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Prevalence of equine herpesvirus-1 infection among Thoroughbreds residing on a farm on which the virus was endemic

James A. BrownVeterinary Medical Teaching Hospital, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Samantha MapesDepartment of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Barry A. BallDepartment of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Aaron D. J. HodderVeterinary Medical Teaching Hospital, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Irwin K. M. LiuDepartment of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Nicola PusterlaDepartment of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Abstract

Objective—To determine the incidence of equine herpesvirus-1 (EHV-1) infection among Thoroughbreds residing on a farm on which the virus was known to be endemic.

Design—Prospective cohort study.

Animals—10 nonpregnant mares, 8 stallions, 16 weanlings, 11 racehorses, and 30 pregnant mares and their foals born during the 2006 foaling season.

Procedures—Blood and nasopharygeal swab samples were collected every 3 to 5 weeks for 9 months, and placenta and colostrum samples were collected at foaling. All samples were submitted for testing for EHV-1 DNA with a PCR assay. A type-specific EHV-1 ELISA was used to determine antibody titers in mares and foals at birth, 12 to 24 hours after birth, and every 3 to 5 weeks thereafter.

Results—Results of the PCR assay were positive for only 4 of the 1,330 samples collected (590 blood samples, 590 nasopharyngeal swab samples, 30 placentas, and 30 colostrum samples), with EHV-1 DNA detected in nasal secretions from 3 horses (pregnant mare, stallion, and racehorse) and in the placenta from 1 mare. Seroconversion was detected in 3 of 27 foals during the first month of life.

Conclusions and Clinical Relevance—Results suggested that there was a low prevalence of EHV-1 infection among this population of Thoroughbreds even though the virus was known to be endemic on the farm and that pregnant mares could become infected without aborting. Analysis of nasopharyngeal swab samples appeared to be more sensitive than analysis of blood samples for detection of EHV-1 DNA.

Abstract

Objective—To determine the incidence of equine herpesvirus-1 (EHV-1) infection among Thoroughbreds residing on a farm on which the virus was known to be endemic.

Design—Prospective cohort study.

Animals—10 nonpregnant mares, 8 stallions, 16 weanlings, 11 racehorses, and 30 pregnant mares and their foals born during the 2006 foaling season.

Procedures—Blood and nasopharygeal swab samples were collected every 3 to 5 weeks for 9 months, and placenta and colostrum samples were collected at foaling. All samples were submitted for testing for EHV-1 DNA with a PCR assay. A type-specific EHV-1 ELISA was used to determine antibody titers in mares and foals at birth, 12 to 24 hours after birth, and every 3 to 5 weeks thereafter.

Results—Results of the PCR assay were positive for only 4 of the 1,330 samples collected (590 blood samples, 590 nasopharyngeal swab samples, 30 placentas, and 30 colostrum samples), with EHV-1 DNA detected in nasal secretions from 3 horses (pregnant mare, stallion, and racehorse) and in the placenta from 1 mare. Seroconversion was detected in 3 of 27 foals during the first month of life.

Conclusions and Clinical Relevance—Results suggested that there was a low prevalence of EHV-1 infection among this population of Thoroughbreds even though the virus was known to be endemic on the farm and that pregnant mares could become infected without aborting. Analysis of nasopharyngeal swab samples appeared to be more sensitive than analysis of blood samples for detection of EHV-1 DNA.

Contributor Notes

Supported by Equine Resident Research Funds from the Center for Equine Health Program of the University of California.

Presented as an abstract at the 25th Annual Meeting of the American College of Veterinary Internal Medicine, Seattle, June 2007.

Address correspondence to Dr. Brown.