Volume 29-15
1 August 2003

[Table of Contents]

OUTBREAK(S) OF EBOLA HEMORRHAGIC FEVER, CONGO AND GABON, OCTOBER 2001 TO JULY 2002

On 17 November, 2001, a cluster of five deaths was reported to regional health authorities by medical personnel at Mékambo Medical Centre in the La Zadié health district (Gabonese Ogooué- Ivindo province) bordering the Congo. All five deaths were associated with signs of bloody diarrhea and occurred within the same family over a 3-week period. An unusually high number of animals, mainly non-human primates (gorillas, chimpanzees, monkeys), found dead in the rainforest of the same district were also reported to the authorities by villagers and nature conservancy organizations.

On 24 November, regional health authorities conducted a preliminary assessment. During the subsequent week, reports of suspected cases of hemorrhagic fever admitted to Mékambo Health Centre and Makokou Regional Hospital prompted a joint mission by the Gabonese Ministry of Public Health and Population (GMoPHP), the Gabonese Ministry of Defence (Army Health Section), the Centre international de recherche médicale de Franceville (CIRMF), and the WHO country office.

On 30 November, blood samples were obtained from two suspect cases and on 7 December were sent to the CIRMF. The clinical suspicion of hemorrhagic fever was confirmed on 8 December, when the CIRMF identified Ebola virus infection in both specimens. The laboratory testing included virus antigen detection, IgG antibody ELISA tests, and reverse transcriptase polymerase chain reaction (RT-PCR). Viral RNA was extracted from both sera, and cDNA strands from the L gene (420 bp) were synthesized, amplified and sequenced, showing that the strain isolated from these first two patients belonged to the Ebola-Zaire subtype. Molecular characterization of RT-PCR products obtained from other Ebola hemorrhagic fever (EHF) cases are in progress at the CIRMF.

On 8 December, WHO head office was notified of the results of laboratory testing. The outbreak of EHF was declared by GMoPHP on 11 December. This report describes control activities implemented in the affected areas to contain the spread of the epidemic and the preliminary epidemiologic findings.

Epidemic Response

The outbreak response in Gabon was organized by MoPHP and the Ministry of Defence in conjunction with WHO and its partners in the Global Outbreak Alert and Response Network. As the initial epidemiologic investigation revealed cases in the neighbouring Cuvette Ouest region of Congo, the Congolese Ministry of Health (CMoH) subsequently joined the response efforts. The international team was involved in field activities for nearly 5 months in the two countries and included over 70 representatives from 17 institutions.

As with previous outbreaks of EHF, control activities centred on surveillance (active case-finding and daily follow-up of contacts for the duration of the maximum incubation period, i.e. 21 days), establishment of isolation wards to ensure strict infection control practices during patient care, implementation of safe burial practices, and social mobilization campaigns to encourage the adoption of practices that would interrupt the spread of disease in the community. Initial laborattory testing of suspect cases was provided by CIRMF (RT-PCR, virus antigen and IgG antibody detection by ELISA testing) and supplemented by the US Centers for Disease Control and Prevention (CDC) in Atlanta, where antigen detection by immunohistochemistry was performed on formalin-fixed postmortem tissues.

Isolation facilities were set up in all affected districts: La Zadié (Mékambo), Ivindo (Makokou), and Mpassa (Franceville) in Gabon, and Mbomo and Kéllé in Congo. Surveillance bases were established in these districts and in others where contacts under follow-up had travelled: Woleu (Oyem) and Libreville. However, the continuity of intensive control efforts throughout the outbreak was hampered by security concerns, leading to the evacuation of the international team on two occasions, and by the remoteness of the affected areas.

Surveillance

On 16 December, 2001, the international team initiated an active surveillance system for EHF in La Zadié District (Gabon). As with previous EHF outbreaks⁽¹⁾, four case notification categories were employed: alert¹, suspect², probable³ and laboratory-confirmed⁴. A modification accounted for the possibility of contact with dead or sick animals as potential risk factors for EHF. A standardized case reporting form was also developed.

The lack of cooperation by the concerned communities, logistic difficulties in accessing the affected areas, and the turnover of surveillance teams operating in the field resulted in incomplete collection of epidemiologic information as well as inconsistencies in applying and recording the notification categories initially used. An analysis of available data is presented in the following section.

Epidemiology

A case of EHF was any probable or laboratory-confirmed case. Major obstacles were encountered during this mission, and - apart from the laboratory-confirmed cases - definitions for the various notification categories were used inconsistently. Probable cases of EHF were classified as such on a case-by-case basis.

