African horse sickness

GENERAL INTRODUCTION: REOVIRIDAE

Introduction

African horse sickness (AHS) is a peracute, acute, subacute or subclinical  non-contagious disease of equids caused by an orbivirus, of which there are nine serotypes, all transmitted biologically by Culicoides midges. The disease is manifested by fever, inappetence and clinical signs and lesions compatible with impaired respiratory and circulatory functions characterized by oedema of subcutaneous and intermuscular tissues and the lungs, transudation into the body cavities, and haemorrhages, particularly of the serosal surfaces. The mortality rate in naive horses, the most susceptible species, may be as high as 95 per cent while donkeys and mules are considerably less susceptible and generally develop only a febrile 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The first known historical reference to a disease resembling AHS is in an Arabian document ‘Le Kitâb El-Akouâ El- KâfiahWaEl Chafiâh’, in which an outbreak of the disease in Yemen in 1327 was reported. 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.^{98, 201}

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 1652.⁹⁸ According to Theiler,²⁰¹ frequent references were 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 1830, Thomas Perry, the District Surgeon at Graaff-Reinet wrote “If the animal is rode during the Sickness, or urged by driving or otherwise to any degree of speed, he falls at once, literally suffocated by the quantity of frothy matter which fills his trachea and issues in abundance from his nostrils”. 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.²⁰¹ The Voortrekkers also suffered severe losses amongst their horses.⁹⁸One of the worst recorded epidemics occurred in 1854/1855 when mortalities were close to 64 850.³² At that time this represented a staggering 40  per cent of the horse population of the Cape and the cost at that time was estimated to be around £525 000. The result was a decline in the growth rate and expansion of the Cape Colony. Horses that survived AHS were considered “salted” and were worth six to ten times more in value.

At first, AHS was confused with anthrax and biliary fever. Dew, grass and cobwebs were suspected as possible causes, where “The distemper was everywhere most destructive in the low grounds and along the watershed of each district; the fatal miasma seeming to be held in suspension during the day, and to descend again at night with the heavy dews”.³² Edington considered the causal agent to be a mushroom.⁶⁹ In 1900, M’Fadyean¹¹⁸ succeeded in transmitting the disease with a bacteria-free filtrate of blood from an infected horse. According to Henning,⁹⁸this 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.

Winged nocturnal insects, such as mosquitoes, were suggested by Watkins-Pitchford to be responsible for the transmission of AHS.¹⁶¹He showed that horses could be protected against infection when housed in mosquito-proof enclosures. However, it was only in 1944 when Du Toit⁶⁶ reported that Culicoides midges were probably vectors of both AHS and bluetongue viruses (see Vectors: Culicoides.)

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. The earliest reasonably successful AHS vaccine was developed by Theiler in 1905 and its use continued until 1933. The ‘vaccine’ consisted of simultaneous inoculation of the virus together with serum obtained from horses that had recovered from the disease. This method provided unpredictable results and was not widely used. Theiler’s pioneering research^{194, 195, 196, 197, 201}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...