Arizona Equine Influenza Outbreak
- Bray Club Arizona
- Apr 8
- 8 min read
As many of you are already aware, there is currently an Equine Influenza A (H3N8) outbreak in Mohave County, AZ. As far as we know as of 4/7/25, there have been three confirmed deaths of donkeys, and one horse. As is often the case with this type of scenario, the game of social media telephone tends to cause confusion and help inaccurate information spread. As countless hours of outbreak training have taught us, in situations like this it’s often best to go to the source for information, so we reached out to UC Davis for what answers we could find. For reference, the ongoing incident was announced on or around April 4, 2025 at the Oatman Burros Rescue and Sanctuary in Mohave, AZ and which will be referred to in this post as “the outbreak” going forward. Here’s a quick list of what we do know:
What disease is responsible for the outbreak? Equine Influenza A H3N8 was found via Polymerase Chain Reaction (PCR) testing of nasal swabs sent to the University of California Davis (UC Davis) lab from an affected animal. This is the most common strain of Equine Flu in the United States.
What is Equine Influenza (EIV)? Per UC Davis “Equine influenza is a highly contagious respiratory disease of horses and other equidae [donkeys, mules, zebras]. It is caused by two subtypes of the influenza A virus, which are related to, but distinct from, influenza viruses in other species. Equine influenza virus spreads easily through contact with infected horses and via contact with infected clothing, equipment, brushes, tack, etc. Some horses do not show any outward signs of infection, but can shed the virus and infect susceptible horses. Outbreaks result in significant economic losses to the equine industry, and are more likely to occur where large groups of horses gather, such as shows, races and events. Increased transportation of horses across state lines and between countries facilitates the spread of the virus.”
Is this bird flu? No. As stated above, EIV is different from influenza viruses in other species. This is not bird flu/avian flu/Highly Pathogenic Avian Influenza/ HPAI. Birds are not typically a vector for this disease.
How is EIV spread? EIV is spread via droplets of saliva, and mucus, much like COVID or influenza in humans. An animal coughing will spray aerosolized droplets to neighboring animals, and sharing feeders, water troughs, transport trailers, wheelbarrows, manure forks, fence rails, stalls, barns, even petting ill animals or walking through their pens can spread infected droplets to others. Sale barns/auctions/kill pens are often a reservoir for EIV, and new animals can introduce EIV to facilities even if they aren’t showing visible signs of illness.
Who is at risk? Young (under 5 years), old, sick, stressed, and most importantly, unvaccinated animals are most at risk. Vaccinated animals may still contract the disease and show symptoms, but the symptoms are typically more mild and have a better prognosis than unvaccinated animals
Is there a vaccine for Equine Influenza A H3N8? Yes. While this is not considered one of the “core” vaccines (tetanus, rabies, west nile virus, eastern equine encephalitis, western equine encephalitis) it IS typically given as a risk-based vaccine, particularly if your animals leave your property or congregate with other equids. It is often referred to by veterinarians as “rhino/flu” because it is combined with a rhinopneumonitis vaccine. It is recommended by manufacturers to be given at least once a year, but many vets will administer the vaccine with both spring and fall shots to maximize protection, especially if animals travel or do events.
What is the Merck live virus vaccine I heard about? There are two types of vaccines for Equine Influenza A H3N8: a killed-virus intramuscular vaccine, and a live-virus intranasal vaccine. The intramuscular vaccine, usually given in the neck or hip, offers protection from this virus for animals who have no known recent exposure, or recent illness. It is manufactured by multiple companies, and may be marketed under names such as Boehringer Ingelheim Calvenza, Merck Prestige, or Zoetis Fluvac Innovator.
The Merck intranasal vaccine is marketed under the name Flu Avert I.N., and is a live virus vaccine given in the nose (think: nasal spray). This vaccine offers a faster immunity or response, and is well suited for animals who may have been exposed to an ill animal, or who have themselves recently been ill.
Does my animal need a booster of the Merck Flu Avert I.N.? If they are fully vaccinated and boosted for EIV, probably not, unless they are high-risk: young, old, in poor health, or have been recently exposed to sick animals. If you are concerned, this is a question best answered by your vet, please call them to discuss your particular case.
My animal has never received a EIV vaccine to my knowledge, which vaccine should I get? This is, again, a question to discuss with your vet. They can go over the risk factors with you and come up with the best vaccination plan. Tell your vet about events or outings you do with your animals, or have recently done, and when they take place. If you routinely gather for hikes or outings with others, share trailers or equipment, or go to places frequented by donkeys or horses, domestic or feral, be sure to let your vet know.
Where do I get these vaccines? Your vet will have access to all of the above-mentioned vaccines. There is no special or unique vaccine for this particular outbreak. If you are a 501(c)3, Merck offers programs and grants to obtain free vaccines. They can be found at here.
