Budl. Org. mond. Sante 1960, 22, 339-371 Bull. Wld Hlth Org. Epidemiology of the Arthropod-borne Encephalitides* J. A. R. MILES, M.A., M.D., F.R.A.C.P.1 Since the recognition that louping-ill, known for well over 100 years as an epizootic disease ofsheep in Scotland, was caused by a virus transmitted by arthropods, many other arthropod-borne viruses capable of causing encephalitis in domestic animals or man have been discovered. The author reviews here the knowledge at present available on these viruses, originally termed " arthropod-borne encephalitides viruses" but now often referred to as " arbor viruses". In this discussion of the host and vector relationships of the two broad groups ofarbor viruses - the mosquito-borne and the tick-borne-and of the distribution, epidemiology and control of the various diseases they cause, the author includes an outline of the types of investigation likely to provide the most useful information, stressing in this connexion the value of ecological surveys. There are numerous viruses which are transmitted into sub-groups A, B and C and there remains a by arthropods and are capable of causing disease large number of ungrouped viruses also. Within in mammals, at least under experimental conditions, this large group the present review is concerned and new viruses belonging to this group are being dis- solely with those viruses which have been proved to covered regularly. More than seventy such viruses cause encephalitis in man and domestic animals are now recognized (Rockefeller Foundation, 1957; and for which we may still use the old term " arthro- Sabin, 1959). The term "arbor viruses" for this pod-borne encephalitides ". Other arbor viruses group is becoming widely accepted despite the valid will be mentioned only where the consideration of criticism that this name tends to suggest trees rather related viruses may help our understanding of the than arthropods to anyone not particularly interested behaviour of those mainly under consideration and in the viruses. in discussing the value of serological methods in Casals & Brown (1954) subdivided the group on epidemiological work on this group. the basis of the characters of the haemagglutinins HISTORY OF THE ARTHROPOD-BORNE ENCEPHALITIDES Certain of the syndromes caused by these viruses confined to the period between January and April. have been recognized for a long time and probably Cleland & Bradley (1917) gave a full description of the first to be recognized was louping-ill, which has the outbreaks and further studies were reported by been known as an epizootic disease of sheep in Cleland, Campbell & Bradley (1919-20). The virus Scotland for well over 100 years; however, the first was isolated in rhesus monkeys by intracerebral of the viruses to be isolated was that known as the inoculation and the histology of the lesions in the virus of Australian " X " disease. In the years 1917 monkey was distinctly different from that of polio- and 1918 epidemics broke out in rural Victoria and myelitis, although some authorities at the time New South Wales in south-eastern Australia; these were under the impression that this was an aberrant epidemics occurred mainly in the summer months outbreak of poliomyelitis. Unfortunately, the strains of February and March, and were almost entirely of the virus isolated in the outbreaks of 1917, 1918 and 1925 were lost and it has not been possible to * Expanded version of paper submitted to Joint WHO/ compare them with the strains of virus which were FAO Expert Committee on Zoonoses, August 1958 1951 1 Professor of Microbiology, Medical School, University isolated from the large outbreaks occurring in of Otago, Dunedin, New Zealand and which have been given the new name of Murray 865 339- 340 J. A. R. MILES Valley encephalitis (MVE). It is, however, virtually There is a further group of closely related viruses certain that the virus of Australian " X " disease transmitted by ticks, and although there appears and that of MVE are the same (Anderson, 1954; to be considerable variation in the biological pro- McLean & Stevenson, 1954; Miles & Howes, perties of these viruses they are all so closely related 1953). serologically that they are regarded as variant Japanese B encephalitis (JBE) originally received strains of one virus. However, for the present that name to differentiate it from type A encephalitis, discussion it will be convenient to use the names which was encephalitis lethargica. Probably the associated with the various syndromes seen in earliest isolations of JBE virus were those made different parts of the world. in 1925 by several Japanese workers (Ito, Tsuchiya It has already been mentioned that the disease & Nakajo; Kojima & Ono; Nishibe; Takaki). louping-ill has been recognized in Scotland for well This virus is one of great importance which is over 100 years, but the etiology of the condition was found over a very wide area of Asia, from southern not