Environmental stressors may cause equine herpesvirus reactivation in captive Grévy's zebras (Equus grevyi)

Abstract

Equine Herpesviruses (EHV) are common and often latent pathogens of equids which can cause fatalities when transmitted to non-equids. Stress and elevated glucocorticoids have been associated with EHV reactivation in domestic horses, but little is known about the correlation between stress and viral reactivation in wild equids. We investigated the effect of an environmental stressor (social group restructuring following a translocation event) on EHV reactivation in captive Grévy's zebras (Equus grevyi). A mare was translocated by road transport from Zoo Mulhouse, France, to join a resident group of three mares in Tierpark Berlin, Germany. We used an indirect sampling method to assess the frequency of EHV shedding for 14 days immediately after the translocation event (termed the 'experimental period'). The results were compared with those from two control periods, one preceding and one subsequent to the experimental period. In addition, we measured fecal glucocorticoid metabolite (fGCM) concentrations daily in all individuals from 6 days before, to 14 days after translocation. We found significantly higher EHV shedding frequencies during the experimental period, compared to each of the two control periods. All animals showed significantly elevated fGCM concentrations, compared to fGCM levels before translocation. Finally, we found that an increase in fGCM concentration was significantly associated with an increased likelihood of EHV shedding. Although the small number of animals in the study limits the conclusions that can be drawn from the study, taken together, our results support the hypothesis that environmental stressors induce viral reactivation in wild equids. Our results suggest that potentials stressors such as group restructuring and translocation should be considered in the management of zoological collections to reduce the risk of fatal EHV infections in novel hosts. Moreover, environmental stressors may play an important role in EHV reactivation and spread in wild equid populations.

Keywords: EHV; Fecal glucocorticoids; Latent infection; Reactivation.

Figure 1. Experimental setup and sampling scheme.

Day 0 indicates the day of the translocation; collected sample types are indicated in the grey-shaded boxes.

Figure 2. EHV shedding prevalence in the experimental and control periods.

The relative prevalence of EHV shedding in the respective period (pre-experimental control period, experimental, and post-experimental control period) is shown (A) for the resident animals, and (B) for all animals.

Figure 3. Individual fecal glucocorticoid metabolite (fGCM) and EHV shedding profiles of Grévy’s zebra mares.

fGCM concentrations time-corrected by one day, from day -6 to day 13, and respective EHV shedding status (+ EHV-positive; − EHV-negative) of each individual (A–D) with day 0 indicating the arrival of the translocated mare (D) at Tierpark Berlin. The shaded area (day 5 to day 9) indicates the time period when the four mares shared one common enclosure.

Figure 4. Relationship between fecal glucocorticoid metabolite concentrations (log-transformed) and detected EHV shedding.

Each point represents an assessment of shedding for one individual on one day with a zero indicating that no shedding was detected and a one indicating that virus shedding was detected. The line shows the estimated increase in the probability of EHV shedding related to fGCM concentrations. (A) original values. (B) time-corrected values.

Figure 5. Phylogenetic relationships among detected EHV viruses.

(A) Phylogenetic maximum likelihood tree generated from nucleotide sequences of a 600 bp fragment of the herpesvirus DPOL gene, using Asinine Herpesvirus 5 (AHV-5, accession no. FJ798319.1) as an out-group. Each tip represents one sample with the host individual’s initial letter and sampling day indicated. Two genotype variants in the clusters Fr and Ki had been identified in “Franzi” and “Kianga”, respectively, in a previous study, with the respective reference sequences labeled “F ref”, and “K ref”, respectively. Clusters “Fr”, “Ki”, and “Ek” are indicated by different frame lines. Colored tips indicate potential transmission events. (B) Genotypes as in Fig. 3. A indicated by frame lines, traced per individual during the two weeks after arrival of the new mare.