Lyme Disease (Borreliosis) in Dogs

History

Lyme disease (borreliosis) is an infectious, tick-borne disease first recognized in dogs in 1985. It is caused by Borrelia burgdorferi, a type of bacteria called a 'spirochete.' The common deer tick Ixodes scapularis (formerly called Ixodes dammini) is the primary carrier of B. burgdorferi in the Northeast and upper Midwest. The tick Ixodes pacificus is the primary carrier in the western United States.
Although Lyme disease was first diagnosed in Connecticut in 1975, there is evidence that it has existed in wildlife for many years. Tissue samples taken from a white-footed mouse in Massachusetts in 1894 have now undergone DNA testing that show the mouse was infected with Lyme disease. Certain environmental factors have caused Lyme disease to be more prevalent in people in the twentieth century. Before 1900, the habitat in the East and upper Midwest was heavily deforested by early settlers. Deer and their associated ticks were greatly reduced. The reforestation of these areas and the resulting booming white-tailed deer population in these areas are probably a big factor in the increase in the incidence of Lyme disease. This coupled with increased awareness and testing capabilities has led to the greatly increased reporting of the disease.

Prevalence of Lyme Disease in the U.S.
U.S. map showing areas where Lyme disease is common
Where is Lyme disease found?
Despite the fact Lyme disease has been diagnosed in people from all 50 states, it is felt that they acquired the disease by traveling to endemic areas (areas where the disease is consistently present). Only a relatively small portion of the United States is endemic for the disease. However, all of the areas where Lyme disease is present are areas with high human and pet populations bringing the total number of people and animals that can be potentially exposed to a high number. In humans, 85% of cases have occurred in the eastern coastal states from Massachusetts to Virginia. 10% of the cases come from Wisconsin and Minnesota and 4% from California. All of the other states account for less than 1% of the disease.

Life cycle of the deer tick

Deer tick compared to a pennyThe deer tick (I. scapularis) goes through several stages in its life cycle. In the spring, the eggs hatch into larvae. During the summer, a larva will feed on a small mammal such as a white-footed mouse. If the mouse is infected with B. burgdorferi, the larva can become infected. This infected larva will then winter over until the following spring when it becomes a nymph (and is still infected) and feeds on another small mammal or a deer, dog, or human. In situations where the nymph was not infected as a larva, it could become infected by biting an infected animal. The nymph then molts into an adult. The infected adult then feeds on a larger mammal such as a deer, dog, or human, and lays its eggs, which will hatch the following spring. In summary, a dog or person usually acquires Lyme disease through the bite of an infected nymph or adult tick.

How is Lyme disease transmitted?

For a nymph to transmit B. burgdorferi, it must be attached to the host for about 48 hours. If a tick dies or is removed before 48 hours, transmission of the bacteria will not occur. Even if a tick is a carrier of B. burgdorferi and it attaches to a dog for more than 48 hours, the dog may not contract the disease. In fact, studies show that only around 10% of dogs that are exposed to B. burgdorferi will contract the disease. There is no evidence to suggest that infected dogs pose a risk to other members of the household except as a reservoir of infected ticks. Once a tick has had a full meal, it will detach and not bite another mammal. The risk comes from ticks that have not gotten a complete meal and are detached. They could possibly seek out a human and bite them causing infection.