New World Screwworm: What Equine Veterinarians Need To Know Now

A practical guide for recognising, reporting and responding to suspected New World screwworm in horses and mixed-species veterinary practice

Last updated: June 4th 2026

The United States Department of Agriculture (USDA) has confirmed a case of New World screwworm (NWS) in a three-week-old calf in Zavala County, Texas, marking the first detection of the parasite in the United States in decades.

New World screwworm is a serious parasitic disease caused by the larvae of Cochliomyia hominivorax. Unlike many fly larvae that feed on dead tissue, screwworm larvae invade and consume living tissue, resulting in progressive wound enlargement, severe tissue damage, secondary infections, and, in some cases, death if left untreated.

The detection has prompted an immediate response from federal and state animal health authorities, including surveillance, containment, movement controls, and eradication measures designed to prevent further spread.

For veterinarians, livestock producers, and animal owners, the case serves as a timely reminder of the importance of early recognition, prompt reporting, and ongoing biosecurity vigilance as authorities work to contain this significant animal health threat.

Should veterinarians and clients panic?

This is not a reason for panic. It is a reason for clinical vigilance.

For many veterinarians in the United States, New World screwworm is something they may have learned about but never expected to see.

For equine veterinarians, this matters because horses are susceptible, horses travel, and horses get wounds. New World screwworm is not only a cattle issue. It can affect livestock, horses, pets, wildlife, birds on occasion, and, rarely, people.

The aim of this article is to give veterinarians a practical, field-ready guide to recognising, reporting and responding to suspected New World screwworm myiasis.

What is New World screwworm?

New World screwworm, Cochliomyia hominivorax, is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals.

That detail is critical.

Many maggots found in wounds are secondary invaders that feed primarily on dead or necrotic tissue. New World screwworm larvae are different. They feed on living tissue, which allows the lesion to enlarge and deepen as the larvae continue to burrow.

This is why New World screwworm myiasis can be so painful, destructive and, if untreated, fatal.

The female fly is attracted to wounds, mucous membranes and moist body openings. She lays eggs on or near the wound. Once the eggs hatch, the larvae enter the tissue and feed. As they mature, they drop from the host into the environment to pupate, continuing the life cycle.

The clinical problem is therefore twofold:

the affected animal requires urgent wound care and pain relief; and
the larvae must not be allowed to escape into the environment and establish locally.

That is why suspected cases must be handled as both a clinical emergency and a biosecurity event.

Why veterinarians should pay attention now

New World screwworm was eradicated from the United States in the 1960s through the sterile insect technique. A later outbreak in the Florida Keys was eliminated in 2017.

The current concern is the northward movement of New World screwworm through Central America and Mexico. Recent confirmed detections in northern Mexico, close to the U.S. border, have increased the urgency of preparedness across veterinary, livestock, public health and regulatory networks.

At the time of writing, official U.S. sources state that New World screwworm is not currently present in the United States. However, the situation is changing quickly, and veterinarians should check current USDA, FDA and CDC updates before making clinical or regulatory decisions.

For equine veterinarians, the key message is simple:

If you see larvae in a wound, especially in a horse with travel history or a wound that appears unusually painful, malodorous, bloody, progressive or deep, New World screwworm must be considered.

Why horses matter

This is not just a cattle disease.

Horses are highly relevant because they are mobile, valuable, frequently transported and commonly affected by wounds. They move for competition, racing, breeding, sales, relocation, referral care and international importation.

In horses, potential risk sites include:

traumatic wounds and lacerations
surgical sites
castration wounds
umbilical sites in foals
vulvar, perineal, preputial or sheath lesions
ear wounds
nasal or oral lesions
draining abscesses
tick-bite sites
wounds hidden under bandages
moist, contaminated or delayed-healing wounds

Equine veterinarians should be especially vigilant in horses that have recently travelled, crossed state lines, come from higher-risk regions, been imported, lived on mixed-species properties, or had wounds managed without close veterinary supervision.

Clinical signs: what to look for

New World screwworm should be considered when larvae are found in a wound or body opening, particularly if the lesion appears aggressive, painful or progressive.

