[Public Health] Tick Tock

The emergence of Lyme disease in the 1980s was a significant public health event. Since that time, the prevalence of the disease has grown substantially. In 2000, more than 18,000 cases were reported to health departments across the United States with 90 percent of these cases occurring in the Northeast United States. The increasing prevalence of Lyme disease is attributed to the expanding white-tailed deer population, which has supported an increase in the deer tick population, and human land-use patterns, such as building homes adjacent to wooded, tick infested areas. More than ever, customers are reliant on efficacious tick control services provided by trained pest control operators to reduce populations of ticks around their properties.

The removal of the Lyme disease vaccine from the commercial market in 2002 has placed further pressure on PCOs to offer tick control services in Lyme disease endemic areas. There are no vaccines currently available in the United States for the prevention of Lyme disease. Although the manufacturer of the previous vaccine sited poor sales as the motivation for removal, the U.S. FDA is investigating complaints that the vaccine allegedly caused the onset of Lyme disease in some cases. Until a new vaccine is developed, PCO tick reduction services and repellant use are the only control methods available to people in Lyme disease endemic areas.

Lyme disease is a bacterial disease spread through the bites of Ixodes ticks, which infects humans and dogs. In the Northeast, the primary vector of the disease is the deer tick, Ixodes scapularis, while in the West I. pacificus, the western black-legged tick, is the main vector. A human’s risk of acquiring the disease is related to the density of ticks in the area, the presence of the Lyme disease bacterium and the extent of person-tick contact, which is related to the type, frequency and duration of a person’s activities in a tick-infested environment.

A study in Connecticut found that most reported cases of Lyme disease are contracted from a tick found on the person’s property. Recreational activities, property maintenance and other occupational or leisure pursuits can become risky activities in tick habitats.

TICK LIFE CYCLE. The tick life cycle is rather complex compared to other pests encountered by PCOs. Proper understanding of the two-year Ixodes life cycle is necessary to achieve efficacious control.

Ticks must take a blood meal from an animal, referred to as a host, in order to molt to their next life stage. In the spring of the first year, disease-free tick eggs are deposited by adult females and hatch in leaf litter on the forest floor. The eggs develop into three-legged larvae that must feed once during the summer, usually on the blood of small mammals like the field mouse. They must stay attached to the host for 36 hours to have a sufficient blood meal. At this time in the life cycle, the ticks are small, around the same size as the period at the end of this sentence. It is during the larval tick’s first blood meal that the Lyme disease bacteria is acquired from the host. After the first blood meal, the ticks overwinter until spring when they molt into nymphs baring four pairs of legs.

In disease endemic areas, such as Connecticut where Lyme disease was discovered, 80 to 90 percent of nymphs are infected with the bacterium. The bacterium can be found in almost 100 percent of white-footed mouse hosts in these areas as well. The nymphs feed once during the summer and molt into adults in the fall. The adults attach themselves to a host, usually the white-tailed deer, where they mate. The male ticks die and the females obtain the blood meal necessary for egg production. After engorging herself on a blood meal, the female tick falls to the leaf litter where she is capable of depositing 3,000 eggs.

Ticks seek hosts through an interesting behavior called "questing." Questing ticks crawl up the stems of grass or perch on the edges of leaves on the ground in a posture with the front legs extended. Ticks sense carbon dioxide, heat and movement from vertebrate hosts, which serves as stimuli for questing behavior. Subsequently, a potential host brushes the vegetation on which the tick quests and the ticks latch on to the host using their extended front legs.

It is the nymph stage that is most likely to bite and infect humans with Lyme disease. This is due to the high percentage of nymphs that are carrying the Lyme disease bacteria and their small size, which makes them difficult to detect during feeding.

IXODES CONTROL. The control of ticks relies on an integrated pest management approach that includes customer education and correct placement of control measures. To successfully control ticks, PCOs must seek out the source of infestations, correctly apply or place control products and advise customers on practices to reduce tick densities in their yards.

Ixodes ticks are found in wooded, brushy or overgrown grassy areas where deer, rodent and bird hosts reside. In a residential setting, the habitat most conducive to ticks and their hosts is leaf litter adjacent to manicured lawns, wood piles and overgrown vegetation. Ticks do not preferentially live on lawns. However as mice, birds and deer enter suburban lawns, often attracted to food from bird feeders or gardens, ticks are moved from adjacent forested areas onto the lawn. Additionally, pets allowed to run in overgrown areas and adjacent forests can pick up ticks and transport them back to the home. PCOs should be aware of these tick habitats to direct control efforts, as well as educate customers about necessary habitat modifications.

Customers should be advised to trim or eliminate dense weed growth and vegetation in the yard, especially at the edges of manicured lawns, to minimize the ground cover attractive to ticks and tick hosts. Bird feeders attractive to tick hosts should be removed from around the home and gardens should be fenced. Pets, especially dogs that can contract Lyme disease, must regularly be treated with a topical flea and tick insecticide. Pets should be prevented from running in densely vegetated areas surrounding the home. If possible, trees around homes should be trimmed to allow sunlight to illuminate and dry the lawn.

Many PCOs have traditionally offered a tick control service that relies on application of granular or liquid pesticides in leaf litter areas where nymph and larvae ticks are found. These applications should begin in the early spring when ticks are emerging and continue into the fall.

As the pressure from ticks and Lyme disease continues in the future, it is important that pest management professionals are able to correctly target nymph and larvae habitats for control and advise customers of changes that can be made around their property to make it less attractive to ticks and their reservoir hosts. Pest control operators in high-risk Lyme disease areas should market tick control services to neighborhoods adjacent to wooded areas and provide brochures to educate customers in these areas.

The author is a Yale Univ. School of Public Health graduate student and a former entomologist with Western Exterminator, Anaheim, Calif.