Poliomyelitis: epidemiology and prophylaxis

BRIEF COMMUNICATIONS Bulletin of the World Health Organization, 56 (4): 649-651 (1978) Poliomyelitis: epidemiology and prophylaxis 6. Geographic synchronism in poliomyelitis epidemics in Kenya D. METSELAAR 1 Abstract In the 1950s, van Loghem drew attention to what he called " synchronism " in the epidemics of polio- myelitis in western Europe. The 1952 epidemic in the Netherlands affected Belgium and neighbouring parts of the Federal Republic of Germany at the same time. A comparable phenomenon is described from Kenya. Notwithstanding the fact that in the three population concentrations ofthe country the three poliovirus types may well be endemic, epidemics of poliomyelitis repeatedly occurredsynchronously in these areas. How- ever, there seems to be no synchronism between polio- myelitis epidemics in Kenya and those in neighbouring East African countries. Tentative explanations are put forward for the similarities and differences between Europe and East Africa. The importance of the pheno- menon as observed in Kenya for control by mass immunization campaigns is stressed. In the 1950s van Loghem (1-3) used the word "synchronism " to describe the fact that epidemics of poliomyelitis frequently affect other geographic areas at the same time. The epidemic in the Nether- lands in 1954 extended into Belgium and the Federal Republic of Germany and possibly even further. Van Loghem (4) regarded rises in the incidence of poliomyelitis as distubances in the balance of a com- mensal type of infection and considered the possi- bility that climatic factors were responsible for the disturbances. Synchronism in epidemics of poliomyelitis appears to be present in Kenya also. Kenya, which is approxi- mately the size of France, is larger than the area in western Europe discussed by van Loghem. There are three densely populated regions in the country: west Kenya with approximately 5 million inhabitants, 1 Formerly Head, Department of Virology, Netherlands Medical Research Centre and National Public Health La- boratory Services of Kenya, P.O. Box 20752, Nairobi, Kenya. Present address: Valkenburglaan 35-D99, 6861 AJ Ooster- beek, The Netherlands. central Kenya and Nairobi with 4.5 million, and the coastal strip with a population of approximately 1 million. The rest of the country is sparsely popu- lated with a total of approximately 1 million inha- bitants. The western and central population centres are approximately 100 km apart, the central and the coastal regions 200 km apart. Since the first epidemic in 1954, poliomyelitis has been of growing public health importance in Kenya (5, 6). Virus isolation has been carried out since 1964. Between January 1965 and June 1977, 2636 specimens obtained from poliomyelitis patients were examined and 1294 poliovirus strains isolated: 1019 of type 1, 137 of type 2, and 138 of type 3. The isolation ratio was, therefore, 7.4:1.0:1.0. Many specimens tra- velled for several days, which explains the low iso- lation rate. In the same period more than 3000 cases of poliomyelitis were reported to the Director of Medical Services; however, the true figure was prob- ably several times higher. The majority of the patients were under 5 years of age, the peak occur- ring in those aged 6-24 months. The isolates of the three virus types were divided according to their place of origin. The three types were isolated practically throughout the observation period. In view of the fact that the specimens were received from only a proportion of the total number of patients, that the majority of poliovirus infections are subclinical, and that the virus is excreted in the faeces of infected persons for 6-8 weeks, and taking into account the isolation ratio for the three types, it can be accepted with confidence that the three types are constantly present not only in Kenya as a whole but in all three areas of dense population. The gaps in the continuity of isolations do not signify that the virus was absent from the respective areas during these periods. However, it must be admitted that we do not know how the picture would have appeared if each of the areas had been completely isolated. It is unlikely that the epidemics in Kenya resulted 3730 649- RE ST - - a - ~COAST -irI- HU-CENTRAL WEST REST a - COAST IU CENTRAL Ci- WEST REST COAST CENTRAL uJL z 0 WEST QUARTERS 13St '3idSt J1S "3rlId 1t id 151 3"' ?' 3 t 3t 5t3d 1" 3rd St YEARS 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 Fig. 1. Isolations of the three types of poliovirus in Kenya between 1967 and 1977 to demonstrate the synchronism among the four centres of population. BRIEF COMMUNICATIONS 651 from a disturbance in the balance of commensal infections and, in retrospect, this may not have been true for Europe either. The striking rhythm in the circulation of type 1 poliovirus in Kenya and its probable causes have been discussed elsewhere (5-7). Fig. 1 shows the obvious synchronism among the four areas, especially for type 1 poliovirus. Despite the fact that each area is endemic for poliomyelitis and therefore should be " autonomous " from the point of view of virus circulation, there exists a reciprocal influence with regard to the timing of the epidemics. When an epidemic starts in one area, the others rapidly become involved. Several factors may be responsible for this syn- chronism. There is a great deal of traffic between the areas; men frequently work far from home and their families stay at home, where the women till the land. During the seasons when the land needs little care, the families travel to visit the men and infected children may shed poliovirus for several weeks. Contrary to the situation in Europe, there was no indication of synchronism between poliomyelitis epidemics in Kenya and those in neighbouring countries. Differences in the age of the virus excre- tors and in travel habits may have been responsible for this contrast. Kenyan children rarely travel outside the country and most adults who travel abroad are immune (5) and therefore not an important factor in the spread of wild polioviruses. On the contrary, in Europe before the introduction of vaccines, wild polio- viruses were excreted by both children and adults and people ofall ages moved freely between countries. The phenomenon of synchronism underlines the need for nationwide mass immunization campaigns when there is a threat of an epidemic. One such campaign, conducted in Kenya, has been described earlier (8). ACKNOWLEDGEMENTS I am grateful to the Director of Medical Services, Kenya, for permission to publish this paper. REFERENCES 1. LOGHEM, J. J. VAN Nederlands tijdschrift voor genees- kunde, 99: 1554-1567 (1955). 2. LoGHEm, J. J. VAN Nederlands tijdschrift voor genees- kunde, 100: 1627-1641 (1956). 3. LOGHEM, J. J. VAN Nederlands tijdschrift voor genees- kunde, 101: 275-276 (1957). 4. LoGHEM, J. J. VAN Nederlands tijdschrift voor genees- kunde, 97: 954-962 (1953). 5. NOTTAY, B. K. & METSELAAR, D. Bulletin of the World Health Organization, 48: 421-427 (1973). 6. METsLAAR, D. Lancet, 1: 174-176 (1976). 7. METSEL4AR, D. ET AL. Bulletin of the World Health Organization, 55: 747-753 (1977). 8. KOINANGE, W. ET AL. Bulletin of the World Health Organization, 48: 543-545 (1973).