Epidemiological studies of rubella virus in a tropical African community

Bulletin of the World Health Organization, 58 (6): 931-935 (1980) Epidemiological studies of rubella virus in a tropical African community M. CLARKE,1 G. C. SCHILD,2 J. BOUSTRED, 3 I. A. MCGREGOR, 4 & K. WILLIAMS 5 The single-radial-haemolysis testfor antibody to rubella virus provides a simple rapid method for carrying out large serological surveys. The availability of a collection of sequential serum samplesfrom inhabitants oftwo Gambian villages, Manduar andKeneba, made itpossible to determine thepattern ofrubella epidemics in these communities between 1966 and 1976. The serological findings indicated that an epidemic had occurred approximately two years before the commencement of the study. There was no further evidence ofrubella infection until approximately 1973 when a large-scale epidemic occurred. Although the communities were monitored throughout the period of study there was no clinical evidence of infection and no cases of congenital rubella syndrome. Although there have been several surveys to deter- mine the age-related prevalence of serum antibodies to rubella virus in Africa (1-5), little is known of the epidemiology of the disease. The single-radial-haemo- lysis (SRH) technique was originally developed for the detection of antibody to influenza virus (6, 7), but has since been adapted for other viruses including rubella (8-10). SRH provides a simple, rapid, and reproducible test system for the detection and assay of antibody in small volumes of sera. The availability of a large collection of sera obtained from residents of two Gambian villages between 1966 and 1976 (11), and including many serial samples obtained from the same individuals, provided an excellent opportunity to carry out a retrospective serological study of rubella in the Gambia. MATERIALS AND METHODS Serial serum samples were collected from the inhabitants of two Gambian villages, Manduar and Keneba, at six-month intervals from March 1966 to March 1976, as part of a long-term surveillance of malaria and other diseases in the Gambian population (11). Manduar village has a population of about 400 Member of scientific staff, Division of Viral Products, National Institute for Biological Standards and Control, Holly Hill, London NW3 6RB, England. 2 Head, Division of Viral Products, National Institute for Biological Standards and Control, London, England. 3 Technical Officer, Division of Viral Products, National Institute for Biological Standards and Control, London, England. 4 Head, Laboratory for Tropical Community Studies, National Institute for Medical Research, Mill Hill, London, England. 5 Chief Technician, Laboratory for Tropical Community Studies, National Institute for Medical Research, London, England. and Keneba about 800. The general epidemiological, socioeconomic, and geographical features of these villages have been described previously (13). SRH testsfor antibody to rubella virus The SRH tests were performed using the method previously described by Clarke et al. (10). Immuno- plates were prepared containing agarose gels incorpor- ating pigeon erythrocytes sensitized with rubella antigen of the GOS-10 strain and guinea pig comple- ment. Wells 2 mm in diameter were punched in the agarose for the serum samples. After preparation, the immunoplates were stored at 4 °C for 24-48 hours until used. Serum samples were maintained at 56 °C for 30 min and then 5-rd aliquots were added to the wells in the immunoplates. The plates were incubated at 37 °C for 18 hours and the diameters of the zones of haemolysis surrounding the wells were measured using a calibrating viewer. The zone diameter is a direct measure of the antibdy concentration (10). Sera which gave haemolysis zones of 4 mm diameter or greater were considered positive for rubella antibody. The British Standard preparation of antirubella serum 69/60 and a negative human serum were incorporated in all tests as controls. Sera were also tested on control immunoplates containing unsensitized erythrocytes and complement to detect non-specific haemolysis. RESULTS Screening of the population In an initial survey of the population, sera collected in Manduar village in 1966, 1971, and 1976 from all 4022 -931- M. CLARKE ET AL. age-groups were tested for rubella virus antibody. A total of 380 individuals were entered in the study. The proportion of individuals of each age-group in whom antibody was detected is shown in Fig. 1. For the various serum collections, 28-61 % of the youngest age-group (0-11 months) had antibody that was presumably of maternal origin. A lower percentage of positive serum samples was detected in the