11 October 2014

What we now know about ebola

Source: WHO.
Update: As of October 20, two nurses who had cared for the index patient have caught ebola. There are allegations that the hospital was not properly prepared for the case, and they did not have proper equipment while they looked after him.

Update: The news broke on October 11 and 12 that a nurse who had cared for the first (index) case of ebola in the US had caught ebola herself. It is worrying because she had worn the full hazmat gear in caring for the patient, and that she cannot remember committing any breach of security, unlike a nurse in Spain who said she had accidentally touched her face with a gloved hand. The US Centers for Disease Control and Prevention is firm that some breach of 'protocol' did occur; the question is where, or whether ebola is not well-understood enough.

The World Health Organization shared a list of ways in which the ebola virus may be transmitted on 6 October. While it's known that the virus is transmitted among humans through close and direct physical contact with infected bodily fluids, the most infectious are blood, faeces and vomit, the WHO said.

The virus has also been detected in breast milk, urine and semen. In a convalescent (recovering) male, the virus can persist in semen for at least 70 days; one study suggests persistence for more than 90 days. This implies that the virus can remain in semen for a longer period, but it remains unclear whether the virus can be transmitted through this method.

WHO also says that saliva and tears may also carry some risk of infection. However, the studies implicating these additional bodily fluids were extremely limited in sample size and the science is inconclusive. In studies of saliva, the virus was found most frequently in patients at a severe stage of illness. 

The whole live virus has never been isolated from sweat. This implies that sweat from a victim cannot transmit the virus.

The ebola virus can also be transmitted indirectly, by contact with previously contaminated surfaces and objects. The risk of transmission from these surfaces is low and can be reduced even further by appropriate cleaning and disinfection procedures.

Ebola virus disease is not an airborne infection. Airborne spread among humans implies that you can catch the virus from inhaling a suspended cloud of small dried droplets. This mode of transmission has not been observed during extensive studies of the ebola virus over several decades.

Theoretically, bigger droplets from a heavily infected individual, who has respiratory symptoms caused by other conditions or who vomits violently, could transmit the virus – over a short distance – to another nearby person. This could happen when virus-laden droplets are directly propelled, by coughing or sneezing (which does not mean airborne transmission) onto the mucus membranes or skin with cuts or abrasions of another person.

However, observation to date is that the spread of the virus via coughing or sneezing is rare, if it happens at all. Epidemiological data emerging from the outbreak are simply not consistent with the pattern of spread seen with airborne viruses, like those that cause measles and chickenpox, or the airborne bacterium that causes tuberculosis.

WHO is not aware of any studies that actually document this mode of transmission. On the contrary, good quality studies from previous ebola outbreaks show that all cases were infected by direct close contact with symptomatic patients.

There are also fears that the disease could change its mode of transmission. The WHO says scientists are unaware of any virus that has dramatically changed its mode of transmission. Speculation that ebola virus disease might mutate into a form that could easily spread among humans through the air is just that: speculation, unsubstantiated by any evidence.

For example, the H5N1 avian influenza virus, which has caused sporadic human cases since 1997, is now endemic in chickens and ducks in large parts of Asia. That virus has probably circulated through many billions of birds for at least two decades. Its mode of transmission remains basically unchanged.

The WHO is calling for more to be done to implement – on a much larger scale – well-known protective and preventive measures. Abundant evidence has documented their effectiveness, it says.