Mosquitoes, Floods and Horse Diseases
By Drs Charlie El-Hage, Josie Holmes and Tias Muurlink, from the Equine Centre
The Equine Centre has treated several horses with confirmed Murray Valley Encephalitis (MVE) recently, following outbreaks of arboviral disease in horses (and people) around Victoria and the Murray River region. Some clinically affected horses were admitted for evaluation and treatment.
Recent heavy rains and flooding throughout the last summer have led to high numbers of mosquitoes around metropolitan and regional Victoria. The most common mosquito-borne (arbovirus) in horses is Ross River Virus (RRV), an alphavirus which sporadically causes musculoskeletal disease in horses.
MVE is a zoonotic arbovirus belonging to the Flavivirus group. Although the majority of infected horses show few clinical signs, severely affected horses may develop encephalitis and in some cases death.
The clinical signs of mildly affected horses admitted to the Equine Centre have included pyrexia, muscle tremors, ataxia, colic and weakness. Treatment of these horses has been supportive and included intravenous fluids, DMSO and Flunixin meglumine.
A five year old TB mare (Figure 1) was admitted for suspected colic due to the sudden onset of severe unremitting distress and discomfort. Upon clinical examination the horse was tachycardic (100 beats per minute), tachypnoeic and exhibited a hypermetric short stepping gait. The mare was euthanased within 24 hours upon welfare grounds due to intractable pain. Although no gross lesions were evident upon post mortem, histopathological findings were significant in the central nervous system.
Perivascular cuffing (Figure 2) by lymphocytes, neutrophils, plasma cells, and macrophages was evident in the midbrain, brain stem, spinal grey matter. A PCR test was positive for MVE. The clinical signs, histopathological findings and virological testing led to a diagnosis of polioencephalomyelitis due to MVE.
Differential diagnoses for these horses include Hendra virus, Equine Herpes Virus Myeloencephalitis, plant toxicities and botulism.
Clinical signs of horses with flaviviral encephalomyelitis are referable to the areas of inflammation within the brain and spinal cord. The Pons and medulla region contains several cranial nerve nuclei hence deficits in swallowing, facial muscle tone and laryngeal function may result. Generalised muscle tremors are often evident; this finding may be attributable to inflammation of the basal nuclei which are largely responsible for fine motor control. Ataxia is generally due to spinal cord inflammation often in the lumbar region. Although theoretically it may be due to vestibular or cerebellar these are less commonly involved.
At present there are no vaccines for arboviruses for horses in Australia. Mosquito control remains pivotal. Regular application of insect repellants for horses is important, horse rugs are available that provide protection for the body, head, belly and neck.
Stabling horses at dusk and dawn, when mosquitoes are most active, will also reduce bites. Installation of fans in stables will reduce mosquito numbers since they prefer still conditions free of wind. It also is advisable to clear any stagnant water that may have pooled around stables or paddocks. If water sources contain large numbers of mosquito larvae, then consider stocking these with fish that feed on such larvae or applying biological and or chemical control measures.
For further information please contact Hinalei Johnston, Marketing & Communications Manager, Faculty of Veterinary Science, University of Melbourne, Tel: 8344 7844 or email: h.johnston@unimelb.edu.au