Ticks
Medical Author: Charles Davis, MD, PhD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
Lyme Disease on Rise
Medical Author: William C. Shiel Jr., MD, FACP, FACR
Lyme
disease is caused by infection with a bacterium called a spirochete (Borrelia burgdorferi) and is transmitted to
humans by infected ticks (Ixodes scapularis and I. Pacificus). Patients with
early stage Lyme disease have a characteristic rash (erythema migrans) accompanied by nonspecific symptoms (for
example, fever, malaise, fatigue, headache, myalgia, and arthralgia). Lyme disease can usually be treated successfully with standard antibiotics.
The U.S. Centers for Disease Control and Prevention
(CDC) released the results of a six-year survey study of Lyme disease from 1992 to
1998. During this period, a total of 88,967 cases of Lyme disease were reported
to CDC by 49 states and the District of Columbia, with the number of cases
increasing from 9,896 in 1992 to 16,802 in 1998. The researchers concluded that
the increase in reported cases is probably a result of both a true increase in
incidence within known high-risk areas as well as more complete reporting as a result of enhanced Lyme disease surveillance. They noted that surveillance capabilities and public awareness of Lyme disease have increased during this period.
Researchers also note that Lyme disease remains underreported with an estimated
seven to 12 cases for each reported case.
What are ticks?
Ticks are scientifically classified as Arachnida (which includes spiders). The
fossil record suggests ticks have been around at least 90 million years. There
are over 800 species of ticks throughout the world, but only two families of
ticks, Ixodidae (hard ticks) and Argasidae (soft ticks), are known to transmit
diseases or illness to humans. Hard ticks have a scutum, or hard plate, on their
back while soft ticks do not.
Ticks have a complex life cycle that includes eggs, larvae, nymphs, and adult
male and female ticks. The larvae, nymphs, and adults all need blood meals.
Usually, the female adult (hard tick) is the one causing the most bites as males
usually die after mating. Ticks do not jump or fly. They simply reach out with
their legs and grab or crawl onto a host. Although some larvae have preferred
hosts, most ticks in the nymph or adult phase will attach a get a blood meal
from several different kinds of animals, including humans. Except for a few
species of larval ticks, the immature phases (larvae, nymphs) usually are even
less selective about where they get a blood meal and are known to bite snakes,
amphibians, birds, and mammals. Larvae are very small (about 1/32 of an inch with
six legs), while nymphs are about 1/16-1/8 inch with eight legs and adults about
3/16-1/4 inch with eight legs. The complex life cycles are described in the last
Web
citation below, and all of the Web citations include pictures of various species
of ticks. Although ticks will die eventually if they do not get a blood meal,
many species can survive a year or more without a blood meal. The hard ticks
tend to attach and feed for hours to days. Disease transmission usually occurs
near the end of a meal, as the tick becomes full of blood. It may take hours
before a hard tick transmits pathogens. Soft ticks usually feed for less than
one
hour. Disease transmission can occur in less than a minute with soft ticks. The
bite of some of these soft ticks produces intensely painful reactions.
Ticks are vectors (transmitters) of diseases for humans and animals. Ticks can
transmit disease to many hosts; some cause economic harm such as Texas fever
(bovine babeiosis) in cattle that can kill up to 90% of yearling cows. Ticks act
as vectors when pathogens in their saliva and mouth secretions get into the
host's skin and blood. Ticks were understood to be vectors of disease in the
mid-1800s, and as investigative methods improved (microscopes, culture
techniques, tissue staining). more information showed the wide variety of
diseases that could be transmitted by ticks.
 |
| Picture of a tick |
What are tick bite symptoms?
Unfortunately, the tick bite is usually painless and remains that way even after
the tick stops the blood meal and falls off of the skin. Later, the bite site
may develop itching, burning, redness, and rarely, localized intense pain (some
soft tick bites) in some individuals. A few individuals may be sensitive or
allergic to tick bites (tick saliva secretions) and develop rash, shortness of
breath, swelling, numbness, or paralysis. However, the majority of individuals
with tick bites develop no symptoms, and many do not remember getting bitten.
Some immediate symptoms that infrequently or rarely develop during or
immediately after a tick bite may be fever, shortness of breath, weakness, vomiting,
swelling, weakness or paralysis, headache, confusion, or palpitations.
Individuals with these symptoms should be seen immediately by a doctor.
Next: What diseases do ticks transmit (act as vectors) to humans? »
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