Serological evidence of latency in cattle experimentally infected with elk herpesvirus

Autor: S. V. Tessaro, S. A. Gilbert, Dirk Deregt
Rok vydání: 2005
Předmět:
Zdroj: Veterinary Record. 156:610-611
ISSN: 0042-4900
Popis: BOVINE herpesvirus type 1 (BHV-1) is the causative agent of infectious bovine rhinotracheitis and infectious pustular vulvovaginitis/balanoposthitis in cattle. In recent years, a group of unique alphaherpesviruses serologically related to BHV-1 has been isolated from ruminants (Engels and Ackermann 1996). In Europe, two viruses belonging to this group have been isolated from cervids, namely, cervid herpesvirus type 1 (CerHV-1) from red deer and rangiferine herpesvirus (RanHV) from reindeer (Inglis and others 1983, EkKommonen and others 1986). More recently, Deregt and others (2000) isolated an alphaherpesvirus, which also belongs to this group, from the semen of a Canadian elk (Cervus elaphus nelsoni); this was named elk herpesvirus (ElkHV). In nature, each herpesvirus is closely associated with a single host species (Davison 2002) and the relationship is characterised by the establishment of latent infection. After primary infection, BHV-1 localises in the sensory ganglia (Ackermann and Wyler 1984). The infection then becomes persistent; it is characterised by periods of viral latency, when no virus is produced, and by the reactivation of virus production (Snowdon 1965). Reactivation of the virus from a latent state is generally thought to be induced by stress, but also occurs following the injection of corticosteroids (Pastoret and others 1980). Inoculation of cattle with CerHV-1 produced no clinical signs of disease, whereas RanHV produced a mild rhinitis, but to the authors’ knowledge there is no reported evidence of latency of either virus in cattle (Reid and others 1986, Nettleton and others 1988). This short communication describes a study to determine whether ElkHV could infect cattle and produce disease. Six Angus cross Hereford calves aged six months were obtained from the laboratory’s BHV-1-free herd and segregated in three isolation pens. Two bull calves (calves 1 and 2) were inoculated intranasally with 2 x 106 TCID50 of ElkHV, two bull calves (calves 3 and 4) were inoculated intrapreputially with the same dose, and two heifer calves (calves 5 and 6) received the same dose intravaginally. An additional five calves served as uninoculated controls. Three days after inoculation, one control heifer calf was introduced to the inoculated heifers and one control bull calf was introduced to each of the other two inoculated groups; the other two control calves remained isolated. The inoculated and the isolated control calves were given 0·1 mg/kg bodyweight dexamethasone by intramuscular injection on five consecutive days starting on day 28 after inoculation. All the calves were monitored twice daily for signs of disease. Their rectal temperatures were measured, and blood, nasal swabs and vaginal/preputial swabs were collected on the day of inoculation and three, seven, 10, 14, 21, 28, 30, 32, 35, 38, 42 and 49 days later. The calves were euthanased with an overdose of barbiturates on day 49 after inoculation, and postmortem examination was carried out on day 50 after inoculation. All the procedures complied with the guidelines of the Canadian Council on Animal Care. For virus isolation, the swabs were placed into tubes containing 2 ml minimum essential medium (MEM) containing 5 per cent horse serum and antibiotics, vortexed and stored at –80°C; 200 μl of the supernatants were inoculated on to confluent susceptible Madin-Darby bovine kidney cells in 24-well plates for one hour. The inoculum was removed and the cells were incubated in fresh MEM containing 2 per cent horse serum and antibiotics for seven days at 37°C. A second passage was performed and the cells were observed for cytopathic effects. A 24-hour virus neutralisation assay without complement was performed on the serum samples as described by Deregt and others (1993), with 100 TCID50 of ElkHV or BHV-1. After inoculation with ElkHV, the calves remained afebrile and none showed signs of disease.Virus was not isolated from swabs taken from any of the calves. All the controls and one bull calf inoculated intrapreputially (calf 4) remained antibody negative. However, the other five inoculated calves seroconverted (Fig 1). Neutralising antibody titres to ElkHV were first observed 10 days after inoculation. On day 28, before the dexamethasone injections were started, the antibody titres remained low (1:2 to 1:8) in four of the five calves. Calf 5, which had been inoculated intravaginally, had the highest titre (1:16). After the calves had received dexamethasone, no disease or febrile response was observed and virus was not isolated. In four of the five seropositive calves, the antibody titres remained within four-fold of the titres obtained before dexamethasone treatment, and none of these produced an antibody titre greater than 1:8. However, the neutralising antibody titres in calf 5 increased eight-fold after dexamethasone treatment to 1:128, evidence of reactivation of the virus from latency (Fig 1). Cross-neutralising antibody titres to BHV-1 were only observed in calf 5 (data not shown). A titre against BHV-1 was first observed 14 days after inoculation. On day 28, before dexamethasone treatment, the titre was 1:2; after the treatment, cross-neutralising titres increased eight-fold to 1:16, in parallel with the increase in titres against ElkHV. PCR for ElkHV was attempted on the sacral ganglia of calf 5 after they had been frozen, but these assays were negative. This may have reflected a low number of genome copies in the ganglia, below the sensitivity of the PCR. The presence of a low number of copies might be supported by the observation that the rise in antibody titre after dexamethasone treatment appeared to be delayed by several days when compared with the findings in BHV-1 latently infected cattle treated with dexamethasone (Masri and others 1996). The authors believe that this is the first time that reactivation of a cross-species latent herpesvirus infection in cattle has occurred experimentally, although cross-species latency without reactivation was previously described in cattle experimentally inoculated with caprine herpesvirus 1 by Six and others (2001). It is probably unlikely that a non-shedding calf latently infected with a cervid herpesvirus would have practical significance for the diagnosis of BHV-1, as natural infection of cattle with cervid herpesviruses appears highly unlikely (Nettleton and others 1988). However, this study does leave open the possibility that there may be Veterinary Record (2005) 156, 610-611
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