Parker County Today September 2017 - Page 66

our expertise: VET COLUMN Equine Herpes - What You Need To Know BY TOM HUTCHINS, DVM, DABVP F ive different herpes virus are commonly found in domestic horses. Equine Herpes Virus (EHV) infection is ubiquitous in the equine population and most horses are infected in the first few months of life. Following infection with an EHV, the virus is able to essentially hide from the immune system in the lymphoid or neurologic tissues; horses are then said to have a “latent infection.” Once infected with EHV, horses are thought to be permanently infected. A latent viral infection can become reactivated via currently unknown mechanisms often during times of stress. Horses with re-activated latent infections are a source of infection for other horses. EHV-1 was one of the first equine herpes viruses to be described. EHV-1 is termed an “alpha herpes virus” that can cause severe pneumonia in newborn foals, mild respiratory disease in young horses and abortion in pregnant mares. The neurologic form of disease is a consequence of damage to blood vessels and the adjacent soft tissues. Depending on what part of the neurologic system is affected (i.e., the spinal cord and brain) clinical signs will vary.  The neurologic form of EHV- 1 infection is generally referred to as Equine Herpes Myeloencephalopathy (EHM) as both the brain and spinal cord can be involved. This form of herpes virus infection can cause clinical signs in any age, breed, or gender of horse. 64 TRANSMISSION: Horse to horse transmission of the herpes viruses is significant when horses are kept in close contact. Contaminated equipment (e.g., water buckets, water hose handles, cleaning and grooming equipment, etc.) can also be a source of infection and people can transmit the virus on their hands or clothes. An infected horse will excrete and aerosolize the virus in respiratory secretions.  All horses with clinical signs are expected to be contagious, although horses not showing any clinical signs can shed EHV.  Neurologic horses shed large quantities of EHV-1 and should be securely isolated. The virus is estimated to be viable for up to 7 days in the environment under normal circumstances but may remain viable for a maximum Dr. Tom Hutchins of one month under perfect experimental conditions. The virus is easily killed in the environment by most disinfectants. DIAGNOSIS: Once exposed and infected with EHV-1 the virus may be detected in blood and nasal swabs for 21 and 14 days respectively in laboratory settings. Virus shedding maybe more transient and difficult to detect in the latter stages of the disease and in clinical outbreaks. Nasal shedding may be prolonged (21+ days) in cases infected with certain neuropathic strains of EHV-1. Horses can develop clinical signs as early as 1 day after exposure to the virus, although clinical signs can be delayed up to 10 days after exposure. Typically EHM develops 5 to 6 days after the primary infection. Infection usually develops following exposure to a horse shedding the virus but in a small percentage of cases, infection occurs by reactivation of latent virus. EHV-1 typically causes a biphasic fever peaking on day 1 or 2 and again on day 6 or 7. With respiratory infections there is often significant nasal and ocular discharge, but not a lot of coughing. There may be some persistent enlargement of submandibular lymph nodes. With the neurologic form there are typically minimal respiratory signs, with fever (rectal temperature greater than 102°F) being the only warning. Neurologic disease appears suddenly and is rapidly progressive reaching its peak intensity within 24 to 48 hours. In horses infected with the neurologic strain of EHV-1, clinical signs may include: nasal discharge,