Ebola Virus Disease (EVD) is a severe illness caused by a group of viruses in the genus Orthoebolavirus, known for causing hemorrhagic fever with a high fatality rate. A frequent query during outbreaks is whether common biting insects, such as mosquitoes, can play a role in spreading the disease. The scientific consensus is definitive on this question.
Defining Viral Transmission Pathways
For any virus to be considered “mosquito-borne,” a complex biological relationship known as biological transmission must exist between the pathogen and the insect vector. In biological transmission, the virus must be able to survive the mosquito’s digestive system, replicate within its cells, and eventually migrate to the salivary glands. This is distinct from simple mechanical transmission.
A pathogen that is simply transferred on the mouthparts of an arthropod without replication is considered mechanical transmission. This is similar to a contaminated needle transferring blood from one person to another. However, mosquitoes do not retain infectious blood droplets on their mouthparts between bites.
To successfully transmit a virus, the pathogen must complete the extrinsic incubation period (EIP) within the mosquito host. The EIP is the time required for the virus to multiply and disseminate from the mosquito’s midgut to the salivary glands. Only after this process is complete can the virus be injected into a new host along with the mosquito’s saliva during a subsequent blood meal.
Why Mosquitoes Cannot Transmit Ebola
The Ebola virus is biologically incompatible with the mosquito’s internal environment, preventing it from completing the necessary transmission cycle. When a mosquito ingests infected blood, the virus must first overcome the midgut barrier. The virus cannot survive the harsh digestive conditions within the mosquito’s gut, which effectively inactivates the viral particles.
Even if the virus survived digestion, it lacks the biological mechanisms needed to infect mosquito cells and replicate efficiently. This lack of replication prevents the virus from reaching the high concentrations needed for dissemination.
Without successful replication, the virus cannot travel to the salivary glands. The inability to infect the salivary glands means the virus cannot be introduced into a new host during biting. The biological pathway required for transmission is blocked at multiple points.
Furthermore, the amount of virus transferred during a bite is extremely small, far less than the high viral load required for Ebola infection. Ebola is spread through direct contact with substantial amounts of infectious body fluids, not the minute amounts transferred by an insect.
The Confirmed Vectors and Spread of Ebola
The actual reservoir and spread of the Ebola virus rely on mammalian hosts and direct human-to-human contact. Scientists believe that African fruit bats of the Pteropodidae family are the most likely natural reservoir for the virus. Transmission to humans occurs through a spillover event involving close contact with the blood, secretions, organs, or other bodily fluids of infected animals.
Spillover events often happen through practices like hunting, handling, or consuming infected animals, such as chimpanzees, gorillas, or forest antelopes. Once a human is infected, subsequent spread occurs via person-to-person transmission through direct contact with the blood or body fluids of a person who is sick or has died from EVD.
These infectious body fluids include urine, saliva, sweat, feces, vomit, breast milk, and semen, which contain high concentrations of the virus. Transmission requires contact with broken skin or mucous membranes, such as the eyes, nose, or mouth. The virus can also spread through contact with contaminated objects or surfaces (fomites), including clothing, bedding, or medical equipment.
The virus remains highly infectious as long as the person is symptomatic. It can persist in immune-privileged sites, such as the testicles, even after recovery. This persistence means the virus can be transmitted through semen for many months. This mechanism of contact transmission dictates the public health response, focusing on infection prevention and control measures, including safe burial practices.