Epidemiology and incidence of plague in the world, 1958 - 79

Bulletin of the World Health Organization, 60 (2): 165 - 169 (1982) Epidemiology and incidence of plague in the world, 1958 79 A. K. AKIEV1 During the period 1958 - 79, 46 937 cases ofhuman plague were recorded in the world, but the general downward tendency ofplague incidence remained, remember- ing that 40 484 human cases were recorded in 1950. The current incidence ofplague is an indication of the epidemicity ofplague in its natural foci. In general, during the period under consideration, plague manifestations followed the universally recog- nized laws of the epidemicity of the disease. Anthroponotic bubonic plague-the most epidemicform ofplague- was noted rarely and in limited areas. A considerable proportion ofhuman plague cases occurred in southern Viet Nam, where the defoli- ation carried out over vast areas during the military operations is thought to have been one cause of the high incidence. Despite the general downward trend in plague incidence, 30 countries of the world were affected by the disease. Therefore, continuous epidemiological surveil- lanceforplague remainsan urgent task confronting the health authorities ofthe coun- tries having known or potential naturalfoci. During the period 1958 - 79, human plague occurred in 30 countries involving 46 937 patients. The highest number of cases (6004) was recorded in 1967; the lowest (286) was reported in 1958. Within the period there were 15 years with the annual total of under 2000 cases and 5 years with a total of more than 4000 cases. It should be noted, however, that a considerable proportion of human plague cases have been in southern Viet Nam; for example, in 1967 94% of the world total of human plague cases occurred in that country. These plague epidemics in southern Viet Nam are known to have been an indirect result of the military operations that continued there for several years. In view of this unusual situation it seems quite justified to consider the trend of plague incidence in the world without taking into account the figures for Viet Nam. Omitting Viet Nam, the number of human plague cases in the world for the period in question was 15 785. The highest number of cases (1390) was recorded in 1962, the lowest (253) in 1973. The annual number of plague patients was about 1000 over a period of 19 years. These data clearly point to a general decrease in the incidence of human plague in the world in the period 1958 - 79, in comparison with the 39 503 cases recorded in 1948 and the 40 484 in 1950.a The main reason for this decrease in the incidence of plague lies in the improvement of living standards in many nations of the world, such that the possibility of outbreaks of anthroponotic bubonic and primary pneumonic plague-the most epidemic forms of the disease-has been reduced practically to zero. During this period, also, epi- demics of human plague in India have ceased and the reasons for this have not yet been determined; however, it is thought that the extensive use of insecticides for malaria control in Indian towns and villages has played an important role. l Director, WHO Collaborating Centre for Reference and Research on Plague; Chief, Department of Epidemiology, Anti- plague Research Institute of the Caucasus and Transcaucasia, Sovetskaja 13, Stavropol, USSR. A French translation of this article will appear in a later issue of the Bulletin. ° FEDOROV, V. N. & KOZAKEVIC, V. P. Sbornik. Prirodnaja oeagovost' i epidemiologija osobo opasnyh infekcionnyh zabolevanij. [Collected papers. Natural focality and epidemiology of particularly dangerous communicable diseases] (in Russian). Saratov, 1959, pp. 18- 39. 4160 -165- A. K. AKIEV DISTRIBUTION AND EPIDEMIOLOGICAL FEATURES Despite the general decline in the incidence of plague in the world, the number of countries affected by plague remains fairly high. In 1958 -79 human plague was recorded in 30 countries, 9 of them (Bolivia, Burma, Brazil, Ecuador, Madagascar, Peru, USA, Viet Nam, and Zaire) being affected by the disease practically every year. In the remaining countries, outbreaks or sporadic cases of human plague occurred during the years when there was a universal resurgence of plague activity. Within the period under consideration there were two increases of plague activity. The first one started in 1961, reached its peak in 1964, and ended in 1966. The second period of activity began in 1973 and ended in 1978. In certain countries the increase in plague incidence occurred between