Equine herpesvirus-1 is a major infectious cause of abortion in mares and is associated with substantial financial costs to breeding farms as a result of losses caused by abortion and the cost of prevention programs. Despite the widespread use of EHV-1 vaccines, abortion secondary to EHV-1 infection is still a concern for mares residing on commercial stud farms.1 The virus can cause lifelong latent infection, with periodic reactivation and shedding important in maintaining EHV-1 in the horse population.2 It is estimated that > 50% of the horse population is latently infected with EHV-1, and it is hypothesized that reactivation in latently infected horses is the major biological source of infective virus.3
Recently, type-specific ELISAs and PCR assays have been used to study the epidemiology of EHV-1 infection in vaccinated and unvaccinated mares and their foals residing on commercial Thoroughbred breeding farms.4–7 Collectively, these studies4,5 have provided direct (PCR assays) and indirect (ELISAs) evidence for infection of foals during the first 5 weeks after birth, with subsequent horizontal spread up to and even after the time of weaning. However, these studies were limited to mares and their foals, included only a random sample of all horses on the farm, followed up a group of horses for a relatively short period by means of serologic testing only, did not include mares in the late stages of pregnancy or other groups of horses residing on breeding farms, or were conducted in the southern hemisphere. As a result, the incidence of viral reactivation and shedding among mares, foals, and other horses residing at Thoroughbred farms in North America on which EHV-1 is endemic is largely unknown. Given the high incidence of latently infected horses and the potential for reactivation, we hypothesized that transmission of EHV-1 within and between resident horses on breeding farms would occur frequently, despite vaccination, and that transmission would be facilitated by less-than-optimal segregation of horse groups. The purposes of the study reported here were to determine the incidence of EHV-1 infection among vaccinated Thoroughbreds and unvaccinated foals residing at a farm on which the virus was known to be endemic and to identify potential temporal associations between individual horses and groups of horses in regard to EHV-1 infection.
Pneumobort-K, Fort Dodge Animal Health, Fort Dodge, Iowa.
Prestige with Havlogen, Intervet Inc, Millsboro, Del.
Procto Swab, Fox Converting Inc, Green Bay, Wis.
Transporter, Healthlink, Hardy Diagnostics, Santa Maria, Calif.
Kendall Monoject, Tyco Healthcare Group LP, Mansfield, Mass.
SNAP foal IgG test, IDEXX Laboratories Inc, Westbrook, Me.
Cryovial, Phenix Research Products, Hayward, Calif.
DNeasy blood kit, Qiagen, Valencia, Calif.
DNeasy tissue kit, Qiagen, Valencia, Calif.
Universal TaqMan Mastermix with AmpErase UNG, Applied Biosystems, Foster City, Calif.
7900 HTA, Applied Biosystems, Foster City, Calif.
Svanovir, Svanova Biotech AB, Uppsala, Sweden.
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