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Rift Valley fever virus (RVFV) is a mosquito-borne zoonotic pathogen which can cause hemorrhagic fever, neurological disorders or blindness in humans, as well as a high rate of abortion in ruminants. Even though this virus is native to sub-Saharan Africa, upon introduction of the virus in certain environment the transmission will occur whenever sheep and cattle are present with abundant mosquito populations.
Between October and December 2006, an unusually heavy rainfall was recorded in eastern Africa, which resulted in floods across several regions of Kenya, Somalia and the United Republic of Tanzania. That event created ideal conditions for the breeding and hatching of flood-water Aedes mosquitoes, which are principal vectors and interepizootic reservoirs for Rift Valley fever (RVF).
Soon after that a serious outbreak ensued which resulted in considerable suffering and death, but also economic hardship across East Africa. Both direct and indirect costs were present, most notably in the domains of patient management, outbreak response, public health control interventions and household losses.
From 30 November 2006 to 12 March 2007, a total of 684 cases (including 155 deaths) of RVF was reported in Kenya, which translates into 23% case-fatality rate. 234 of those cases (or 34%) were confirmed in the laboratory by the presence of IgM antibodies or by employing reverse transcription polymerase chain reaction (RT-PCR).
The majority of cases occurred in young herdsmen with a history of contact with sick ruminant animals. Most of the cases were confirmed in North Eastern Province (333), followed by Rift Valley Province and the Coast Province (183 and 141, respectively), and only a fraction of them were recorded in Central Province and the Eastern Province (14 and 13, respectively).
The response to the outbreak was initially organized by the Kenya Field Epidemiology and Laboratory Training Program and Kenyan Ministry of Health, in collaboration with the Kenya Medical Research Institute, World Health Organization, CDC's Emerging Infections Program and Médecins Sans Frontières International.
From 19 December 2006 to 20 February 2007, a total of 114 cases (including 51 deaths) of RVF were reported in Somalia, which translates into 45% case-fatality rate. Nevertheless, merely three cases were laboratory confirmed, either by IgM antibody detection or RT-PCR.
Cases were geographically distributed in the Lower and Middle Juba region, Gedo region, Hiran region, as well as Middle and Lower Shabelle region. During the outbreak response, the strenuous security situation has repeatedly hampered surveillance and control activities in the affected regions.
Apart from heavy rainfall, other factors were likely responsible for the occurrence of RVF outbreak in Kenya and Somalia, most notably high Aedes mosquito populations, flat topology of the area and water-retaining soil that supports flooding, dense bush cover, and high livestock populations.
Albeit comparison of the genetic composition of RVFVs involved in the 2006–2007 epidemic with the older isolates from the period between 1944 and 2002 did not reveal any major genetic changes in this pathogen, it is important to be aware of the possibility that the virulence of the virus may be altered by substitution of only a few fundamental amino acids.
It is clear that an integrated human, animal and environmental health system is necessary to address requirements for adequate preparedness. Instead of pursuing entirely separate health systems, there should be intentional and functional collaborations, which represent an interdisciplinary concept also known as 'One Health'.