Lyme disease is caused by infections of the bacterium Borrelia burgdorferi (stained blue in inset, among red blood cells) which is transferred to humans via blood-sucking bites from the tick, Ixodes scapularis. Lyme disease can cause flu-like symptoms in the short term and more serious illnesses in the long term, if not treated. Why is Lyme disease on the rise in eastern North America? Deer are hosts of ticks but are not reservoirs of B. burgdorferi—their immune systems detect and kill the bacterium. But a tick with a prior bacterial infection can consume a blood meal from a deer and then bite and infect a human. The fitness effects of a deer on B. burgdorferi can be summarized as: a. + because the deer supplies the bacterium with food. b. − because the deer kills the bacteria in its blood. c. + because the deer feeds and disperses the tick that can then infect a human with the bacterium. d. − or + depending on whether the deer is the tick's last host.

Verified step by step guidance

1. 1. The first step is to understand the relationship between the deer, the tick, and the bacterium B. burgdorferi. The deer serves as a host for the tick but does not act as a reservoir for the bacterium because its immune system can detect and kill the bacterium. However, a tick that has previously been infected with the bacterium can feed on a deer and then bite and infect a human.
2. 2. The second step is to analyze the fitness effects of a deer on B. burgdorferi. The deer provides a positive effect (+) because it supplies the bacterium with food through the blood that the tick consumes.
3. 3. The deer also has a negative effect (−) because it kills the bacteria in its blood, preventing the bacterium from multiplying and spreading.
4. 4. The deer has another positive effect (+) because it feeds and disperses the tick that can then infect a human with the bacterium. This increases the chances of the bacterium being transmitted to humans.
5. 5. The effect of the deer can be negative or positive (− or +) depending on whether the deer is the tick's last host. If the deer is the last host, the tick will not have the opportunity to infect a human, making the effect negative. However, if the deer is not the last host, the tick can go on to infect a human, making the effect positive.

Key Concepts

Lyme Disease Pathogenesis

Lyme disease is an infectious disease caused by the bacterium Borrelia burgdorferi, which is transmitted to humans through the bite of infected ticks, primarily Ixodes scapularis. Understanding the life cycle of the tick and the role of the bacterium in causing disease is crucial for comprehending how Lyme disease develops and spreads. The disease can manifest with flu-like symptoms initially and lead to more severe health issues if left untreated.

Tick Ecology and Host Interaction

Ticks are ectoparasites that require a blood meal from a host to develop and reproduce. In the case of Lyme disease, deer serve as hosts for ticks but do not harbor the bacterium, as their immune systems eliminate it. This interaction is significant because while deer support tick populations, they do not contribute to the spread of Borrelia burgdorferi, highlighting the complex dynamics between hosts and pathogens in disease ecology.

Fitness Effects in Ecology

In ecological terms, fitness effects refer to the impact of one species on another in terms of survival and reproduction. In the context of Lyme disease, the fitness effects of deer on Borrelia burgdorferi can be both positive and negative. While deer provide a food source for ticks, they also eliminate the bacterium from their system, influencing the overall dynamics of tick populations and the potential for disease transmission to humans.