Equine herpesvirus 1

Equine herpesvirus 1 (EHV-1) is a virus of the family Herpesviridae that causes abortion, respiratory disease and occasionally neonatal mortality in horses. Initial spread of EHV-1 by a newly introduced horse through direct and indirect contact can lead to abortion and perinatal infection in up to 70 percent of a previously unexposed herd.^[1] Abortion usually occurs in the last four months of gestation, two to four weeks after infection of the mare.^[2] Perinatal (around the time of birth) infection can lead to pneumonia and death. Encephalitis can occur in affected animals, leading to ataxia, paralysis, and death. There is a vaccine available (ATCvet code QI05AA11 (WHO)), however its efficacy is questionable.The virus varies in severity from sub-clinical to very severe.^[3] Most horses have been infected with EHV-1 but the virus can become latent and show no signs and never be an issue.^[4] In 2006, an outbreak of EHV-1 among stables in Florida resulted in the institution of various quarantine measures.^[5] The outbreak was determined to have originated with several horses imported from Europe via New York, and then shipped to Florida.^[6]

Strains

EHV-1 has two main strains that have been isolated. D752 is the strain that is more correlated to the neurological outbreak of this virus and the non-neurological outbreaks are more closely associated with N752 . Some horses do show the neurological effects without the D752 isolate but the majority are with this strain.^[7] A mutation results in both these two different main strains, D752 is the presence of aspartic acid and the N752 is the presence of asparagine. 80–90% of neurological disease is caused by the D752, and 10–20% for N752.^[3] The neurological strain has been named Equine Herpesvirus Myeloencephalopathy, or EHM. There is currently no licensed vaccine against EHM and no treatment other than palliative care.^[8] The disease rose to worldwide public prominence in early 2011, due to an outbreak centered on the National Cutting Horse Association (NCHA) Western National Championships held in Ogden, Utah from April 29 to May 8, 2011. The outbreak lasted approximately two months, generating 90 confirmed cases of the disease, spread over 10 American states, and resulting in the deaths of 13 horses before it was declared contained by the US Department of Agriculture.^[9]

Latency

The EHV-1 virus can latently infect horses and can reactivate if appropriate conditions appear. These conditions include high levels of stress, immunosuppression, transportation,^[10] sale barns, competitions,^[11] geological and management practices. Current studies that have been done are showing that a majority of horses are actually latently infected with EHV-1. The consequence of latency is that in fact, it can cause an outbreak of EHV-1 in a closed group of horses, without any external source. Latent infected horses do not shed EHV-1 through nasal secretions and therefore are not infectious and are called clinically normal.^[11]

Signs, Symptoms and Transmission

The signs the occur with EHV-1 and the EHM strain include things such as decreased coordination, urine dribbling, fever, hind limb weakness, leaning against things to maintain balance, lethargy and the inability to get off the ground.^[4] More signs of the infection of this virus include depression, anorexia, nasal and ocular discharges. Fever is the most common clinical sign of EHV-1,^[10] some horses that appear perfectly healthy can still spread the virus from nostril secretions^[4] and also from secretions from coughing. Horses or humans that have been in contact with aborted fetuses, fetal fluids and placentae can easily spread the virus.^[3] When a human walks through secretions or walks in fetal fluid and gets it on their boots it will be transmitted to wherever they walk. It is extremely easily transmissible. Most mature horses build some type of immunity through repeated natural exposure, but will not build immunity to the EHM strain.^[3]

Risk Factors

There is a strong correlation between the risk factors and infected horses. The main risk factors that are directly related to EHV-1 include age, confined vs pasture, higher traffic of people and horses in and out of stables, use of common equipment, training and competition, the sex of the horse, the gender and even what season it is. Horses that are stabled are more prone to EHV-1 because of stress, those in pastures have less stress. Older horses have a higher susceptibility, and during the time of winter and spring are all risk factors.^[11] The horses past exposure has a huge increase in risk factor, transporting and hauling frequently also increases the risk.^[7]

Diagnosis, treatment and prevention

Treatment

The current diagnosis for EHV-1 is PCR, polymerase chain reaction, which copies the DNA so once can see if a virus is in the DNA segment. PCR can detect the viral load in the DNA, telling if the horse is very infected or below detection levels.^[7] To get samples for PCR a veterinarian may want to have an uncoagulated blood sample and also a nasal swab. There are a few downfalls of PCR one being that it is very time consuming and takes a while to get back samples and results and the other downfall is how the results are interpreted. Horses can have a latent virus but not show clinical signs and be called clinically normal, except the PCR results will give positive test result when it’s not so straightforward ^[10] The one other downfall of PCR is that there is not standardized protocols between the laboratories.^[7] Treatment for EHV-1 is limited at the time and there isn’t one definitive treatment that is guaranteed to work. The treatments now include intravenous fluids, I.V’s, or anti-inflammatory drugs.^[4]

Prevention

Vaccines exist to control the virus but not to prevent it. The inactivated vaccine contain a low antigen load and are made to help protect against the respiratory symptoms, the performance of the inactivated vaccines is variable are doesn’t work for all.^[3] The modified live vaccine is made to vaccinate healthy horses 3 months or older, to help prevent the respiratory symptoms caused by EHV-1. Vaccinations should be given in 6-month intervals, and different horses will need to be vaccinated at different times.^[3] Pregnant mares should be vaccinated during the fifth, seventh and ninth months of gestation, with the inactivated EHV-1 vaccine. Foals should be vaccinated in a series of 3 doses starting at 3 months in 4–6 week intervals.^[3] Even though a horse has been vaccinated, infection and clinical disease still continues to occur. New vaccines to help prevent the spread of the virus are being studied.^[4]

To prevent the spread of EHV-1, there are a few steps that should be taken. Most important is to stop horse movement and transportation; not allowing horses that have been exposed to EHV-1 to be in contact with unexposed horses; and to isolate animals that are showing symptoms of the virus.^[4] Ideally, horses that are showing symptoms should be completely quarantined. The recommended quarantined period is at least 21 days.^[10] Another facet of EHV-1 control is to be aware of the potential for spreading the virus via equipment, and transmission, via human contact, between horses. Since people can and do transfer this virus via their hands and clothing, people need to take sanitary precautions when handling a sick horse. Disinfecting footwear and wearing gloves can help minimize the risk of spreading.^[4] Routinely cleaning and disinfecting the barns and buildings where horses have been is important for preventing transmission.^[10] In the case of an outbreak, one should also take precautions to minimize stress on the horses. As stated previously, stress will trigger a latent virus to reactivate, and it can allow an uninfected horse to be more easily infected.^[10]

References