understood until Pool, Brownlee & Wilson India through Japan and out to the islands in the first isolated the agent in 1930 and Alston & Gibson Pacific, and is responsible for many severe cases of in 1931 demonstrated that the agent was a filtrable encephalitis. virus transmissible to mice. The disease is a disease A third virus, related to the previous two, suddenly of sheep of very considerable economic importance became epidemic in 1933 in St. Louis and other cities in certain areas, but in Great Britain the number of of the USA. This virus was fully investigated at that human cases is quite small and only very few have time by the US Public Health Service and the out- been recognized until recently. Now it is realized break was fully described in a report (Publ. Hlth that this virus can cause a paralytic disease resem- Bull (Wash.), 1935). Since the first big outbreak bling poliomyelitis, and more cases are being dis- infections with the virus of St. Louis encephalitis covered since virus diagnostic techniques are more (SLE) have been recognized widely over the USA advanced and are being more widely used. and have been extensively studied. The next tick-borne encephalitis virus to be The first of the equine encephalomyelitis viruses isolated was that of Russian far-eastern (spring- to be recognized was that virus now known as summer) encephalitis, which was first isolated by western equine encephalomyelitis (WEE), the more Silber and co-workers in 1937 (see Silber & Shub- widespread and possibly the more important of the ladze, 1945). This virus caused a disease mainly two North American equine encephalitides. This in forest areas and was responsible for disease in virus was first isolated by Meyer, Haring & Howitt man rather than in domestic animals. The disease (1931). In 1933 Tenbroeck & Merrill and Giltner was a highly fatal encephalitis and the maximum & Shahan (1933) isolated a different type of ence- incidence was in the spring and summer. A further phalomyelitis in the eastern states of the USA and virus of this group was isolated in Czechoslovakia this virus is known as the virus of eastern equine in 1948 (Hloucal, 1953; Krej'i, 1949). This virus encephalomyelitis (EEE). These two viruses together causes a biphasic meningo-encephalitis and can be have been responsible for a great deal of disease transmitted by milk as well as by ticks. More and Miller (1945) states that in the worst-affected recently similar viruses have been isolated in the year, 1938, there were 184 162 cases in equines. European USSR and several Eastern and Central Large numbers of human cases have also occurred. European countries. All these viruses belong to A further type of equine encephalomyelitis virus either group A or group B of Casals & Brown, and is found in South America. It was isolated in Vene- the way in which they will be treated in this review zuela and is therefore known as Venezuelan equine will bear some relation to this grouping, but will be encephalomyelitis (VEE) (Beck & Wyckoff, 1938). primarily based on the main vectors for the trans- This virus is highly infectious for man in the labora- mission of the different viruses. First the equine tory, but not so many human cases have been encephalomyelitides will be considered. These reported in the field as there have been of the North viruses all belong to group A. Then the Japanese American equine encephalomyelitides, but this B group of viruses will be considered. These are difference may in part be due to differences in mosquito-borne viruses belonging to group B. After reporting. that the tick-borne viruses will be discussed separa- All the viruses which have been mentioned to tely, although these strains also belong to group B date are viruses normally transmitted by mosquitos. of the arbor viruses. EPIDEMIOLOGY OF THE ARTHROPOD-BORNE ENCEPHALITIDES 341 DISTRIBUTION, OCCURRENCE OF EPIDEMICS, AND TRANSMISSION WESTERN EQUINE ENCEPHALOMYELITIS (WEE) cases have been recorded-for example, Richter's (1942) Chicago case-where it was reasonably Distribution certain that the patient had been infected in a metro- The distribution of WEE in the USA and Canada politan area. Cases of the disease are particularly has been carefully studied. In the USA, the disease common in well-watered and irrigated areas. In the has repeatedly been present in man and domestic studies of Kern County already cited (Buss & animals in the western States and in the Middle Howitt, 1941) all the cases were centred in the West, west of the Mississippi, and occasional out- farming and irrigated areas of the county and breaks have been recorded in States further to the none was reported from the mountain and desert; east and even along the eastern seaboard. In Canada similarly, the Yakima valley in the State of Wash- the disease has been recorded only east of the Rocky ington, a hot irrigated valley, has had numbers of Mountains in Saskatchewan, Manitoba and Ontario.