Hendra Virus

This week another case of Hendra virus was confirmed on a property at Tewantin in QLD. Hendra virus was first isolated in 1994 after 13 horses died as a result of infection with the previously unknown disease. During this outbreak two people who had close contact with these horses became infected with the disease and one of these cases was fatal. Since then there have been sporadic outbreaks of disease that have resulted in the deaths of both horses and people. I realise that I have covered this topic in a previous newsletter, however, in light of this weeks developments, I thought it was a good time to review what we already know and present some of the recent information that has come to light about this devastating virus.

Fruit bats (Pteropus spp.), commonly known as flying foxes, are the natural host of Hendra virus. Antibodies to Hendra virus have been isolated in all four species of fruit bats occurring in Australia. Since the first outbreak in 1994, more than 30 cases of Hendra virus in horses have been detected on or east of the Great Dividing Range from Cairns to northern New South Wales. However, because antibodies to Hendra virus have been detected in fruit bats in all parts of Australia (including Victoria), Hendra virus could potentially occur wherever there are fruit bats.

To date, spillover events from fruit bats to horses have only happened in paddock situations where fruit bats are present. However, once an affected horse enters the stable environment, disease spread between horses is fast and efficient. It is known that transmission of Hendra virus is via contaminated body fluids like respiratory secretions, blood and urine. It is also thought that Hendra virus can survive on items like buckets and feedbins for a period of hours under mild conditions.  Research has shown that horses, humans, cats and guinea pigs can become infected with Hendra virus. However, to date natural infections have only been reported in horses and humans. Our current understanding is that infection is from bat to horse, from horse to horse and from horse to human. Horses are infected via the nasal/oral route. Human infections have occurred after close contact with blood and other body fluids (especially respiratory secretions, saliva and urine). To date, there is no evidence to suggest that human to human or human to horse transmission occurs.

It is suspected that spillover events are associated with pregnancy and lactation in fruit bats and Hendra virus has been identified in the placenta and foetal tissues of experimentally infected fruit bats. However, this most recent case does not coincide with the birthing season of fruit bats, so this association may be weaker than originally thought.

We suspect that the incubation period in horses (time from exposure to first clinical signs) is between 5 and 16 days. Worryingly, research conducted by the Australian Animal Health Laboratories showed that a Hendra virus infected horse can potentially excrete Hendra virus through nasal secretions from 2 days after exposure to Hendra virus up to and including the time that it shows clinical signs.

Once clinical signs are evident, disease progression is very fast and the mortality rate in affected horses is in excess of 70%. Affected horses can present in many different ways. Some horses will present with respiratory or neurological signs and others will present with a combination of both. Many horses will have an increased rectal temperature and an increased heart rate and all horses will deteriorate rapidly. This clinical presentation, in conjunction with proximity to fruit bats is highly suggestive of Hendra virus infection.

In relation to the risk posed by Hendra virus to Victorian horse owners, our advice is to be aware but not alarmed. All properties with Hendra virus cases have reported some level of fruit bat activity in the vicinity and initial cases have typically been horses paddocked or kept outside in areas that were attractive to fruit bats. Now is a good time to make a note of whether fruit bats are present and active around your horse property. If you have seen or heard fruit bats on your property, we recommend that you place feed and water for your horses under cover. This is probably the most proactive step you can take to mitigate the risk to your horse at the present time. 

When calling one of our veterinarians out to see a sick horse, please inform them if fruit bats have been sighted in the vicinity of the property. The attending veterinarian will assess the level of risk and may decide to use personal protective equipment to examine the horse and/or take samples for Hendra virus. To date, this situation has not arisen, however, should this situation arise, we ask for your patience and understanding whilst we ensure the safety of all humans and horses on the property. 

Position of the head is not associated with change in horse vision.

The results of recent research undertaken in the Czech Republic will be of great interest to our dressage and showjumping clients. These researchers sought to test the hypothesis that a horse cannot see directly in front of itself when its nose is lowered and must instead rely upon its rider.

It has recently become accepted in certain scientific circles that position of the head is associated with changes in the horses vision. This belief was based upon the fact that the area of the retina (part of the eye that transmits visual information to the brain) with the highest cell density coincides with the area of the retina responsible for binocular (field of) vision. These scientists believed that the horses field of vision was related to the position of it’s head.

The Czech researchers sought to disprove this theory, by showing that the horse can in fact keep it’s eyeball in the optimal position for vision in any position of the head, thereby ensuring vision remains uninterrupted.

The Czech researchers assessed a number of horses by recording the angle of the pupil in reference to the ground whilst the horse’s head was placed in different positions. They found that the horizontal axis of the eyeball of the horse was similar when the position of the head was raised or lowered. So, by keeping the position of its eyeball, regardless of the position of it’s head, the horse’s visual perception remains the same at all times.

The researchers concluded that it was unlikely that horses had limited vision in relation to their head position when ridden by a rider.

Bartos, L., Bartosova, J. and Starostova, L. (2008) Position of the head is not associated with changes in horse vision.  Equine vet. J. 40 (6) 599-601.