these two periods of plague activity. Thus, in the period under consideration, cycles of plague activity of two types were seen: 3 - 5 and 8 - 10-year cycles. It was between the main periods of activity that the localized outbreaks of anthroponotic bubonic plague took place. The incidence of the disease at any one time is, in fact, an indication of the epidemicity of plague in its natural foci, in which epizootiological factors become active periodically. In some countries, this process involves the population of domestic rodents and results in the occurrence of localized outbreaks of anthroponotic bubonic plague. On the other hand, the occurrence of human plague in 30 countries at the same time is evidence of great activity in the world's natural foci of plague. Africa During 1958 -79 the incidence of plague in Africa was very low (about 6%o of the world total). However, in this continent plague occurred in many countries, particularly in those situated within the arid zone, and the relation of the periodic activity of the natural foci of plague to the occurrence of human infections was clearly demonstrated. Out of 30 countries that reported plague to WHO during the period, 16 are located in the continent of Africa. The disease occurred only once in Uganda (1959), Cameroon (1961), Malawi (1963), Mauri- tania (1967), Angola (1975), and Sudan (1979), whereas there were two outbreaks in Equa- torial Guinea (1969, 1970) and Southern Rhodesia (now Zimbabwe) (1974, 1975) and numerous outbreaks (3 -7) in Kenya, Lesotho, the Libyan Arab Jamahiriya, Mozambique, South Africa, and the United Republic of Tanzania. The periodicity of human plague in Africa varied from country to country. In Madagas- car and Zaire it was 2 -4 years; in South Africa and the United Republic of Tanzania, 5 -9 years; in Equatorial Guinea and Lesotho, 11 - 13 years; in Angola, Cameroon, Southern Rhodesia (Zimbabwe), and Uganda, 16- 18 years; and in the Libyan Arab Jamahiriya, 24 years. In 1978 there was an outbreak of bubonic plague in Kenya, where no human plague had occurred since the cessation of the sporadic cases recorded in 1961 - 67. According to pre- liminary information, the first plague cases in this outbreak were seen at the end of August in Kajiado District, Rift Valley Province. A total of 61 patients was detected in this district and additional cases were detected during November and December in the Eastern Coast and Central Provinces. The outbreak occurred in arid regions in the presence of large numbers of the multimammate rat, Mastomys natalensis, which readily colonizes human dwellings. Human plague continued to occur in 1979: of 227 cases notified in Kenya, 183 cases were reported from Kiambu District of the Central Province, 13 cases from Taita- Taveta District of the Coast Province, 1 case from Kitui District of the Eastern Province, and 30 cases from Kajiado District of the Rift Valley Province. The fact that human plague 166 INCIDENCE OF PLAGUE, 1958-79 167 reappeared in Kenya indicates that there may also have been a resurgence of the infection in some other African countries, particularly in those hit by the drought that started in 1970. Throughout the period under review, human plague was reported regularly from Mad- agascar and Zaire, with the disease occurring in settlements in the same provinces every year. In Zaire the cases occurred in Kivu and Upper Zaire Provinces, and in Madagascar, in Antananarivo and Fianarantsoa. In these two countries the epidemics were characterized by the occurrence of sporadic cases of human infection and seem to have been restricted by the flea and rodent control measures carried out wherever human cases, or plague in rodents and their fleas, were reported. Thus the epidemiological situation in Africa is very complicated, in particular since all the known types of plague epidemicity have been reported, including the anthroponotic bubonic and primary pneumonic forms. An outbreak of plague occurred in early March 1979 in the Juba rural council area of Equatoria Province in Sudan. A total of 226 cases (no deaths) was reported during the period from 5 March to 22 May. The disease presented clinical symptoms of fever with inflamed and painful swollen lymph glands. Although bacteriological investigations carried out by local authorities at the beginning of the outbreak confirmed the presence of Yersinia pseudotuberculosis subsp. pestis in six positive cases, the sera subsequently obtained from patients and examined by different laboratories (CDC, Atlanta, USA; Pasteur Institute, Paris, France) showed negative results in the haemagglutination test. In view of the fact that no previous cases of human plague had been reported from Sudan, and that natural foci of the disease existed in adjacent parts of neighbouring coun- tries, epidemiological investigation and flea/rodent control were considered to be indispen- sable in the Juba rural area. The evidence available concerning plague in Africa indicates that the infection is enzootic throughout the continent and that under certain conditions cases of human plague might occur in any African country. Americas In the Americas, during the period under consideration, 7382 cases of human plague (15.7% of the world total) were reported from 7 countries. Human plague was constantly present in Bolivia, Brazil, Ecuador, Peru, and the United States of America, and occurred periodically in the form of sporadic cases in Colombia and Venezuela. In the Americas, sporadic cases occurred fairly regularly in many of the western states of the USA, and this is evidence of persistent plague activity in the population of wild rodents. The clinical forms of plague seen in Ecuador and Peru (anthracoid and tonsillar plague) indicate that the fleas of human dwellings also carry plague. In the last 2 - 3 years there has been a sharp decline in the incidence of plague in the Americas, probably as a result of active epidemiological surveillance in the natural foci of plague. Asia In Asia there have been outbreaks in Burma, and in southern Viet Nam. In southern Viet Nam the incidence of plague began to increase during the period 1966 - 72, when the disease was reported from 38 provinces and districts. According to Velimirovicb a significant part " VELIMIROVIC, B. Zentralblatt fur Bakieriologie, Parasitenkunde, Infektionskrankheiten und Hygiene. 1. Abi. Orig. A, 228: 482- 508; 228: 509- 532 (1974). 168 A. K. AKIEV of the increase was related to movements of the population as a result of military oper- ations. However, the war in Viet Nam started in 1945 and it had not influenced the incidence of plague before 1962. Without denying the impact of human migrations upon the spread of communicable diseases of man in general, they would be most likely to increase the activity of factors associated mainly with anthroponotic infections. In my opinion, an important cause of the increased incidence of plague in Viet Nam was the extensive chemical defoliation, which according to Velimirovic started in 1961 and was carried out on a large scale between 1966 and 1969. It is evident that defoliation, particularly when repeated, can result in ecological conditions highly favourable for rodents living in the wild. These changes then lead to expansion of plague enzootic areas. In fact, 8607o of the total number of human cases in southern Viet Nam in 1966 - 70 occurred in the provinces subjected to major defoliation. This may be of practical importance in view of the fact that defoliation is being practised in agriculture in many countries of the world, in particular since little is known about the effects of defoliation on the ecosystem. In other respects the epidemiological features of human plague in Viet Nam were in accordance with the known laws governing the occurrence of human infection in the presence of epizootics among wild and commensal rodents. Although cases of secondary plague pneumonia have been not infrequent in Viet Nam, these have very seldom given rise to outbreaks of primary pneumonic plague. In Burma, before 1974, the annual incidence of human plague was at a low level typical of the epidemicity of plague in the rat foci. However, during 1974 the incidence began to rise and outbreaks were recorded in places that had been free of the infection for a long time, demonstrating the enzootic character of plague throughout most of the territory of Burma -from Kachin State in the north to the towns of Rangoon and Pegu in the south; from Lashio District in the north-east (Shan State) to the town of Akyab (Arakan State) in the south-west. Nevertheless, human plague occurs mainly on the occasion of epizootics in wild and commensal rodents. No outbreaks of anthroponotic bubonic and primary pneumonic plague have been recorded. In contrast, the 1967-68 outbreaks of bubonic plague in Nepal were associated with bites of Pulex irritans-the flea of human dwellings. Thus, these outbreaks were anthro- ponotic in character. There is also evidence that natural foci of wild plague exist in Nepal, since the Y.pseudotuberculosis