Borrelia burgdorferi is the bacterium responsible for Lyme disease. It is a spirochete, which means it is a spiral-shaped bacterium that is longer than it is wide. B. burgdorferi has very specific growth requirements in the laboratory, so it is not routinely cultured.
The two primary reservoir hosts for B. burgdorferi are white-footed mice and whitetail deer. Even though the ticks bite the deer, the deer do not become infected, but they serve as means of survival and transportation for the ticks. Humans and other animals, such as dogs, cats, horses, and cattle, can also become infected with B. burgdorferi.
Ticks transmit B. burgdorferi through their bites, from one host to another. The organism lives in the intestinal tract of the tick and can be passed from host to host when the tick feeds. It takes 24 to 36 hours after a tick becomes attached to its host before it starts releasing B. burgdorferi.
The tick life cycle has three active stages after the egg hatches: larva, nymph, and adult (see Appendix 1). The larva must have a blood meal before it can molt to the nymph stage, and the nymph must have a blood meal before it can molt to the adult stage. A tick at each active stage can become infected when it takes a blood meal. The bacteria stay with the tick when it molts to the next stage. As it feeds before molting, a tick at each stage can infect another animal by depositing B. burgdorferi at the site of its bite. Typically the larva will feed on small rodents, like the white-footed mouse. If the mouse was infected, the larva becomes infected. Nymphs prefer feeding on many animals, such as small rodents, dogs, cats, horses, cattle, and humans. Similarly, if any of these animals were infected, the nymph would become infected. Adult ticks prefer to feed on whitetail deer. The life cycle takes 2 years to complete. Ticks can feed on people at any of the three stages (Figure 26).
Two ticks that can transmit Lyme disease to humans have been identified: the black-legged deer tick, Ixodes scapularis, in the northeastern and north central United States, and the western black-legged tick, Ixodes pacificus, on the Pacific coast of the United States. The larvae of both black-legged ticks are the size of a pin head and are tan colored. The nymphs are the size of a poppy seed and are beige or semitransparent. The adults are about the size of an apple seed and are black or reddish (Figure 27). For Lyme disease to occur, there must be B. burgdorferi, white-footed mice, and whitetail deer in an area.
Not all black-legged ticks are infected with B. burgdorferi. In some areas less than 1% of the ticks are infected, but in other areas over 50% are. B. burgdorferi can infect other species of ticks, but none of them have been shown to transmit Lyme disease to humans.
Cats that develop Lyme disease usually display fever, anorexia, lethargy, lameness, and irregular breathing. There is nothing about these symptoms that is specific to Lyme disease, so many cases will go undiagnosed.