Staying Calm, Prepared, and Informed About EHV
News of an EHV outbreak can strike fear in horse owners, but following preventative measures and biosecurity protocols can keep your horse — and others — healthy.
By Sarah Coleman
There is constant concern in the horse world about the spread of Equine Herpesvirus (EHV). Social media content and news reports often amplify the anxiety as equestrians debate the best way to avoid exposure once a positive EHV case or outbreak has been confirmed.
As we learned during the COVID-19 pandemic, preparation, prevention, and following biosecurity protocols is far more valuable than panic when it comes to tackling a highly infectious disease.
Causes of EHV
Though the term “equine herpes virus” is often used without an identifying number, EHV is actually a family of equine viruses named EHV-1 through EHV-5. The most common equine herpes viruses in horse populations worldwide are EHV-1 and EHV-4.
Nearly all horses will be exposed to EHV over the course of their lifetime, many at a young age, and most will show no serious symptoms from the virus. Equine herpes virus is ubiquitous in the environment, so there is no way to keep a horse from being exposed to the pathogen.
Once the horse inhales the virus from their environment, it attacks and replicates within the cells lining the horse's respiratory tract. Immune cells pick up the virus and carry it to the lymph nodes to eliminate it, but instead the virus replicates and enters the bloodstream.
“After a horse has been exposed to the virus, equine herpesvirus can become latent in nerve tissue and immune cells, effectively ‘hiding’ in the body until something activates it,” said Dr. Ben Buchanan, equine internist and critical care specialist at the Brazos Valley Equine Hospital in Navasota, Texas. “No one knows what triggers it.”
Once the virus is activated, a horse will shed the virus from its respiratory tract into the environment.
“Hence, we say EHV is always around as apparently healthy horses are shedding the virus, and sick horses are shedding a higher level of virus,” explained Dr. Katie Flynn, US Equestrian Senior Veterinarian for Equine Health and Biosecurity.
Symptoms of EHV-1
A horse with EHV may look perfectly healthy and offer no signs that they are shedding the disease. Alternatively, EHV-1 can cause respiratory disease, said Dr. Buchanan. “Horses often have a snotty nose, runny eyes, or a cough. They may also have a fever and feel generally off.” It’s important to note that these symptoms can be attributed to multiple other issues, including allergies.
“Once a horse is exposed to the virus, it takes two to 10 days for the virus to incubate in them before clinical signs may appear,” said Dr. Flynn. A fever (a temperature of over 101.5 degrees Fahrenheit) is often the first sign a horse is sick.
Older competition horses seem to be more at risk of EHV-1 than younger horses, and all competition horses are certainly at more risk than horses who don't leave the farm. “Most reports we see show horses over 10 years old having more problems [with EHV-1] than the younger horses,” Dr. Buchanan said.
“Herpes symptoms are often short-lived, but the impact on a training schedule can be substantial,” he continued. “Even when a horse appears clinically recovered, it may take several weeks before they are ready to resume full work. More severe cases can require months before returning to previous levels of performance.”
Diagnosis and Treatment
The common test for EHV is a Polymerase Chain Reaction (PCR) test. Done via nasal swab or blood, this test identifies viral DNA, meaning the horse is actively shedding the virus. A PCR test is very sensitive; it can detect tiny amounts of DNA and can often catch infections early. The sensitivity of PCR tests is why they are not recommended for every horse who has been around another horse who has tested positive for EHV-1. Healthy horses may intermittently shed a low level of virus, making detection an incidental, unrelated finding.
“For the purposes of testing, we only care about the horse who is contagious in that moment,” Dr. Buchanan explained. “That's why we don’t ask that every horse who was on show grounds with an EHV-positive horse be tested.”
The primary treatment for a horse who with EHV-1 is supportive therapy, such as anti-inflammatory drugs like flunixin meglumine (Banamine) or phenylbutazone (bute) to reduce fever, pain, and inflammation.