Possible findings include:

visible maggots in a wound
larvae in the nose, ears, mouth, genitalia, umbilicus or other body openings
foul odour
bloody discharge
purulent discharge
wounds that become deeper or larger
pain that seems disproportionate to the wound
depression or irritability
inappetence
isolation from other animals
rubbing, stamping, head shaking or self-trauma depending on lesion location
deterioration despite routine wound care

The wound may initially resemble ordinary traumatic myiasis. The key distinction is that New World screwworm larvae feed on living tissue, not just necrotic debris.

If the lesion is deepening, bleeding, malodorous, unusually painful or full of larvae, do not dismiss it as “just maggots.”

Immediate actions: the first 24–48 hours after a suspect case

The first 24-48 hours after a suspected New World screwworm case are critical.

The veterinarian must balance two priorities:

immediate animal welfare, including pain relief and wound care; and
preservation of diagnostic evidence and containment of larvae.

Use the following order of operations.

1. Isolate and contain the animal

Immediately move the affected horse or animal into the most secure, enclosed and fly-controlled area available.

Ideally, this should be:

a fully enclosed stall
a fly-screened treatment bay
a secure hospital stall
an enclosed barn area with minimal insect access

The aim is to reduce the risk of larvae dropping from the wound into bedding, soil, manure or the surrounding environment where they could pupate.

Do not move the animal unnecessarily.

Once New World screwworm is suspected, the animal should remain on the immediate premises until guidance is received from state or federal animal health officials. The client should be prepared for a temporary on-site regulatory hold until official clearance is provided.

For horses scheduled to travel for shows, racing, breeding, sales, referral care or interstate movement, this can be inconvenient. However, containment is essential.

2. Notify the correct authorities

New World screwworm myiasis is a foreign animal disease concern in the United States.

If suspected, contact:

your State Animal Health Official; and
the USDA APHIS Area Veterinarian in Charge for your state.

If the animal has recently been imported, APHIS Live Animal Import should also be notified.

This should not be managed as a routine wound case without official notification. In many situations, a Foreign Animal Disease Diagnostician or other official animal health representative may be involved in sample collection and investigation.

3. Collect diagnostic samples before applying topical treatment

Before applying topical products, flushing aggressively or debriding the wound, preserve diagnostic evidence.

Wear appropriate PPE, including disposable gloves.

If directed to collect specimens, use forceps to remove larvae from:

several areas of the wound
different depths within the lesion
the deepest part of the wound whenever possible

This is important because secondary fly larvae may be present near the surface, while diagnostically important larvae may be deeper in the lesion.

If possible, collect larvae of different sizes.

Place larvae into a tightly closing vial or tube and submerge them in 70% ethyl or isopropyl alcohol at room temperature.

Do not discard all larvae before official identification has been considered.

4. Proceed with wound care after sampling

Once appropriate samples have been collected, proceed with animal welfare and wound management.

This may include:

mechanical removal of remaining visible larvae
careful wound lavage
debridement where clinically indicated
antiseptic wound care
analgesia
systemic antimicrobials where secondary bacterial infection is suspected
topical and/or systemic larvicidal treatment where authorised and appropriate

Treatment decisions should be based on the latest FDA-authorised product information and guidance from state and federal animal health officials.

5. Maintain a strict on-site hold

The affected animal should not leave the premises until official guidance is received.

The clinical priority is no longer only the individual wound. It is containment, confirmation, treatment, premises management and prevention of local establishment.

Sample collection: details that matter

If you are instructed to collect samples, technique matters.

Practical field points include:

use disposable gloves
use forceps to remove larvae gently
collect larvae from several sites within the wound
collect larvae from different depths
prioritise larvae from the deepest part of the lesion
collect larvae of different sizes if possible
place specimens in a tightly closing vial or tube
preserve specimens in 70% ethyl or isopropyl alcohol
keep specimens at room temperature
submit samples as directed by APHIS or state animal health officials

Do not rely on visual identification alone.

Do not collect only superficial larvae.

Do not place all larvae in formalin.

