Ebola hemorrhagic fever is considered a classic zoonotic disease with persistence of the Ebola virus in a reservoir species. The exact source of Ebola infections remains an open question, and despite all the efforts, we still lack clear indicators about the reservoir of the virus. As each country must strengthen their preparedness and response system to prevent eventual Ebola outbreaks, adequate knowledge about the ecology and transmission of this disease is of utter-most importance.

Animal hosts, reservoirs and sources of infection

Although a lot of effort has been put into identifying the natural reservoirs with every large outbreak of the disease, potential hosts or arthropod vectors for Ebola virus have not been found. Apes, humans and possibly other mammalian species susceptible to Ebola infection are regarded as the end hosts, and not as the reservoirs of infection.

Bats and rodents have been considered as potential reservoir species for a long time. Such idea was supported by experimental studies in African plants and animals that resulted in the transmission of Zaire Ebola virus to African fruit and insectivorous bats, although a conclusive link could not be established. Unfortunately, aside for Zaire ebolavirus, we have no data for potential reservoirs of other four Ebola virus species.

The presence of Zaire ebolavirus in naturally infected fruit bats was first documented when viral RNA and antibodies were detected in three different bat species of the Pteropodidae family: Hypsignathus monstrosus, Epomops franqueti and Myonycteris torquata. As a result, there is an overlap of geographic Ebola virus distribution with the range of the fruit bats, although Zaire ebolavirus has not been successfully isolated from naturally infected animals.

Still, successful isolation of Marburg virus from the cave-dwelling species of fruit bat Rousettus aegyptiacus lends support to the proposed idea of bats as a reservoir species for all filoviruses. This idea is further corroborated by Marburg virus outbreaks that have been associated with caves or mines heavily infested with bats.

Transmission of Ebola from the reservoir species to humans or other potential end hosts might be an infrequent event, taking into account the restricted distribution and scarce contact with the reservoir species. However, bats are frequent residents in equatorial Africa, where they are even hunted for food. As Ebola virus in the reservoir might present in a subclinical or asymptomatic way, there is a possibility of sporadic activation via appropriate stimuli (stress, changes in food patterns, infection with other pathogens) demonstrated in experimental conditions. This could explain the sporadic nature of infection and periodical outbreaks of this disease in Africa.
 

Non-human primates represent a source of infection for humans, but they are considered an accidental host (akin to humans) and not a reservoir of infection. Ebola outbreaks with Zaire ebolavirus and Taï Forest ebolavirus have been observed in gorillas and chimpanzees since 1994. Reston ebolavirus is dangerous for crab-eating macaque (cynomolgus) monkeys which were farmed in Philippines and subsequently imported into the USA and Italy.

Although the latter species was also a suspected cause of a deadly outbreak in pigs in People's Republic of China and Philippines, experiments have shown that it does not cause disease in porcine populations. Still, pigs can act as an amplifying host, thus potential transmission routes need to be further addressed.

Transmission of the Ebola virus

Infection with Ebola virus in Africa has occurred through the handling of infected gorillas, chimpanzees, monkeys, fruit bats, forest antelopes and porcupines. The disease is introduced into the human population via close contact with the blood, bodily secretions, organs or other fluids of infected animals. Once in humans, Ebola can spread efficiently in the community through human-to-human transmission.

Most human infections during outbreaks usually occur by direct contact with infected patients or cadavers. Healthcare workers, family and friends in close contact with the diseased are at the highest risk of getting infected, due to the close contact with blood or body fluids. Characteristic burial ceremonies where mourners have a direct contact with the body of the deceased can also represent an important path of the transmission.

The virus enters the host through mucous membranes, breaks, cuts and abrasions in the skin, or by parenteral introduction. Viral RNA and even infectious particles are found in genital secretions, semen, urine, saliva, feces, vomit and in the skin of infected patients. Various objects (for example needles and syringes) contaminated with infected body fluids represent a threat as well; hence they should be properly sterilized.

It is important to note that the airborne transmission of Ebola has not been documented (in comparison with respiratory viruses such as influenza virus). Although the virus is not transmitted via food or water, and the proper cooking of foods should inactivate infectious particles, ingestion of contaminated food may represent a possible route of exposure in natural infections.

The current West African outbreak across Guinea, Nigeria, northern Liberia and eastern Sierra Leone is still evolving. It has proven very difficult to control, resulting in flight cancellations, international travel advisories and closure of borders. Although previous African Ebola outbreaks mostly occurred in smaller towns and rural areas, this one struck well-connected urban centers – including capital cities of the affected countries. Concerns about the international spread of the disease accentuate the extraordinary need for improved control measures.