We know that nature and health are tied together intrinsically but there’s more to it than fresh air and clean water. The way that wildlife habitat protects humans from disease—and the connection between the destruction of wildlife habitat and the emergence of disease—is vastly under reported but essential to human health.
In nature, diseases and animals develop and evolve together, so the diseases often have little to no effect on the animals that carry them. However, when transmitted to humans, these diseases can be devastating. These kinds of diseases, which can be transmitted from animals to humans, are called zoonotic diseases or zoonoses. According to the Centre of Public Health and Zoonoses at the University of Guelph, about 60 percent of infectious diseases are zoonotic and about 75 percent of emerging infectious diseases, which have recently been discovered, increased in incidence or spread in reach, are zoonotic.
Disease is a normal and necessary part of biodiversity, balance and resilience in the natural world but zoonotic disease has been on the rise for the last hundred years and has jumped in the last couple decades. The way zoonotic diseases function has mostly remained the same but, during this time, humans have changed the environment in which these diseases operate.
Habitat destruction, degradation and fragmentation, as a result of human activities, such as urban expansion and agriculture, has forced wildlife out of their natural habitat areas and into more urban areas. This creates more opportunities for animals and humans to interact and presents zoonotic diseases with opportunities to escape their natural habitat. Destroying the habitat area of species that carry zoonotic diseases increases the chances of these species coming in contact with humans and transmitting the diseases that they carry. For instance, research has shown that the risk of Lyme disease increases when urban and suburban housing developments cause habitat fragmentation.
In Canada, Lyme disease and West Nile virus are two zoonotic diseases that have been on the rise in recent years, due to the range expansion of ticks and mosquitos. Range expansion is what occurs when a species expands the habitat area that it occupies, often as a result of habitat loss or climate change.
Lyme disease is a zoonotic disease that is transmitted from ticks to humans through the bite of infected ticks. Reported cases of Lyme disease in Canada have increased from 64 cases in 2005 to 987 cases in 2016, although these numbers under represent true infection rates as most cases are not reported. Lyme disease and other tick-borne diseases have been on the rise in Ontario due to the range expansion of ticks from the United States to Canada. Tick habitat range has been expanding faster than predicted and is expected to expand even faster in the coming years as climate change continues to impact habitat conditions.
West Nile Virus
West Nile virus is another zoonotic disease that is transmitted from birds to humans through the bite of infected mosquitoes. West Nile virus first emerged in Ontario in 2001 and then again in 2012. During this time, there were 5,339 West Nile virus cases reported in Canada, although these numbers under represent true infection rates as most cases are not reported. West Nile virus and other mosquito-borne diseases have been on the rise in Ontario due to climate change, as warm winters and wet springs become more common, and destruction of bird and mosquito habitat.
Another zoonotic disease often found in Ontario is rabies, which is spread to humans through contact with the saliva of infected animals, either through bites or scratches. After almost a decade without any cases, rabies returned to Ontario in 2015 when a number of cases were reported in the Hamilton area. This spread of rabies then reached raccoons and skunks in Burlington, where eight cases were confirmed in 2016 and 18 cases were confirmed in 2017. This recurrence in rabies prompted the government of Ontario to conduct rabies vaccine baiting for raccoons, skunks and foxes.
Some other zoonotic diseases include hantavirus, which spreads to humans through infected rodent urine, feces and or saliva, tularemia, which spreads through infected deer flies and ticks that have been on dead rodents, such as rats and rabbits, raccoon roundworm, which is an intestinal parasitic infection that spreads through infected raccoon feces and histoplasmosis, which is a respiratory disease that spreads through infected bird and bat feces. Histoplasmosis is often associated with pigeons in urban and suburban areas as the disease can be transmitted by breathing in the fungal spores that grow on the pigeon droppings that often accumulate on sidewalks and building walls, roofs and balconies
Just as destroying wildlife habitat increases the chances of humans coming in contact with animals carrying zoonotic diseases, protecting wildlife habitat can decrease these chances. In this way, wildlife habitat acts as a barrier—or at least a buffer—to prevent zoonotic diseases from escaping natural areas and spreading to humans.
Wildlife habitat also creates a “dilution effect.” Those species that are higher on the food chain are usually found in small numbers and are most sensitive to food source changes. When habitat areas become degraded or fragmented, predator species, such as hawks and coyotes, often become extinct in those areas, which results in an increase in prey species, such as insects and mice. These prey species tend to be more prone to carrying disease, which increases the chances of the disease being transmitted to humans. However, in larger wildlife habitat areas, there is a more diverse range of species—including both predator and prey—which dilutes the population of small prey species that tend to carry disease.
In Ontario, throughout Canada and around the world, more and more resources are being put into monitoring, preparing and responding to zoonotic diseases. Of course, these efforts are necessary to protect human health but it is worth remembering that the emergence of these diseases could be prevented through the protection of wildlife habitat.
As our populations rise, communities grow and climate change worsens, it is more important than ever for us to protect wildlife habitat—if not for our planet, then for our health.
Last modified: April 6, 2018