Equine herpesvirus type 1 (EHV-1) is the herpesvirus associated with abortion. The virus has also been associated with perinatal foal death; rhinopneumonitis in foals, growing horses, and some adult horses; and encephalomyelitis in adult horses. The virus is distinct from EHV-4, which is the major cause of rhinopneumonitis in foals and is only rarely isolated from equine abortions.

EHV-1 infection is acquired via inhalation, with the virus attaching to, penetrating, and replicating in upper airway mucosal epithelial cells. If local immunity fails to overcome infection, the virus breaches the basement membrane to invade the lamina propria of the respiratory mucosa and infects T lymphocytes and endothelial cells. The resulting viremia disseminates virus throughout the body. Abortion is the result of ischemia consequent to vasculitis of uterine vessels that disrupt the uteroplacental barrier. Lymphocytes resident within the endometrium are also thought to potentially transfer virus directly to uterine endothelium and result in abortion. This latter mechanism has been proposed to explain abortion of single mares in a group and abortions that occur many weeks or months after viremia.

Viral latency also occurs with EHV-1 infection, with periodic reactivation of latent virus resulting in asymptomatic shedding from the respiratory tract that may result in infection of in-contact horses. If local immunity has waned, reinfection and viremia can recur, again placing the fetus at risk. Although vaccinations do not eliminate preexisting latent EHV-1 infections, if they stimulate sufficient local immunity to prevent shedding, transmission of virus to other in-contact animals may be prevented.

Vaccination timing and efficacy against EHV-1 abortion remains controversial. Pneumabort-K +1b (Wyeth Animal Health, Guelph, Ontario) is a killed-virus preparation approved for use to protect against EHV-1 abortions in mares, with administration recommended during the fifth, seventh, and ninth months of gestation. Nonpregnant mares that may come in contact with pregnant mares should have vaccine administered at the same time as pregnant mares. Rhinomune (Pfizer, Animal Health, New York, NY) is an attenuated live virus preparation approved for use in preventing respiratory disease caused by EHV-1. Although the product label makes no claim for provision of protection against abortion, it does state that no adverse reactions have been reported in pregnant mares vaccinated with this product and further recommends vaccination of pregnant mares after the second month of gestation and at 3-month intervals thereafter. Prestige II with Havlogen (Intervet/Schering-Plough Animal Health, Whitehouse Station, N.J.) is a killed-virus preparation that contains EHV-1, EHV-4, and equine influenza subtypes A1 and A2; the product label makes no claims concerning provision of protection against abortion. Prodigy (Intervet/Schering-Plough Animal Health, Whitehouse Station, N.J.) is a killed-virus preparation of EHV-1 labeled for the prevention of abortion. Vaccination with this product is recommended at the fifth, seventh and ninth months of gestation. Recommendations for frequency of administration of booster vaccines, although they vary with the product used, are notably at frequent intervals because herpesviruses typically do not stimulate long-lasting immune protection (even immunity from natural infection wanes in 3 to 6 months). Although the efficacy of vaccination in the face of an abortion outbreak from rhinopneumonitis is unknown, Pneumabort-K +1b is labeled for this use.

Research regarding changing administration between vaccine types or brands during gestation is lacking. Some practitioners believe that switching vaccines during pregnancy leads to vaccine breaks in which EHV-1 abortion is more likely to occur. Until this phenomenon is studied, we caution against changing products during pregnancy in gestating mares.

Prevention and control of EHV-1 abortion cannot rely solely on a vaccination program because vaccination provides limited protection against viral shedding and the disease and properly vaccinated mares occasionally abort. One should use unerring management procedures in concert with a vaccination protocol to reduce mare exposure to the virus. Pregnant mares should be separated from the rest of the farm population. Permanent resident mares should not be allowed contact with transient boarders that normally reside elsewhere. Stress should be minimized to reduce the risk of activation of EHV-1 virus that may already be present in the mare. Mares that have aborted as a result of EHV-1 should be isolated from the rest of the herd. In addition, all mares that have been in contact with aborting mares should be segregated from those not yet exposed to the virus, and booster vaccines may be administered to in-contact mares in an attempt to stimulate immunity. Strict hygienic measures should be instituted to minimize spread of infection to the rest of the mares on the premises.

