Pathogens ecology drives host adaptation: the case of theileriosis

“We hope that our research will have an impact, because it addresses some major issues regarding food security in Africa over the medium and long term”, says Stéphane Joost, who works in EPFL’s Laboratory of Geographic Information Systems (LASIG), and is senior scientist of the study together with Paolo Ajmone Marsan from Università Cattolica del Sacro Cuore, Italy. The research was recently published in the scientific journal Frontiers in Genetics.

Theileriosis is a tick-borne cattle disease killing thousands of animals each year in eastern and central Africa. According to the Food and Agriculture Organization (FAO) of the United Nations, the disease imposes financial losses of 170 million dollars per year in the region. Those losses lead to food insecurity, and are partly responsible for rural depopulation. Current prophylaxis damages also environment, being based on pollutant acaricides. However, particular indigenous cattle populations are able to survive infection in endemic areas.

The method and the results

The study, in which LASIG post-doctoral researcher Elia Vajana was first author, started with the characterization of the parasite and vector ecological niches in Uganda, where tolerant cattle are reported. Maps have been derived by relying on epidemiological data from 823 georeferenced indigenous cattle to depict the risk of being bitten by a tick and the risk of infection by the parasite. Then, a "landscape genomics" approach has been undertaken to identify candidate genes for theileriosis tolerance, by combining risk information with cattle genetic characteristics. In particular, the research documents the possible involvement of two genes, by possibly paving the way for sustainable breeding programs exploiting such a natural disease tolerance. The results can help local veterinarians – who took part in the sampling campaign – to identify the riskiest areas, and to inform farmers’ associations about which breeds to raise, depending on the region considered.

Why cross-breeding is a mistake

For the researchers, the study shows the importance of preventing indigenous cattle from crossing with industrial breeds, more productive but not adapted to the local habitats and their endemic diseases. In the case of theileriosis, indigenous zebu populations, that migrated from India to Africa around 4,000 years ago, show a natural resistance towards the tick bite, whereas European breed are unable to counteract tick burden, and die just a few weeks after the parasite injection. “Our study shows that the cross-breeding carried out in Africa in the last few years – pushed by European corporations with the promise of higher yields in terms of milk and meat – is a mistake based on a short-term approach. Diluting the gene pool of indigenous breeds is reducing the cattle’s natural tolerance to this disease,” say the researchers.

The NextGen project

The study is based on a strict collaboration between EPFL, Università Cattolica del Sacro Cuore, and Makerere University, and was carried out in the context of the European Union’s research project NextGen (FP7/2010-2014). According to Joost: “The study involved large amounts of environmental data produced by meteorological stations and satellites, and large amounts of genetic and epidemiological data collected on the ground as part of the NextGen project. Extensive IT resources were also used to develop the statistical models and produce the results. This field of ‘Geocomputational Molecular Ecology’ combines some of the unique environmental engineering skills that we have developed here at EPFL”.