Do not discard larvae before official identification is considered.

Do not submit samples without first contacting the appropriate animal health officials.

Quarantine and movement control

A suspected or confirmed case may require quarantine under the supervision of a USDA-accredited veterinarian and/or an official state or federal veterinarian.

Animal health officials will determine the appropriate quarantine premises and level of control based on the case, location, risk and availability of suitable facilities.

For equine veterinarians, this is especially important because horses move frequently. A suspect animal should not be transported to another property, event, clinic or state without official guidance.

If referral care is required for welfare reasons, movement should be coordinated with animal health authorities.

Treatment principles

Treatment should be coordinated with state and federal animal health officials and checked against the latest FDA-authorised product information.

General treatment principles may include:

removal of visible larvae and eggs
thorough wound cleaning
antiseptic wound management
topical treatment where authorised and appropriate
systemic medication with larvicidal activity where authorised and appropriate
analgesia
systemic antimicrobials where secondary infection is present
re-examination after treatment to ensure no live larvae remain
ongoing monitoring and possible re-treatment
treatment and disinfestation of the surrounding premises where directed

The animal should be re-examined carefully after treatment. If live larvae remain, further removal, monitoring and possible re-treatment may be required.

Treatment and regulatory boundaries

Veterinarians should be careful not to treat New World screwworm as a routine extralabel drug-use scenario.

FDA has issued Emergency Use Authorizations and conditional approvals for specific products because New World screwworm presents a significant animal health, public health and food-supply threat.

These authorisations differ by:

product
species
route of administration
indication
prevention versus treatment
age restrictions
food-animal status
withdrawal periods
milk discard requirements
who is authorised to use or administer the product
whether use is restricted to an infested zone or adjacent surveillance zone

The key point is this:

Do not assume that a product authorised for one species, one indication or one regulatory pathway can automatically be used in another species or situation.

Current FDA-authorised or conditionally approved products include products relevant to cattle, horses, sheep, goats, swine, dogs, cats, deer, donkeys, domestic hybrid equids, captive wild or exotic mammals, birds and other species groups, depending on the specific product.

Examples include:

F10 Antiseptic Wound Spray with Insecticide and F10 Antiseptic Barrier Ointment with Insecticide: Explicitly authorised under recent 2026 Emergency Use Authorizations for the prevention and treatment of NWS myiasis in multiple species, including horses, donkeys, domestic hybrid equids, and captive exotics.
Topical Permethrin and Coumaphos Powders: Authorised for treatment and prevention in cattle, swine, sheep, goats, and horses.
injectable doramectin products with species-specific indications
oral or topical ectoparasiticides authorised for dogs and cats, which may be relevant on mixed-species properties

For equine veterinarians, one important example is Dectomax/Dectomax-CA1. FDA has issued an emergency use authorisation that includes horses, but the indication is specific. Veterinarians must check the current product fact sheet before use and should not assume that a product is authorised for both prevention and treatment in horses.

A second important boundary is medicated feed.

No medicated feed is currently authorised for the prevention or treatment of New World screwworm myiasis in any species. Federal law does not permit extralabel use of animal drugs in feed.

Before prescribing or recommending any product for New World screwworm, veterinarians should check:

the live FDA Animal Drugs for New World Screwworm page
the product-specific fact sheet
the species covered
whether the indication is prevention, treatment or both
any age restrictions
food-animal restrictions
withdrawal periods or milk discard requirements
whether the product is available over the counter or only under specific authorisation
whether use is limited to particular zones or official response situations

Treat the animal, but do not treat the regulatory details casually.

One Health and staff biosecurity

New World screwworm is a One Health concern.

Although veterinarians are focused on animal patients, Cochliomyia hominivorax can also infest people. The fly can lay eggs in wounds or mucous membranes, and the larvae can feed on living human tissue.

Veterinary teams must treat suspected cases as an urgent personal safety issue. The scale of the current outbreak is historic; as of mid-2026, health officials have recorded over 170,000 animal cases and nearly 2,000 confirmed human infestations, including several fatalities, across Central America and Mexico.

