Foot and mouth disease outbreaks in Zambia

Transcription

1 Rev. sci. tech. Off. int. Epiz., 1983, 2 (1), Foot and mouth disease outbreaks in Zambia E. OVERBY* and G.C.N. ZYAMBO** Summary : The recorded foot and mouth disease ouîbreaks in Zambia are confined to three zones. The disease is caused by SAT 1 and SATc2 virus in the two zones A and B. In national game parks close to Zone B, all three SAT types have been isolated from wildlife. In Zone C (close to the Tanzanian border) outbreaks of FMD were caused by virus type O. In the latest recorded outbreaks in zones B and C the virus strains have behaved mildly. It is believed that the rinderpest panzootic of 1896, which wiped out a large part of the ruminant population, probably also eliminated foot and mouth disease (FMD) in the Southern African territories (Condy, 1979). This may be the reason why no outbreaks of the disease were reported in cattle in Zambia during the first third of this century. FMD has been seen in the country from 1933 and since then it has been recorded during 29 of the 50 subsequent years. Four virus types have been isolated : SAT 1, SAT 2, SAT 3 and O of which SAT 3 has been found in game animals only. FMD outbreaks up to 1974 have been reported before (Zyambo, 1975). This paper deals with the latest surveys and reports on FMD in Zambia. EPIZOOTIOLOGY It can be concluded from the reports of FMD in the cattle population (Table I) that there seems to be three main high risk areas where the disease may be endemic. These are : Zone A : The Southern part of Western and Southern Provinces with borders on to the Caprivi Strip of Namibia and Botswana. * Epizootiologist (DANIDA), Zambia Institute of Animal Health, P.O. Box 237, Mazabuka, Zambia. ** Director of Veterinary and Tsetse Control Services, P.O. Box 50060, Lusaka, Zambia.

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3 192 Zone B : The border of Central and Southern Provinces on the Kafue River (Kafue Flats) with extensions (most extreme in the latest outbreak) to Kalomo, Namwala, Monze, Mazabuka, Choma and Mumbwa Districts. Zone C : The border of Northern Province with Tanzania with a minimal extension to Mbala and Isoka Districts. The dry season outbreaks in Zone A have been considered as flare-ups or continuations of preceding wet season, SAT 2 outbreaks (Zyambo, 1975). This was probably also the case with outbreaks in the area in 1975, but not with the occurrence of FMD in this zone in The disease was then first diagnosed in winter during the month of July 1979, and the virus was confirmed to be a different strain, SAT 1. FMD in Zone B has been considered to have originated from wildlife in Kafue National Park and Kafue Flats (Zyambo, 1975). The latest outbreak of SAT 2 in the area occurred first in the Monze, Kalomo and Namwala Districts during the months of October and November 1981 and was confirmed in cattle returning to home villages after having been grazing on the Kafue Flats during the dry season. The disease later spread to Mazabuka and Choma Districts in the following months and, unfortunately, outbreaks were also confirmed in Mumbwa District (north of the Kafue River) in March TABLE I Foot and mouth disease outbreaks Year Month Virus Location Untyped A 1934 Untyped C 1942 May Untyped B 1944 Untyped B January Untyped B September SAT 2 B 1950 February SAT 1 B July SAT 1 B 1955 July SAT 1 B 1956 May SAT 2 A 1957 February SAT 2 A 1958 February SAT 2 A March July SAT 2 SAT 2 B B 1965 June SAT 2 B January SAT 2 A June January SAT 1 SAT 2 B A February O C June SAT 1 A October SAT 2 B September O C

4 193 A new zone of FMD infection (Zone C) was confined to part of Mbala District in the Northern Province in However, the disease had been recorded 42 years ago in the same area. Eleven villages with about 800 head of cattle were infected and the outbreak was believed to be an extension of FMD outbreaks in Tanzania. The virus was determined to be type O. Recently, in September 1982, a fresh type O outbreak was discovered in the area. The outbreak occurred in Isoka District, far from the last one, but close to the Tanzanian border. This outbreak is also believed to be an extension of outbreaks in Tanzania. FMD IN WILDLIFE FMD in game animals has been studied by Hedger and Dillman and results are described in the Final Report, Veterinary Wildlife Research Section, German Agency for Technical Cooperation (Dillman, 1976). Preliminary investigations into FMD in buffalo and lechwe were started in 1973 on the Kafue Flats in the area of Blue Lagoon (North bank). Probang* and serum samples from 30 Kafue lechwe and 13 buffaloes were collected and sent to AVRI, Pirbright, for examination. The study was followed up by collection of serum and probang samples from 20 Kafue lechwe at Lochinvar (South bank of Kafue Flats). Foot and mouth disease virus (FMDV) was isolated from 2 buffaloes (15%) and all 13 buffaloes showed positive serum titres to SAT 1. From the 50 Kafue lechwe samples, no virus was isolated but a few sera showed doubtful titres. TABLE II Foot and mouth disease results of specimens from wildlife Year Species Serology Virus isolation 1967 Buffalo SAT 1, 2, Elephant 1967 Hippopotamus 1973 Buffalo SAT 1 SAT Lechwe Doubtful 1974 Buffalo SAT 1, 2, 3 SAT 1, 2, 3 The isolation of FMDV from the buffaloes in the Blue Lagoon area encouraged further investigations in the Kafue National Park. In 1974, buf- * Detailed procedure is indicated in field notes from Pirbright. The probang is formed as a cup with a sharp edge all round. Attached to its base is a strong flexible wire about one metre long. The probang is pushed into oesophagus to obtain pharyngeal scrapings as FMD virus is believed to perpetuate well in the mucous membrane of the pharynx.

