The influence of raw milk exposures on Rift Valley fever virus transmission

Abstract

Rift Valley fever virus (RVFV) is a zoonotic phlebovirus that can be transmitted to humans or livestock by mosquitoes or through direct contact with contaminated bodily fluids and tissues. Exposure to bodily fluids and tissues varies by types of behaviors engaged for occupational tasks, homestead responsibilities, or use in dietary or therapeutic capacities. While previous studies have included milk exposures in their analyses, their primary focus on livestock exposures has been on animal handling, breeding, and slaughter. We analyzed data from multiple field surveys in Kenya with the aim of associating RVFV infection to raw milk exposures from common animal species. Of those with evidence of prior RVFV infection by serology (n = 267), 77.2% engaged in milking livestock compared to 32.0% for 3,956 co-local seronegative individuals (p < 0.001), and 86.5% of seropositive individuals consumed raw milk compared to 33.4% seronegative individuals (p < 0.001). Individuals who milked and also consumed raw milk had greater odds of RVFV exposure than individuals whose only contact to raw milk was through milking. Increased risks were associated with exposure to milk sourced from cows (p < 0.001), sheep (p < 0.001), and goats (p < 0.001), but not camels (p = 0.98 for consuming, p = 0.21 for milking). Our data suggest that exposure to raw milk may contribute to a significant number of cases of RVFV, especially during outbreaks and in endemic areas, and that some animal species may be associated with a higher risk for RVFV exposure. Livestock trade is regulated to limit RVFV spread from endemic areas, yet further interventions designed to fully understand the risk of RVFV exposure from raw milk are imperative.

Fig 1. Study area.

Kenya is identified in the red square in the upper left map of Africa and surrounding geographical areas. The main map is a close-up map of Kenya that specifies the locations of each region or village wherein participants were enrolled and surveyed to be included in the original studies. Each village region is indicated by a red circle and labeled with the region or village name. Created in QGIS 2.18.11 using MapBox.

Fig 2

Forest plot of milking exposure (A) and raw milk consumption (B) by animal type. Odds ratios reported with CI₉₅. Any milking (A) or consumption (B) includes exposure to any or all animal types detailed in the questionnaire. No other animal types were reported. Sheep or goats reported for milking exposures were conflated into a category described as “shoats” in the questionnaires administered to participants in Busia, but sheep and goats were referred to separately in questionnaires utilized in all other regions. Therefore, all respondents reporting milking behavior with “shoats”, “sheep”, or “goats” were confounded in an inclusive category of “sheep or goats” for the purpose of this analysis.