“There is some evidence that anti-thrombocytes [blood thinners to prevent blood clots] may prevent the neurologic form of EHV, but it's difficult to recreate, so few studies have been done,” said Dr. Buchanan.
Some veterinarians will administer heparin, a blood thinner, subcutaneously for three days to a horse who has a confirmed EHV-1 diagnosis in an effort to prevent the neurologic form of the disease, he explained.
In addition, valacyclovir is an anti-viral medication that can be administered to reduce the amount of virus in the bloodstream. “If I have a client with a horse with EHV-1, we discuss whether we want to use valacyclovir or heparin — or anything at all — knowing that 90% of horses with EHV-1 will not become neurologic,” said Dr. Buchanan.
Many horse owners opt to not use either drug, simply letting the virus run its course.
“Unfortunately, I can’t predict the outcome of a horse infected with EHV-1," said Dr. Flynn. “I have had healthy, young horses succumb to severe neurologic disease, and I have had 25-year-old horses with extremely high fevers and high viral loads stand there and look perfectly healthy. What I have learned is that EHV affects each horse differently and there is no predicting the outcome.”
Dr. Flynn stressed that immediate attention and treatment of a clinical horse is essential, and early treatment and intervention typically leads to the best outcome of less exposed and clinical horses.
Prevention
EHV is not considered a core vaccine by the American Association of Equine Practitioners (AAEP), but this doesn't mean that horse owners should simply dismiss the vaccine's validity as a vital part of their horse's healthcare, especially if the horse travels off their home farm.
An EHV vaccine is a risk-based vaccine, meaning that the horse’s treating veterinarian will assess the risk a specific horse has of contracting a disease. If the horse travels frequently or co-mingles with horses who do, an EHV vaccine is recommended every six months.
“An EHV vaccine is inexpensive and incredibly safe,” said Dr. Buchanan. Few horses have any reaction to the vaccine, even when administered twice per year, as is recommended for most competition horses who travel extensively.
“We know that if the horse is vaccinated for EHV, it's going to shed less virus for a shorter amount of time,” he continued. “If the virus moves from the respiratory tract into the horse's blood [viremia], it won’t be as high or as long if they do become sick, and their neurologic risk is less.”
Protecting your horse also helps protect all the other horses they come in contact with. “If everyone vaccinates their horse, a potential outbreak is smaller and shorter,” explained Dr. Buchanan.
It's worth reiterating that a vaccine is not a guarantee that a horse will not get sick. “Just like the human flu vaccine, the EHV vaccine reduces the nasal shedding and amount of virus in horse. With less virus in the horse, the horse is more likely to successfully overcome the mild disease symptoms,” Dr. Flynn explained.
“Vaccination is a critical tool in the toolbox of protecting equine health, but it must be used in conjunction with routine biosecurity practices to best protect your horse,” she said.
Horse owners should always use individual buckets and nose rags at shows, and take care to not let a hose end rest inside every bucket when filling. EHV remains contagious in water longer than on dry surfaces, so care should also be taken in wash racks to not spray the horse directly in the face with the hose or let them drink from it.
“We also can't rule out some component of virus spread from human deliveries,” said Dr. Buchanan, such as from braiders, farriers, riders, and health professionals. “Hand sanitizers are great, as is a reminder not to touch every horse.”
Opinions vary about whether or not to vaccinate a horse who has been potentially exposed to an EHV-positive horse. “My advice is to vaccinate,” Dr. Buchanan stated. “The data is mixed regarding if vaccinating a horse with exposure is harmful, and there's logic that it may slow down virus transmission enough that it [the virus] will burn itself out.”
Competing With Care
Assistance with developing and implementing biosecurity protocols for competition facilities is available to all USEF competition organizers in a newly-released guide that includes everything from best practices on entry procedures to disease response to communication templates. If an infectious disease outbreak does occur, USEF works with the competition organizer and state animal health officials to assist with any contact tracing and advise on the risk assessment and epidemiological investigation that state animal health authorities conduct.
