Risk factors, patterns and seropositivity of inter-epizootic Rift Valley fever virus of cattle in northern Tanzania
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Background
Rift Valley Fever (RVF) is a zoonotic viral disease affecting both animals and humans, with significant public health and economic implications. In Tanzania, a series of RVF outbreaks in livestock have been reported. The epidemiological information about hosts’ exposures status, time, location and risk factors that contribute to virus maintenance between one epizootic and the other, is important for preparedness and intervention plans. This study investigated the seroprevalence and potential determinants of RVFV exposure in cattle in Babati, Hai, and Moshi Districts between the last RVF outbreak in 2007 and the anticipated El Niño rains of 2024, which could precipitate the next outbreak.
Methods
A total of 1,627 cattle serum samples (790 from Babati, 518 from Moshi, and 319 from Hai), were collected between August and October 2023 from both urban and rural areas, were analyzed using a competitive ELISA to detect RVFV-specific antibodies. Data on cattle biodata, management practices, environmental factors, were obtained through structured questionnaires and field records. Sero-prevalence was determined by descriptive statistics, while risk factors were quantified by multivariable logistic regression and RVF seropositivity was plotted on administrative boundaries of Babati, Moshi, and Hai districts by using QGIS.
Results
The overall RVFV seroprevalence was 15.0% (95% CI: 13.26 – 16.73). Seropositivity was 22.2% (95% CI: 18.62-25.78) in Moshi, 12.2% (95% CI: 8.63-15.82) in Hai and 11.4% (95% CI 9.18-13.61) in Babati. Age (OR = 1.22, 95% CI: 1.15–1.3, p<0.001), sex (OR = 0.53, 95% CI: 0.36–0.79, p = 0.0017), herd size (OR=0.2201; 95% CI: 0.095–0.5101; p=0.0004), water access along irrigated crop fields (OR=3.7181; 95% CI: 1.5868–8.7121; p=0.0025), and geographical location (OR = 2.23, 95% CI: 1.63–3.06, p < 0.001) were significant predictors of seropositivity. Spatial visualization revealed clustering of seropositive herds in villages near lake Manyara and Tarangire National parks in Manyara region, also seropositive clusters in villages near irrigated areas, Nyumba ya Mungu dam and floodplains in lower Moshi.
Conclusion
This study highlights ongoing inter-epizootic exposure of cattle to RVFV in northern Tanzania. The RVFV is not randomly distributed during inter-epizootic periods, but rather forms seropositive clusters. Most significant determinants of seropositivity includes age, sex, herd size, water access along irrigated crop fields and geographical location Moshi showing higher seropositivity.
