Presently, there is no solid evidence that pigs have any role in the past outbreaks of Ebola virus disease, according to Christine Atherstone, a researcher with the International Livestock Research Institute (ILRI), based in Uganda. She is the lead author of a new paper investigating the role of pigs in the spread of the Ebola virus to people in that country.

Since the discovery of the Ebola virus, there have been four outbreaks of the disease in Uganda. There has been a very rapid increase in pig keeping in the country, such that the domestic swine population increased from 190,000 to 3.2 million in the 30 years to 2008. This increase and the fact that pigs are the only domestic species known to be naturally infected with the virus led Atherstone and her co-authors to investigate the role of pigs in these outbreaks through examination of published papers, interviews with people with relevant expertise and spatial analysis comparing pig density with Ebola cases.

Two viruses are known to cause Ebola disease in people - the Reston Ebola virus (REBOV) occurs naturally and causes mild respiratory symptoms in pigs, while the Zaire Ebola virus (ZEBOV) leads to fever and severe lung disease following artificial infection.

Bats have been linked to outbreaks in people but other mammals living in proximity to man are also known to carry the virus. These include pigs,but the limited serological data do not link pigs to past outbreaks of Ebola.

Not all human index cases – the first cases in a particular area – have been linked to contact with the likely virus carriers, bats and non-human primates, leading to the conclusion that the virus was transmitted by another host but pigs were not identified in any of the cases.

Domestic pigs inhabit the same areas as some potential Ebola virus zoonotic hosts and may interact with them, especially over food sources, particularly fruits, the authors suggest. Interactions at the human–pig–wildlife interface could support transmission through saliva or injuries resulting from fighting over food, they found.

It also emerged that symptoms of fever in pigs are commonly reported by farmers and that most cases are undiagnosed.

There is also a correlation between the timing of Ebola virus outbreaks and periods of peak pork consumption, which occur during festivals and holidays. However, the authors suggest this could be related to the gatherings of people as well as to the food consumed.

Atherstone and her co-authors conclude from their study that pigs may be amplifying hosts in the transmission of the Ebola virus but they are not reservoir hosts. They recommend that their work is continued in order to gain further knowledge so that any risk can be communicated with minimum impacts on the production chain, poverty and rural livelihoods.

The paper, by C. Atherstone, E. Smith, P. Ochungo, K. Roese and D. Grace and entitled “Assessing the potential role of pigs in the epidemiology of Ebola virus in Uganda” appears in early online edition of the journal, ‘Transboundary and Emerging Diseases.’