Epidemiological Investigation of a Rift Valley Fever Outbreak in Humans and Livestock in Kenya, 2018
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Am. J. Trop. Med. Hyg., 103(4), 2020, pp. 1649–1655 doi:10.4269/ajtmh.20-0387 Copyright © 2020 by The American Society of Tropical Medicine and Hygiene Epidemiological Investigation of a Rift Valley Fever Outbreak in Humans and Livestock in Kenya, 2018 Abdala Hassan,1† Mathew Muturi,2*† Athman Mwatondo,2 Jack Omolo,2 Bernard Bett,3 Solomon Gikundi,4 Limbaso Konongoi,5 Victor Ofula,5 Lyndah Makayotto,6 Jacqueline Kasiti,7 Elizabeth Oele,1 Clayton Onyango,8 Zeinab Gura,1 Kariuki Njenga,9 and Peninah Munyua8 1Field Epidemiology and Laboratory Training Program, Ministry of Health, Nairobi, Kenya; 2Kenya Zoonotic Disease Unit, Nairobi, Kenya; 3International Livestock Research Institute, Nairobi, Kenya; 4National Public Health Laboratory Service, Nairobi, Kenya; 5Kenya Medical Research Institute, Center for Virus Research, Nairobi, Kenya; 6Division of Disease Surveillance and Response, Ministry of Health, Nairobi, Kenya; 7Directorate of Veterinary Services, Central Veterinary Laboratory, Nairobi, Kenya; 8Division of Global Health Protection, Centers for Disease Control and Prevention, Nairobi, Kenya; 9Washington State University Global Health Program-Kenya, Washington State University, Pullman, Washington Abstract. On the last week of May of 2018, a community-based syndromic surveillance system detected mass abortions and deaths of young livestock in northeastern Kenya. Two weeks later, Rift Valley fever (RVF) was confirmed in humans presenting with febrile illness and hemorrhagic syndrome in the same region. A joint animal and human response team carried out an investigation to characterize the outbreak and identify drivers of disease transmission. Here, we describe the outbreak investigation and findings. A total of 106 human cases were identified in the months of May and June 2018: 92% (98) and 8% (8) of these cases occurring in the northern and western regions of Kenya, respectively. Seventy-six (72%) were probable cases, and 30 (28%) were laboratory confirmed by ELISA and/or PCR. Among the confirmed cases, the median age was 27.5 years (interquartile range = 20), and 60% (18) were males. Overall, the case fatality rate was 7% (n = 8). The majority of the confirmed cases, 19 (63%), reported contact with livestock during slaughter and consumption of meat from sick animals. All confirmed cases had fever, 40% (12) presented with hemorrhagic syndrome, and 23% (7) presented with jaundice. Forty-three livestock herds with at least one suspect and/or confirmed animal case were identified. Death of young animals was reported in 93% (40) and abortions in 84% (36) of livestock herds. The outbreak is indicative of the emergence potential of RVF in traditionally high- and low-risk areas and the risk posed by zoonosis to livestock keepers. INTRODUCTION contaminated animal products. Human infection through bites of infected mosquito, although rare, has been reported.10 Rift Valley fever (RVF) is a vector-borne zoonosis caused by Acute human infections present as a self-limiting febrile illness Phlebovirus in the family Phenuiviridae. Globally, epidemics of with nonspecific symptoms; however, a small proportion of RVF are most frequent in the horn of Africa’s Rift Valley region cases proceed to a more severe disease that may present with and the Arabian Peninsula.1 Outbreaks, however, have been ocular complications, encephalitis, hemorrhagic fever, jaun- reported in islands off the East African coast: Madagascar and dice, and other signs of liver malfunction.11 French Island of Mayotte; parts of West, South, and North In Kenya, RVF is a priority zoonotic disease because of the Africa; and in the Arabian Peninsula countries of Saudi Arabia high morbidity and mortality, frequency of outbreak events, and Yemen, an indication of the potential for spread and global and socioeconomic impacts during outbreak events.12 The health security importance of the virus.2 In Kenya, outbreaks – are associated with heavy rainfall and flooding, which provides last major outbreak in 2006 2007 resulted in approximately ideal conditions for mosquito vector multiplication and, con- 340 human cases, 90 human deaths, and economic losses of sequently, disease emergence.3,4 As such, RVF epizootics are more than US$32 million in direct livestock mortality and in- 10,13 cyclic and periodic in nature, sometimes occurring as explo- direct losses, partly due to impediment to trade. As such, sive outbreaks that cause significant morbidity and mortality in early detection and response to RVF outbreaks in animals humans and animals.5 Mosquitoes of the Aedes species are before spillover to humans is a primary objective of the considered to be the primary vectors, whereas the Culex and country’s animal health surveillance system. Anopheles species and other biting flies have been reported to In May 2018, the Kenya Meteorological Department re- be the main secondary, amplifying vectors that propagate ported that some regions of the North Eastern region of Kenya transmission after