People usually get Rift Valley fever through contact with blood, body fluids, or tissues of infected animals, mainly livestock such as cattle, sheep, goats, buffalo, and camels. This direct contact can occur during slaughter or butchering, while caring for sick animals, during veterinary procedures like assisting an animal with giving birth, and when consuming raw or undercooked animal products.
People can also get RVF through bites from infected mosquitoes and, rarely, from other biting insects. Infection with the RVF virus (RVFV) has occurred in laboratories when someone has inhaled virus that was in the air (known as aerosol transmission). Spread from person to person has not been documented, and no transmission of RVF to health care workers has been reported when standard infection control precautions have been put in place.
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 allows more mosquito eggs to hatch.
- As mosquito populations increase, the potential for the virus to spread to animals and people 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.
Several mosquito species can spread RVFV, most commonly the Aedes and Culex mosquitoes, and these vary by region. Environmental conditions, particularly rainfall, are an important risk factor for outbreaks in both animals and people. RVF outbreaks are most often linked to years of unusually heavy rainfall and flooding, because mosquitoes spread the disease and heavy rainfall allows more mosquito eggs to hatch.