age-group 1-4 years, while over 50% of children aged between 5 and 10 years had antibody. For individuals aged over 10 years, over 93% had antibody in each ofthe years in which a survey was conducted. Table 1 shows the titre (SRH zone diameter) of antibody in sequential serum samples from selected individuals in Manduar village. It is seen that, except in the case of the youngest subjects, in whom antibody detected in the March 1966 sample was probably of maternal origin, antibody levels remained fairly constant for the ten-year period of the study. There were no significant differences between males and females in the frequency or titre of antibody. Study of children under 10 years ofage It was evident from the initial survey that the identi- fication of periods in which rubella was epidemic could only be made by a detailed study of the young age-groups of the population. Table 1 shows that children born after April 1964 had not experienced rubella infection up to March 1972, but had acquired antibody to rubella by March 1976. Sera collected in Manduar from children aged 0-10 years, during each year from 1966 to 1976, were there- fore tested. A distinct pattern emerged. The propor- tion of children with rubella antibody in the 1966 survey was 83.5% and in 1972 was 32.8%. The pro- portion began to rise in 1973 and in 1974 it was 91.4%. Fig. 2 shows the age-related distribution of anti- body in the children in selected years between 1966 and 1976. Of the children born in 1965, only 14.3% were found to have antibody to rubella virus in the 1966 survey, while 77.8% of the children born in 1964 had acquired antibody by 1966. Thus a rubella epidemic almost certainly occurred in the period 1963-64. There was no serological evidence that rubella infec- tions occurred between 1964 and 1972, since children born after 1966 remained serologically negative during this time. Table 2 shows antibody levels in indi- vidual children born after 1966 from whom sequential I,0 1966 ¶EIr2HHHHHFHhIWLE - i- 'O 8 100_ March 196 60 40 2 March 1972100 -- 60ri x 1x966 i"lUn m20x~x1971 ____________________ C)------0O 1976 March 1974100 60 20 Age Fig. 1. The percentage of inhabitants of Manduar villas with rubella antibody by age-group (years) in 1966, 197 and 1976. March 1676100 40204H lH H nH o a;% 0 ulI I I I l a .- a V' a r. * *> Agele Fig. 2. Age-related incidence of rubella antibody in children aged 0-10 years from Manduar village. 932 0 4 CIO U0 RUBELLA IN AFRICA 933 Table 1. Diameter of the zone of haemolysis (mm) produced by sequential serum samples from selected individuals Serum samples Date of Age in 1966 March Nov. March March Nov. March March birth (years/months) 1966 1966 1967 1971 1971 1972 1976 1903 63/- 11.6 10.0 12.2 11.8 12.2 11.6 11.6 1926 40/-B 7.2 5.4 8.0 7.4 8.4 6.8 7.0 1936 30/-a 8.2 6.8 6.6 8.2 8.4 9.0 8.2 1946 20/-a 6.8 8.8 9.6 10.0 10.8 10.4 9.8 1956 10/ -a 11.8 10.4 12.2 10.4 10.2 9.8 9.4 1961 5/ a 10.3 9.0 9.0 9.8 11.4 10.8 8.6 10 Nov. 62 3/4 9.8 10.2 11.2 8.4 8.8 8.6 8.2 28 Apr. 63 2/11 0 0 0 0 0 0 9.6 21 Dec. 63 2/3 12.2 10.6 10.0 8.8 10.0 13.6 8.0 26 Jan. 64 2/2 6.6 7.6 8.4 9.8 9.6 9.6 7.8 14Apr. 64 1/11 0 0 0 0 0 0 9.4 12Dec.64 1/3 0 0 0 0 0 0 11.0 20Feb.65 1/1 0 0 0 0 0 0 9.4 26 Nov. 65 0/4 6.0 0 0 0 0 0 10.0 13 Jan. 66 0/2 5.0 0 0 0 0 0 10.4 8 Exact age in months not known. Table 2. Diameter of the zone of haemolysis (mm) in sequential serum samples from children born between 1967 and 1974 Date of serum sample Date of birth March March March March March March 1971 1972 1973 1974 1975 1976 16 May 67 0 0 0 9.6 8.8 10.4 9 June 67 0 0 0 10.4 9.6 10.8 21 Feb. 68 0 0 0 8.8 8.4 9.0 May 68 0 0 9.2 9.0 8.6 8.6 23 Feb. 69 0 0 0 11.2 10.2 10.6 2 Nov. 69 0 0 10.2 10.4 9.8 10.4 20 July 70 0 0 0 10.2 9.2 8.0 5 Apr. 71 - 0 0 8.1 10.6 9.4 28 Nov. 71 - 4.8 0 10.4 8.8 8.8 2 May 72 - - 0 10.1 9.4 11.4 29Jan. 73 - - 9.8 11.4 11.0 9.2 4Apr. 73 - - - 0 0 0 18July73 - - - 0 0 0 7 Aug. 73 - - - 0 0 0 7 Feb. 74 - - - 7.0 0 0 3Jan. 74 - - - 7.6 0 0 19 June 74 - - - - 0 0 M. CLARKE ET AL. serum samples were collected annually between 1971 and 1976. In 1973, a few children showed evidence of antibody while by March 1974, a high proportion of children aged one year and over had developed antibody. Thus an outbreak of rubella infection appears to have occurred between March 1972 and March 1974. From the latter half of 1973 up to the completion of the study in March 1976 there was no further evidence of rubella infection in this community, since children born between April 1973 and March 1976 remained seronegative. A small-scale survey of children aged 0-2 years living in Keneba village revealed a similar epidemio- logical pattern for rubella. The prevalence