PPR risk associated with herd management system in Tanzania

A study published in Epidemiology and Infection provides an updated picture of the prevalence of Peste-des-petits-ruminants virus (PPRV) in northern Tanzania.

According to the research team, livestock managed in a system where they are the sole source of an owners’ livelihood are more likely to become infected with PPRV than livestock managed in a system where the owners’ livelihood is supplemented by agriculture. Additionally, the presence of cattle may affect infection risk, even though they are not typically considered important hosts for the virus.

The study was conducted by researchers at Pennsylvania State University, the University of Glasgow, and the Nelson Mandela African Institute of Science and Technology.

The researchers surveyed livestock across 20 villages in northern Tanzania for evidence of past infection, comparing rates in herds from pastoral villages, where people rely almost solely on livestock, and from agropastoral villages, where people rely on a mix of livestock and agriculture.

Catherine Herzog, first author of the paper, said, “According to our models, herds from pastoral villages had 3.8 times the risk of becoming infected and developing detectable antibodies compared to those from agropastoral villages. If you look only at herds of sheep or only at herds of goats instead of mixed-species herds, that risk increases to 9.4 or 9.5 times higher, respectively, for pastoral systems when compared to agropastoral systems.”

Female sheep and goats also had 1.5 times higher risk of becoming infected than males. Understanding prevalence and infection risk could improve the researcher’s ability to predict how the disease will spread and to refine management techniques to minimize disease risk.

“Next we plan to investigate which aspects of the livestock management system within a pastoral herd may be driving this increased risk,” said co-author Ottar Bjørnstad, who led the research team. “For example, herd size, herd age structure, contact rates among livestock and wildlife, and access to veterinary service may all play a role in infection risk. Knowing which animals are at greater risk of infection may affect how we allocate resources to manage prevention strategies.”

Although PPRV is generally considered to be a sheep and goat virus, the research team also considered cattle in their surveys and analysis.

“Cattle are often managed alongside sheep and goats and can become infected, although it is unclear if cattle can transmit the disease,” said Herzog. “We found that the rate at which animals become infected — force of infection — was related between sheep and cattle and between goats and cattle. This is consistent with the idea that cattle play a role in transmission to sheep and goats, or, alternatively, that an unknown external factor is affecting transmission in each species in a similar way.”

To further explore the role of cattle in transmission, the researchers plan to perform experimental transmission trials with all three species, working with collaborators in Ethiopia.

Article: Herzog, C.M., de Glanville, W.A., Willett, B.J., Kibona, T.J., Cattadori, I.M., Kapur, V., Hudson, P.J., Buza, J., Cleaveland, S., Bjørnstad, O.N. (2019). Pastoral production is associated with increased peste des petits ruminants seroprevalence in northern Tanzania across sheep, goats and cattle. Epidemiology and Infection, 147:e242, doi: 10.1017/S0950268819001262

[SOURCE: Pennsylvania State University]