Original Research
Neuropathogenicity of Equine Herpesvirus 9 in Cattle

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Abstract

The pathogenicity of equine herpesvirus 9 (EHV-9), a neurotropic equine herpesvirus isolated from a herd of Gazella thomsoni, was studied in cattle. Seven calves were inoculated intranasally with 105 and 107 plaque-forming units of the EHV-9 P19. Three animals showed brain lesions consisting of glial reactions and perivascular cuffings in the olfactory bulb and the frontal and temporal lobes. Additionally, the animal that was inoculated with 107 plaque-forming units showed neuronal degeneration and loss, as well as nuclear inclusions compatible with herpesvirus. EHV-9 was isolated from the brain of calf 6 and the lungs of calves 1 and 2. The results suggested that cattle are susceptible to experimental infection with EHV-9 and at risk from natural infection from reservoir hosts.

Introduction

Equine herpesvirus 9 (EHV-9), a highly neurotropic equine herpesvirus, was first described in an outbreak of disease in Thomson’s gazelles (Gazella thomsoni) that died of fulminant encephalitis [1], [2]. EHV-9 is most closely related immunologically to equine herpesvirus 1, but its deoxyribonucleic acid (DNA) fingerprint is different from those of equine herpesvirus 1 and other equine herpesviruses [1]. The affected animals had encephalitis characterized by neuronal necrosis with intranuclear inclusion formation and perivascular cuffings [2]. Although the natural host of EHV-9 is still uncertain, some members of the family Equidae are suspected to be reservoirs because Burchell’s zebras showed relatively high seroprevalence (60% among 45 animals) for EHV-9 at the Serengeti National Park in Tanzania [3]. More recently, EHV-9 was detected in a polar bear with progressive encephalitis, and the source was traced to members of a potential equid reservoir species, Grevy’s zebras [4]. In addition, EHV-9 was found in an aborted Persian onager in a zoo [4]. It was also detected in a giraffe in a zoo [5]. These reports indicated an increasingly broad range of hosts for natural infection by EHV-9, raising fears of emerging infections in various wild and domestic animal species.

Herpesvirus infection in natural hosts is often mild and is usually followed by a latent infection [6], [7]. Some herpesviruses may cross the interspecies barrier, inducing severe and fetal disease in such hosts [8], [9]. One example is B-virus which produces a very mild disease in Macaque monkeys, but, if infection is transmitted to human beings it leads to fetal encephalitis [9]. Many previous experimental studies have been conducted in rodents and domestic animals to ascertain the infectivity of EHV-9. Fetal acute encephalitis occurred in mice, rats [1], hamsters [10], goats [11], pigs [12], dogs, and cats [13], [14] that were inoculated with EHV-9 through the nasal route, whereas horses [15] exhibited moderate nonsuppurative encephalitis characterized by perivascular cuffings of mononuclear cells and gliosis in the olfactory and limbic systems. Encephalitis was induced even when using different inoculation routes, similar to the case of the pigs that were orally infected with EHV-9 [12]. No information is available regarding the susceptibility of large ruminants to EHV-9. In this study, calves were inoculated intranasally with EHV-9 to determine the infectivity of calves to EHV-9.

Section snippets

Animals

Seven 3-month-old Holstein calves (six females and one male) that were purchased from a private farm and proven to be free from infectious diseases were used for induction of the experimental infection by EHV-9 P19. The animals were provided with a commercial diet for livestock and tap water ad libitum.

Viruses

Madin–Darby bovine kidney (MDBK) cells were used for propagation of the working seed of EHV-9 P19. The inoculums were prepared by culturing the virus from the original seed stocks of EHV-9 (P19,

Clinical Signs

Calves 2 and 4 had a fever of 40°C or greater on day 8 postinoculation (PI). Other calves showed no clinical signs of fever. Suppurative nasal discharges were observed in calf 3 on day 7 PI, and in calf 4 on days 1, 3, and 4 PI. Blood analysis showed a slight decrease in the WBC count on day 7 PI in calf 1, who also showed a slight increase in cerebrospinal fluid.

Virus Isolation and PCR Results

The EHV-9 was isolated from the frontal lobe of the brain of calf 6 and from the lungs of calves 1 and 2. No virus was isolated from

Discussion

Cattle are a mainstay of the livestock industry, and an outbreak of infectious disease in cattle could lead to huge economic losses, similar to that seen with foot-and-mouth disease in European countries. EHV-9 provoked fulminant encephalitis in a herd of gazelles in a Japanese zoo [2], and experimentally similar fulminant encephalitis was induced in goats inoculated with EHV-9 through the nasal route [11]. EHV-9 has recently been detected in collections from animals in zoos, including a polar

Acknowledgments

This study was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Health, Labour, and Welfare of Japan (Emerging-General). The authors are grateful to Miss Cate Swift for her proofreading of this manuscript.

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