A jersey cow definitively diagnosed as having Bovine Ephemeral Fever was also found to have evidence of a mild, multifocal neuropathy with Wallerian degeneration in the Casino district of Northern NSW, in May 2016.
This area, within the North Coast Local Land Services (LLS) experienced exceptionally high numbers of cattle affected by Bovine Ephemeral Fever (3-day-sickness) during the summer of 2015-2016.
Clinical signs associated with suspected infections during the BEF virus (BEFV) epizootic ranged from mild lameness, fever, ptyalism, recumbency, prolonged ataxia and occasionally death.
This outbreak of BEF also coincided with a prolonged period of good rainfall following several years of draught in the Casino area.
During a property visit just outside of Ettrick for investigation of a separate incident, a district veterinarian was alerted to the existence of 3 cows showing neurological signs after recovering from the acute phase of suspected infection with Bovine Ephemeral Fever.
One animal had been recumbent for a period of approximately 10 days. The farmer had provided the cow with basic supportive care. When the animal was eventually able to again self-ambulate, it displayed an unusual gait.
Two other animals had shown similar neurological deficits in the hind-legs, however, without an observed period of being recumbence.
The property owner kept two of the animals aside during milking on a subsequent visit by the district veterinarian. They were observed by the veterinarian to be in appropriate body condition, bright alert, and normally responsive. One cow showed mild proprioceptive deficits of the hind limbs and a second cow displayed quite marked proprioceptive deficits with a wide based stance in the hind limbs and a hypermetric hind limb gait. A third cow was being kept in a holding paddock close to the dairy, and was observed to collapse in the hind region on excitation.
Samples were obtained post mortem from 2 of these animals. Both cows were seropositive for Bovine Ephemeral Fever. One cow was also PCR positive for Bovine Ephemeral Fever Viral Antigen, from fresh spleen. Histopathological results from this animal revealed mild Wallerian degeneration within the white matter tracts of the thoracic spinal cord.
Bovine Ephemeral Fever (BEF) is an economically important disease in cattle1-3. The disease is caused by an arbovirus. Specifically, a single stranded RNA virus from the family Rhabdovirus and genus Ephemerovirus4. It is believed to be spread by Culex and Anaopheles mosquitoes and Culicoides biting midges in Australia.5
Outbreaks of BEF occur seasonally as vector populations increase, resulting in high rates of virus transmission to susceptible cattle.5-6
There are a range of clinical signs associated with BEF, including; acute or peracute fever, anorexia, depression, ocular and nasal discharge, ptyalism, muscle stiffness, lameness, ruminal stasis, sternal recumbence, paralysis and death.7-8
Production losses can be attributed to decreased milk production in dairy cows, abortion, loss of body condition, temporary infertility in bulls and deaths.1,4,8
There have been many reports of Wallerian degeneration (and subsequent chronic ataxia) in some animals post recovery from BEF, however, the pathogenesis of this very specific lesion remains undetermined.9-11 One hypothesis suggests the changes are resultant of trauma from compression injuries suffered during prolonged recumbency or as result of misadventure.
Another hypothesis is a suspect neotropism of BEFV in cattle whereby the virus enters the CNS and persists, causing chronic inflammation, resulting in Wallerian degeneration.9
Non-steroidal anti-inflammatory drugs (NSAIDs) and parental calcium supplementation may be administered as indicated. General supportive care including shade, water and feed, are essential for animals that experience prolonged recumbence.4
In situations like that experienced on the NSW North Coast in 2015/16, when a major outbreak of BEF had not been experienced for several years, the disease can be more serious, as older, and heavier naïve animals will be most susceptible. Under such conditions vaccination may be warranted.4
Both live and inactivated vaccines are available, and with either vaccine, two doses are required four weeks apart to achieve adequate immuno-protection.8 This interval can be reduced to 2 weeks when protection is required urgently.4
Animals that recover from natural infection with BEFV develop long lasting immunity to the disease.4
The higher morbidity and case fatality rates experienced in the described epizootic indicates that the above average rainfall experienced in the region was favourable to vector populations and that the cattle population was relatively susceptible to BEFV.
This case study also demonstrates that infection with BEFV can potentially cause Wallerian degeneration leading to chronic ataxia. It also demonstrates that BEF viral antigen can be detected on PCR of the spleen several months after an animal has recovered from the acute phase of infection with BEFV.