Causative agent: Ovine herpes virus Type 2 (in North America)
The virus that causes malignant catarrhal fever has never been isolated and propagated. The principal host for this virus is sheep, in which the virus causes no disease. In a serologic survey in the USA, 61% of goats and 53% of sheep were seropositive to MCF virus by competitive-inhibition ELISA testing (32).
MCF causes disease in dead end hosts such as cattle, bison and deer species (9). The virus is thought to be transmitted from sheep and possibly goats to dead end hosts (bison?), but not from dead end host (bison) to dead end host (bison) (9). The mechanism of transmission of the virus is not known..
Clinical signs:
Malignant catarrhal fever has been reported in bison (33,34,35,36,64). In the past, the disease has always been considered to be fatal in bison (35). Bison can develop an acute fatal form of disease in which they die in 7 to 10 days, a chronic fatal form in which they die in periods of up to 156 days, and a recovered form in which they remain persistently infected for an unknown length of time (36,64).
Clinical signs in bison are reported to include: corneal opacity, conjunctivitis, ocular discharge, nasal discharge, excess salivation, anorexia, diarrhea, melaena , hematuria , multi focal ulceration of the oral mucosa, fever, circling, ataxia, behaviors suggestive of blindness, lameness, and difficulty urinating (33,34,35,36,64). The most common presenting clinical signs are anorexia, depression and bloody diarrhea (64).
The disease is usually seen in the winter (64). The disease is usually seen in bison over 6 months of age (64). Herd mortalities have ranged from 3 to 100% (34,35,64).
Postmortem findings:
Postmortem findings include corneal opacity, reddened conjunctiva, ulcerations and erosions of the mouth, esophagus, rumen, abomasum, intestinal tract, and trachea, hemorrhages in the urinary bladder, and enlarged lymph nodes(34,35,36).
Diagnosis:
The diagnosis may be made from blood samples and nasal swabs that are submitted to a diagnostic pathology laboratory for PCR testing. Herds may be monitored for infection by submitting a serum sample to a laboratory for antibody detection with a competitive inhibition ELISA test. Postmortem examination and submission of tissue samples to a diagnostic pathology laboratory for histopathology and PCR testing may be used to establish a diagnosis.
Treatment:
There have been no successful treatment protocols established for treating MCF in bison. Treatment with corticosteroids may prolong the course of the disease.
Control:
In Canada and the United States there are no commercially available vaccines for the control of MCF in any species. Additions to a herd should be tested with a PCR test or the competitive inhibition ELISA test before being introduced to the herd. Introduced bison should be quarantined for at least one month before introduction into the herd.
Since a large percentage of normal sheep and goats carry MCF virus, it would be advisable to avoid any contact between bison and sheep or goats. The safe distance between bison and sheep or goats may be greater than one mile. If goats are to be used as surrogate mothers for bison calves they should be tested for the presence of MCF virus before they are purchased.