The 2014 Ebola outbreak in West Africa is the largest in history. As of mid-October, there have been approximately 9,000 cases and 4,500 deaths. The World Health Organisation warned that the infection rate could reach 5000 to 10000 new cases a week by the end of the year.
The virus is primarily transmitted from sick to healthy people by blood or body fluids (including but not limited to urine, saliva, sweat, feces, vomit, breast milk, and semen). In addition, objects contaminated with the virus (including needles and syringes) and direct contact with infected animals also play a role. Given this knowledge, it is not an unreasonable question to ask if blood feeding mosquitoes could spread the virus from infected people or animals.
Mosquitoes are just flying syringes aren’t they?
Mosquitoes are not flying syringes. They don’t transmit pathogens by transferring small infected droplets of blood. There is a complex biological process between the mosquito and the pathogen that must be completed before transmission can occur. In addition, there are ecological questions regarding the diversity, abundance, distribution and host-feeding patterns of local mosquitoes that can all influence the importance of mosquitoes in outbreaks of disease.
Unraveling these biological factors can be a complex process.
What happens inside the mosquito?
The mouth parts of a mosquito are made up of small tubes that either suck or spit. For a mosquito to effectively transmit a virus, the virus must make its way from the mosquito gut to the mosquito saliva.
If a mosquito takes a blood meal from an infected animal that contains the virus, the virus must infect the cells of the gut and then pass through to the body of the mosquito, replicate and then disseminate throughout the mosquito until the salivary glands are infected. This process is known as the extrinsic incubation period and can take anywhere from a few days to over two weeks. Once the salivary glands are infected, the mosquito may pass the virus to a new host through the saliva she injects while taking a blood meal.
There can be many barriers in this process. It may simply be the case that the virus cannot survive long enough in the gut of a mosquito. If it does survive, the virus may not “escape” the gut of the mosquito. In this case, the pathogen is excreted and the mosquito does not become infected. Even if most of the body of the mosquito becomes infected, the salivary glands may remain uninfected and the pathogen is not transmitted through the bite of the mosquito.
Experiments to determine the ability of individual mosquito species to transmit pathogens are known as “vector competence” experiments. Hundreds of these have been conducted in many countries to assess the ability of local mosquito species to transmit endemic and exotic pathogens. These studies typically involve the exposure of mosquitoes to an infected blood meal and then testing, at various times following infection, the legs, wings and body of the mosquito (to determine infection) and salivary glands or saliva specifically.
There are very few published vector competence studies on Ebola. In one, three species of mosquito (Aedes albopictus, Aedes taeniorhynchus, and Culex pipiens) were infected with Ebola Reston virus but no virus replication was recorded. What was interesting about this study was that, by attempting to inoculate the mosquitoes through intrathoracic injection rather than orally (i.e. via an infected blood meal), the researchers were able to bypass the mid-gut barrier. It gave the virus the best chance of infection. However, the lack of virus replication in the mosquitoes suggests they are unlikely to be natural hosts of the virus.
What happens outside the mosquito?
When assessing the role of mosquitoes in outbreaks of disease, it is important to look at not only how competent the mosquito is at becoming infected and transmitting a virus. What happens in the laboratory may not reflect what is happening in the field.
For example, a mosquito that preferentially feeds on birds may be an effective vector but will play a minor role in transmission of the pathogen to humans as it will rarely, if ever, bite a person. These mosquitoes, however, may play an important role in spreading the pathogens amongst wildlife and this may indirectly increase the risk of exposure to humans. There may also be mosquitoes that are effective vectors but are naturally found at such low densities (due to reliance on specific environmental conditions) they rarely bite humans.
The natural reservoir host for Ebola virus appears to be bats. However, it is suspected that one of the most likely routes of transmission from bats to humans could be via the spread of the virus to primates (who are infected by bat droppings or bodily fluids of diseased bats) and then to humans through expose to the infected primate. There are likely to be plenty of mosquitoes that readily bite both bats and various primates in regions where Ebola is endemic but there is no evidence that mosquitoes play a role in these endemic enzootic transmission cycles.
During an outbreak of Ebola in Kikwit (Democratic Republic of the Congo) in 1995, researchers collected approximately 35,000 arthropods and tested them for the presence of the virus. 15,118 mosquitoes were tested and no Ebola virus was detected.
While mosquito-borne pathogens have emerged (or reemerged) in various parts of the world, this has primarily been driven through the movement of vectors and infected individuals through international travel. There is no evidence that mosquitoes have played a role in the transmission of any of the emerging zoonotic pathogens that have jumped from animals to humans in recent times.
There are also other factors to take into consideration. One critical factor with regard to the current Ebola outbreak is the epidemiological data. As described in this review of transmission, human cases are driven by direct contact with the blood or other body fluids of infected patients. If mosquitoes were capable of transmitting Ebola virus, there would likely be a very different distribution of cases with many people becoming infected who hadn’t had prior contact with an infected person.
Unfortunately, while the mosquitoes in West Africa are not transmitting Ebola virus, they are transmitting malaria parasites. Malaria will kill many times more people in West Africa than Ebola this year. Perhaps the most significant public health impact of the Ebola outbreak in West Africa will be the disruption of anti-malaria campaigns?
The photo at the top of this piece is taken from European Commission Humanitarian Aid & Civil Protection.