What is a tick? Ticks are not insects but Arachnids, a class of Arthropods, which also includes mites, spiders and scorpions. They are divided into two groups – hard bodied and soft bodied – both of which are capable of transmitting diseases in the United States.
Ticks are parasites that feed by latching on to an animal host, imbedding their mouthparts into the host’s skin and sucking its blood. This method of feeding makes ticks the perfect vectors (organisms that harbor and transmit disease) for a variety of pathogenic agents. Ticks are responsible for at least ten different known diseases in humans in the U.S., including Lyme disease, Rocky Mountain spotted fever, babesiosis, and more recently, anaplasmosis and ehrlichiosis.
The Deer Tick Life Cycle The deer (or black-legged) tick in the East and the related western black-legged tick are the only 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. Detailed descriptions of this life cycle and the seasonal timing of peak activity, as they occur in these regions, are provided below.
The above graph shows the host-seeking behavior of I. scapularis ticks according to life-stage and season. Larval activity peaks in August, nymphs are active during the summer months, and adults are active during the spring and fall. People primarily acquire Borrelia burgdorferi (the causative agent of Lyme disease) from infected nymphs because of their small size. Host-seeking larvae are not infected. Infected adults are large enough to be noticed and are usually removed by people before B. burgdorferi is transmitted.
Consequently, very few Lyme disease cases are reported during spring and fall. Click here to learn about the probability of B. burgdorferi transmission according to how long an infected tick feeds on a person. Stage 1: Larva – As shown in the upper left corner of the life-cycle diagram to the right, 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 over a few days, engorges (swells up) with blood.
If the host is already infected with the Lyme disease spirochete (a form of bacterium) 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 natural reservoirs of infection. Larval ticks are not born infected, they cannot transmit Lyme disease to nimal or human hosts. Instead, “reservoir” hosts 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. Therefore, larvae do not, in themselves, pose a threat to humans or pets.
Stage 2: Nymph – Larvae, after feeding, drop off their hosts and molt, or transform, into nymphs in the fall. The nymphs remain inactive throughout the winter and early spring.
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 four or five 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 and upper midwest, 25% of nymphs have been found to harbor the Lyme disease