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Lyme Disease - An Increasing Threat to Dogs and Their Human Companions

by Dr. Cody W. Faerber and Dr. S. Mario Durrant

In 2002, 23,763 cases of human Lyme disease were reported to the Centers for Disease Control and Prevention (CDC). Ninety-five percent of these cases were from the states of Connecticut, Delaware, Rhode Island, Maine, Maryland, Massachusetts, Minnesota, New Jersey, New Hampshire, New York, Pennsylvania, and Wisconsin. The real startling statistic is that the actual number of human cases may be as much as 10 times the number reported. For the U.S. canine population, the threat is even greater!

What causes Lyme disease? This disease is caused by the spirochete Borrelia burgdorferi. Spirochetes are very small microorganisms that are free-living in the environment. For Lyme disease to exist in an area, at least three closely interrelated elements must be present in nature: (1) the Lyme disease bacteria B. burgdorferi (2) Ixodes ticks that can transmit the bacteria, and (3) mammals such as mice and deer to provide a blood meal for the ticks through their various life stages. Lyme disease can occur in dogs, cats, horses, wild animals, and humans.

The deer tick (Ixodes scapularis). Starting from left to right: adult female, adult male, nymph, and larva on a centimeter scale.
Where do Ixodes ticks live? In the United States, ticks of the genus Ixodes transmit the Lyme disease bacteria, B. burgdorferi, to animals and humans. Known as the deer tick or black-legged tick, Ixodes scapularis is commonly responsible for transmitting Lyme disease in the northeastern and north-central United States. In eastern states, ticks are associated with deciduous forest and habitat containing leaf litter. Leaf litter provides a moist cover from wind, snow, and other elements. Importantly, research demonstrates that tick populations are reduced 72-100% when leaf litter is removed. In the north-central states, I. scapularis is generally found in heavily wooded areas often surrounded by broad tracts of land cleared for agriculture. On the Pacific Coast, the bacteria are transmitted by the western black-legged tick (Ixodes pacificus) and habitats are more diverse. Here, ticks have been collected in habitats with forest, north coastal scrub, high brush, and open grasslands. Coastal tick populations thrive in areas of high rainfall, but ticks are also found at inland locations.

What is the life cycle of a tick? Knowing the complex life cycle of Ixodes ticks is important in understanding the risk of acquiring Lyme disease and in finding ways to reduce this risk. For this example, the two year life cycle of an I. scapularis deer tick located in a northeastern state will be described. Life cycles may vary slightly for other ticks located in different regions of North America.

The life cycle requires 2 years to complete. Adult female ticks lay eggs on the ground in early spring. By summer, eggs hatch into larvae. Larvae feed on mice, other small mammals, deer, and birds in the late summer and early fall, molt into nymphs, and then are dormant (inactive) until the next spring. Nymphs feed on rodents, small mammals, birds, dogs, and humans in the late spring and summer and molt into adults in the fall. In the fall and early spring, adult ticks feed and mate on large mammals (especially deer) and bite dogs and humans. The adult female ticks then drop off these animals and lay eggs in spring, completing a 2-year life cycle.

What signs will I see in my dog? This organism causes a fever, decreased appetite, and arthritis. The arthritis is often associated with the joints of the front limbs, and commonly causes swelling, heat, and pain in these areas. These signs will often come and go. Problems with the kidneys, heart, and nervous system have also been reported.
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