People are infected with RVF virus (RVFV) through contact with blood, body fluids, or tissues of RVFV-infected animals, mainly livestock. This direct contact with infected animals can occur during slaughter or veterinary procedures, like assisting the animal with giving birth. Infection through breathing in droplets contaminated with RVFV (aerosol transmission) has occurred in the laboratory setting. Less commonly, people can be infected with RVFV from bites of infected mosquitoes and, rarely, from other biting insects that have the virus on their mouthparts. Spread from person to person has not been documented.
Several mosquito species can spread RVF virus and these vary by region. Environmental influences, particularly rainfall, are an important risk factor for outbreaks in both animals and people. RVF outbreaks have been observed during years with unusually heavy rainfall and localized flooding.
The transmission cycle of RVFV can look like this:
- The virus can be spread from female mosquitos to their offspring through the eggs (vertical transmission).
- In the eggs, the virus remains viable (infectious) for several years during dry conditions.
- Excessive rainfall enables more mosquito eggs to hatch.
- As mosquito populations increase, the potential for the virus to spread to the animals and people also increases
- RVFV outbreaks in animals, most commonly livestock, lead to increased handling of infected animals, which then increases risk of exposure to the virus for people.