion animals? 

The best way to prevent screwworm infestations is to be knowledgeable and aware of risks, especially if traveling to endemic areas. Properly treat and manage any wounds and seek care if there are any nonhealing wounds or if fly larvae (maggots) are noticed on a wound. Keep animals indoors, if possible, to minimize exposure to egg-laying female flies. Additionally, if going outdoors, use insect repellents and note that, although off-label, systemic insecticides may also be helpful in limiting potential infestations.^3,4

Who should be alerted to a potential infestation? 

Veterinarians should contact their state animal health official or veterinarian as well as the USDA Animal and Plant Health Inspection Service (aphis.usda.gov/labs/diagnostic-testing). Suspected samples can be sent to the National Veterinary Services Laboratory in Ames, Iowa.^3,4