A horse getting a positive test for a transmissible disease at a competition doesn’t automatically mean that all horses there are at risk, although it is essential to follow strict biosecurity measures to prevent an outbreak.
“As a state animal health official for over 20 years, I have managed over 40 equine herpesvirus outbreaks, as well as numerous other equine infectious disease outbreaks,” said Dr. Flynn. “From my vast experience, I can confidently say that if a sick horse is immediately removed and the exposed horses are monitored, the virus spread can be stopped almost immediately. However, any delays in removal of the sick horse or lapses in biosecurity precautions, or the failure to properly monitor temperatures, guarantees disease spread.”
One of the best ways to tell if a horse is not well is by taking their temperature at least twice a day; if they have a fever, something is amiss. Thermal microchips can be helpful, but Dr. Buchanan stresses that these may not reflect an accurate core temperature if the horse has just worked hard or is lying in the sun. It's important that a caretaker is willing to also take a rectal temperature if the microchip reads high. The mandatory fever reporting rule in place at USEF competitions was driven by the need to identify any potential infectious disease cases early. This rule ensures sick horses are evaluated, and those suspected of an infectious disease are isolated in accordance with the USEF-required isolation plan.
Dr. Flynn said she is confident in the USEF biosecurity protocols implemented at USEF-licensed competitions to protect equine health, while ensuring early detection and prompt response to an infectious disease incursion.
“Furthermore, we have successfully managed — in collaboration with the competition veterinarian, competition management, and state veterinarian — several infectious disease incursions on the show grounds and continued competition with the unaffected population while exposed horses were quarantined,” she explained. “In all cases, no additional confirmed cases were identified. This is evidence that USEF biosecurity protocols work.”
It’s easy to worry that an EHV case could spread through the show grounds like wildfire, but actually, Dr. Buchanan said that EHV outbreaks are often in a single barn. If there are multiple cases at a competition venue, they are not in a sequential line. “Herpes is more random,” he added.
"In my experience, identifying exposure is dependent upon numerous factors and is not as simple as the horses in the same part of the barn,” said Dr. Flynn.
Above all, Dr. Flynn says, it’s important to keep perspective. EHV cases pop up all over the country throughout the year, and in the vast majority of situations, they don’t spread to other states – and sometimes not even to other barns. The best defense for infectious disease is always a good offense in the form of consistent biosecurity practices before there’s a disease positive.
For more
information on equine health and biosecurity please visit https://www.usef.org/learn/equine-health or email [email protected].
To access the USEF Biosecurity Measures guide, please scan the QR code with your smartphone.
Risk of Equine Herpesvirus Myeloencephalopathy (EHM)
While the biggest fear is the progression of EHV-1 infection to cause the neurologic form of the disease (called Equine Herpesvirus Myeloencephalopathy or EHM), less than 10% of cases progress to that stage.
It is not known why EHV sometimes develops into EHM, Dr. Buchanan said. “It is believed that the higher the amount of virus circulating then the higher the incidence of EHM, but that has not been proven,” he explained. In cases where it does progress, the virus causes inflammation along the blood vessels of the spinal cord, which is like a mini-stroke, resulting in neurologic damage.
“EHM should be considered in horses presenting with acute fever followed by ataxia (poor muscle control or a wobbly gait), weakness, urinary dysfunction, reduced tail tone, or recumbency,” Dr. Flynn stated. Clinical signs range from mild hind limb incoordination to rapid progression, to the horse being unable to rise.
“The prognosis is dependent upon the individual horse and the severity of signs. A mildly affected horse may recover from EHM with intensive supportive care, whereas recumbent horses have a substantially poorer prognosis,” she continued. “Prompt identification, isolation, biosecurity measures, diagnostic testing, and supportive management are critical to limiting transmission and improving outcomes.”
Photos: US Equestrian