When the words “public health” are mentioned, we think different thoughts. In current terms, we think of the outbreak of measles and the anti-vax movement throughout the United States. In the future, however, we consider the increasing prevalence of the super fungi and super bacteria, who are resistant to almost all antifungals and antibiotics, respectively. Public health encompasses so many aspects that every answer is the right answer, but rarely do people think of ‘biodiversity’ or ‘the environment’ when they think of public health.
And in one way, rightfully so. The words ‘biodiversity’ or ‘the environment’ are frequently used in contexts that are distant from the stereotypical image of public health - such as conservation, or climate change.
But here’s why biodiversity is important in public health. Diseases are spread in various ways, and many can spread by a vector. A vector is an agent that spreads pathogens to another living organism. It can range from a flea to something as notable as a mosquito. A vector borne disease is caused when these organisms come in contact with humans, and spread pathogens into our system. The World Health Organization expects that 17% of all infectious diseases are vector borne, and account for more than 700,000 deaths per year.
Here’s the worrying part: these vectors usually thrive in warm and humid conditions. The same article by the World Health Organization states that people living in tropic and subtropic conditions are the most vulnerable because of this. We shouldn’t celebrate too early, however, as the climate changes into a warm tropical climate, these vectors are finding themselves into our part of the globe.
However, these vectors are also part of nature; vectors such as fleas, ticks, and mosquitoes rely on animals to feed on and reside in. Each and every host vary in their ability to harbor a pathogen or a vector. Vector borne diseases are thought to be heavily reliant on vector demography and feeding rates. If a vector never comes into contact with a human, or if a vector never gets infected with a pathogen, the disease is effectively suppressed with little to no human effort.
This suppression is called the dilution effect. The dilution effect is a phenomenon in which a presence of vertebrate hosts with low capacity to feeding vectors (incompetent hosts) “dilute” the effect of highly competent vectors, thereby reducing disease risk.
An example of a disease affected by dilution is Lyme disease. Lyme disease is caused by black-legged ticks infected by the bacterium Borrelia burgdorferi. During the nymphal stage, these black-legged ticks reside and feed on white-footed mice, where the tick nymphs are infected with Borrelia. These tick nymphs can then feed on humans to transmit Borrelia burgdorferi to us.
Fortunately, there is proof that the dilution effect can reduce the frequency of this disease. The Cary Institute found that habitats with highly diverse community are at a lower risk of Lyme disease, and that poorly diverse communities are at a higher risk of Lyme disease. The Center for Disease Control’s incidence reports from 2017 show that Lyme disease is extremely prevalent in urban communities, where biodiversity is almost nonexistent.
Maintaining high biodiversity is imperative to safeguard public health. Although the dilution effect does not completely eradicate vector borne diseases, its absence has dire consequences in human health. Nonetheless, we should not lose hope that the relationship between public health and biodiversity become more apparent.