Because Cochliomyia hominivorax does not differentiate between animal and human tissue, strict biosecurity is mandatory.

Any staff member, handler, or owner with open cuts, fresh surgical wounds, ulcers, draining skin lesions, or exposed dermatitis must be strictly barred from handling the suspected animal, collecting larvae, cleaning the wound, removing bedding, processing samples, or assisting with premises decontamination.

Any staff member, handler or owner with open cuts, fresh surgical wounds, ulcers, draining skin lesions, exposed dermatitis or other compromised skin should not handle the suspected animal, collect larvae, clean the wound, remove bedding, process samples or assist with premises decontamination.

Clinic and field teams should use appropriate PPE, including disposable gloves, and should avoid direct contact with wound exudate, larvae, contaminated bedding and contaminated equipment.

On mixed-species or farm properties, veterinarians should also consider the people on the premises, including:

farm workers
grooms
handlers
owners
farriers
technicians
children
anyone involved in wound care or bedding removal

If a person develops a suspicious wound, sees larvae in a wound or body opening, or has recent exposure to an affected animal, they should seek medical care promptly and inform the clinician about possible New World screwworm exposure.

Whole-property and multi-species risk guide

A suspected case in one horse should trigger a wider property-level assessment.

New World screwworm is not species-specific. Equine veterinarians should look beyond the index horse and ask what other animals are present, whether any have wounds, and whether there has been recent travel, importation, birthing, castration, surgery, dehorning, branding, trauma or tick infestation.

Species	Clinical Context & Risk Factors	Current Outbreak Relevance
Bovine	Deep tissue strikes, navels of newborns, castration sites, dehorning or branding wounds, tick-bite irritation.	Major livestock concern and a central focus of surveillance, border control and sterile fly response efforts.
Canine	Guard dogs, farm dogs, working dogs, imported pets and dogs moving between rural and domestic environments.	Important companion-animal and premises-level risk, especially on farms, ranches and mixed-species properties.
Equine	High mobility across state lines for sport, breeding, racing, sales and referral care; frequent lacerations and surgical wounds.	Requires aggressive wound vigilance, especially in travelling horses and mixed-species premises.
Small ruminants	Castration wounds, kidding or lambing sites, ear wounds, foot lesions and traumatic injuries.	Important sentinel species on small farms, hobby farms and mixed livestock properties.
Wildlife and exotics	Traumatic wounds, predator injuries, capture injuries, antler wounds, neonatal sites and limited handling capacity.	Relevant to wildlife interfaces, zoos, sanctuaries, rehabilitation centres and extensive grazing systems.
Cats and other companion animals	Bite wounds, abscesses, surgical wounds and outdoor exposure.	Relevant on mixed-species premises and in households or farms with animals travelling from risk regions.

During the property audit, ask specifically about:

cattle, sheep, goats, pigs, dogs, cats, horses, donkeys, mules, wildlife and exotic animals
newborn animals
recent castrations, dehorning, branding or surgery
traumatic wounds or abscesses
animals with ticks or tick-bite irritation
animals recently imported or moved from higher-risk regions
dogs or pets that have travelled internationally
horses arriving from competitions, breeding farms, sales or transport hubs
carcass disposal and wildlife access
fly pressure and current insect control measures

The practical message is simple:

Do not stop at the wound in front of you. If New World screwworm is possible, assess the property.

Premises management

If a case is confirmed, the environment matters.

Larvae can leave the wound and enter the environment to pupate. Stalls, pens, bedding, trailers, stocks, wash bays, treatment areas and other holding areas may require specific attention.

Premises management may include:

collecting expelled larvae
killing larvae in alcohol before disposal
insecticide treatment of holding areas where directed
careful bedding removal and disposal
cleaning with warm water and soap after insecticide application
fly trapping or monitoring
ongoing surveillance after the affected animal has been released

These steps should be directed by APHIS and state animal health officials.

The aim is not only to help the affected animal. It is to prevent establishment of the fly in the local environment.