of maternal antibody was similar to that found in Manduar village. There was no evidence of an outbreak of rubella infection around 1964, although a slight increase in the proportion of 2-year-old children with rubella antibody in 1968 appeared to indicate some transmission of virus. A full-scale outbreak of infec- tion in 1972-73 was indicated by the fact that, in 1974, 95.8% of two-year-olds had rubella antibody. DISCUSSION It is evident from previous serological surveys (1-5, 14, 15) that rubella infection is widespread on the African continent. The high incidence indicated by the present study is similar to that found in the Korekoro tribe of northern Zimbabwe (14) and the Turkana people of northern Kenya (15). However, as a clinical entity it has not given rise to concern and possibly for the greater part passes undiagnosed. In the present study, the serum samples collected from a population at regular intervals provided an opportunity to cor- relate serological evidence of infection with clinical disease. Although epidemic periods for rubella were clearly identified by our serological investigations, there was nothing in the clinical records to suggest an outbreak of overt rubella. A retrospective examination of clinical records of the MRC Unit in the Gambia failed to reveal any evidence of rubella-like syndrome in neonates. This clinical entity has, however, been described elsewhere in Africa (16-18). It is therefore likely that the strains of rubella circulating in Africa are potentially terato- genic but that the congenital rubella syndrome is infrequent in the Gambia because of the high level of immunity in adolescents and adults as indicated by the present survey. Our serological studies clearly defined two major periods of rubella virus transmission in the Gambia, in 1963-64 and in 1973-74. The 10-year interval between epidemics is in line with experience in other parts of the world (19). The present findings indicate that antibody to rubella virus was well maintained in inhabitants of the Gambia. This was evident even throughout the inter- epidemic period when transmission of rubella virus was absent or at a low level, as evidenced by lack of seroconversion in children. Thus, maintenance of antibody levels does not seem to be dependent upon repeated reinfection. The findings for rubella virus are in contrast to our observations (12) in the same Gambian population that antibody to the haemag- glutinin of influenza A/Hong Kong/68(H3N2) virus was maintained for only a short time with many indi- viduals becoming reinfected with the same or a closely related virus strain one or two years after their initial infection. The well-maintained serum antibody levels for rubella virus in inhabitants of the Gambia, despite evidence of high decay rates for influenza antibodies and reported rates ofimmunoglobulin catabolism that were 6-7 times greater than those found in Caucasians (20), may reflect differences in the immunobiological characteristics of rubella and influenza viruses. UM ETUDES EPIDEMIOLOGIQUES DU VIRUS RUBEOLIQUE DANS UNE COLLECTIVITE D'AFRIQUE TROPICALE L'epreuve d'hemolyse radiale simple pour la detection et le titrage de l'anticorps anti-virus rubeolique constitue une methode rapide et facile permettant d'effectuer des enquetes serologiques de grande envergure. Grace au prelevement d'echantillons successifs de serum chez des habitants de deux villages gambiens, Manduar et Keneba, il a e possible de suivre l'apparition et l'evolution des epidemies de rubeole dans ces collectivites entre 1966 et 1976. Cette etude a porte sur un total de 380 individus. On a constate qu'en general 28 a 61% des enfants de moins d'un an possedaient un anticorps antirubeolique, qu'on a suppose etre d'origine maternelle. Dans le groupe d'age 1-4 ans, le pourcentage de serums positifs etait plus faible, alors que plus de 50% des enfants de 5 a 10 ans possedaient des anticorps. En ce qui concerne les sujets ayant depasse 10 ans, plus de 93% d'entre eux avaient des anticorps. Afin de determiner les periodes oil se sont produites des epidemies de rubeole, on a eprouve des serums recueillis chaque annee de 1966 a 1976 chez des enfants de moins de 10 ans. Les resultats ont montre qu'une epidemie etait survenue environ deux ans avant le debut de 1'etude. II n'y a pas eu d'autres preuves d'infection rubeolique jusqu'A 934 RUBELLA IN AFRICA 935 environ 1973, annee oil une vaste epidemie s'est produite. Bien que ces collectivites aient e surveillees pendant toute la periode de 1'etude, il n'a pas e observe de signes cliniques d'infection, ni aucun cas de syndrome rubeolique conge- nital. 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