5 194 faloes from the game reserve were immobilised by darting on foot and from a helicopter and 67 serum and probang samples were sent to Pirbright for examination. FMDV was isolated from 35 of the 67 buffaloes (52%), which was of a similar order to the percentage of virus isolations made in Botswana in 1969 and 1972 (Hedger, 1972). Although SAT 3 infections had not been recorded in domestic stock in Zambia before, the strain was recovered in 6 buffaloes. Furthermore, SAT 1 was isolated in 19 buffaloes and SAT 2 in 10 of the animals. Again, all 67 buffaloes showed positive titres to South African types of FMDV. During the immobilisation of the buffaloes, their ages were established by checking teeth and horn growth. The virus isolates were mainly found in young animals. The investigators concluded that the buffalo population in the Kafue area presents a reservoir of all 3 SAT virus types, and that FMD is being perpetuated in the buffalo population mainly in the absence of clinical disease. During the clinical inspections of the immobilised animals, old lesions on the tongue were noted in 2 buffaloes. One was more in the nature of an erosion in an animal which was scouring. The discovery of SAT 3 in buffalo was not entirely unexpected by Pirbright Institute since similar results had been recorded from other areas, even as far north as Uganda. FMD OUTBREAKS SINCE 1975 The disease broke out in January 1975 in Zone A and was due to SAT 2. About 30% of the cattle in the infected area contracted FMD, showing high morbidity with negligible mortality. Both ring and barrier vaccinations reinforced by stringent zoo-sanitary measures were used to control the disease and in October the area was declared free of FMD. Surprisingly, in June 1979, an outbreak occurred in Zone A, which was identified as SAT 1. Outbreaks in this zone have all previously been confirmed to be SAT 2 and usually occur in the wet season. The May 1956, dry season outbreak in this area was an exception, otherwise other dry season outbreaks were considered as flare-ups or continuations of preceding wet season outbreaks. Almost 100% morbidity was noted with the 1979 SAT 1 outbreak, and severe lesions typical of FMD were found. The outbreak continued into 1980, extending into Sesheke District of Western Province. Finally, it was eliminated in March In October 1981, FMD was diagnosed in Zone B (Kafue Flats area), and was recognised as an SAT 2 outbreak. The disease extended into 6 districts, the largest extension of FMD from that area since The clinical course of the outbreak was extremely mild in the beginning and, surprisingly, the disease mainly affected old animals such as working oxen. The morbidity was in general low and sheep and pigs were not showing pathological lesions.

6 195 Erosions of small areas of epithelium of gums, lips and tongue, commonly on the tip and edge, but not normally on the upper part, were the only visible signs in the mouth. Small erosions were usually seen in the interclavian cleft, generally without formation of vesicles. However, with the extension of the outbreak into areas where FMD has not been seen for many years, the clinical course was found more severe with higher morbidity as calves were also infected. In some of those cases, pathological lesions were also noted on the upper part of the tongue. In all cases, mortality was low as death mainly occurred in young calves due to inadequate milk supply from dams. In many of the affected animals the feet were seriously infected due to secondary bacterial infections. After 42 years of freedom from FMD in the Northern Province an outbreak occurred in Mbala and Isoka Districts (Zone C) in February The outbreak was caused by type O virus. It is believed that this outbreak, in a few villages about 20 km from the Tanzanian border, was an extension of the outbreak in January 1976 that broke out in the Sumbawanga region of Tanzania, which was of the same virus type. The disease was generally mild in the affected animals. Vaccination of all susceptible cattle within the prescribed area was carried out using a quadrivalent vaccine (types A, O, SAT 1 and 2) from Wellcome Laboratories, Kabete. This outbreak was contained in the original area where it broke out and, by October 1976, the quarantine restrictions were lifted. During the following years the zone was under strict surveillance and, in November-December 1977, about 100 probang and sera samples from the area were submitted to AVRI, Pirbright for examination. Type O was isolated from 7 of the 100 cattle, quite a high virus isolation rate 19 months after infection (Hedger, 1978). However, in September 1982, a fresh type O outbreak occurred in the zone, involving about 600 head of cattle in 20 different herds, all in the area of Nakonde, the border village at the main road to Tanzania. The veterinary authorities started a vaccination campaign against strains O and A but unfortunately, in January 1983, the disease jumped about 20 km towards the west to a mission station where about 800 head of cattle got involved. The whole outbreak, which is believed to be an extension of outbreaks in the Tanzanian Mbozi District, came under control in February 1983, when the first round of vaccination was completed with about 42,500 head of cattle vaccinated. The morbidity in infected herds was in general surprisingly low and the lesions small and easy to ignore. CONTROL MEASURES Following the current outbreaks, adequate control measures were adopted. Prophylactic measures were massive ring inoculations with commercial vaccines to all cattle at risk. Additional measures included : ordering cattle owners to stop all cattle movement, effective road-blocks sometimes with further reinforcement by

