As of February 2011 an increase in muscle soreness, joint pain and neurological disease was reported in NSW horses. Similar cases were also reported from Victoria, South Australia and southern Queensland. The incidence peaked in early April and declined steadily thereafter. The alphavirus, Ross River virus (RRv) has been identified in many of the musculoskeletal cases while the flaviruses; Murray Valley encephalitis virus and Kunjin virus have been implicated in the majority of the neurological cases. These infections are believed to be arboviral in origin, transmitted by blood sucking insects. This is consistent with the pattern of infection and the declining incidence with the onset of cooler weather.
Disease from or exposure to Murray Valley encephalitis virus and Kunjin virus was identified on multiple properties in the Orange area. This case study outlines a local horse that is believed to have developed clinical signs from exposure to Kunjin virus.
An 8 year old Miniature mare presented with sudden onset incoordination and tremors. She had been maintained in a stable herd and with no recent exposure to other horses. Diet consisted of pasture and small amounts of Lucerne hay and grain, and had not been altered at all prior to the onset of illness. No other mares in the group of fifteen were affected, and all other horses on the property (140 in total) were clinically normal.
On physical examination the mare was noted to have consistent muscle tremors and fasciculation of the facial muscles. She circled to the right intermittently, had a slight head tilt and had significant difficulty prehending food. Mucous membranes, heart rate, lung and gut sounds were normal. The temperature was elevated at 39.4 degrees Celsius. She had slight faecal staining around the perineum and soft faeces were noted in the stall. She displayed severe hyperaesthetic reactions to touch and sound. Despite her clinical symptoms she appeared relatively bright - attempting to eat and drink, and interacting consistently with nearby horses.
Blood was taken in serum and lithium heparin tubes from the affected mare and six other members of her herd, none of which were clinically symptomatic. The affected mare had minimal changes on routine blood testing aside from a slight increase in her neutrophil count, but she and four normal mares all returned high viral titres for Kunjin virus. Repeat testing at one and three week intervals indicated recent infection rather than past exposure.
The affected mare was treated symptomatically with flunixin meglumine and trimethoprim- sulfa. She improved rapidly over the next three days and was clinically normal at a recheck examination one week later. No other horses on the property developed clinical signs.
Clinical arboviral disease in the Orange area was relatively rare during the reported peak season of incidence. However, ongoing herd testing indicated that the numbers of infected horses significantly exceeded those with clinical signs. It is likely that many cases were subclinical or undiagnosed due to vague presentations and rapid recovery from illness, and so the prevalence in this area may have been underestimated.
Like other regions of southern Australia, the increased risk of equine aborviral disease in the Orange area in early 2011 was likely from a very wet season that resulted in high insect numbers.
The clinical improvement of the horse in this case study is consistent with other reports. Most horses recovered gradually over a period of 1-3 weeks.
Further information about Flavivirus disease in NSW horses can be found at: www.dpi.nsw.gov.au