Rift Valley Fever
Rift Valley Fever (RVF) is a bunyavirus that is predominately transmitted via
mosquitoes to domestic livestock and human hosts. It was first recognized in
the 1900's in Kenya, Africa among domestic sheep. In 1931, the agent causing RVF
was isolated from infected sheep.
RVF is endemic to parts of Africa with tropical and sub-tropical environments.
Outbreaks typically occur or peak in late summer especially after periods of
heavy or unusual amounts of rainfall. In 1997-1998, Kenya experienced the
largest outbreak reported. The outbreak involved 89,000 human cases and 478
human deaths. The first outbreak outside of Africa occurred in Saudi Arabia in
2000-2001, which resulted in 683 human hospitalizations and 95 deaths. More
recently, in 2003 Egypt experienced an outbreak that consisted of 45 cases and
17 deaths. In 2006-2007, Tanzania and Somalia experienced 1000 human cases and
300 deaths from an outbreak that was associated with increased amounts of
rainfall in the region.
Map of Rift Valley Fever endemic areas (from CDC)
Those primarily affected are domestic livestock and humans in endemic areas
of Africa. Mosquitoes act as the vector for RVF and transmit the virus by
feeding on the potential host. Once a host is infected they have the potential
to infect other mosquito vectors via a blood meal. Aedes species mosquitoes are
the primary vector. However, the Anopheles, Culex, and possibly other mosquito
species can also become infected and act as vectors for the virus. Mosquitoes
are not the only method of transmission for humans. Humans can also become
infected through close contact with infected animals. This typically occurs via
animal slaughter, cleaning, obstetrical exams or birthing assistance,
un-pasteurized animal products, or aerosolized virus particles.
Aedes aegypti
Once a human host is infected the incubation period to the onset of illness
is about 2-6 days. Humans experience flu-like symptoms, which may include fever,
weakness, weight loss, dizziness, headache, nausea, vomiting, and muscle and
joint pain. However, about 1% of those infected experience more severe symptoms.
These symptoms include retinal inflammation, brain inflammation, or hemorrhagic
fever. Onset of hemorrhagic fever results in 50% mortality in about 3-6 days
after infection of the host. Treatment typically includes supportive care and
the use of experimental antiviral drugs such as Ribavirin. At this time the United
States does not have a licensed human or animal vaccine.
The virus is typically recognized in domestic animal populations through
symptoms consisting of elevated temperatures, increased rates of abortions, and
high mortality rates among young animals.
Although RVF is currently not present in Florida, the state of Florida has
optimal conditions to support such a virus. Florida has tropical and
sub-tropical areas throughout the state, where increased amounts of rainfall can
aid in increasing vector populations. Florida also has the same species of
mosquito vectors that cause the endemic rates of RVF in parts of Africa. Methods
of RVF introduction into Florida can be through trade routes and may include
importation of infected animals or an infected mosquito vector. Due to the
potential for RVF to become endemic if introduced into Florida, surveillance of
this and other diseases is important to prevent the spread of disease.
Methods of prevention of RVF and other mosquito borne diseases outbreaks include
taking proper protective measures against mosquitoes.
Resources
Florida Agricultural Response Team:
http://www.flsart.org/rvf/
Centers for Disease Control:
http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/Fact_Sheets/Rift Valley
Fever Fact Sheet.pdf
World Health Organization:
http://www.who.int/csr/disease/riftvalleyfev/en/index.html
Sissoko, Daouda. et. al. Rift Vally Fever, Mayotte, 2007-2008. Emerging
Infectious Disease. Vol.15, No.4; 568-570: 2009.
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