subsp. pestis isolates belong to the glycerol-positive variant. Until recently, plague has been highly epidemic in India, and the incidence of the disease there determined the epidemiological situation in the world. For example, in 1953, 780o of the total number of human plague cases in the world occurred in India. Since 1967, the country has been free from plague largely as a result of the large-scale application of insecti- cides in populated areas, primarily for the purpose of malaria control. However, it seems unlikely that these measures could have resulted in complete eradication of the plague natural foci, and the causative agent undoubtedly continues to exist in the wild rodent population. It follows that sporadic cases and even small outbreaks of bubonic plague are likely to occur in India. For this reason, it is most important to investigate carefully all out- breaks of feverish illness of unidentified etiology. Incidentally, such an outbreak occurred in June 1969 in two villages of Bhir District, Maharashtra State.c On the whole, then, this survey has shown that during the period 1958 - 79, the occur- rence of plague has followed the universally recognized laws of the epidemiology of the disease as a zoonotic infection with natural foci.d Epidemics of all types, including anthro- ponotic bubonic and primary pneumonic forms, occurred in practically all continents of the world. SANT, M. V. Indian journal of medical sciences, 26: 480- 484 (1972). " Weekly epidemiological record, 48: 149 - 160 (1973). INCIDENCE OF PLAGUE, 1958 - 79 PLAGUE EPIDEMIOLOGICAL SURVEILLANCE In countries with known or potential natural plague foci, continuous epidemiological surveillance remains a most important task confronting the health authorities concerned. In many countries, however, plague surveillance is at present inadequate, as demonstrated by the recent mortality from plague. Use of the highly effective, modern, therapeutic drugs depends upon prompt detection and early treatment of suspected plague patients. According to information reported to WHO, 2067 people died of plague in the period 1966 - 78, out of the 38 634 cases recorded (5.35%o). The impression is that plague mortality is relatively low, but analysis of the data by country shows that, in fact, the mortality remains high. Thus, in Burma the average mortality was 10.80/o, and mortality was, in general, very high in those Asian countries in which the disease broke out periodically. Out of the 102 patients with plague during the 1968 epidemic in Indonesia, 43 (42.207o) died. In Nepal in 1967 - 68, 29 (78 0o) of the 37 plague patients died. Plague mortality was also high in the Americas-the mean over several years being 13.90%o. During the period 1966- 77, the number of human cases and deaths in the United States of America was 97 and 14, respectively-a mortality rate of 14.40/o. The mean mortality rate for the continent of Africa was 22.150/o. In this case, it was high not only in those countries in which plague occurred sporadically, but also in Madagascar and Zaire known to be endemic for plague. The average mortality rate in Madagascar was 34.2%/o and that in Zaire 19.807o. There were significant differences in mortality rate among the provinces of these two countries. Thus, 3 out of 28 patients died (over 10%/o) in Kivu Province of Zaire in 1974- 75, while in Upper Zaire Province 11 out 16 people (690%o) died in 1976- 77. During 1974 - 77 in Fianarantsoa Province, Madagascar, 23 out of 79 patients died (about 30%/o), whereas in Antananarivo Province only 9 (160/o) of the 55 plague patients died within the same period. CONCLUSIONS Despite the general downward trend in the incidence of human plague observed through- out the world during the period 1958 - 79, most important tasks remain in the field of epidemiological suveillance of plague, including: (1) the radical reduction of the incidence of human plague in Burma and in Viet Nam through the control of urban plague; (2) the improvement of epidemiological surveillance of plague, particularly as regards early detection and treatment of suspected plague patients; (3) the definition of a mandatory complex of measures to be carried out in the course of the epidemiological surveillance of plague in countries having natural foci of infection; (4) the organization of special studies on the particular features of the natural focality and epizootiology of plague in tropical countries, in order to be able to formulate energetic measures for the prevention of plague. 169