Equine Herpesviruses 1 and 4

Equine Herpesviruses 1 and 4

 Dr C.E. Foote

Equine Consulting Services

Importance of EHV-1 and EHV-4 to industry

Equine herpesvirus 1 (EHV-1) and equine herpesvirus 4 (EHV-4) are major causes of respiratory disease in horses worldwide. EHV-1 also causes epidemic and sporadic abortion in mares, most commonly during the last trimester of pregnancy. Neurological disease associated with EHV-1 infection is rare but can affect horses of all ages, including unweaned foals. Viral respiratory disease has been reported to be a common cause of lost training time, second only to lameness and orthopaedic disease. Like other herpesviruses, the virus is able to produce a latent infection that persists for the lifetime of the host, complicating the designs for the control of EHV-1 and EHV-4 by vaccination. As they say…herpes is for life!!

Diagnosis of infection

Lab based tests are available to assist in diagnosing infection, however results may be confused due to the latent nature of the disease. It is often difficult to determine if antibodies present in blood samples are due to an active infection, or due to a previous infection. Swab samples can be taken from nasal discharge and also tested, however as the virus only excretes for a short period of time, this is also not 100% reliable. Tests can be done on aborted fetuses if EHV-1 infection is suspected.

Epidemiology of EHV-1 and EHV-4

Studies indicate a dissemination of EHV-1 and EHV-4 throughout Australia and worldwide in countries including Japan, Great Britain, New Zealand, Canada, the United States of America and China.    The major segment of the horse population susceptible to respiratory disease caused by EHV-1 and EHV-4 is the annual foal crop however, the majority of studies have concentrated on horses older than 12 months of age. 

Mares and young foals

Prior to the introduction of vaccination in Australia

Studies have provided strong serological evidence of EHV-1 infection of young, suckling foals. Infection has been suggested to increase from rates of 27% at 12 hours to 82% at 90 days of age. This serological data suggested that new cases of EHV-1 infection occurred in foals in their first six months of life. A peak incidence of new cases of EHV-1 infection occurred when the foals were 126 days of age. It was proposed that lactating mares were the source of the EHV-1 from which their foals were infected and these foals then infected other foals by close contact. A silent cycle of EHV-1 infection was proposed and illustrated that some foals were infected prior to weaning and that these foals spread EHV-1 to other foals within their group both before and after weaning (Fig. 1).   

After the introduction of vaccination in Australia

Studies continue to demonstrate the presence of EHV-1 and EHV-4 in samples taken from vaccinated mares and their unweaned, unvaccinated foals.   The youngest foal identified as being EHV-1 positive was 22 days of age, and the youngest EHV-4 positive foal was only 11 days of age. The results suggest that foals are being infected with EHV-1 much earlier than first thought and support the continuation of the cycle of infection in vaccinated populations. The vaccine used in these mares has been reported to reduce clinical signs and the occurrence of abortion following experimental challenge. It has been acknowledged that many systems of farm management, such as routine handling of mares and foals, and the mixing of paddock groups are innocently contrived to encourage the introduction and spread of this disease.  While there may be some benefit in vaccinating mares late in gestation to assist in reducing the chances of abortion, as foals are being shown to be infected with EHV-1 at such an early age, the efficacy of vaccinating weanling foals becomes questionable when costs of vaccination are considered. 

Weanling foals

Prior to introduction of vaccination in Australia

In a study of EHV-1 and EHV-4 in unvaccinated populations of foals aged between six and 12 months of age, EHV-1 and EHV-4 was detected from weaned foals over a six month period. Asymptomatic foals were just as likely to be shedders of EHV-4 virus as symptomic foals and the month of March was identified as an important risk factor. Interestingly, EHV-4 was more likely to be shed by colts rather than fillies and foals with a history of respiratory disease which required veterinary treatment were more likely to be shedders of EHV-4.

Following introduction of vaccination in Australia

One study provided serological evidence of infection in a vaccinated population of weanling foals.   Of the twenty foals, eight (40%) were classified as having responded to vaccination and cases of EHV-1 and EHV-4 infection were identified in seven vaccinated foals (35%). All foals identified as cases of infection had lymph node enlargement and a mucopurulent nasal discharge was observed in six of the seven foals.

Summary and implications

Taken together, these studies describe the continued spread of both EHV-1 and EHV-4 from mare-to-foal, and subsequent spread of infection from latently infected foals who had been previously infected to other foals during weaning.   The continuation of the cycle of infection ensures the survival of EHV-1 in the population as lifelong latently infected fillies enter the broodmare herd. 