7 196 mobile veterinary staff at each block to prevent cattle movements through by-paths, adequate disinfection measures, frequent inspection of suspected cattle herds and where new cases were found, specimens were collected for determination of the virus strain. These measures brought infected areas under quarantine with restrictions at cordon lines. Pickets were not removed until a month after the second vaccination. The two vaccinations were generally given to all cattle at risk with an interval of 2-4 months between the two vaccinations. This has also been the case with the latest outbreaks. However, control measures were delayed by difficulties in obtaining sufficient vaccine at the right time. The vaccination regime in a current outbreak is to follow the first vaccination between 2-4 weeks with a second and a third cover within 4 months of the second. The level of immunity is also monitored by serum samples from the vaccinated animals. DISCUSSION The behaviour of the SAT 2 virus strain in the outbreak of FMD in Zambia seems to be mild and atypical as described above. However, in some of the last reported cases there was a more severe clinical course which might have been due either to FMD outbreaks in areas which have been free for years, or to a strengthening of the virus strain through passages in the same species. The origin can be traced to the time when large cattle herds were brought together for dry season grazing on the Kafue Flats. There is, therefore, a possibility that livestock density (Hugh-Jones, 1972) could have an important bearing on FMD attack rates. With such large herds of cattle coming together, the chance of having a carrier animal shedding an hypervirulent strain in sufficient quantities to initiate an outbreak is quite great. Wildlife is abundant in the area so that carrier animals can be found in game (Dillman, 1976). However, in Zimbabwe, susceptible cattle were kept in close contact with buffalo carrying FMD virus, over a period of two and a half years, without transfer of the infection from buffalo to cattle, although buffalo to buffalo infection took place (Condy and Hedger, 1974). Nevertheless, the behaviour of the virus strain favours transference of infection from wildlife to cattle. * * * LES FOYERS DE FIÈVRE APHTEUSE EN ZAMBIE. E. Overby et G.C.N. Zyambo. Résumé : En Zambie, des foyers de fièvre aphteuse ne sont constatés que dans trois zones. Dans les zones A et B, la maladie est causée par les types SAT 1 et SAT 2 du virus. Dans les parcs nationaux proches de la zone B, les trois types SAT ont été isolés chez les animaux sauvages. Des foyers de type O ont été signalés dans la zone C, proche de la frontière tanzanienne.

8 197 Au cours des derniers foyers constatés dans les zones B et C, les souches du virus se sont montrées moins pathogènes. * LOS FOCOS DE FIEBRE AFTOSA EN ZAMBIA. E. Overby y G.C.N. Zyambo. Resumen : En Zambia, sólo se registraron focos de fiebre aftosa en tres zonas. En las zonas A y B, la enfermedad está producida por los tipos SAT 1 y SAT 2 del virus. En los parques nacionales próximos a la zona B, se aislaron los tres tipos SAT en los animales salvajes. Se registraron focos de tipo O en la zona C, próxima a la frontera tanzaniana. En los últimos focos comprobados en las zonas B y C, las cepas del virus se mostraron menos patógenas. * * * REFERENCES 1.CONDY J.B. and HEDGER R.S. (1974). The survival of foot and mouth disease virus in African buffalo with non-transference of infection to domestic cattle. Res. vet. Sc., 16, CONDY J.B. (1979). A history of foot and mouth disease in Rhodesia. Rhod. vet. Journ., 10, Department of Veterinary Services, Annual Reports, Government Printers, Lusaka, DILLMAN J.S.S. (1976). Final report, German Agency for Technical Cooperation. 5. HEDGER R.S. (1972). Foot and mouth disease and the African buffalo (Syncerus caffer). J. comp. Path., 82, HEDGER R.S. (1978). Unpublished letter from AVRI, Pirbright to Director of Veterinary Services, Zambia. 7. HUGH-JONES M.E. (1972). Epidemiological studies on the foot and mouth disease epidemic. Attack rates and cattle density. Res. vet. Sci., 13, ZYAMBO G.C.N. (1975). Foot and mouth disease outbreaks in Zambia. Bull. Off. int. Epiz., 83 (1-2),