For the first case of EHF, identified retrospectively, symptoms began on 25 October. From 25 October, 2001, to 18 March, 2002, 124 cases of EHF, of which 37 (30%) were laboratory-confirmed, were identified in Congo (Figure 1), where EHF was reported for the first time, and in Gabon. Given the high mobility of the population across the border between the two countries, the geographic distribution of cases is presented according to the location where patients were cared for and/or died: 65 cases were noted in Gabon (47 [38%] in La Zadié district, 17 [14%] in Ivindo district and 1 [1%] in Mpassa district) and 59 in Congo (33 [26%] in Mbomo district and 26 [21%] in Kéllé district). Ninety-seven deaths were reported, corresponding to a case-fatality ratio (CFR) of 78%. The median time from onset of symptoms to death was 6 days. Sixty- two cases (50%) were female. Ages ranged from 0 to 85 years (median 26 years); 34 (27%) were aged < 15 years.

All but two cases (122) were epidemiologically linked to recognized chains of transmission. There was epidemiologic evidence of at least six different introductions (four in Gabon and two in Congo) of Ebola virus into human communities during this epidemic, each related to a hunting episode. The genetic sequencing and the comparisons of viruses detected from human cases in the various chains of transmission, performed by the CIRMF and the National Institute for Virology (NIV), from South Africa, will help to analyze these findings.

The first index case was probably infected during a hunting party near Mendemba village on 21 October, 2001, and the last case infected near Grand Itoumbi village on 23 February, 2002. Human index cases of EHF had reported contacts with gorillas, chimpanzees, monkeys, forest duikers and porcupines. Ebola virus was detected in the carcass of a gorilla butchered by one of the index cases shortly before onset of illness; this was the only case for which it was possible to demonstrate Ebola virus in the incriminated animal.

During this epidemic, the vast majority of secondary cases were related to community-based transmission. All cases observed in the Ivindo district were linked to two imported cases from La Zadié, which were admitted to Makokou Regional Hospital. Three health care workers were infected: one in Mékambo Health Centre (La Zadié district, Gabon), one in Makokou Regional Hospital (Ivindo district, Gabon) and one in Olloba Health Centre (Mbomo district, Congo).

Case Description

Among the 37 patients with laboratory-confirmed EHF for whom information was available, the most commonly reported signs and symptoms included fever (30/31), headache (22/28), nausea and vomiting (23/31), anorexia (23/29), diarrhea (28/31), fatigue/asthenia (26/30), abdominal pain (20/28), muscular or joint pain (20/27), difficulties in swallowing (16/28), difficulties inhaling (9/28), and hiccups (5/28). Signs of bleeding were observed in only about 52% of these patients, primarily involving the gastrointestinal tract.

Ecological Studies

The high number of animals found dead in the rainforest, of which two gorillas were positive for Ebola, appeared to indicate intense viral activity in the wildlife populations. The situation therefore provided an excellent opportunity for conducting ecological studies to search for the still unknown natural reservoir of Ebola. In February 2002, mammals and birds were collected and sampled by a team from CIRMF, CDC, NIV, the Wildlife Conservation Society and the Institut de recherche en écologie tropicale de Makokou. The laboratory investigations are currently under way.

End of the Epidemic

On 6 May, 2002, 48 days after the death of the last registered case, the outbreak of EHF in Gabon was declared over by the Minister of Health. Although there has been no official declaration from the Government of the Republic of Congo, the last case was notified on 18 March, and no further  cases were reported for a period exceeding twice the maximum incubation period for Ebola (42 days).

Control efforts in Congo and Gabon were hampered by the remoteness of the affected areas, making efficient communication difficult to establish, as well as lack of transport for personnel and insufficient materials for barrier nursing. Problems in securing full cooperation among the affected communities in identifying and hospitalizing cases and in reporting contacts further emphasized the need for more effective social mobilization strategies and activities at the onset of Ebola outbreaks.

Outbreak of Suspected EHF in Congo and Gabon,
June 2002

On 6 June, 2002, CMoH reported six suspected cases of EHF, including five deaths, in Mbomo district (Cuvette Ouest region). A small team from CMoH and WHO investigated these six cases. The epidemiologic pattern of this cluster and the clinical signs of the cases were consistent with EHF. The first two suspected cases of EHF, with onset of symptoms on 17 May, were hunters who had been in contact 4 days earlier with a chimpanzee and a pangolin found dead in the rainforest south of Olloba.

