The black-legged tick, Ixodes scapularis, is the most common carrier of Lyme disease in the mid-west and eastern states. I. pacificus is known to be the vector in the west. Other species of ticks such as the dog tick or wood tick, the lone-star tick and the rabbit tick, and biting insects such as mosquitoes, deer flies and horse flies have been shown to carry the Lyme disease bacterium. However, their ability to transmit the disease is not known at this time. Recent investigations have uncovered populations in Southwest Michigan and studies are continuing to determine the extent of the Lyme tick population (see Michigan Map).
The black-legged tick has a complex life cycle involving development through three active stages: larva, nymph, and adult. This process usually takes two years or more. Larvae and nymphs require blood to proceed to the next development stage, and adult females need blood to lay their eggs.
Seasonal Life Cycle in Michigan
Studies of the life cycles of disease vectors are important in many ways:
· They provide a predictable base for seasonal public information campaigns; by knowing when the specific stage of tick will be present in the environment; the public can be educated on tick avoidance and personal protective measures.
· In areas of high Lyme disease transmission concern, control efforts can be effectively targeted at times when the ticks are susceptible.
· Understanding the ecology of the ticks can help researchers better predict the risk of Lyme disease to the public.
The life cycle of the black-legged tick has been studied in detail in many regions. The most recognized life cycle pattern is based on studies conducted in the Northeast United States. Life cycles, however, differ in many areas based on local and regional habitat and climactic variation. Recent longitudinal (long term, repeatable) surveys in Southwest Michigan have provided detailed seasonal information about the black-legged tick in our region. Below is a graphic example of the black-legged tick life cycle in Michigan.
The peak of adult activity in Michigan is during the early spring and autumn months. Female and male ticks search out their preferred host, the white-tailed deer, for feeding and reproduction. Female ticks require a large blood meal to develop large egg batches. The male ticks, which rarely feed, locate the female ticks at their feeding sites on the host by pheromones (reproductive chemical cues). After mating and engorging on blood, the female tick drops off the host and begins to lay eggs, usually in a protective bed of leaves, and the male dies. The mother tick, if infected, rarely passes Lyme disease infection to her offspring. Upon completing egg laying, the female tick dies.
Larval ticks begin to hatch from their eggs in the early spring, although in Michigan, there may be two larval hatching periods. Larvae will hatch in early spring from eggs that were laid by females in the autumn months. Eggs that were laid in the early spring will hatch in the early summer months.
Larval ticks are very small (approximately 1 millimeter in size) and may bite people. Their preferred hosts, however, are small mammals that live in forested environments (mice, chipmunks, squirrels, etc.). It is at this stage where the tick can acquire the Lyme bacteria. Small mammals may be carriers of the bacteria, previously infected by nymphal ticks, which have fed during the spring. The Lyme disease cycle is dependent on the nymphal ticks emerging and feeding before the larvae to successfully pass infection from one generation to the next. The small mammals remain carriers for a defined period (usually several weeks to months) and show no serious illness. Note: The chance of becoming infected by the bite of a larva is low.
Once the larva has successfully fed, it drops to the forest floor and finds a sheltered environment to ‘hibernate' and to molt (transform) to the nymphal stage. The nymph will remain inactive until the next spring.
Nymphs emerge in the early spring and prefer to feed on small and medium sized mammals. These nymphs may have been infected the previous year while in the larval stage. When they feed, if infected, they will pass on the Lyme bacteria to the mammal host. This continues the cycle, creating infected hosts for newly emerged larvae to feed on. The nymphs are the most dangerous stage for human contact. They will readily attach themselves to people, and because of their small size (appx. 1.5 millimeter), are difficult to see or notice. Once the nymph has completed its blood meal, it drops from the host and finds a sheltered area (preferably forest) to undergo transformation to the adult stage.
The ability of the tick to transmit Lyme bacteria is dependent on the amount of time it is attached to the host. Studies have shown that removal of infected, feeding ticks prior to 48 hours significantly reduces the potential for infection.
The Lyme disease bacteria are ingested from the host in a blood meal and the bacteria then reside on the midgut (stomach) lining of the tick. The bacteria survive through the tick's transformation to the next life stage. When the tick begins to feed again a complex set of interactions occur in the tick's body. The bacteria must detach and penetrate through the ‘stomach' lining into the ticks hemocoel (body cavity). From there, the bacteria migrate to, and enter the salivary glands of the tick. They are then ‘injected' into the host along with the salivary fluid. This set of interactions takes some time to complete, and research has shown that prior to 24 hours attachment it is unlikely that infection will occur. Prompt removal of attached ticks can provide protection even past 48 hours.
Please see "Suggestions for Michigan's Public" for more personal protective information against ticks and Lyme disease.
Figure: Model of the Lyme bacteria, Borrelia burgdorferi.