Wolves shown to help control tuberculosis in Asturias
A study published in a prestigeous journal Nature (Tanner et al., 2019) found that wolves perform a crucial function in the ecosystem – by selective predation, they can keep the levels of disease prevalence at bay. Thus, in Asturias, northern Spain, there is a much lower level of tuberculosis in wild boars where wolves are present. In this way, there is also less transmission of this deadly disease to livestock.
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Tuberculosis is an infectious chronic bacterial disease that can be transmitted not only between different mammal species, but also to humans. The symptoms only appear after a few months and include weight loss, night fever, cough and weakness. In animals, the disease is transmitted through close contact or in water or food. Deer, wild boar and badgers are frequent reservoirs of the disease.
The study used field data from 2000 to 2014 and modelling methods to compare how wild boar populations and tuberculosis levels change through time in areas with and without wolves. in the 14-year period of field data, the wolf density increased, but this did not prevent the wild boar density to increase as well. At the same time, in areas with wolves, the tuberculosis prevalence decreased from 17% in 2000 to just 3.8% in 2014. This points to the fact that the higher predation due to greater wolf density was not limiting wild boar growth. That is likely because mortality from wolf replaced the deaths caused by tuberculosis.
Increase in tuberculosis without wolves
In the part of Asturias where wolves are not present, the wild boar populatoin tripled in the same time period. There, the disease prevalence increased from 3% to 7%. This, with the accompanying increase in density, means that transmissions to livestock are about 500% more likely.
A model was created to compare how wild boar populations and tuberculosis prevalence change if wolf population is stable from 2014, or if it declines to extinction by 2042. If the population is stable, the boar density would increase slightly but would soon stabilise due to wolf predation. The disease levels would be very low. On the other hand, if wolves declined as a result of deliberate removal, the boar populations would first increase. Soon, however, the population would start declining again as a result of mortality from tuberculosis. In the end, the boar population size with and without wolf would be very similar.
Selective predation an important ecosystem service
This positive effect wolves have on the reduction in tuberculosis is a result of their selective predation. They most often prey on sickly individuals, therefore those transmitting tuberculosis. The other prey group are piglets, who tend to be more susceptible to disease than mature individuals.
When tuberculosis spreads from wildlife to livestock, it causes great costs as the animals need to be slaughtered. As a result, a lot of efforts are put into preventing tuberculosis not just in Spain, but also in many other countries. Therefore, disease control by wolves is a vey important ecosystem service. While the wolves occasinally depredate livestock and cause economic damage in this way, the compensation payments add up to only a quarter of what is invested into tuberculosis prevention schemes. Therefore, conserving the wolf may be the most cost-effective way of disease prevention.
Tanner, E., White, A., Acevedo, P., Balseiro, A., Marcos, J., & Gortázar, C. (2019). Wolves contribute to disease control in a multi-host system. Scientific Reports, 9(1), 1–12. https://doi.org/10.1038/s41598-019-44148-9