On 21 June, GMoPHP reported two suspected cases of EHF, including two deaths in the village of Ekata (La Zadié district in Ogooué-Ivindo province). Both patients had fallen ill in Oloba (Mbomo district) and were epidemiologically linked to the suspected EHF cases identified there. Lack of community cooperation prevented follow-up of identified contacts, thorough investigation, or the collection of samples to confirm the etiology of the disease.

From 17 May to 25 July, 2002, 11 suspected cases of EHF with 10 deaths (CFR 90%) were identified in Congo (9 cases, 8 deaths) and in Gabon (2 cases, 2 deaths). During the same period, Écosystèmes forestiers d'Afrique centrale(ECOFAC), a nature conservancy organization, investigated the deaths of wild animals in the affected areas and concluded that a very large epidemic with a high mortality rate had occurred among gorillas and chimpanzees (and others animals) in the forest south of Olloba.

The human population in the areas where the last EHF outbreaks have occurred relies heavily on hunting for its food and economy. In the event of an EHF outbreak in animals, the local population would be at risk if they were to have contact with sick or dead animals. WHO is therefore strongly encouraging the government to develop health information and education messages to prevent Ebola virus infection and transmission, and to improve awareness in the population about the disease.

Editorial note

Congo, Gabon and others countries in Central Africa should ensure, as a priority, the design and implementation of national plans to improve preparedness for epidemic-prone diseases, including Ebola, and to strengthen an integrated disease surveillance system.

The international outbreak response team, working with the ministries of health, the ministries of defence and the forestry and wildlife ministries of Congo and Gabon, under WHO's leadership, comprised partners in the Global Alert and Response Network. The international team included the International Federation of Red Cross and Red Crescent Societies, Médecins Sans Frontières (Belgium and Holland), United Nations Children's Fund and teams from the following countries: Belgium (Epicentre, Brussels), Canada (Field Epidemiology Training Program, Health Canada, Ottawa), Congo (ECOFAC, Brazzaville), European Community (European Programme on Intervention Epidemiology Training, Paris), France (Institut Pasteur, Lyon and Epicentre, Paris), Gabon (Centre International de Recherches Médicales de Franceville, Franceville; ECOFAC, Libreville; Institut de recherche en écologie tropicale, Makokou, and the Wildlife Conservation Society, La Lopé), South Africa (National Institute of Virology, Johannesburg), United States (CDC, Atlanta), and United Kingdom (National Health Service).

Reference

1. World Health Organization. Outbreak of Ebola haemorrhagic fever, Uganda, August 2000-January 2001. Wkly Epidemiol Rec 2001;76:41-46. URL: <http://www.who.int/wer/pdf/2001/ wer7606.pdf>.

Source:    Adapted from Outbreak(s) of Ebola haemorrhagic fever, Congo and Gabon, October 2001-July 2000. Wkly Epidemiol Rec 2003;78(26):217-28.

1. An alert case was any person with sudden onset of high fever OR sudden death OR bleeding or bloody diarrhea or blood in urine. The use of the Alert category was restricted to the community. If laboratory samples were obtained at an appropriate time during the illness, the previous notification categories were reclassified into "Laboratory-confirmed" cases or "Not a case". Laboratory-confirmed cases were either positive for Ebola virus antigen or Ebola IgG antibody; use of the "Not a case" category meant that there was no Ebola specific detectable antibody or antigen. Individuals identified with isolated IgG antibodies without symptoms were not included as cases.

2. A suspect case was any person, dead or alive, with contact with a probable or confirmed EHF case or contact with a dead or sick animal AND fever OR fever and at least three specific symptoms (headache, anorexia, fatigue, myalgia or arthralgia, dyspnea, vomiting or nausea, diarrhea, abdominal pain, dysphagia, hiccups) OR any unexplained bleeding.

3. A probable case was a person with symptoms compatible with EHF, as evaluated by a clinician OR a dead person with an epidemiologic link with a confirmed case.

4. If laboratory samples were obtained at an appropriate time during the illness, the previous notification categories were reclassified into "Laboratory-confirmed" cases and "Not a case". Laboratory-confirmed cases were either positive for Ebola virus antigen or Ebola IgG antibody; use of the "Not a case" category meant that there was no Ebola specific detectable antibody or antigen. Individuals identified with isolated IgG antibodies without symptoms were not included as cases.

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