Australian Animal Health Laboratory Helps Understand Mystery Horse Disease

In September 1994 in Hendra, a suburb of Brisbane, Australia, an apparently new disease resulted in the deaths of 13 horses and one human from a severe respiratory disease. This resulted in a decision to temporarily stop all horse movements in Queensland. The in contact horses were destroyed and the disease was contained. A huge research effort to establish the cause of the disease was begun. Subsequently a new virus was isolated which has been called the Hendra virus (HeV, formerly equine morbillivirus). However, about one year later, a farmer developed fatal encephalitis (brain inflammation) that appeared to be the result of exposure to two Hendra virus infected horses that had died more than one-year earlier. There was an urgent need to further investigate this new virus disease and RIRDC approved funds for Dr. Peter Hooper of the Australian Animal Health Laboratory to lead a team to further investigate how this disease was spread.

In work undertaken by the Queensland Department of Primary Industries, blood samples were taken from a large number of horses and other species to determine if there was evidence of exposure to the Hendra virus. These samples showed no evidence of exposure to the virus but subsequently, fruit bats (flying foxes, Pteropus spp) were found to have a high prevalence of positive blood tests, which indicated a possible wildlife reservoir of the virus.

In the RIRDC-supported project, the following experiments were undertaken:

  • Investigation of a model for an outbreak of HeV in a group of stabled horses.
  • Determining whether infected cats can transmit the disease to horses.
  • Evaluation as to whether fruit bats could be infected with HeV, develop disease and be a source of infection for horses.
  • Development of a more efficient blood test, an ELISA, which could be easily used by all laboratories.

The ELISA test was successfully developed and has been made available to all major veterinary diagnostic laboratories in Australia. The experimental studies showed that horses can be infected by the mouth and nose and can excrete virus in their urine and saliva. The disease is not highly transmissible from horses, cats or fruit bats and there is extremely little likelihood of spread by the respiratory route. However, the presence of virus in a horse that had recovered from the disease indicates that a carrier state is possible and the study showed that fruit bats can be infected with the virus. While many questions remain unanswered about this important disease, the studies undertaken with support from RIRDC have shown that, while serious, this new disease is not a major threat to the horse industry. However, continuing research, surveillance and monitoring is important to ensure that future problems do not develop. The project showed the importance of having a well-resourced equine research and development program, to enable new diseases such as this to be effectively investigated and contained.

—Rural Industries Research & Development Corporation (RIRDC)

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