First Laboratory-Confirmed Outbreak of Human and Animal Rift Valley Fever Virus in Uganda in 48 Years

Rift Valley fever (RVF) is caused by RVF virus (RVFV), a single-stranded RNA virus in the Bunyavirales order, Phenuiviridae family, and Phlebovirus genus. It is an emerging epidemic disease of humans and livestock and an important endemic problem in sub-Saharan Africa.1 Rift Valley fever virus is transmitted to livestock and humans by the bite of infected mosquitoes or exposure to tissues or blood of infected animals.2 Interepidemic virus maintenance is thought to occur either transovarially in Aedes species mosquitoes or through cycling of low-level transmission between domestic livestock or wild ungulates and mosquitoes.3 After periods of heavy rainfall, Aedes mosquitoes rapidly emerge, resulting in extensive amplification of the virus through infection of livestock.2 Presentation of RVF in animals can vary among species with a range of clinical severity. Livestock, particularly cattle, sheep, and goats are highly susceptible to RVFV and present with symptoms of fever, loss of appetite, weakness, low milk production, nasal discharge, vomiting, and diarrhea. During large epizootics, “abortion storms,” particularly in sheep and cattle, have been identified. High newborn mortality (80–100%) and adult mortality (5–20%) may also be observed.3 Humans infected with RVFV typically have mild, self-limited febrile illness, but can present in a small number of cases (< 8%) with severe jaundice, rhinitis, encephalitis, and hemorrhagic manifestations. Retinal degeneration (5–10% of cases), hemorrhagic fever (< 1%), or encephalitis (< 1%) may also develop.4,5 Outbreaks of RVF have been reported most frequently in East Africa, especially Kenya, Somalia, and Tanzania, with the last major outbreak in the region recorded in 1997–1998. However, outbreaks have also been reported in other African countries, including Egypt, South Africa, Madagascar, and Senegal.6 In 2000, the first RVF outbreak outside of Africa was reported in Saudi Arabia and Yemen.5,7 More recently, Kenya and Tanzania experienced a large epizootic of RVFV in 2006–2007 and Sudan in 2010.6 The emergence of RVFV in East Africa resulted in widespread livestock morbidity and mortality, with hundreds of laboratory-confirmed human cases, and likely thousands of asymptomatic or mild RVFV human infections going undetected.8,9 Rift Valley fever virus in Uganda was first detected in mosquitos collected in Semliki forest, Western Uganda, in 1944,10 and has since been detected several times by the East African Virus Research Institute (EAVRI), now the Uganda Virus Research Institute (UVRI). Human cases were recorded during outbreaks occurring near Entebbe in 1960 and 1962.11,12 Since then, serological evidence of human and livestock RVFV infections in Uganda have been intermittently reported,13,14 but the last published description of acute human RVFV infection was seven cases occurring near Entebbe in 1968.15 On March 10, 2016, UVRI and the Uganda Ministry of Health (MOH) received a report of a suspected viral hemorrhagic fever (VHF) case presenting to Kabale Regional Referral Hospital (KRRH) in Kabale district in southwestern Uganda. The initial case was of a 48-year-old male butcher who had been working in a local abattoir, where livestock were brought for slaughter. The patient presented with a history of fever, vomiting, diarrhea, headache, and hemorrhagic symptoms (bleeding gums, epistaxis, bloody urine, and stools). A blood sample was taken and sent to the UVRI VHF laboratory for testing. On March 11, a second suspected VHF case presented to KRRH with similar symptoms; a blood sample was collected for testing. This patient was a 16-year-old male student first reported by Kabale District Health Office from the Uganda–Rwanda border village of Katuna. Both samples were tested for hemorrhagic fever viruses, including Ebola viruses, Marburg virus, Crimean–Congo hemorrhagic fever virus, and RVFV. Both specimens were found to be positive for RVFV by reverse transcriptase polymerase chain reaction (RT-PCR) and IgM serology.