This is a collaborative post by myself and my colleague, Dan Peach.
Lately, we’ve been inundated with updates about the Ebola outbreak: news, pictures, videos, facts, symptoms and more. This is no surprise, as there are over 14,000 confirmed cases as of November 12, 2014. We’d like to shed some light on the virus and speculate on the natural transmission cycle and the potential for insects, specifically flies such as filth-breeding flies, to act as vectors of the disease – an area which remains a challenge for scientists.
There are four species of Ebola virus – all of which cause disease in vertebrates. It is a hemorrhagic fever belonging to the family filoviridae that was first recognized in Africa in 1976. It is readily transmitted via contact with infected bodily fluids, and can survive on surfaces contaminated by such fluids for at least several hours in dark conditions (Sagripanti et al. 2010). Usually, outbreaks in human populations follow that of outbreaks in chimp or gorilla populations (Leroy et al. 2004, Lahm et al. 2007). This happens (at least partially) because humans consume bush meat and contract the disease. There are normally a few outbreaks every few years in rural Africa, usually 2-4 months after the end of a wet season (Lahm et al. 2007), but the remote locations and small numbers contain them. This time it was different because it got into some of the larger population centres, such as Monrovia, and overwhelmed the healthcare system. For this reason, the 2014 outbreak has become the largest in history.
One of the areas where the Ebola virus really stands out compared to other pathogenic outbreaks (like measles, whooping cough, smallpox, and the 1918 influenza pandemic) is its reliance on the animal reservoirs. The Ebola virus can be incubated in pigs, primates, antelopes, dogs, and bats. These (or other yet-to-be determined species) can form a reservoir for Ebola to survive if it has been eradicated from the local human population. Viruses like smallpox and measles have no animal reservoirs that we know of. So despite being highly contagious, they’ve been relatively easy to eradicate from human populations because humans are the only organisms one needs to worry about in breaking the chain of transmission.
For a long time fruit bats have been implicated in the transmission of the disease because unlike pigs, primates, antelopes, or dogs, they can be infected with the virus and survive without symptoms (also known as asymptomatic). However, their role in maintaining the virus in nature is under debate.
We would like to bring forth the hypothesis that filth-breeding flies may be responsible for transmitting the disease. There are many kinds of biting and filth-breeding flies, which are quite common around farms, residences, and food establishments (like restaurants or meat-packaging plants). Many of these flies have lapping sucking mouthparts, so they only consume liquids (like bodily fluids resulting from Ebola) and, as they move from food source to food source, they sample and eat food by regurgitating liquid and dropping it on the food to liquefy it. More importantly, because filth-breeding flies are scavengers (on carrion, feces, bodily fluids, and/or decomposing organic material) they could be capable of transmitting diseases to animals, as well as humans.
Another reason we believe filth flies may be responsible for transmitting the disease is because outbreaks in animals (such as gorillas or pigs) usually precede human outbreaks (Leroy et al. 2004, Lahm et al. 2007). While consumption of infected animals has been implicated in many human outbreaks, infected animals or animal carcasses could provide easy, undefended food resources for flies. If these flies then land on human food or on or around an orifice of a human, and deposit infected fluids from a previous meal, the potential may exist for infection. Mosquitoes that feed on an infected animal or human could also potentially transmit the virus.
Ebola is an RNA virus, so the best evidence for insect vectored Ebola would be if insects were found to vector RNA viruses. One study found that house flies could successfully transmit a pathogenic RNA virus from infected pigs to uninfected pigs (Otake et al. 2003). The virus was detected in the house flies up to 6 hours after exposure, plenty of time for a pathogen-carrying fly to expose multiple unexposed hosts.
Finally, many filth flies are exceptional travelers, and individuals have been reported to fly distances of up to 65 km. Therefore having the potential to distribute the Ebola virus both long distances as well as within the immediate vicinity of their feeding and/ breeding site.
There are many many many many flies out there, orders of magnitudes more than any vertebrate deemed responsible for Ebola. Even if a small fraction of the fly populations would be carriers of Ebola, they probably would still be the dominant vector of spreading the disease.
Currently sanitation is the only way to control these flies; there are no effective traps or baits, and pesticides may not be available in developing countries in rural Africa. However there may be hope in monitoring these flies for Ebola by extracting DNA samples. When a fly feeds on tissue or bodily fluids from an animal carcass, it may pick up genetic material from that animal. If the tissue or fluid in that meal is infected with large enough amounts of Ebola, then DNA from Ebola may be present along with animal DNA. This genetic material would likely only be present for a short time before it is degraded by the fly’s digestive system, however, the potential exists to capture flies and survey for the presence of Ebola as well as what they have been feeding on. This would of course also depend on the level of Ebola infection in the tissue consumed. A team of researchers are currently sampling DNA from blow fly meals in southern Guinea to determine whether large mortality events of certain species have occurred in the area (Vogel, G. 2014).
Only after this most recent outbreak have scientists really started to ask the question “what is the role of insects in vectoring the Ebola virus”. In fact, some the most brilliant minds in Medical, Urban, and Veterinary Entomology (MUVE) will be gathering to brainstorm the possibilities this week at the Entomological Society of America. So, it won’t be too long until we have an answer. In the meantime, we’re willing to bet on flies playing an important role.
There are many filth-breeding flies, but here are 5 culprits for potential vectors of Ebola virus:
House Fly (Family Muscidae):
Blow Fly (Family Calliphoridae):
Flesh fly (Family Sarcophagidae):
Mosquito (Family (Culicidae):
Eye Gnat (Family Chloropidae):
Buss, L. Adult Liohippelates Sp [Photo] 2011. University of Florida. Web. 13 November 2014. http://entnemdept.ufl.edu/creatures/livestock/flies/liohippelates.htm
Getty Images. Digital Image of Ebola Virus [Cover Photo] 2014. New York Post. Web. 16 November 2014. http://nypost.com/2014/09/30/ebola-in-the-usa-texas-patient-has-deadly-disease/
Lahm, S., Kombila, M., Swanepoel, R., and Barnes, R. 2007. Morbidity and mortality of wild animals in relation to outbreaks of Ebola haemorrhagic fever in Gabon, 1994-2003. Transactions of the Royal Society of Tropical Medicine and Hygiene 101: 64-78
Leroy, E., Rouquet, P., Formenty, P., Souquière, S., Kilbourne, A., Froment, J-M., Barmejo, M., Smit, S., Karesh, W., Swanepoel, R., Zaki, S., and Rollin, E. 2004. Multiple Ebola transmission events and decline of central African wildlife. Science 303: 387-390
Sagripanti, J-L., Rom, A., and Holland, L. 2010. Persistence in darkness of various alphaviruses, Ebola virus, and Lassa virus deposited on solid surfaces. Archives of Virology 155(12): 2035-2039
Vogel, G. 2014. Are bats spreading Ebola across sub-saharan Africa? Science 344: 140