African horse sickness

African horse sickness

J A W COETZER AND A J GUTHRIE

Introduction

African horse sickness (AHS) is a peracute, acute, subacute or mild infectious but non-contagious disease of equids caused by an orbivirus, of which there are nine serotypes, all transmitted by Culicoides midges. The disease is manifested by pyrexia, inappetence and the clinical signs and lesions compatible with impaired respiratory and circulatory functions that are characterized by oedema of subcutaneous and intermuscular tissues and of the lungs, transudation into the body cavities, and haemorrhages, particularly of the serosal surfaces. The mortality rate in horses, the most susceptible species, may be as high as 95 per cent while donkeys and mules are considerably less susceptible and generally develop milder disease. The disease occurs regularly in most countries in sub-Saharan Africa and is regarded as one of the major scourges of the continent.

The first known historical reference to a disease resembling AHS is in an Arabian document ‘Le Kitâb El-Akouâ El- KâfiahWaEl Chafiâh’, in which an outbreak of the disease in Yemen in 1327 is reported.69 Another early reference to what was probably AHS in horses imported into East Africa from India is by Father Monclaro in his account of the journey of Francisco Baro to East Africa in 1569.69, 131

Horses and donkeys were introduced into South Africa shortly after the arrival of the first settlers of the Dutch East India Company in the Cape of Good Hope in 1652.69 According to Theiler,131 frequent reference was made to the disease in the records of the Dutch East India Company. In 1719, for instance, nearly 1 700 horses succumbed to the dreaded ‘perreziekte’ or ‘pardeziekte’ in the Cape of Good Hope. The ability of frost to arrest outbreaks was recognized at this time.

In 1850 Gordon Cumming and other hunters reported AHS among their horses while Livingstone was unable to use horses on some of his journeys in southern, central and East Africa and was forced to travel on foot or on the back of an ox.131 The Voortrekkers also suffered severe losses amongst their horses.69

At first AHS was confused with anthrax and biliary fever. In 1900 M’Fadyean82 succeeded in transmitting the disease with a bacteria-free filtrate of blood from an infected horse. According to Henning69 this finding was confirmed independently by Theiler and by Nocard a year later, and by Sieber in 1911. From these experiments it was concluded that the disease was caused by a virus.

One of the reasons why Sir Arnold Theiler sited the now Onderstepoort Veterinary Institute at Onderstepoort in 1908, was the high prevalence of AHS in the immediate area. Theiler’s pioneering research122, 124, 126, 127, 131 suggested that there was a plurality of ‘immunologically distinct strains’ of AHS virus (AHSV) since immunity acquired against one ‘strain’ did not always afford protection against infection by ‘heterologous strains’; 26 to 81 per cent of horses reacted when challenged with heterologous strains. This work was expanded on by Alexander and co-workers,5, 8, 11, 12 who employed antisera from horses recovered from AHS in cross-neutralization tests in mice.

A major advance in research on AHSV came in 1933 and 1935 when Alexander4, 5 showed that viscerotropic isolates of AHSV became neurotropic but did not lose their immunogenicity after serial intracerebral passage in mice. Alexander in 19389 was the first to succeed in propagating AHSV in chicken embryos by inoculation of the chorio-allantoic membrane, and he showed that the virus became attenuated during passage in embryonated hens’ eggs.

The possibility that AHS may be transmitted by biting insects was offered by Pitchford and Theiler in 1903;106 they showed that horses could be protected against infection when housed in mosquito-proof enclosures. In 1944 Du Toit46 reported that Culicoides spp. were probably vectors of both AHS and bluetongue viruses (see Vectors: Culicoides.)

Although mortality in horses as a result of AHS occurs every year in South Africa, major epidemics prior to 1953 seem to have occurred at intervals of roughly 20 to 30 years. Severe losses were reported in 1780, 1801, 1839, 1855, 1862, 1891, 1914, 1918, 1923, 1940, 1946 and 1953. The outbreak of 1855 is still considered to have been the most severe; nearly 70 000 horses, making up more than 40 per cent of the entire horse population of the Cape of Good Hope, died.20

Before the advent of mechanized transport the horse played a very important role in transportation, was vital in military operations, and was used for draught power in agriculture and mining. Heavy losses as a result of AHS were therefore very disruptive and deeply felt. Although horses are nowadays seldom used for these purposes, many haveconsiderable monetary value as performance horses or animals used for other forms of recreation. Furthermore, exportation of horses from areas where AHS occurs can only be accomplished following strict quarantine and testing procedures.2, 3

Since a polyvalent vaccine against AHS has been available in southern Africa, severe losses due to the disease have largely ceased although it continues to occur in individual or small groups of horses. However, epidemics of the disease in recent years in countries outside the endemic regions in Africa15, 64, 81 serve as a warning that the disease may spread to continents hitherto free of...

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