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Lyme disease and other tick-borne diseases: a review.

Ticks are important because they can function as vectors of diseases that affect humans, livestock and wildlife (1). As Lyme disease has proliferated in recent years, concern about tick-borne diseases has also increased. In the United States, there are 12 species of ticks (seven kinds of hard ticks and five of soft ticks) that can carry disease agents, can cause paralysis or a painful bite, and are a nuisance to humans (2, 3). Worldwide, there are approximately 800 species of ticks, about 100 of which carry diseases (2).

Ticks are arachnids, within the larger group of arthropods. The ticks' only food is the blood sucked from their hosts, which are usually warm-blooded animals. Occasionally, humans are among these hosts and tick-borne illnesses are transmitted to them (2, 3, 4).

The life cycle of ticks consists of four distinct stages: egg, larva, nymph and adult. Eggs hatch into six-legged larvae, known as seed ticks. A larva attaches to a host, sucks blood, and when full drops to the ground. Within a few days the larva changes form, becoming a nymph. The larger nymph, now with eight legs, finds another host for a blood meal. Nymphs may be dormant over winter, then seek their blood meal the following spring. Some nymphs feed only once and molt to an adult, while others feed several times, molt to another nymph before feeding and molting to an adult, which then finds a host. The life cycle of a tick usually takes one to two years (2, 3).

Despite the popular notion, ticks really do not bit their hosts. Rather, they insert their barbed mouthparts through the skin of the host to draw blood (3). As the tick feeds, disease agents such as bacteria, viruses, rickettsiae and protozoa, which develop inside the tick, can be injected into the host (2).

Several characteristics make ticks efficient vectors: they attach firmly; are persistent bloodsuckers; feed very slowly; and may go unnoticed for a period of time. They have few natural enemies and most species have a wide range of hosts. The larvae and nymphs usually feed on birds and small mammals, while the adults feed on larger mammals, including humans. Severe infestations on wildlife may cause anemia, loss of weight and even death. Temperature can be a factor in their development, with unfed ticks enduring cold well; engorged ticks and eggs are less resistant. Sunlight and drying have adverse effects on them (3, 5).

In general, harmful features of tick infestation include:

* local irritation

* blood loss

* cutaneous wounds, with possible secondary infections

* disease transmission

* tick paralysis

Disease syndromes

Lyme disease (Lyme borreliosis) (2, 6, 7, 8)

Transmission -- By the deer tick in the eastern United States and the Midwest, and by the western black-legged tick in the
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Author:Reed, George H.
Publication:Journal of Environmental Health
Date:Jun 1, 1993
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