Practical checklist for equine veterinarians

When examining a wound, ask:

Are larvae present?
Is the wound unusually painful?
Is there a foul odour?
Is there bloody or purulent discharge?
Is the wound enlarging or deepening?
Has the animal travelled recently?
Has the horse come from or through a higher-risk region?
Has the horse been imported?
Are other animals on the property affected?
Are there cattle, small ruminants, dogs, wildlife or feral animals nearby?
Could this wound have been hidden under a bandage, rug or boot?
Has the horse recently had surgery, castration, breeding trauma or a draining abscess?

If New World screwworm is possible:

isolate the animal in a secure, fly-controlled area
protect yourself and staff with PPE
contact your State Animal Health Official
contact the USDA APHIS Area Veterinarian in Charge
collect samples only as directed
collect larvae from deep within the wound
preserve larvae in 70% alcohol
avoid unnecessary movement of the animal
manage the wound after preserving diagnostic evidence
check current FDA product-specific guidance before treatment
assess the whole property for other at-risk animals

Do not:

dismiss larvae as “just maggots”
discard all larvae before identification
apply topical treatment before preserving samples if sampling is required
move the animal unnecessarily
rely on social media treatment protocols
assume a product authorised for one species applies to another
assume a product authorised for prevention is authorised for treatment
use medicated feed for NWS prevention or treatment
allow staff with open wounds or skin lesions to handle the animal or samples

What equine practices can do now

Every equine practice can prepare without alarming clients.

Recommended steps include:

Save contact details for your State Animal Health Official and USDA APHIS Area Veterinarian in Charge.
Keep 70% alcohol, leak-proof tubes, gloves, labels and forceps available.
Add New World screwworm to wound differential lists and internal protocols.
Train veterinarians, interns, technicians and reception staff on when to escalate a wound case.
Add travel and import questions to wound history templates.
Review the FDA Animal Drugs for New World Screwworm page regularly.
Prepare a short client-facing wound vigilance message.
Encourage prompt assessment of wounds containing larvae.
Pay close attention to horses that travel, compete, breed or live on mixed-species properties.
Review fly-control and wound-management advice with clients before summer travel and competition periods.

Preparedness does not need to be complicated. The aim is to avoid delay if a suspicious case appears.

What to tell horse owners

Horse owners do not need to panic, but they should take wound care seriously.

A useful client message is:

“New World screwworm has not been detected in the United States, but because it has been moving north through Mexico, we are asking horse owners to check wounds carefully. Please contact your veterinarian promptly if you see maggots, notice a foul smell, see bloody discharge, or if a wound appears to be getting deeper, larger or more painful.”

Owners should be encouraged to inspect:

traumatic wounds
surgical sites
castration wounds
umbilical sites in foals
sheath, vulvar and perineal areas
ears and nostrils
wounds under bandages
horses that have recently travelled
horses on mixed-species properties

The message is not fear.

The message is early recognition.

Early recognition is important

If New World screwworm enters an area, delay matters.

A missed case gives larvae time to mature, leave the wound, enter the environment and continue the life cycle. Early reporting allows animal health officials to identify the parasite, treat the affected animal, manage the premises, trace potential movements and implement control measures.

Equine veterinarians are part of the early warning system.

A horse with a suspicious wound may be the first sign that something has changed. Knowing what to look for, who to call and how to preserve samples could make a significant difference.

New World screwworm is not currently established in the United States.

The aim is to keep it that way.

Official resources for veterinarians

Veterinarians should consult the latest guidance from:

Screwworm.gov (The official, centralized U.S. federal portal linking live updates from USDA APHIS, the FDA, and the CDC)
USDA APHIS New World Screwworm current status updates
USDA APHIS Area Veterinarian in Charge contact list
State Animal Health Officials
FDA Animal Drugs for New World Screwworm
FDA New World Screwworm: Information for Veterinarians
CDC New World Screwworm outbreak situation summary
USDA APHIS Standard Operating Procedure for possible detections of New World screwworm in animals
USDA APHIS New World Screwworm veterinarian fact sheet

Because the situation is changing quickly, this article should be treated as a living resource and reviewed regularly against official USDA, FDA and CDC updates.