The Management of Sheep-Associated Malignant Catarrhal Fever in a Zoological Ruminant Collection
Between February 1994 and February 1998, seven nondomestic ruminants of five species in the Fort Worth Zoo’s collection died with strong histopathologic evidence of malignant catarrhal fever (MCF). This paper reviews the epidemiology of the disease in this collection and the use of multiple diagnostic assays to manage and attempt to eradicate MCF from the collection.
Losses to MCF occurred in axis deer (Cervus axis) (two), hog deer (Cervus porcinus) (two), tufted deer (Elaphodus cephalophus) (one), white-tailed deer (Odocoileus virginianus) (one), and dama gazelle (Gazella dama) (one). Clinical signs ranged from acute onset, rapidly progressing weight loss, and death within 5 days, to chronic weight loss, bluing of the corneas, ocular and nasal discharge with death several weeks after onset of clinical signs. All deaths occurred as isolated events within single species herds.
Blood and/or tissue samples from 116 animals representing 16 species in the Zoo’s ruminant collection were screened by polymerase chain reaction (PCR) for the WC-11 sequence from wildebeest-associated MCF. All were negative. This same group was tested by PCR for a purported sequence of OHV-2, sheep-associated MCF. Fifty-seven tested positive, including five clinically ill animals that subsequently died of MCF. Fifty-two positive animals consisted of clinically healthy mouflon (41), aoudad (9), domestic sheep (1) and goats (1). Samples for PCR testing were not available from two of the seven dead animals (one axis deer and the white-tailed deer.) At no time did a clinically healthy nondomestic ruminant (excluding aoudad and mouflon) test positive for OHV-2 (48 animals representing 11 species).
Of the 116 animals screened by PCR, 72 animals representing 14 species were also screened retrospectively by competitive ELISA. Although the date of the two assays did not always coincide, there was exceptional agreement of PCR and cELISA results. Excluding domestic goats, 64 of 65 animals tested by both methods had complete agreement of results. Validation of the cELISA for domestic goats is ongoing (pers comm, T. Crawford). This population of ruminants was also screened by the immunoperoxidase test (IPT) and serum neutralization (SN) methods. Positive titers on the IPT were common, with 119 of 146 animals testing positive at dilutions of 1:100 or 1:20. Six of six animals that died of MCF were tested and were all positive. The presence of neutralizing antibodies in this population of ruminants was rare, with only nine animals positive by SN of 96 tested. Six of the seven animals that died of MCF were tested and were negative for neutralizing antibodies. Evaluation of the specificity and sensitivity of IPT and SN in this population by statistical analysis is pending.
Mouflon and aoudad are suggested to be the point source of sheep-associated MCF at the Fort Worth Zoo. The herds were 93 and 82% positive by PCR, respectively. At no time did individuals of either species show clinical disease. They were housed adjacent to each other and directly uphill, approximately 30 yd from the pens of six of the seven animals that died of sheep-associated MCF. The white-tailed deer that died of MCF was housed at the opposite end of the zoo from the mouflon and aoudad, but in close proximity to the domestic sheep and goats, which might represent another point source of OHV-2 in the collection. There is no overlap of staff, traffic, or equipment between the white-tailed deer herd and the mouflon and aoudad herds.
PCR and cELISA appear to be sensitive screening tools for the presence of OHV-2 in nondomestic ruminants. However, the existence of a carrier state of OHV-2 in nondomestic ruminants has not been confirmed or refuted by this study. Mouflon and aoudad were eliminated from the Zoo’s collection in 1996. A tufted deer died of MCF in March 1997. A hog deer that tested negative in April 1997 tested positive and died of clinical disease in early 98. It is not known whether these animals harbored latent virus undetectable by PCR, or if an unknown source of infection existed in the collection after removal of the aoudad and mouflon. Herd mates of the affected hog deer were euthanatized and screened by PCR and cELISA. Both animals were negative. No animals have been lost to MCF since February 1998, including herd mates of the dama gazelle and tufted deer that had intimate exposure to clinically ill animals.
The authors thank Mr. Rick Tucker for his assistance with retrospective data collection and Dr. Tim Crawford of WADDL for his consultation on the cELISA assay.