Dr Liesel van der Merwe is a small animal medicine specialist. Send her your questions: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Dr Liesel van der Merwe

We all know how the current strain of Covid-19 initially was found in bats. There are other viruses that can cross the species barrier; African horse sickness (AHS) being one of them. And currently AHS is a problem.

It is caused by a virus of the family Reoviridae, genus Orbivirus. The disease has both a seasonal (late summer/autumn) and a cyclical incidence, with disease associated with drought followed by heavy rain, such as we have experienced this year.

Horses need to be vaccinated yearly with all the different strains, in three separate vaccinations, to prevent them from becoming ill. Still, 70% to 90 % of affected horses will die. Infection in zebras is subclinical and they are the reservoir for the infection.

Dogs can have a fatal infection. These cases have severe accumulation of fluid on the lungs within 12 to 24 hours, and literally drown. In milder cases symptomatic treatment may be effective.

There is no rapid diagnostic tool for AHS in dogs. The causes of acute onset breathing issues are heart failure, electric shock, bleeding on the lungs, severe allergic reactions, viral infections with affinity for lung tissue and some types of acute bacterial infections. General sepsis can also cause the lung to become leaky as a ‘bystander’ injury.

Treatment of these conditions is empirical as exact cause may take some time to confirm. So, basically we treat what symptoms we see.

These cases have been diagnosed intermittently for many years, but a peak in numbers presented about four years ago led to some research on the topic at the Onderstepoort Veterinary Hospital.

It was found that a small number of dogs have antibodies to African horse-sickness and have never shown any clinical signs. In fact, one of my own dogs, whose blood was stored in the data base, tested positive. So, there is a prevalence in the normal canine population in built up areas.

Initially it was thought that dogs were infected by eating horsemeat. However, it is now highly suspected that the midges can also bite dogs and transmit the infection.

Before everyone panics – the incidence of dogs which show this disease is exceptionally low in the canine population in comparison to the number of horses getting horse sickness. We may see up to two confirmed cases in a normal year and about five to 10 in a bad year from the entire region. These numbers are very, very small.

We have no idea what sets off the infection in dogs. Affected dogs – where we have some history – were not specifically associated with horses. It is just an interesting observation that viruses are well able to cross the species barrier and have been doing so for a very long time.