PT Mwamba¹, KF Tshioko ¹, A Moudi ¹, V Mukinda², GN Mwema³, D Messinger³, L Okito⁴, D Barakymfyte⁵, P Malfait⁶, R Pebody⁶, M Szczeniowski⁷, K Esteves⁷, D Heymann⁷, Y Hutin⁸ , RJ Williams⁸, AS Khan⁸, JJ Esposito⁸

1. WHO representative, World Health Organization (WHO)-Zaire, Kinshasa, Zaire
2. Médecins Sans Frontières
3. Institut National de Recherche Biomédicale, Kinshasa, Zaire
4. Ecole de Santé Publique, Kinshasa, Zaire
5. WHO African Regional Office, Brazzaville, Congo
6. European Programme for Intervention Epidemiology Training, Brussels, Belgium
7. Emerging and other Communicable Diseases Surveillance and Control, WHO, Geneva, Switzerland
8. Epidemiology Program Office, Special Pathogen Branch and Poxvirus Section, Viral Exanthems and Herpesviruses Branch, Division of Viral and Rickettsial Diseases, National Centers for Infectious Diseases, CDC

This report is published jointly in Morbidity and Mortality Weekly Report 1997 (Vol. 46, No 14 of 11 April) and Weekly Epidemiological Report.(Vol.72, No 15 of 11 April)

Monkeypox is an orthopoxvirus with enzootic circulation in the rainforests of central and western Africa; the virus can be transmitted to humans and cause a syndrome clinically similar to smallpox (e.g., pustular rash, fever, respiratory symptoms, and in some cases, death). From February to August 1996, a total of 71 clinical cases of monkeypox including 6 deaths, occurred in 13 villages in Zaire located in the Katako-Kombe health zone (1996 combined population: 15 698), Sankuru Sub-region, Kasai Oriental Region (figure 1) (1). During the initial investigation of this cluster of human cases, specimens of serum and/or crusted scab or fluid from vesicles were collected from 11 patients and monkeypox virus infection was confirmed in all 11 patients by the WHO Collaborating Centre for Smallpox and Other Poxvirus Infections at the Centers for Disease Control and Prevention (CDC), Atlanta, Georgia, United States of America. Preliminary DNA phylogenetic studies of this strain of virus indicated only minor genetic variation compared with other strains of monkeypox virus from Zaire collected during 1970 -1979. Because of reports by local public health officials of ongoing disease transmission, the Zairian Ministry of Health and WHO organized a follow-up investigation in February 1997 to characterize the magnitude of the outbreak. This report summarizes the preliminary results of the ongoing multidisciplinary investigation, which suggest that person-to-person transmission accounted for most monkeypox cases investigated in 1996 and 1997; in contrast, during previous years, reports concerned primarily sporadic cases that resulted from animal-to-human transmission.

As part of the follow-up investigation, during 23-27 February, a dwelling-to-dwelling active case search was conducted in 12 villages (1997 combined population: 4057) including some of the villages of the initial investigation. A possible monkeypox case was defined as a vesicular, pustular or crusted rash, not diagnosed as chickenpox by the family or the health-care provider, that occurred since January 1996 in a resident of the Katako-Kombe zone. A total of 92 (7 with typical active vesiculo-pustular skin lesions) possible monkeypox cases were identified (attack rate: 2.3%). Fifty-one (55.4%) case-patients were male and 25 (27.2%) were aged 15 years or older. In Akungula, the village with the highest attack rate (11.3%), the 45 reported cases were clustered in 8 of the 44 housing compounds. Of the 84 case-patients for whom vaccination data were available, 15 (17.9%) had a vaccination scar on the upper left arm suggesting receipt of vaccinia vaccine. Of these, 13 (86.6%) were aged 25 years or older. Three of the 92 case-patients died (case-fatality rate 3.3%) ; all were aged under 3 years and died within 3 weeks of disease onset. The other three deaths originally reported were either not monkeypox cases or occurred in a village where no active case search was performed during the follow-up investigation.

Of the 89 case-patients for whom data were available, 65 (73.0%) reported contact with another case-patient 7-21 days before the onset of illness and thus were considered secondary cases. The number of suspected cases identified per month increased during February- August 1996 and decreased gradually in the following months (figure 2). However, in February 1997, the number of reported cases increased again . The number of secondary cases was highest in August 1996, during the peak of the outbreak.

