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EHV-1 and EHV-4 introduction

AKA rhinopneumonitis, seen primarily in young horses at the time of weaning and beginning of training, 80-90% of horses are exposed to the virus by the age of 2, majority of horses run a mild respiratory disease (like a common cold), disease is acute, treatment is palliative (ban amine, fluids etc.) and prognosis is favorable, natural transmission happens by contact with another infected horse, aerosolized respiratory secretions or virus-contaminated fomites


types of EVH

EHV 1-5 infect horses, EHV 6-9 infect wild horses and zebras, EHV 1 and 4 are the most important, closely related but antigenically distinct with different disease profiles


distinguishing features of EHV 1 and EHV 4

EHV 1 infects a variety of cell types: respiratory, epithelial, endothelial, neuronal, lymphoid cells, EHV 1 is more virulent and can cause neurologic disorders, abortion (due to vasculitis), neonatal death, ocular disease and death
EHV 4 is more commonly isolated from mild respiratory cases and prognosis is favorable


similarities of EHV 1 and 4

both are enveloped viruses that make them highly susceptible to destruction by common disinfectants


main reservoir of EHV 1 and 4

latently infected horses (horse that is carrying the virus and begins shedding it due to stress), at stud farms foals acquire the virus from adult mares that shed the virus asymptomatically


How is EHV spread?

lives in the environment an average of 7 days, spread through direct horse to horse contact and indirect contact (fomites, personnel), very contagious, morbidity can reach 100%


risk factors for EHV outbreaks

overcrowding, heavy parasite burden, poor nutritional state, climatic extremes, concurrent disease and the intermingling of animals from different social groups (separate animals based on age/use), even if you have a closed herd, you can still have outbreaks


EHV latency

large majority of recovered horses carry a latent EHV infection for life in the t lymphocytes and the neurons of the trigeminal ganglia (cranial nerve that aids in feeling/movement of jaw), reactivation of the virus occurs after transport, rehousing, weaning, inclement weather, surgery, corticosteroid administration etc., reactivation is not accompanied by respiratory clinical signs, but the shedding of virus particles may infect other horses, reactivation may cause abortion due to destruction of blood vessels or endothelial cells in uterus


EHV 1 pathogenesis

replicates in upper respiratory tract, infects endothelial cells, lymphocytes and monocytes, viremia (virus reaches the blood stream and goes all over the body including the uterus and CNS), causes vasculitis and ischemia in the CNS and uterus, resulting in late term abortionthe virus reaches the trigeminal neurons 48 hours after infection and establishes latency


EHV 4 pathogenesis

replicates in the upper respiratory tract, has low affinity for endothelial cells, does not establish viremia so it does not cause abortion or neurologic disease, respiratory disease that may have started with EHV (1 or 4) predisposes the affected horses to opportunistic bacterial infections leading to bacterial pneumonia


EHV 4 clinical signs

mild respiratory disease but may lead to secondary bacterial pneumonia, young animals have a more serious disease-depression, anorexia, biphasic fever (days 1-2 and 6-7), nasal discharge and ocular discharge
older animals or previously exposed show very mild respiratory disease or no sign of disease


EHV 1 clinical signs

uncommon to cause clinically apparent respiratory disease, except for very young foals infected with a highly virulent strain, mares abort, but without previous signs of respiratory disease


incubation period of EHV

up to 10 days


EHV vs flu clinical signs

in EHV cough is not a major clinical sign, except if horse is kept in unclean environment or is not rested from training/performance activities



laboratory: virus isolation, CSF, serology
very difficult to detect latently infected horses



NSAIDs for fever, maybe antibiotics, maybe fluids, mares that abort generally don't have any further complications and have no problems getting pregnant again



8 different commercially available killed vaccines, responsible for reducing abortion rate in mares by 75% over a 20 year period in KY, vaccines generally don't prevent respiratory disease, but diminish shedding, there is a vaccine only for abortion that has a much larger viral load than the regular vaccination, mares are generally vaccinated at 3, 5, 7 and 9 months of pregnancy


management of EHV

no stress, don't mix age groups, cleanliness, isolate new horses, isolate sick horses, don't mix new horses with pregnant mares in the last 1/3 of pregnancy, don't mix resident groups with itinerant group