emergence.6,7 Susceptible livestock, pri- had received three times the expected annual rainfall in the marily sheep, goats, cattle, and camels, are infected through period between March and May 2018.14 In the first week of bites of infected mosquitos and through mechanical trans- May 2018, the Kenya Directorate of Veterinary Services mission by biting flies.7 On infection, the disease in animals is (KDVS) activated a community-based, syndromic surveillance characterized by fever, abortion storms, and high mortality (SS) system to monitor occurrence of RVF-associated syn- rate, especially among young livestock.8,9 Spillover of in- dromes in livestock. This system had been previously used in fection from animals to humans occurs through direct contact 2015–2016 in 22 RVF high-risk counties in Kenya.15 with infected animal fluids and tissues and consumption of On May 25, 2018, reports of mass abortions and mortality of young sheep, camels, and goats were reported to the KDVS by the animal health services in Eldas subcounty, Wajir * Address correspondence to Mathew Muturi, Kenya Zoonotic County. By June 4, 2018, four human mortalities from the Disease Unit, P.O. Box 20811-00202, Nairobi, Kenya 00202. E-mail: [email protected] same subcounty, with a history of febrile and hemorrhagic † These authors contributed equally to this work. illness, were reported to national human health services. On 1649 1650 HASSAN, MUTURI, AND OTHERS FIGURE 1. Spatial distribution of human Rift Valley fever cases (probable and confirmed) by counties, Kenya, May–June 2018 (n = 106). This figure appears in color at www.ajtmh.org. June 7, 2018, patient samples were submitted for laboratory RVF was any person presenting in health facilities in any of the testing at the Kenya Medical Research Institute (KEMRI), affected counties with a fever (> 37.5°C) or a 2- to 6-day history Nairobi. Two of three patients tested positive for RVF virus by of fever of unknown origin and sudden onset of flu-like symp- reverse transcription–PCR (RT-PCR). A national outbreak of toms such as muscle pain, joint pains, and headache with or RVF was declared on June 7, 2018. And, consequently, a without ocular disease, meningoencephalitis, or hemorrhagic multidisciplinary field investigation team was deployed on fever. A probable case was a suspect case with close contact June 13, 2018, to investigate the outbreak and assist with with sick or dead livestock (cattle, goats, sheep, and camels) at response and control efforts. Here, we report findings from the least 14 days before the onset of illness or a person who died field investigation carried out among humans and livestock in with hemorrhagic-like signs between April and June 2018. Wajir, and subsequent outbreaks in humans in Marsabit and Contact with livestock was defined as drinking of unpasteurized Siaya counties, Kenya. milk or any activity that results in exposure to animal blood and body tissues including and not limited to slaughtering, care of METHODS sick animals, veterinary procedures, disposal of products of abortions, and assisted animal birthing procedures. Study site. The study was conducted in Wajir and Marsabit A confirmed case was a suspect case with laboratory counties in northeastern Kenya and Siaya County in south- confirmation of the presence of anti-RVF virus IgM by ELISA or western Kenya from May to June 2018 (Figure 1). RVF RNA by RT-PCR. Case detection and case ascertainment. The investigation We reviewed medical records for the 1-month period pre- case definition was adapted from the WHO.16 A suspect case of ceding the outbreak and conducted human active case finding RVF OUTBREAK IN HUMANS AND LIVESTOCK IN KENYA 1651 using community-level key informants and door-to-door case (anti-RVF hyperimmune mouse ascitic fluid) added to each ascertainment in affected areas in the three counties. Addi- well and incubated for 1 hour at 37°C. Plates were washed and tional cases were identified through snowball sampling, that 100 μL of goat anti-mouse IgG, heavy and light chain is, already identified suspected cases assisted with re- specific conjugate (Kirkergard & Perry, catalog 074-1806) cruitment of other people with similar symptoms within the added in all the wells and incubated for 1 hour at 37°C. The village. For livestock, the case definition was adapted from plates were washed and 100 μL of the ABTS substrate (Kir- the World Organization for Animal Health (OIE) terrestrial kergard & Perry, Cat. No. N8 50-62-00) added followed by 30- manual.17 A suspect livestock case was any animal presenting minute incubation at 37°C. The optical density (OD) value was or with a history of abortions or deaths of young animals read with a spectrophotometer at 405 nm and the adjusted (< 3 months old) or hemorrhagic syndrome in individual ani- OD calculated by subtracting the OD of the negative/mock mals. A confirmed herd was a suspected herd with a labora- antigen–coated wells from the positive antigen–coated wells. tory confirmation of the presence in serum of anti-RVF virus The OD cutoff was calculated as the mean of the adjusted OD IgM by ELISA in any individual animal within the herd.