44 Transmission

There are multiple modes of transmission that allow zoonotic diseases to jump between animal and human populations. Direct contact involves directly touching saliva, blood, urine, feces, or other bodily fluids from an infected animal or human carrier. Ebola is often transmitted from human to human through direct contact with fluids – interacting with an infected individual’s bedding provides a high risk of contracting the disease. Zoonoses can also spread through vectors – animals who are not themselves infected, but somehow carry infectious material with them. These vectors are typically mosquitoes, like the ones who carry malaria throughout the world’s tropical regions. Finally, zoonoses can be spread indirectly through food or the air. Contaminated foods, like unpasteurized milk, undercooked meat or eggs, or even unwashed fruits and vegetables can become a source of infection. (NC)

Controlling these methods of transmission is important to reducing the incidence of new disease in humans. One of the most important ways that this can be achieved is by reducing human-animal interactions. Habitat destruction and urbanization have been driving an increase in these potentially disease-spreading interactions. As human populations increase, we seek to increase our land footprint to build housing, farms, and other services necessary to support that growth. This inevitably expands into wildlife habitat, displacing organisms. Sometimes, these animals adapt, finding ways to live in cities. This further increases the chance of human-animal contact, risking transmission of dangerous pathogens. Decreasing forest cover has been linked to an increasing incidence of disease, particularly in biodiverse tropical areas. (NC)

Specifically, human encroachment into wild places is increasing the prevalence of lyme disease. Lyme disease is spread to humans by deer ticks, who live in forested areas and feed on the blood of small rodents, like chipmunks and squirrels. These animals become reservoirs for the disease, not becoming infected but allowing each tick that feeds on them to become a carrier. Through this mechanism, more ticks become infectious and can pass the disease to humans who enter their habitats. Historically, these rodent populations have been kept under control

by large predators, like foxes, decreasing the prevalence of Lyme disease in small animal populations. As humans have expanded and developed suburban areas, large forests have become fragmented. These forests, now significantly reduced in size, still house lots of small rodents but can no longer support predatory species. Thus, rodent populations rapidly grow, increasing the reservoir for lyme. When humans enter these environments or when ticks spread close to their homes, they have a significantly increased risk of contracting the disease. (NC).