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Just by living in the world, human beings are
susceptible to disease. Many diseases — for
example, influenza and tuberculosis — are spread
when bacteria or viruses pass from one person to
another. Other diseases are acquired genetically
from ones parents, such as cystic fibrosis. Some
diseases, such as heart disease and osteoporosis,
develop as humans age. Others humans get from the
environment; examples are lead poisoning and skin
cancer due to exposure to the sun. Finally, there
are diseases that can be transmitted from animals
to humans.

Lyme disease is one such disease. Cases
of Lyme disease, which is transmitted to humans
and pets by the bite of a tick, are steadily
increasing in areas such as the Northeast region
of the United States. At present, there is no
vaccine for humans; the best way to prevent Lyme
disease is to take precautions against being
bitten by a tick and allowing the tick to remain
attached to the skin long enough for the infection
to be transmitted. The story of Lyme disease in
the United States began in 1975, when two mothers,
Polly Murray and Judith Mensch, alarmed by the
great number of cases of joint inflammation in the
their communities of Lyme and Ease Haddam,
Connecticut, contacted public health authorities
(7:5). The health department contacted Allen
Steere and his colleagues at Yale University, in
New Haven. Steere believed the outbreak may
provide a clue about the infectious agent or
environmental toxin that was responsible for
arthritis (2:26).

One early observation made by
Steere was an association between the arthritis
and a prior skin rash. A connection was then made
between this rash and a similar one called
erythema migrans, which comes from the bite of the
sheep tick, Ixodes ricinus and is frequently found
in northern Europe (7:5). After field studies and
patient surveys were carried out the researchers
released three essential findings which later led
to the discovery of the infectious agent. First,
the disease was seasonal, occurring most commonly
in the summer and much less so in the middle of
the winter. In geographic areas such as
Connecticut, these findings suggest that the virus
was either a summer virus or an infection carried
by and insect or a tick (7:35). Second, the
disease did not spread from one person in a family
to another.

Summer viruses were commonly spread
from person to person, especially those living in
the same household. When a summer virus was
eliminated, the focus shifted to the involvement
of arthropods (7:37). Third, the disease was much
more prevalent on one side of the Connecticut
River than the other. Since the Connecticut River
bisects the state, this became an important factor
in finding the cause of the disease. The
researchers found a good correlation with the
frequency of a certain tick now named Ixodes
scapularis. When furthered questioned, the
affected people remembered being previous bitten
by a tick (2:27).

At first, the infectious agent
producing Lyme disease was thought to have been
either a virus, protozoan, fungi, or bacteria.
Among the possible agents, bacteria was the
highest on the list. European physicians had been
treating patients with medicines that were
effective against bacteria. However, these
findings were not originally accepted in the
United States. It was only after some of the
patients in Steeres study were successfully
treated with antibiotics, which are effective
against bacteria, but not against viruses,
protozoan, or fungi, that bacteria was determined
to be the infectious agent (14:1015). The specific
bacteria was found by Willy Burgdorfer, an expert
on a variety of tick-borne diseases. While working
at Montanas Rocky Mountain Laboratories,
Burgdorfer received a shipment of ticks from New
York.

After looking at the contents of the ticks
through a microscope, Burgdorfer found a wavy form
of a spirochete that had never been seen in the
Ixodes group of ticks before. After determining
that the same spirochetes were present in ticks
taken from Long Island and New Jersey, the newly
named Borrelia burgdorferi was confirmed as the
bacteria that caused Lyme disease (1:47). The tick
that transmits Lyme disease is called Ixodes
scapularis. In the northeastern and north-central
United States it is often called the deer tick
because it is found so frequently on deer. The
southern form of I. scapularis is usually called
the black legged tick.

The southern form of the
species poses less of a threat of infection. Fewer
of the southern ticks are infected and they tend
to feed on other animal hosts rather than humans
(2:43). The ticks that transmit Lyme disease
generally live about two years. I. scapularis
larvae, which are not much larger than the period
at the end of this sentence, hatch in the summer
from eggs laid by the adult female that spring.
They usually feed on a field mouse or other rodent
host that summer and into the early fall. The
larvae then change into nymphs the first year;
these nymphs pass through the winter without
feeding.

the following spring and summer the
nymphs feed on a rodent or other small animal. At
least three out of four Lyme disease cases in the
United States are from the bite of a nymphal tick
that occurs sometime between May and August. Most
of the bites go unnoticed because the nymphs are
so small–about the size of a poppy seed (2:45).
In the second year, the nymph changes to an adult
after feeding. The larger ticks, especially the
females, are more likely to be noticed by people.
I. scapularis adults bite deer and other large
mammals, such as humans. The adults feed later in
the year than the nymphs and larvae and may remain
active even as temperature drop to just above zero
in the late fall.

Lyme disease infection occurring
in the fall, especially in the northeast, can
usually be attributed to the bite of an adult
(2:45-46). The Lyme disease spirochetes can remain
active and even multiply inside a ticks body, but
seldom are they passed from an adult female to her
offspring. In order for the spirochetes to spread
in nature, an infected tick must feed on another
animal, thereby passing the spirochete from that
animal to other feeding ticks. Many types of
mammals and birds are capable of hosting the
bacteria and of passing it on to other ticks, thus
completing the vector-reservoir-vector cycle.
Because about 99 percent of the larvae of I.
scapularis do not carry the bacteria even if their
adult mother of father did, the larvae must
acquire the bacteria by feeding on an infected
host. In the case of deer ticks this host is
usually a wild field mouse, called Peromyscus
leucopus. In some regions more than half the mice
are infected with Lyme disease bacteria, thus
providing a continuous reservoir of the
spirochetes for many ticks.

In high-risk areas for
Lyme disease, such as New Jersey, the chances that
a larva will become infected is as least one in
four (13:36). The first sign of the disease in 60
to 80 percent of the cases is a rash–a reddish
blotch or bulls eye pattern, often no more than 2
1/2 inches across. If left untreated it may, in
two weeks to a month, expand to four times that
size. The rash does not always occur at the site
of the bite (11:9). Often it is found at the
armpit, groin, or the back of the knee. However,
in many cases of Lyme disease no rash occurs;
therefore it may be necessary to look at other
factors before making a diagnosis (16:41).

Other
common symptoms include chills, fever, fatigue,
and other ….

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