Horse sickness outbreak in dogs Print E-mail
News - Rubrieke
Tuesday, 27 June 2017 06:17
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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

In the last year or so there have been increasing reports of dogs which have shown severe breathing problems and death, ascribed to African Horse Sickness.

African Horse Sickness (AHS) is caused by the AHS virus and it is a disease primarily of horses: the horse is most severely affected, while donkeys and zebras are more resistant. The disease occurs regularly in sub-Saharan Africa and is spread among horses in South Africa by biting midges (Culicoidesspp). In areas where heavy frost occurs, outbreaks are highly seasonal.

It has been proposed that AHS is endemic to the eastern parts of the Mpumalanga and Limpopo provinces, where the virus occurs in zebra ‘reservoirs’ and from where it spreads southwards during the summer months.

Horses show a ‘dikkop’ form where the head becomes very swollen, and a ‘dunkop’ form where there is more fluid on the lungs and the head is less swollen. The importance of the disease in susceptible horses is due to the high mortality rate (70% – 95%), as well as restrictions on the movement of horses to disease-free areas in South Africa and the rest of the world.

Antibodies and/or the virus have been detected in camels, cattle, African elephants, spotted hyenas, lions, cheetahs, African wild dogs, jackals and genets. However, the domestic dog is the only other species known to get the severe form of the disease.

Various outbreaks in dogs have been reported since 1906. In most of the reported field outbreaks in dogs there was a well-documented history of ingestion of horse meat. Transmission of the AHS virus to dogs by midges was not considered to be important in the spread of AHS as it has been shown that midges do not readily feed on dogs.

It is clear that the midges do not feed on dogs to the extent that they feed on horses and livestock, although it is possible that dogs could be an incidental host for the midges. Other potential carriers such as mosquitoes and ticks have also been suggested and in some cases shown to be capable of transmitting AHS.

Seroprevalence is the presence of antibodies against a virus in the blood of the normal population.

Seroprevalence to the AHS virus in dogs have been reported as follows: 4% in Kenya, 8% in Botswana, 9% in Nigeria and 1% in South Africa. Generally, domestic dogs have a lower seroprevalence of AHS virus antibodies than wild carnivores and this is attributed to the greater frequency with which wild carnivores eat or scavenge zebra meat.

Mortality in dogs infected with AHS virus ranges from 20% – 78% in previously recorded outbreaks.

The clinical course of AHS in dogs after ingestion of infected horse meat is one to two weeks, although peracute and chronic forms (more than three weeks) have also been reported. Not all dogs infected with the AHS virus become clinically ill; some dogs only display a transient fever reaction.

Symptoms reported in dogs include: fever, increased rate of breathing, fluid build-up in the lungs, white foam around nostrils; pharyngitis; coughing; diarrhoea; and convulsions.

Post mortem changes in dogs tend to correspond to the ‘dunkop’ form of the disease in horses, including: severe fluid accumulation in and around the lungs; inflammation and congestion of the stomach and intestines; blood-stained faeces; and enlarged spleen and liver. In chronic cases, dogs only display emaciation on post-mortem examination.

In the current outbreak the majority of cases are showing a very rapid onset of symptoms, a rapid deterioration and death due to lung failure caused by the severe inflammation and fluid accumulation in and around the lungs. A smaller percentage of cases showed a slower progression and only a few responded to supportive therapy.

The data of cases presented or diagnosed at the Veterinary Faculty is currently being evaluated at Onderstepoort in an effort to get information about this new outbreak. The patients we have seen have all been in relatively close contact with horses and none were fed horse meat. Transmission by insects, probably the midges, is most likely.

To prevent exposure we suggest that animals are kept inside in the mornings and evenings, when midge activity is high. Topical sprays with insect repellent activity as well as Advantix® and Exspot® spot-on formulations can also be used. 

Source: Van Sittert, SJ, Drew, TM, Kotze, JL, Strydom, T, Weyer, CT & Guthrie, AJ, 2013, ‘Occurrence of African horse sickness in a domestic dog without apparent ingestion of horse meat’, Journal of the South African Veterinary Association 84(1)


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