Several research groups have looked in a wide range of people and other primates worldwide for antibodies that specifically recognise three distinct filoviruses: Marburg (strain Musoke), Ebola Zaire (strain Mayinga) and Reston virus. People who have been exposed to a virus may have elevated levels of antibodies in their blood serum that specifically bind to the components of that virus. This is known as seroconversion (from seronegative to seropositive). Antibodies may provide protection against future exposure to the virus if the levels of antibody suffice to neutralize the amount of new virus received. The new virus must share structural elements with the original one and the antibodies must bind an important part of the virus rather than just the most obvious and superficial part.
Monkeys surviving filovirus infection clear the virus from their bodies and are not infectious (Fisher-Hoch et al. 1992#1). The same seems true of human survivors. Unlike the case of the unrelated HIV infection, the presence of anti-filovirus antibodies does not indicate the people currently carry the filovirus. The high levels of antibodies raised in response to fighting off the initial filovirus challenge are not predictably successful in protecting monkeys against a second challenge with Ebola Zaire (Fisher-Hoch et al. 1992#2).
21.3% of serum samples collected from people in 5 different zones of the Central African Republic were positive for antibodies recognising Ebola virus and 3.2% for antibodies against Marburg virus (Johnson et al 1993 #1). Occupation or ethnic background influenced exposure among people living in the forest. For example, 37.5% of male hunter-gatherers were seropositive while 13.2% of male subsistence farmers had antibodies to Ebola virus (Johnson et al 1993 #2). Exposure to filoviruses must therefore be quite common in some regions, and not all exposure causes fatal disease.
43.3% of sera from monkeys originating in several countries, including China, the Philippines and Uganda, were positive for antibodies against at least one filovirus (Becker et al. 1992). Surprisingly, the same authors found 6.9% of a sample of 1288 people living in Germany had antibodies against at least one filoviral antigen. Sub-clinical infection with filoviruses may be widespread and filoviruses may not be restricted to the African continent. This conclusion is supported by the presence of anti-Ebola antibodies in 4.5% of a sample of people living in Madagascar, an isolated ³micro-continent² with remarkably different flora and fauna (Mathiot et al 1989).
At least two distinct Ebola-like filoviruses have been reported in non-human primates at quarantine facilities in the USA; in Reston, Virginia in 1989, and Pennsylvania in 1990, (Mahy et al. 1991). Separate groups of filovirus-infected monkeys were identified in 1990 in Virginia and Texas. 26 animal handlers were considered to be exposed to these filoviruses, 5 have developed antibodies against the viruses but none experienced any illness.
Filoviruses are present worldwide and vary in their ability to cause disease. Some strains are more virulent than others, hosts vary in their susceptibility to disease (Fisher-Hoch et al. 1992 #2). Whether or not individuals have previously been exposed to closely related viruses might possibly influence resistance to new infection. The mechanism of transmission, and dose transmitted also influence the probability of infection.