Equine herpesviruses (EHV) are found in the majority of horses. Most horses have been exposed to the virus and have had no clinical signs or side effects. There are nine EHV that we have been able to identify worldwide, but EHV 1, 3, and 4 are the strains that have caused clinical disease in horses.
EHV-1 specifically has multiple manifestations of disease in horses: neurological form, respiratory form, and can cause abortion and neonatal death. There is also some evidence that it may cause chorioretinopathy (a disease process in the eye). Research has shown that greater than 80% of horses may be latently infected with the EHV-1 virus, but not all horses will develop clinical signs.
The Science Behind EHM
EHV-1 myeloencephalopathy (EHM) is still being studied today and we are constantly acquiring new information. There are multiple reports of theories describing how the virus causes neurological disease. A single point mutation in the DNA polymerase gene has been associated with neurological disease, causing the presence of aspartic acid (D) or asparagine (N) at position 752. AAEP reports that 80-90% of neurological disease cases are caused by D752 isolates, and 10-20% by N752 isolates.
Recently, there has also been a proven association with a single nucleotide polymorphism at position 2254 in the DNA polymerase gene and the occurrence of EHM.
The virus can become reactivated and furthers cell-associated viremia delivering the virus to endothelial cells in the uterus and central nervous system. Clinical signs occur due to the vasculitis and cellular damage caused by the virus.
Infection of Horses
Horses are affected through contact with respiratory secretion, aborted fetuses/placentas, or by fomites (infected objects, such as grooming tack). Following infection, a viremia is established as the virus circulates the body in infected cells. Following this, virus-infected cells are latently infected. Horses can shed virus in nasal secretions as early as day 1 of infection, and can continue to shed the virus for approximately 28 days. The virus can cause disease in infected horses or can re-activate from latency causing clinical disease in the horse. The virus can typically survive in the environment for up to 7 days, but this can vary based on the environmental conditions.
Signs of EHM
The incubation period of EHV is typically 4-6 days, but has the potential to be longer. Some clinical signs associated with the neurological disease may include:
Loss of tail tone
Inability to rise
Clinical signs (neurological signs and fever) and isolation of the virus are how we confirm EHM. Isolation of the virus can be done using quantitative polymerase chain reaction (qPCR) from nasal swabs and blood collection.
Treating and Preventing EHM
After confirming EHM, implementing a strict quarantine and biosecurity measures under supervision of local and state veterinarians is key. Treatment options include anti-inflammatories and supportive care.
Vaccination and biosecurity protection are the two ways to help avoid disease outbreak. No current EHV-1 vaccine has a claim to prevent EHM. Some EHV-1 vaccines can reduce nasal shedding and possibly viremia. By vaccinating your horses, you induce a strong immune response to the virus without inducing clinical disease.
It is important to clean tack, equipment and the environment to inactivate the virus. Cleaning first with a detergent or soap to allow for removal of organic material (such as soil) is recommended in order to prevent disinfectants becoming inactive when they make contact with organic material. There are multiple effective disinfectants that will kill the virus, including 1:10 dilution of bleach to water.
Other key points for understanding EHM prevention include:
Immunity following infection or vaccination offers limited protection
Boosting (routine vaccination) contributes to herd immunity to help protect individual horses
It is difficult for the body to establish immunity due to “evasion” properties of this virus
Early recognition and diagnosis is key
Quarantine and close monitoring of suspected cases is important
Exposed horses should have their temperature taken twice daily
Please note that this virus is still being investigated and new research is constantly being release as we try to better understand EHM.
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