Compendiums

Disease and the wildlife-livestock interface: some animals should not touch

New diseases in naive populations

When Europeans began to colonize North America, they brought many diseases such as smallpox, measles, chickenpox, and whooping cough along with them. As Europeans moved across the continent and encountered indigenous tribes, they introduced these foreign pathogens. Europeans were able to tolerate these diseases because they had been
exposed to them for hundreds of years and had developed strong immunities. Native Americans had never been exposed to any of these diseases and therefore, had no immunities, which led to massive epidemics and deaths of indigenous people. On average, 25 – 50% of native tribe members lost their lives when an epidemic hit a tribe. Though a
devastating part of history, this illustrates an important concept in disease ecology. The introduction of diseases, particularly by a population that is largely immune to them, can be devastating to populations that have never encountered
the disease.

Disease transmission between wildlife and livestock

One of the biggest concerns with wildlife and livestock interacting with each other is the introduction of a deadly pathogen into a naïve herd. Both livestock and wildlife can act as pathogen reservoirs, where a herd carries pathogens and parasites but isn’t negatively
affected. Built-up immunities allow a reservoir population to be a healthy host of pathogens the same way Europeans were able to carry smallpox and measles without evident die offs. When an individual from a clean population encounters a reservoir population, the clean individual is now infected and may infect the rest of
the clean herd.
Disease transmission between wildlife and livestock is bi-directional and can have consequences on both groups. Diseases can be transmitted between wildlife and livestock in two main ways: vector transmission and direct transmission. How a disease is transmitted influences our management options for the disease.

How are disease transmitted?

Direct transmission occurs when animals come close enough to spread the disease through direct contact.

 

Vector transmission occurs when a “vector” such as tick or hookworm carries a disease from one animal to another.

Examples of diseases transmitted between livestock and wildlife

What does this mean?

A pneumonia infected lung of a bighorn sheep lamb. Pneumonia was initially spread to bighorn sheep from domestic sheep and can have devastating effects of wild sheep populations.

The outcomes of disease infections vary between ungulate species. Although some
transmission results in ulcers and sores that an animal can recover from, other
transmission can cause complete die-offs of a herd. Outbreaks are a huge concern
for conservation and economic livelihood, particularly with increasing contact
between wildlife and livestock that results from increasing human development
and expansion.

There are multiple ways we can help reduce the risk of wildlife-livestock disease transfer. The most effective strategy is to completely eliminate contact between wildlife and livestock herds, especially for directly transmitted diseases; however, doing so is often impossible. When not possible, we can strive to mitigate transmission risk through, for example, fencing livestock, testing and vaccinating livestock (this option is less feasible for wildlife), implementing grazing allotment closures on a seasonal basis, and lethally removing wildlife that have contacted livestock. Nevertheless, each of these options requires large economic investments and is a large workload for livestock producers and wildlife managers. Through continued collaboration of biologists, wildlife managers, livestock owners, policy makers, and the general public, we can work to reduce likelihood of problematic disease transfers.

BIOLOGICAL WARFARE

The introduction of foreign pathogens to a naïve population can be so detrimental it historically has been used as a weapon of mass destruction. Examples include Mongols catapulting bodies of plague victims over the city walls of Caffa (1346) and Confederates selling clothing from smallpox patients to Union troops. During World War II, Japan killed hundreds of thousands of Chinese by dropping plague bombs onto cities. Many treaties and legislation attempt to prevent biological warfare; however, it still occurs.