When facilities are limited for separating at-risk from nonexposed mares, the practitioner can perform polymerase chain reaction (PCR) testing on nasopharyngeal washes and whole blood samples collected from incoming mares. Procedures for nasopharyngeal washes are described by Conboy (2005). Submitting these samples to a diagnostic laboratory that can perform PCR testing for EHV-1 (e.g., University of Kentucky Livestock Disease Diagnostic Center, Lexington, Ky.) results in timely reporting of results, which can be used as a screening measure for either keeping animals isolated or allowing them to be moved into different locales on a farm. Animals with positive results on PCR testing of nasopharyngeal washes should at least be considered to have been exposed to the virus (but may not be actively infected, nor shedding the virus), and those with positive results in blood should be considered to be viremic and therefore likely to be shedding the virus. Some practitioners require maiden mares arriving at a breeding farm to be PCR negative for EHV-1 before they are allowed to mix with other mares at the farm or before they are allowed to be sent to a stud farm for breeding. Such a screening protocol may prove to be valuable in controlling an outbreak of EHV-1 respiratory or neurologic disease (although the test does not specifically identify the variant that causes neurologic disease).

Tetanus toxoid administration should be mandatory in all vaccination programs because of the incidence and life-threatening consequences of the disease for the dam and foal. The initial series of injections in unvaccinated horses consists of a two-dose series, with the second dose given 4 to 6 weeks after the first. For the pregnant broodmare, booster vaccines are given 4 to 6 weeks before the date of expected foaling to provide passive protection from colostrum intake by the newborn foal.

This insect-transmitted neurologic disease is caused by viruses of the Togaviridae family, of which Eastern, Western, and Venezuelan encephalomyelitis viruses are most pathogenic. Horses in endemic areas should be immunized with a suitable inactivated-virus vaccine before the mosquito season each year, which corresponds to the foaling season. In areas where mosquito resurgence occurs in late summer or fall, a second annual dose should be given in late summer, just as for the inactivated WNV vaccine. Pregnant mares are routinely administered a booster vaccination 4 to 6 weeks before the date of expected foaling in an attempt to provide passive protection to the newborn foal from colostrum intake.

This insect-transmitted neurologic disease is also caused by a virus transmitted mainly by mosquitoes, and outbreaks have occurred throughout the United States and worldwide. Three vaccines are currently available for horses: (1) an inactivated vaccine that requires an initial two-dose primary immunization series; (2) a recombinant canarypox vaccine that requires an initial two-dose primary vaccination series; and (3) a flavivirus chimera vaccine that requires only a single dose for primary immunization. Revaccination in late summer before mosquito population resurgence has been recommended for both the inactivated and the recombinant products. All products are thought to be safe for vaccination of the pregnant mare, but recommendations have been to provide the primary course of vaccination to previously unvaccinated mares while they are nonpregnant. However, a recent Texas study revealed no adverse effects when the inactivated vaccine was administered to previously nonimmunized pregnant mares at all stages of gestation. Pregnant mares are routinely administered a booster vaccination 4 to 6 weeks before the date of expected foaling in an attempt to provide passive protection from colostrum intake by the newborn foal.

The risk of rabies is widespread across the United States. Because of the associated mortality and public health risks, immunization against this disease should be recommended for all horses. A single dose is recommended for primary immunization. Pregnant mares can be administered a booster vaccination 4 to 6 weeks before foaling in an attempt to provide passive protection from colostrum intake by the newborn foal. However, because of the relatively long duration of immunity, some authors recommend the vaccine be given after foaling but before breeding to reduce the number of prepartum vaccines given to a mare.

Rotavirus is considered to be the most common infectious cause of diarrhea in foals, and farm outbreaks can affect a large proportion of the foals on a farm and become endemic. Foals of very young age are susceptible to adverse effects of rotavirus infection, which causes a profuse watery diarrhea. One of the better ways to provide protection to newborn foals is to vaccinate pregnant broodmares. A three-dose series (1 month apart) of vaccine administration is recommended, with the first dose given at 8 months of gestation. Thus, the last dose is given approximately 1 month before foaling to optimize colostral immunoglobulin concentration.

Any deworming program should be suited to the individual requirements of a farm or stable, with evaluation of program success by examination of feces at regular intervals to monitor parasitic egg levels (eggs per gram [epg]). Sound pasture management (e.g., low stocking density, regular pasture rotation, and pasture harrowing) should be used in concert with deworming protocols to establish an effective antiparasitic program. An example of one health program, including a deworming schedule, for broodmares is presented in Box 9-2.