Neurological disease in horses

Arbovirus infections have been attributed to the alphavirus Ross River virus (RRv) in many musculoskeletal cases, and to flavivirus infection with Murray Valley Encephalitis virus (MVEv) and Kunjin virus in the majority of neurological cases.

An increased number of horses showing muscle and joint soreness, and neurological signs have been reported in southern states of Australia since February 2011. Reported cases peaked from mid-March to early April and numbers have steadily declined.

Cause

An arbovirus infection.

Distribution

There are indications that cooler weather in south eastern Australia has led to a reduction in mosquito numbers, and a consequent reduction in the number of new cases being reported in horses.

Hosts

Horses

Affected animals

  • horses

Symptoms

There seem to be 2 distinct syndromes to this neurological disease in horses:

  • muscle and joint pain
  • nervous signs.

Common clinical signs include:

  • reluctance to walk
  • stiff gait
  • ataxia
  • depression
  • tremors
  • weakness and lethargy.

Less common clinical signs include:

  • increased responsiveness to touch and sound
  • facial paralysis
  • hypermetria
  • muscle fasciculation.

Impacts

Most cases recovered gradually over 1-3 weeks and 10-15% of affected horses died or were euthanased for welfare reasons.

How it is spread

Arboviruses are a large group of viruses, spread mainly by blood sucking insects, and include alphaviruses and flaviviruses.

Monitoring and action

Interpretation of laboratory results is difficult and time-consuming, and often requires a range of tests.

Many horses can become infected with arboviruses but only a small number become ill. This means that a positive blood test must be carefully interpreted. Repeat testing is crucial to show that antibody levels are rising and allow a definitive diagnosis.

Control

The horse is usually a 'dead-end' host for mosquito-borne arbovirus infections, and is not considered a likely source of new infection for people or other horses.

You can prevent mosquito bites in the field by:

  • covering up with light-coloured, loose-fitting clothing and covered footwear
  • regularly applying effective repellent containing diethyl‑meta‑toluamide on exposed skin.

Horse owners should reduce the exposure of their animals to insect bites. They can do this by using registered repellents, rugging, using fly masks (especially in the mornings and early evenings when more insects are present) and removing sources of stagnant water.

Further information