EHV-1 General Information

The most recent outbreak of EHV-1 was reported to occur on January 11, 2012, in Orange County, where a gelding displaying neurologic signs was confirmed positive for the neuropathogenic strain. Additional horses from the same premises subsequently have been confirmed positive. Information updates will be posted on our Facebook page.

Last year's major EHV-1 outbreak appeared to be related to initial cases at a cutting horse show in Ogden, Utah, which was held from April 29 - May 8, 2011. Horses at that event may have been exposed to this virus and subsequently spread the infection to other horses. There were subsequent unrelated outbreaks in August and September 2011 but of a much smaller scale.

In 2006, a number of racetracks, private veterinary clinics and university teaching hospitals outside of California were shut down to limit the spread of an EHV-1 infection. There were additional cases in 2007.

Information on EHV-1 will continue to be posted on this site to provide awareness to horse owners regarding the virus and infection.

What is EHV-1?

EHV-1 (equine herpesvirus-1) is one of a large group of DNA viruses causing potentially serious disease in horses and other species. EHV-1 has two forms: one that causes abortion in mares and one that causes respiratory infection and neurological symptoms. The above cited outbreaks have involved the EHV-1 respiratory/neurological form of the virus causing a condition known as Equine Herpes Myeloencephalopathy (EHM).

EHV-1 is contagious and is spread by direct horse-to-horse contact, by contaminated hands, equipment and tack, and, for a short time, through aerosolization of the virus within the environment of the stall and stable.

What are the Clinical Signs of EHV-1?

The initial clinical signs of the infection may be nonspecific and include fever of 102°F or greater. Fever may be the only abnormality observed. Other presenting signs may be combinations of fever and respiratory symptoms of nasal discharge and cough. Some horses have reddish mucous membranes.

Horses with neurological disease caused by EHV-1 infection can soon become uncoordinated and weak and have trouble standing. Difficulty urinating and defecating may also occur. Often the rear limbs are more severely affected than the front. Signs of brain dysfunction may occur as well, including extreme lethargy and a coma-like state.

The incubation period of EHV-1 infection is HIGHLY VARIABLE, depending on the host, on the virulence of the virus, and on environmental and other factors such as stress. The AVERAGE incubation period is 4 to 7 days, with the majority of cases being 3 to 8 days, but with some taking up to 14 days. When neurological disease occurs, it is typically 8 to 12 days after the primary infection involving fever. In most cases, horses exposed to EHV-1 will develop a fever and possibly nasal discharge and then go on to recover.

Center for Equine Health, School of Veterinary Medicine, UC Davis, Tel (530) 752-6433