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Transmission


The spread of Ebola between people occurs only by direct contact with the blood or body fluids of a person after symptoms have developed. Body fluids that may contain ebolaviruses include saliva, mucus, vomit, feces, sweat, tears, breast milk, urine, and semen. Entry points include the nose, mouth, eyes, or open wounds, cuts and abrasions. Contact with objects contaminated by the virus, particularly needles and syringes may also transmit the infection. The virus is able to survive on objects for a few hours in a dried state and can survive for a few days within body fluids. Ebola virus may be able to persist in the semen of survivors for up to seven weeks after recovery, which could give rise to infections via sexual intercourse. Otherwise, people who have recovered are not infectious. The potential for widespread infections in countries with medical systems capable of observing correct medical isolation procedures is considered low. Usually when someone has symptoms, they are sufficiently unwell that they are unable to travel without assistance.

Handling infected dead bodies is a risk, including embalming. Because dead bodies are still infectious, traditional burial rituals may spread the disease. Nearly two thirds of the cases of Ebola infections in Guinea during the 2014 outbreak are believed to have been contracted via unprotected (or unsuitably protected) contact with infected corpses during certain Guinean burial rituals.

Healthcare workers treating those who are infected are at greatest risk of disease. This occurs when they do not wear appropriate protective clothing such as masks, gowns, gloves and eye protection. This is particularly common in parts of Africa where the health systems function poorly and where the disease mostly occurs. Hospital-acquired transmission has also occurred in African countries due to the reuse of needles. Some healthcare centers caring for people with the disease do not have running water. In the United States, spread has occurred due to inadequate isolation.

Airborne transmission has not been documented during EVD outbreaks. Transmission among rhesus monkeys via breathable 0.8–1.2 μm aerosolized droplets has been demonstrated in the laboratory. That airborne transmission does not appear to occur in humans may be due to there not being high enough levels of the virus in the lungs. Spread by water or food other than bushmeat has also not been observed, nor has spread by mosquitos or other insects.

Initial case
While it is not entirely clear how Ebola initially spreads from animals to human, it is believed to involve direct contact with an infected wild animal or fruit bat. Wild animals other than bats capable of being infected include: a number of monkey, chimpanzees, gorillas, baboons and duikers. In Africa wild animals, including fruit bats are hunted to eat, being known as bushmeat.

Animals may become infected when they eat fruit already partially eaten by bats carrying the virus. Fruit production, animal behavior, and other factors may trigger outbreaks among animal populations.

It does appear that both domestic dog and pigs can also be infected with ebola viruses. Dogs when the carry the virus do not appear to develop symptoms, while pigs appear to be able to transmit the virus to at least some primates.

Reservoir
The natural reservoir for Ebola has yet to be confirmed; however, bats are considered to be the most likely candidate. Three types of fruit bats (Hypsignathus monstrosus, Epomops franqueti, and Myonycteris torquata) have been found to possibly carry the virus without getting sick. Whether or not other animals are involved in its spread is not known as of 2013. Plants, arthropods, and birds have also been considered as possible reservoirs as well.

Bats were known to reside in the cotton factory in which the first cases of the 1976 and 1979 outbreaks were observed, and they have also been implicated in Marburg virus infections in 1975 and 1980. Of 24 plant species and 19 vertebrate species experimentally inoculated with EBOV, only bats became infected. The bats displayed no clinical signs and is evidence that these bats are a reservoir species of the virus. In a 2002–2003 survey of 1,030 animals including 679 bats from Gabon and the Republic of the Congo, 13 fruit bats were found to contain EBOV RNA fragments. Antibodies against Zaire and Reston viruses have been found in fruit bats in Bangladesh, thus identifying potential virus hosts and signs of the filoviruses in Asia.

Between 1976 and 1998, in 30,000 mammals, birds, reptiles, amphibians and arthropods sampled from outbreak regions, no Ebola virus was detected apart from some genetic traces found in six rodents (Mus setulosus and Praomys) and one shrew (Sylvisorex ollula) collected from the Central African Republic. Further efforts; however, have not confirmed rodents as a reservoir. Traces of EBOV were detected in the carcasses of gorillas and chimpanzees during outbreaks in 2001 and 2003, which later became the source of human infections. However, the high lethality from infection in these species makes them unlikely as a natural reservoir.