Tick Biology

This is a brief review of tick biology. Most of the review will focus on Ixodes scapularis, the black-legged tick, and its potential for transmitting infectious agents into its hosts. One such infectious agent is Borrelia burgdorferi, the causative agent of Lyme disease. At the end of the article are some common - and not so common - questions I've been asked about ticks.

Life cycle of the black-legged tick, Ixodes scapularis (formerly I. Dammini,) aka deer tick:

In the U.S. Northeast and Midwest, the black-legged tick has four stages (egg, 1arva~, nymph and adult) in its usual two-year life cycle. Throughout this cycle, various hosts are parasitized, but the tick feeds only once during each stage.

Eggs are fertilized in the fall, deposited in the leaf litter in the spring, and emerge as larvae in August. The larvae must first find a blood meal. They crawl onto low-lying vegetation or wait around on the forest floor for their first victim. This victim is commonly, but not exclusively, the white-footed mouse, Peromyscus leucopus. This small mammal is the source (the technical term is reservoir) of the Lyme disease bacterium for the tick population. Once attached to their host, the tick larvae embed their mouth parts into the animal and feed off the blood for two to three days. This feeding period is crucial, because if the larvae feed on a mouse that is infected with the Lyme disease bacterium, they will probably become infected as well. After becoming sated, the larvae feeding on the mice drop off into the leaf litter and become dormant until the following spring.

The larvae are very small and usually go undetected by humans. Tick to human transmission of Lyme disease is probably low at this point, since only 1 percent of black-legged tick larvae are infected at hatching. It should be noted that nymphs infected as larvae the previous year can infect a new host, whether it be mouse, chipmunk, or human, with the Lyme disease bacterium. In areas where Lyme disease is endemic, up to 70% of the white-footed mice carry the spirochete.

During the spring and into the early summer, the tick larvae of the previous year molt into nymphs and emerge to find a new host. As in the previous year, a white-footed mouse often serves the purpose, but other mammals - deer, birds and, unfortunately, humans - can be the victims.

Nymphs are commonly found on the forest floor, inhabiting the leaf litter. Most black-legged tick bites occur during nymph season.

When examined under a microscope, it is apparent that the tick is quite complex. First, the mouth parts are intricately designed to suck blood. The most prominent part of the blood-drawing apparatus is the hypostome. Consisting of a tapered lance with evenly spaced recurved ridges, the ridges act as barbs to maintain a good hold on its host. Second, ticks have their own version of an advanced warning system consisting of carbon dioxide sensors on their front legs that can detect the breath of a possible victim. A questing tick, as it is called, just hangs around on some low-lying vegetation with its two front appendages waiting to clasp onto a warm, moving object that is exuding carbon dioxide.

Overall, the greatest host-seeking activity of nymphal black-legged ticks takes place in spring and summer. This time period coincides with the highest incidence of Lyme disease cases, which occur between the months of May and August, although they may be reported well into the fall season. After feeding for a few days in the summer, the nymphs fall off of their hosts and molt into adults, usually in late summer or autumn.

The adult black-legged tick now climbs upon some vegetation, approximately one meter above the ground, and waits for another host. In the East, this is usually a white-tailed deer. Fortunately, adult ticks are larger than the nymphs and larvae and thus are easier for humans to see.

Adult ticks feed on the blood of their host for 5 to 7 days, but sometimes for as long as 11 days. The goal of the female tick is to feed and mate, as opposed to the male tick, which - most experts believe - only mates and does not feed. The female will engorge with blood in order to nourish the developing eggs.

The peak of adult host-seeking activity takes place in late October and early November. After mating, it is thought that the male drops off of the host and dies. The female drops off into the leaf litter, becomes dormant for the winter, and lays her eggs - between 1,000 and 3,000 of them -the following spring.

