Lyme disease is caused by infections of the bacterium Borrelia burgdorferi (stained blue in the 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. Step 1: Understand the relationship between the deer, the tick, and the bacterium Borrelia burgdorferi. The deer serves as a host for the tick, providing it with a blood meal, but the deer's immune system kills the bacterium if it enters the deer's bloodstream.
2. Step 2: Analyze the role of the deer in the life cycle of the tick. The deer provides food for the tick, allowing it to survive and reproduce. This indirectly benefits the bacterium because the tick can later infect a human with Borrelia burgdorferi.
3. Step 3: Consider the fitness effects of the deer on Borrelia burgdorferi. While the deer kills the bacterium in its own blood, it also supports the tick population, which can carry and transmit the bacterium to humans.
4. Step 4: Evaluate the options provided in the question. Option (a) suggests a positive effect because the deer supplies food, but this is not directly true for the bacterium. Option (b) suggests a negative effect because the deer kills the bacterium, which is partially true. Option (c) suggests a positive effect because the deer supports the tick, which can spread the bacterium, and this aligns with the indirect benefit to the bacterium. Option (d) suggests variability based on the deer's role as the tick's last host, which is not directly relevant to the bacterium's fitness.
5. Step 5: Conclude that the most accurate answer is (c), as the deer indirectly benefits Borrelia burgdorferi by feeding and dispersing the tick, which can then infect humans with the bacterium.

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 spreads and the symptoms it can cause, ranging from flu-like symptoms to more severe long-term health issues.

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 highlights the complex dynamics between ticks, their hosts, and the pathogens they carry, which is essential for understanding the epidemiology of Lyme disease.

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, depending on whether the deer serves merely as a food source for ticks or if it also contributes to the transmission of the bacterium to humans. This dual role is critical for understanding the rising incidence of Lyme disease in certain regions.