Equine Biosecurity Reminder
A reminder that we are entering a time of peak herpes virus risk. Think Delta variant of covid and we are facing the same spread dynamics for this virus. Good personal sanitation between the handling of different horses and farm biosecurity including judicious vaccination is the only way to protect against disease spread.
Equine herpesvirus-1 (EHV-1) is one of the most important and prevalent viral pathogens of horses and a major threat to the equine industry. EHV-1 primarily causes respiratory disease but viral spread to distant organs enables the development of more severe outcomes such as abortion and neurologic disease.
Extensive work investigating the transmission cycle of EHV-1 has identified mare and foal populations as important reservoirs enabling virus transmission before and after weanling, with infection in foals occurring within the first 30 days of life. The trend now is of increasing numbers of outbreaks of a devastating form of EHV-1, equine herpesvirus myeloencephalopathy (EHM).
Broodmares undergo recrudescence of latent viral infection as a result of stress, resulting from pregnancy and foaling, then shed virus exposing their foals to infection. Progression of disease leading to abortion is usually dependent on certain factors including the virulence of the EHV-1 strain involved, the level and magnitude of viremia, and the hormonal state of the pregnant mare. Neurologic symptoms may be simultaneously present with or without respiratory disease or abortion. The observed clinical signs in infected horses are a culmination of the vasculitis, edema, hemorrhage, ischemia, and necrosis resulting from the viruses’ predilection for blood vessel walls.
There seems to be no satisfactory scientific explanation for the variable incidence of EHM and different clinical manifestations observed during outbreaks of EHV-1. Several factors including sex, age, immune status of the horse, the reproductive status of the mare (including the stage of gestation), the severity of infection, the type of strain, and perhaps the route of transmission determine the resulting disease outcome.
The nervous signs appear mainly in the hind limbs. In some cases, there are signs of bladder dysfunction with accompanying urinary incontinence and scalding of the perineal area or urinary retention which may lead to colic. Horses that are standing have a good prognosis unlike horses that are unable to stand who may suffer additional complications such as pneumonia, colic or bladder rupture and are generally euthanised.
This disease condition is a major concern for the horse industry due to its negative impact on economic health. NZ had its first outbreak of EHM on one property in 2014 with further cases diagnosed since then.
The associated risk factors for the increased incidence of EHM in other countries are still poorly defined. However overseas outbreaks have been reported mostly at places such as racetracks, riding schools, and veterinary hospitals where horses from different origins congregate. The high stocking density of stabled horses during events such as horse racing may facilitate the quick spread of EHM by direct contact when outbreaks occur. Young foals that come in close contact with EHM-infected mares and stallions are at high risk of developing ocular disease associated with EHV-1
There is no specific drug effective against disease caused by EHV-1.. However, good hygiene and management practices together with symptomatic treatment of infected horses may help curtail the spread of the viral infection. Symptomatic treatment with a range of drug regimes is suggested in the veterinary literature. The currently available inactivated vaccine reduces the clinical incidence of disease however, the effectiveness of vaccination in preventing abortion or EHM is still unclear. Nonetheless, it is recommended to vaccinate every horse that is at risk of exposure to EHV-1 to help reduce the severity of disease.
Control and Notification
Control measures aim at reducing viral spread to susceptible horses and at preventing the reactivation of virus in latently infected horses.
Infected sick horses are primary sources of infectious EHV-1, and as such, should be quarantined to prevent direct contact with un-infected horses. Also, infected materials such as aborted foetus or uterine contents, including placenta, should be disposed of appropriately to limit exposure and spread.
High-level biosecurity measures should be put in place on horse farms and all visitors should be encouraged to use a footbath and wash their hands before entering or leaving. Infected equipment must be disinfected and disposed of, and different equipment and workers must be used on affected and unaffected horses to prevent horizontal transmission of the disease.
Movement of horses and visitors should be restricted onto and off the infected farm premises until laboratory tests indicate negative results for EHV-1 infection. Newly acquired horses should be quarantined from the rest of the herd for at least 3 weeks and must be certified negative for EHV-1 before being allowed to join the resident population.
Horse owners should involve their veterinarian whenever neurological symptoms are seen in their animals. Nervous signs may be the result of many disease processes. It is important that your vet is involved to workup the case, carry out appropriate diagnostic testing through the routine commercial veterinary laboratories, and guide owners on appropriate biosecurity measures such as quarantining sick animals and use of foot-baths, disposable gloves and site specific clothing, to reduce the risk of spread until a diagnosis is available. Veterinarians concerned about a potential case of EHM should seek advice from the NZ Equine Health Association in the first instance. Depending on the presenting situation MPI should be informed, especially if there are multiple cases or commercial lab screening indicates presence of EHV-1 and specialist testing is required at MPI’s Animal Health Laboratory. MPI’s exotic disease and pest freephone: 0800 80 99 66.
Official controls around EHV-1/EHM would not be implemented currently as EHV-1 is not considered a Notifiable/Unwanted Organism under the Biosecurity Act. The owner/vet would be expected to work alongside the NZEHA with assistance from the MPI’s Investigation/Lab team to implement voluntary controls, including movement restrictions and biosecurity measures, and wider surveillance/testing (if required).
NZEHA has a code of practice that should be followed to safeguard the equine community in the case of any EHM outbreak. Click here to go to the full code for the control of contagious disease.
For more information go to the article below.