Tick is the common name for the small
that, along with mites,
order Acarina. Ticks are
ectoparasites (external parasites), living by
hematophagy on the
Ticks are an important vector of a number of human and animal diseases.
Carios kelleyi, a species of soft tick. (Credit: Center
for Disease Control & Prevention Public Health Image Library).
The major families of tick include the Ixodidae or hard ticks, which have
thick outer shells made of chitin, and Argasidae or soft ticks, which have a
membraneous outer surface. Soft ticks typically live in crevices and emerge
briefly to feed, while hard ticks will attach themselves to the skin of a host
for long periods of time. Tick bites look like mosquito
bites, but can also sometimes bruise or resemble a bullseye.
Ticks as disease vectors
Hard ticks can transmit human diseases such as
relapsing fever, Lyme disease, Rocky Mountain spotted fever, tularemia, equine
encephalitis and several forms of ehrlichiosis. Additionally, they are
responsible for transmitting livestock diseases, including babesiosis and
Diseases such as HIV/AIDS and malaria
can be transmitted by soft ticks.
tick-borne diseases correspond to a specific tick-host combination, and are
limited in their geographical extent.
According to the
Rhode Island Department of Health, roughly 70% of people who develop Lyme
disease catch it from ticks in their own yard.
Ticks are often found in tall
grass, where they will rest themselves at the tip of a blade so as to attach
themselves to a passing animal or human. It is a common misconception that the
tick can jump from the plant onto the host. Physical contact is the only method
of transportation for ticks. They will generally drop off of the animal when
full, but this may take several days. Ticks contain a structure in their mouth
area that allows them to anchor themselves firmly in place while sucking blood. Pulling a tick out
forcefully may squeeze contents of the tick back into the bite and often leaves
the mouthpiece behind, which may result in infection.
male tick size comparison
Dermacentor variabilis, the American
dog tick, is
perhaps the most well-known of the North American hard ticks.
Ixodes dammini, the
deer tick, is
common to the eastern part of
North America and is known for spreading Lyme disease.
I. pacificus lives in the western part of the continent and is
responsible for spreading Lyme disease and the more deadly Rocky Mountain
spotted fever. It tends to prefer livestock as its adult host.
In some parts of Europe,
tick-borne meningoencephalitis is a common viral infection.
Australian tick fauna consists of approximately 75 species, the majority
of which fall into the Ixodidae, hard tick, family. The most medically
important tick is the Paralysis tick, Ixodes holocyclus. It is found in a
20-kilometre band that follows the eastern coastline of Australia. As this
is where much of the human population resides in New South Wales, encounters with these parasites are relatively common.
Although most cases of tick bite are uneventful, some can result in life
threatening illnesses including paralysis, tick typhus and severe allergic
Deer (black-legged) tick
The deer (or black-legged) tick, and the related western black-legged tick,
are the primary known transmitters of Lyme disease in the United States. Both
are hard-bodied ticks with a two-year life cycle. Like all species of ticks,
deer ticks and their relatives require a blood meal to progress to each
successive stage in their life cycles.
The life cycle of the deer tick comprises three growth stages: the
larva, nymph and adult. In both the northeastern and mid-western U.S., where Lyme disease has become
prevalent, it takes about two years for the tick to hatch from the egg, go
through all three stages, reproduce, and then die. A detailed description of
this life cycle and the seasonal timing of peak activity, as they occur in these
regions, is provided below.
Eggs laid by an adult female deer tick in the spring hatch into larvae later
in the summer. These larvae reach their peak activity in August. No bigger than
a newsprinted period, a larva will wait on the ground until a small mammal or
bird brushes up against it. The larva then attaches itself to its host, begins
feeding, and engorges with blood over several days.
If the host is already infected with the Lyme disease spirochete
from previous tick bites, the larva will likely become infected as well. In this
way, infected hosts in the wild (primarily white-footed mice, which exist in
large numbers in Lyme-endemic areas of the northeast and upper mid-west) serve
as spirochete reservoirs, infecting ticks that feed upon them. Other mammals and
ground-feeding birds may also serve as reservoirs.
Visible tick damage on a kitten
Because deer tick larvae are not born infected, it is believed that they
disease to their human hosts. Instead, "reservoir" hosts, as mentioned
above, can infect the larvae. Having already fed, an infected larva will not
seek another host, human or otherwise, until after it reaches the next stage in
its life cycle. It is not completely known whether larvae, in themselves, pose a
threat to humans or their pets.
Most larvae, after feeding, drop off their hosts and molt, or transform, into
nymphs in the fall. The nymphs can remain active throughout the winter and early
In May, nymphal activity begins. Host-seeking nymphs wait on vegetation near
the ground for a small mammal or bird to approach. The nymph will then latch on
to its host and feed for 4 or 5 days, engorging with blood and swelling to many
times its original size. If previously infected during its larval stage, the
nymph may transmit the Lyme disease spirochete to its host. If not previously
infected, the nymph may become infected if its host carries the Lyme disease
spirochete from previous infectious tick bites. In highly endemic areas of the
northeast, at least 25% of nymphs have been found to harbor the Lyme disease
Too often, humans are the hosts that come into contact with infected nymphs
during their peak spring and summer activity. Although the nymphs' preferred
hosts are small mammals and birds, humans and their pets are suitable
substitutes. Because nymphs are about the size of a poppy seed, they often go
unnoticed until fully engorged, and are therefore responsible for the majority
of human Lyme disease cases.
Adult deer tick.
Once engorged, the nymph drops off its host into the leaf litter and molts
into an adult. These adults actively seek new hosts throughout the fall, waiting
up to 3 feet above the ground on stalks of grass or leaf tips to latch onto deer
(its preferred host) or other larger mammals (including humans, dogs, cats,
horses, and other domestic animals). Peak activity for adult deer ticks occurs
in late October and early November. Of adults sampled in highly endemic areas of
the northeast, at least 50% have been found to carry the Lyme disease
As winter closes in, adult ticks unsuccessful in finding hosts take cover
under leaf litter or other surface vegetation, becoming inactive when covered by
ice and snow. Generally, winters in the northeast and upper mid-west are cold
enough to keep adult ticks at bay until late February or early March but not
when temperatures begin to rise. At this time, they resume the quest for hosts
in a last-ditch effort to obtain a blood meal allowing them to mate and
reproduce. This second activity peak typically occurs in March and early April.
Adult female ticks that attach to deer, whether in the fall or spring, feed
for approximately one week. Males feed only intermittently. Mating may take
place on or off the host, and is required for the female's successful completion
of the blood meal. The females then drop off the host, become gravid, lay their
eggs underneath leaf litter in early spring, and die. Each female lays
approximately 3,000 eggs. The eggs hatch later in the summer, beginning the
two-year cycle anew.