While it has been reported that the current vaccination strategy results in a reduction in the period of excretion of virus, clearly improved vaccination and management strategies are required if the aim is to eliminate the ongoing threat due to continued circulation of EHV-1 and EHV-4 in horse populations. Further studies are currently in progress examining alternative vaccination strategies utilizing recombinant proteins and DNA technologies.

References

Allen, G. P. & Bryans, J. T. (1986). Molecular epizootiology, pathogenesis and prophylaxis of equine herpesvirus-1 infections. In Progress in Veterinary Microbiology and Immunology, pp 78 - 144. Edited by R. Pandey. Basel: S. Karger.

Foote, C. E., Love, D. N., Gilkerson, J. R., Whalley, J. M. (2004). Detection of EHV-1 and EHV-4 DNA in unweaned Thoroughbred foals from vaccinated mares on a large stud farm. Equine Veterinary Journal, 36, 341-345.

Foote, C. E., Gilkerson, J. R., Whalley, J. M., Love, D. N. (2003). Seroprevalence of equine herpesvirus 1 in mares and foals on a large Hunter Valley stud farm in years pre- and post- vaccination. Australian Veterinary Journal 81, 283-288.

Foote, C. E., Love, D. N., Gilkerson, J. R., & Whalley, J. M. (2002). Serological responses of mares and weanlings following vaccination with an inactivated whole virus equine herpesvirus 1 and equine herpesvirus 4 vaccine Veterinary Microbiology 88, 13-25.

Foote, C. E., Love, D. N., Gilkerson, J. R., Whalley, J. M. (2001). Serological responses of mares and weanlings following vaccination with an inactivated whole virus Equine Herpesvirus 1 and Equine Herpesvirus 4 vaccine. Australian Equine Veterinarian 19, 68-69

Gilkerson, J. R. (1998). Seroepidemiological studies of equine herpesvirus 1 and equine herpesvirus 4 in Thoroughbred foals and racehorses. PhD thesis.

Gilkerson, J. R., Jorm, L. R., Love, D. N., Lawrence, G. L. & Whalley, J. M. (1994). Epidemiologic investigation of equid herpesvirus 4 (EHV 4) excretion assessed by nasal swabs taken from Thoroughbred foals. Veterinary Microbiology 39, 275-283.

Gilkerson, J. R., Love, D. N., Drummer, H. E., Studdert, M. J. & Whalley, J. M. (1998). Seroprevalence of equine herpesvirus 1 in Thoroughbred foals before and after weaning. Australian Veterinary Journal 76, 677-682.

Gilkerson, J. R., Love, D.N. & Whalley, J.M. (1997). Serological evidence of equine herpesvirus 1 (EHV-1) infection in Thoroughbred foals 30-120 days of age. Australian Equine Veterinarian 15, 128-134.

Gilkerson, J. R., Teague, N., Whalley, J. M. & Love, D. N. (1999a). A prospective cohort study of upper respiratory tract disease in one and two-year old racehorses. Serological evaluation of the role of equine herpesviruses 1 and 4 (EHV-1 and EHV-4) in respiratory disease. Australian Equine Veterinary Journal 17, 76-81.

Gilkerson, J. R., Whalley, J. M., Drummer, H. E., Studdert, M. J. & Love, D. N. (1999b). Epidemiological studies of equine herpesvirus 1 (EHV-1) in Thoroughbred foals: a review of studies conducted in the Hunter Valley of New South Wales between 1995 and 1997. Veterinary Microbiology 68, 15-25.

Gilkerson, J. R., Whalley, J. M., Drummer, H. E., Studdert, M. J. & Love, D. N. (1999c). Epidemiology of EHV-1 and EHV-4 in the mare and foal populations on a Hunter Valley stud farm: are mares the source of EHV-1 for unweaned foals. Veterinary Microbiology 68, 27-34.

Heldens, J. G. M., Hannant, D., Cullinane, A. A., Prendergast, M. J., Mumford, J. A., Nelly, M., Kydd, J. H., Weststrate, M. W. & van den Hoven, R. (2001). Clinical and virological evaluation of the efficacy of an inactivated EHV1 and EHV4 whole virus vaccine (Duvaxyn EHV 1,4). Vaccination/challenge experiments in foals and pregnant mares. Vaccine 19, 4307-4317.