Possible natural immunity to Ebola?

          Possible natural immunity to Ebola?

A surprisingly high proportion of the Gabonese population could have immunity against Ebola. Antibodies to the virus were found in 15.3% of rural communities, whereas these people had never had haemorrhagic fever or other specific symptoms of the disease (such as severe diarrhoea or vomiting). IRD researchers and their partners 1 recently discovered this large number of healthy carriers among Gabonese people, even in areas where there has never been an Ebola outbreak. The scientists consider that these people have somehow come into contact with the virus, probably present in fruit contaminated by saliva from Chiroptera (fruit bats) 2.

Ebola fever epidemics have been striking central Africa for more than 30 years. Today the researchers know that Chiroptera, suspected of being the natural reservoir of the virus, can contaminate humans directly. However, the exact mechanisms of human contamination are still uncertain. This study sheds new light on the circulation of Ebola in the wild and on the threat it poses for humans, which could prove to be less severe than predicted.

Since the first recorded case of Ebola in 1976, outbreaks of the disease occasionally strike humans and great apes in Gabon, the Republic of Congo and the Democratic Republic of Congo, the Sudan and Uganda. The natural reservoir of the virus (bats) and the origin of most epidemics are known. But how exactly did humans become contaminated? What determines whether or not an epidemic breaks out? A surprising fact has emerged: a high proportion of the Gabonese population is immunized against the disease: 15.3% possess antibiotics against Ebola, even in areas where no epidemic has ever been recorded . This was found recently by IRD scientists and their partners 1 The results mean that the persons involved have been exposed to the virus but have never developed clinical symptoms of the infection.

Antibody search

For a better understanding of how Ebola circulates and spreads to humans, a three-year serological study was conducted, the most extensive to date concerning this virus. A multidisciplinary team (doctors, epidemiologists, viruses, veterinary surgeons and laboratory technicians) screened the rural population across the whole of Gabon, a country with a variety of types of terrain: some where epidemics occur, others which are spared by the disease. More than 4000 blood samples were analysed to detect the presence of antibodies against Ebola. The prevalence rate 3 varied significantly between regions, but the average found was 15.3%, the highest value so far reported for this disease. The savannah areas and the plains revealed intermediate rates of 10.5 and 12.4% respectively. In the Lakeland areas, only 2.7% of inhabitants possessed antibodies, whereas in the forest zones the rate reached 19.4%, even as high as 33.8% in some villages.

A real immunity to Ebola

The research team next looked into the immune status of people carrying antibodies, the first such investigation concerning this disease. They first showed that the antibodies react specifically against one or more proteins of the virus. These individuals had indeed developed specific antibodies against Ebola. In vitro tests subsequently brought evidence of a significant rise in the number of T8 lymphocytes (white blood cells which destroy infected cells) producing cytokine IFN-g, a substance involved in the immune system. This immune memory specifically concerning the Ebola virus is similar to that generated by vaccines whose effectiveness against Ebola in animals has been shown in previous studies. This similarity prompts the researchers to wonder if these people are naturally protected against new infection.

 

IRD/Pierre Becquart

High populations of bats live in Gabon’s forest areas, especially near villages.

Mild or unnoticeable forms of the disease

The high immunity rates are the biological proof that populations have been in contact with the Ebola virus. In order to develop antibodies, these healthy carriers must have been exposed to the virus in the past. They report that they have never suffered from the disease or in any case live in a non epidemic area. Ebola does not always provoke haemorrhage, but it triggers high fever, diarrhoea and vomiting, with a 90% mortality rate. It is highly unlikely that such symptoms had gone unnoticed. The researchers therefore deduce that the people with antibodies probably developed a mild form of the disease or an infection which did not produce symptoms.

Are healthy carriers contaminated by bats?

Given the extremely high mortality rate produced by full-blown Ebola, the high antibody prevalence observed excludes the theory that the healthy carriers could be survivors of past epidemics. Moreover, the fact that no serious clinical form of the disease had developed means that any idea of a human-human transmission (by way of blood, vomit or diarrhoea which contain the virus) must also be discarded.

No particular risk factor could be identified (such as sex, age, hunting, contact with wild animals), which means that there is a common origin of exposure located near or in the villages. Only three animal groups are known to be naturally infected with Ebola: chimpanzees, gorillas and bats. The great apes live deep in the forest and only rarely come into physical contact with humans. They probably do not contribute to the prevalence rates. Chiroptera, however, are particularly abundant in the forest areas of Gabon, where the immunity rates are indeed at their highest. Great numbers of bats perch in the trees, eat fruit, particularly in sectors near the villages. The local people could therefore come into contact with the virus when they gather and eat fruit contaminated by these animals’ saliva.

How and why have people who have never manifested clinical symptoms of infection or living in areas free of the disease been able to develop immunity against Ebola remains uncertain. However, these investigations help better understand the epidemiology of the virus, the processes involved in human contamination and the generation of epidemics, with a view to developing improved prediction and prevention strategies.

1. These research investigations were conducted jointly with scientists from the Centre International de Recherches Médicales de Franceville in Gabon.

2. Recent studies have shown the presence of Ebola virus in body fluids such as saliva or sweat.

3. The prevalence of an infection is the number of persons in a population who are contaminated at a given moment. It is usually expressed as a percentage.