Is this outbreak affecting the wild donkeys in Oatman? No, there is no evidence this outbreak is affecting the wild donkeys of Oatman at the time of this writing.
Is this what caused the deaths of the wild donkeys earlier in the week near Blythe? While those deaths are still under investigation, there is currently nothing suggesting the deceased wild donkeys were affected by EIV. If that changes we will update.
Is this what caused the deaths of 25 burros in holding in Utah recently? No. Per the press release from BLM, the burros that recently died in Axtell holding facility had equine gammaherpes virus. This virus presents with similar symptoms as EIV, but there is no vaccine for equine or asinine gammaherpes virus. The only way to prevent the spread of either type of gammaherpes virus is biosecurity and quarantine of sick animals. Please note these animals had a secondary infection of Streptococcus equi ssp. Zooepidemicus, however strep zoo (not to be confused with strangles, strep equi equi) is present in all equids’ natural microflora, but may cause illness in animals with an already depressed immune system, such as those fighting off another infection.
Are there other sick animals in the state/area? Possibly. At this point the only reporting on the disease is self reporting to the community, so it’s difficult to say for sure where the disease has spread. If you are concerned about your animals contracting the disease, you may choose to eliminate non-essential travel for your animals. If your animal was exposed to the disease or if they are showing symptoms, it is essential you call your vet immediately and do not move your animals unless specifically directed by your vet. Don’t be a vector! The more an infected or exposed animal travels, the more likely they are to spread the disease and increase the outbreak.
How do I prevent my animals from getting sick? Ensure they are fully vaccinated, practice good biosecurity when leaving your property, practice responsible quarantining procedures when obtaining a new animal, and practice good farm hygiene and husbandry (ensure animals are healthy and vaccinated, keep manure picked up and removed, keep water buckets cleaned and sanitized, keep feeders cleaned and sanitized, don’t visit areas with sick animals).
How long are animals contagious? Per the AAEP “Respiratory shedding typically lasts for 7-10 days post infection in naïve [unvaccinated] animals; much shorter shedding periods occur in partially immune (previously vaccinated) horses. The period of live virus shedding does not always correspond to clinical illness, and horses may continue to shed virus after resolution of fever and clinical signs.”
How long does the virus last on surfaces? “[The] virus can remain viable for up to 2 days on contaminated fomites and solid environmental surfaces, e.g. grooming supplies, stall latches, etc. [The] Virus can survive in aerosols for several hours and on hands for a few minutes. In water, virus viability has been reported up to 3 days. Virus survival in water is temperature dependent and may be longer than 3 days in cold water.”
What do I do if I think my animals are getting sick? First call your vet. After you speak to your vet, follow this guide for what to do if you think your animals may be ill. Remember: time is of the essence.
My animals were or may have been exposed to the virus, what now? Call your vet immediately. If you are waiting for your call to be returned, monitor their temperature twice daily. Remember, donkey’s temperature should be 98.5*, do not use horse temperature guides. Then, per the AAEP “any donkey showing clinical signs of any respiratory disease (coughing, nasal discharge, fever) should be immediately isolated and standard respiratory biosecurity guidelines should be followed until a diagnosis is confirmed. Donkeys housed in the same barn with an animal showing respiratory clinical signs should be isolated for 14 days. Because many animals in an individual facility may be affected simultaneously or are incubating infection when the first case is recognized, it may be best to isolate all equids together by quarantining the entire facility rather than moving them to individual isolation facilities. All animal movement on and off the premises should be suspended during the quarantine period. During an influenza outbreak, affected and exposed animals should be isolated from susceptible animals, preferably in a different air space. Coughing donkeys can aerosolize the virus and transmit infective virus particles for 35 yards, and potentially farther depending on housing conditions and ventilation.”
Key Takeaways:
-Vaccinate your animals. Be sure you talk to your vet every spring and fall about your herd’s current vaccination needs.
-Properly quarantine (and vaccinate, when needed) new animals. Be sure new animals are kept at a distance from the rest of your herd until they are confirmed to be healthy and vaccinated against communicable disease.
-Practice biosecurity. Don’t congregate with other animals of questionable health status, do not share water buckets, tack, brushes, manure rakes, community pens, etc with others at events. Be cognizant of biological hazards when visiting sale barns, other properties, or show grounds, and take steps not to transmit possible disease on your clothes, shoes, body, or vehicle.
-Stay put. When there is a high risk of disease transmission in your area or community, stay home and don’t become an unwitting vector.
-Have a vet. Maintain a good working relationship with a veterinarian in your area. Stay up to date on preventative care, pay bills promptly, and care for your animals in a way so as to minimize the likelihood of emergencies and crises.
Where can I find more information?
For information on Equine Influenza:
For more information on Merck Flu Avert I.N.
For more information on biosecurity
For more information of quarantine and isolation protocols
