Patterns of Rift Valley fever virus seropositivity in domestic ruminants in central South Africa four years after a large outbreak

Abstract

Rift Valley fever (RVf) is a mosquito-borne viral zoonosis showing complex epidemiological patternsthat are poorly understood in South Africa. Large outbreaks occur in the central interior at long, irregular intervals, most recently in 2010-2011; however, the level of herd immunity of ruminant livestock, a key determinant of outbreaks, is unknown. During 2015-2016 a cross-sectional study on 234 randomly-selected farms investigated the prevalence, patterns of, and factors associated with, antibodies to RVF virus (RVFV) in livestock in an area heavily affected by that outbreak. A RVFV inhibition ELISA was used to screen 977 cattle, 1,549 sheep and 523 goats and information on potential risk factors was collected using a comprehensive questionnaire. the estimated RVfV seroprevalence, adjusted for survey design, was 42.9% in cattle, 28.0% in sheep and 9.3% in goats, showing a high degree of farm-level clustering. Seroprevalence increased with age and was higher on private vs. communal land, on farms with seasonal pans (temporary, shallow wetlands) and perennial rivers and in recently vaccinated animals. Seropositivity amongst unvaccinated animals born after the last outbreak indicates likely viral circulation during the post-epidemic period. the current level of herd immunity in livestock may be insufficient to prevent another large outbreak, should suitable conditions recur.Rift Valley fever (RVF) is an arthropod-borne viral zoonosis caused by RVF virus (RVFV), a member of the Phlebovirus genus, family Phenuiviridae of the recently established order Bunyavirales 1 . RVF is endemic primarily in sub-Saharan Africa but has crossed several barriers including the Sahara to Egypt, the Red Sea to the Arabian Peninsula and the Indian Ocean to the Comoros, Mayotte and Madagascar 2-4 . Spread through infected animals and mosquitoes to countries where the disease is not endemic is increasingly possible, with more frequent export of livestock from Africa to other countries and the presence of known or potentially competent vectors in those countries 4,5 . RVFV causes sporadic outbreaks with high morbidity and mortality, characterized by abortion storms and high mortalities in neonatal sheep, cattle and goats 6,7 . Human infection generally occurs concurrently with disease outbreaks in domestic ruminants 2,5,8 , where humans work or live in close contact with livestock.Outbreaks tend to occur following above average rainfall and localized flooding 9 . These climatic conditions favour breeding of floodwater mosquitoes that are the proposed maintenance vectors of this virus via transovarial transmission 10 . The floodwater mosquitoes considered to be vectors on the interior plateau of South Africa are those in the Aedes subgenera Ochlerotatus and Neomelaniconion 11 . Outbreaks may then be amplified by epidemic vectors, of which Culex theileri is considered the most important on the interior plateau 12 . Risk factor studies conducted during and after outbreaks in both humans and animals have identified several other environmental, ...

First Serological Evidence of Crimean-Congo Hemorrhagic Fever Virus and Rift Valley Fever Virus in Ruminants in Tunisia

“…Our results agree with a previously reported study in northern Somalia, suggesting that young sheep are more susceptible to RVFV infection [ 80 ]. In contrast, other studies have shown that RVFV seroprevalence is higher in adult animals [ 81 , 82 , 83 ].…”

Risk factors associated with exposure to Crimean-Congo haemorrhagic fever virus in animal workers and cattle, and molecular detection in ticks, South Africa

“…As no comprehensive list of farms was available, farms were randomly selected within the study area using random geographic coordinates generated in ARCGIS 10.2 (Esri, Redlands, CA, USA) and projected on Google Earth (Google LLC, Mountain View, CA, USA). The coordinates were generated with probability proportional to the number of livestock-owning households for each small area of the 2011 National Census [ 25 , 26 ]. The nearest livestock farm to each selected point was approached for participation in the study.…”

“…Up to nine cattle per farm were sampled from the jugular or coccygeal veins while restrained in a head gate by the veterinary team. Systematic random sampling was used, or when not possible a combination of haphazard and convenience sampling, to select three apparently healthy animals from each of the following age groups where possible: 6 months to 2 years, 2–4 years and >4 years [ 26 ]. Hyalomma ticks were collected from their predilection sites on the perineum and the tail tip using thumb forceps and then transferred to a 15 ml pierced vial.…”

Silent Circulation of Rift Valley Fever in Humans, Botswana, 2013–2014

“…Both IgG-and IgM-seropositive samples were found across sampled years, but no significant differences could be detected by year of testing (IgG and IgM χ 2 = 0.27; p = 0.60) or season (seasonal data for IgG only available for 2013; χ 2 = 0.98; p = 0.32). However, all IgM-positive samples (n = 7) were obtained during the wet season (November 2013-February In South Africa, large RVFV outbreaks in livestock have occurred every 20-30 years, and concomitant infection in humans have occurred during these periods (10). In Botswana, no human infections have been recorded, nor have large outbreaks in livestock occurred, suggesting that the dynamics of RVFV transmission and persistence differ between these countries.…”