According to recent findings by environmental scientists at the University of Missouri-Columbia and the University of Illinois at Urban-Champaign, decreasing biodiversity might mean an increase in Borrelia Burgdorferi, a bacteria species responsible for inducing the condition of Lyme disease in humans. The study suggests host abundance decreases with increasing species diversity. This means people are at greater risk of being diagnosed with Lyme disease in places where biodiversity is lower.
This effect of using biodiversity to quantify disease risk is referred to as the dilution effect hypothesis. Despite being highly controversial, the hypothesis is being used to predict whether or not a person is at high risk of encountering a pathogen infected vector within a certain area. A vector is any organism that carries and transmits a disease carrying organism from one host to another. In the case of Borrelia Burgdorferi, that organism is often the Black-legged Tick also referred to as the Deer Tick (Ixodes scapularis).
In North America the White-footed Mouse (Peromyscus leucopus), is one of the most common natural reservoir hosts for Borrelia Burgdorferi. Differently from other hosts, a reservoir host is one in which the pathogen (Borrelia Burgdorferi) can reproduce while not actively being transmitted to new hosts. Despite being a natural carrier for the pathogen, infected mice don’t show any life-threatening systems, nor can infected mice infect their own or other species. In order for Borrelia Burgdorferi to be transmitted from reservoir host to host a vector is required. In the case of Lyme disease, the vector is the Deer Tick.
When adult ticks or newly hatched nymphs parasitize upon a White-footed Mouse, Borrelia Burgdorferi is transmitted from reservoir host (mouse) to vector (tick). Similarly, to the White-footed Mouse, infected Deer Ticks don’t display any abnormal behavior or negative health effects, but unlike the White-footed Mouse, the Deer Tick can transmit the pathogen to other species. When an infected tick finds another bloodmeal it transmits Borrelia Burgdorferi to the new host. In certain cases, this next blood meal or host might end up being a human being. In cases where an infected tick feeds upon and successfully transmits Borrelia Burgdorferi to a human being, a condition referred to as Lyme disease can be induced.
According to the Center of Disease and Control (CDC) Lyme Disease is a condition wherein “typical symptoms include fever, headache, fatigue, and a characteristic skin rash called erythema migrans”. The symptoms are often mild at first, but if left untreated the CDC warns the “infection can spread to joints, the heart, and the nervous system”. In severe cases Lyme Disease can cause life-long damage to the immune and nervous system. Dr. Richard Price, an ecologist and climate scientist at the University of East Anglia, developed Lyme disease as a result of being bitten by a tick while on an expedition in Northern Canada. “There were few all be it very mild symptoms, and I never developed the bulls-eye rash, but eventually after a few weeks the symptoms got more and more severe” says Dr. Price. As a result of the infection Dr. Price suffers with damage to his nervous system that reduces his ability to control muscles around his neck and eyes at times.
Lyme disease is not something you have to go to the remote wilderness such as Northern Canada to contract however. According to the study, people are at greater risk of encountering infected ticks along woodlots, farm fields, roadsides, and even in our own backyards. This is because these human induced habitats also make great breeding and feeding grounds for the White-footed Mouse, and where ever there are mice there are ticks and Borrelia Burgdorferi. When using computational models’ researchers found that species richness can be used to predict the density of infected ticks or DIN. Species richness simply refers to the number of different species that make up a landscape.
In places with high species richness, such as old growth forests, competition from other organisms such as chipmunks, squirrels, and shrews can drive down mouse populations via competition of resources. Predators such as snakes, foxes, and owls can also drive down mouse populations. With less mice, the models suggest DIN decreases, thus lowering the disease risk to humans. Many of these predators and or competitors occur in fewer numbers or are absent all together in these human induced habitats that support less species richness. As a result, in these habitats the models suggest DIN is greater, and so is the disease risk to people.
While it’s worth noting that these are only models, and it is hard to model all the variables within a landscape, the models can be used as a way to help better manage land to decrease vector-borne zoonotic disease transmission to humans. Going forward, hopefully an increase in field data can help further strengthen these models, and help protect people from what’s the most common vector-borne illness in the United States.
Halsey, S. J., and Miller, R. J. 2020. Maintenance of Borrelia Burgdorferi among vertebrate hosts: a test of dilution effect mechanisms. Ecosphere: 11(2)