Arboreal squirrels of the Funisciurus (Thomas' and Kuhl's tree squirrels) and Heliosciurus (sun squirrels) spp. have been previously implicated as probable reservoir hosts in Zaire based on antibody data and a single viral isolate from a Funisciurus anerythrus (2). In an attempt to assess the potential role as a reservoir for monkeypox virus and to estimate the seroprevalence in wild caught species, animals were obtained through the hunting efforts of the local villagers and supplemented with live trapping of small mammals and rodents by the study team. Over 4 days, 84 animals representing 16 species were captured. All animals were examined for lesions and serum specimens were collected from 64 (76%). Except for one squirrel from which skin biopsies were collected, suspect lesions were not present on any other animal. The majority of the animals captured and processed were Funisciurus sp. (22 [34%] out of 64) and Cricetomys emini (Gambian rat) (15 [23%] out of 64). Virus isolation and antibody studies are ongoing.

Discussion

Monkeypox virus, first identified in 1958 as a pathogen of cynomolgus monkeys, was associated with human illness in Zaire and West Africa during 1970-1971. The number of human monkeypox cases associated with the epidemic described in this report exceeded the total of 37 sporadic cases previously detected in the Sankuru Sub-region, Kasai Oriental Region during active surveillance activities conducted during 1981-1986 (2). The outbreak was unrecognized until the end of July, when an abrupt increase in the number of cases prompted a preliminary investigation by public health officials in Zaire (1). One person in a single village was the likely primary case-patient who may have been the source of infection for a cascade of person-to-person transmission to 8 members of his family from February to July. During this period, some monkeypox cases were also identified in persons who reported no contact with any other case-patient suggesting introductions of monkeypox into the human population through contacts with wild animals.

In a previous study (2), the secondary attack rate of monkeypox within households was low, suggesting low potential for person-to-person transmission and that the infection could not sustain itself in a human population. However, this outbreak - with active cases continuing to occur in February 1997- differs from previously described monkeypox episodes. First, this outbreak represents the largest cluster of monkeypox cases ever reported. Second, the proportion of case-patients aged 15 years or older (27.2%) was substantially higher than previously reported (7.5%) (2). Third, the proportion of secondary cases (73.0%) was substantially higher than previously reported (29.6%) (3). Fourth the clustering of cases by household compounds, and the previously undescribed prolonged chains of transmission suggest that person-to-person transmission accounted for most of the cases during this outbreak. Finally, the case-fatality proportion (3.3%) was lower than that previously reported (9.8%). Cessation of vaccinia vaccination (which is protective against monkeypox infection) (2) in the late 1970s has resulted in an increase in the number of persons susceptible to monkeypox and could account for the magnitude of the outbreak and the higher proportion of case-patients aged 15 years or older.

Local measures to interrupt disease transmission are ongoing and include education of health-care providers and distribution of health messages such as limiting contact with wild caught animals and restricting contact with suspected cases to a single person (preferably the oldest member of the household who has either recovered from monkeypox or has a vaccinia vaccination scar). Cohort studies of persons who had household or other close contact with monkeypox case-patients were interrupted during the investigation because of civil unrest in Zaire. These studies are needed to quantify the newly observed person-to-person transmission potential and to evaluate whether monkeypox infection can be sustained in a human population without the occurrence of new cases acquired through contact with wild animals. Analytical studies should also provide information about the natural history of monkeypox infection in humans and animals based on changing demographics and increased human interaction with the flora and fauna of the rainforest. The results of such studies will determine the need for additional risk-reduction measures, possibly including consideration of vaccinia vaccination under select circumstances.

References:

1. WHO. Monkeypox, Zaire. Wkly Epidemiol Rec 1996;71:326.

2. Jezek Z., Fenner F. Human monkeypox. Melnick JL, ed. Monographs in Virology, Vol. 17. Basel, Switzerland: Karger, 1988.

3. Jezek Z, Marennikova SS, Mutumbo M, Nakano JH, Paluku KM, Szczeniowski M. Human monkeypox. A study of 2,510 contacts of 214 patients. J Infect Dis 1986;154:551-55.