Mosquitoes

Louis Magnarelli and colleagues published a paper in 1986 where they tested field isolates of (Connecticut) mosquitoes and deer and horse flies for the spirochete using a Lyme spirochete-specific antibody. They found that 9% of 402 deer and horse flies were positive for the spirochete. Of 66 mosquitoes tested, 7.6% were positive for the spirochete. But one must remember that it was not shown whether these spirochetes were viable or not. In addition, this data does not lend any evidence as to the competence of these insects as vectors of the Lyme spirochete. Competence means the ability of a vector to acquire, maintain, and transmit a particular pathogen. In a laboratory feeding experiment published in 1987, Magnarelli and colleagues fed mosquitoes from membranes containing blood (as opposed to feeding from live animals) "spiked" with B. burgdorferi. They showed that a few mosquitoes (7%) could maintain the Lyme spirochete in their digestive tracts for 14 days. Before one can conclude that mosquitoes transmit B. burgdorferi to humans, it must be shown that (1) mosquitoes can become infected by biting an infected white-footed mouse or other suitable reservoir and (2) that mosquitoes can transmit the spirochete to susceptible hosts. The bottom line is that, yes, a minority of a given mosquito population may harbor the bacterium (in low numbers), but they may not be able to transmit them to humans. To this end, Magnarelli and colleagues tried to infect hamsters by subjecting them to B. burgdorferi-infected mosquitoes. No spirochetes were isolated from the hamsters, but one animal did produce antibodies to the spirochete, suggesting a transient infection. To complicate matters, different species of mosquitoes feed off of different hosts. If the mosquito is vector competent, in order to qualify as a vector of B. burgdorferi to people, the mosquito must be able to feed on mice and humans.

A survey of mosquitoes was performed by a group from the Czech Republic and published last year (Halouzka, et al., 1998). They surveyed 6 species of mosquitoes over the course of three summer seasons (1993-1995) and found an overall infection rate of 1.7%. Not all species of mosquito were infected. In a series of three winter samplings, they found that Culex pipiens molestus had an overall infection rate of 7.4%. This species showed only a 3.4% infection rate in the summer samplings. They successfully isolated B. afzeilli from an unfed Aedes vexans.

Flies

A physician in Old Lyme (CT) published a short report regarding the transmission of the Lyme disease spirochete by a biting fly (Luger 1990). The patient claimed to experience an EM rash as a result of the bite from a fly. In my opinion, this is circumstantial evidence that the fly transmitted B burgdorferi, but it is not direct conclusive evidence The fly was not retrieved and therefore not tested.

Fleas

The possibility that fleas carry B. burgdorferi was investigated by Teltow and colleagues (1991) and took place in Texas. They surveyed many different arthropods (i.e. ticks, fleas) from different sources (feeding on a host, some not found on hosts). In surveying different hosts for the cat flea (Ctenocephalides felis), fleas were found on 58 different hosts. They pooled each collection of fleas from each animal, resulting in 58 pools of fleas. Only 1 pool, (1.7%) originating from a dog, was infected with B. burgdorferi.

Other Species of Ticks

Mather and Mather (1990) looked for B. burgdorferi in three species of host-seeking ticks (ticks that were not feeding on a host) in Rhode Island. They found that of 761 scapularis (26%) were infected. 0f 76 lone star ticks (Amblyomma americanum), none were infected. Of the 44 dog ticks they tested, none were infected. In addition, they experimentally demonstrated that only I. scapularis and not D. variablis and A. americanum could transstadially carry (as larvae molting to nymphs) and subsequently transmit B. burgdorferi to P. luecopus. Mukolwe, et al., (1992) showed that only I. scapularis and not D. variablis or A. americanum can become infected with B. burgdorferi if feeding on white rabbits.

Do other species of ticks transmit B. burgdorferi? Terry Schulze has shown that in southern New Jersey in 1984, five percent of adult and 22% (2 out of 9) of nymphal lone-star ticks (A. americanum) were positive for B. burgdorferi when analyzed by microscopy. In addition, two patients from southern New Jersey each had female lone-star ticks removed from their Lyme rash (erythema migrans). This data suggests that this species of tick may serve as a secondary vector of B. burgdorferi

There have been more recent reports (in the southern US) of A. americanum containing spirochetes distinct from the Lyme spirochete There is increasing evidence that A. americanum may be transmitting a spirochete that is very similar to the B. burgdorferi spirokete that is transmitted by I. scapularis. This clinical entity is referred to in the literature as STARI (southern tick-associated rash-like illness). Kirkland and colleagues in 1997 investigated a series of erythema migrans-like rashes at a camp in North Carolina. In summary, there were 14 cases of this rash in which all patients had removed ticks. Only one tick was positively identified as A. americanum. When the vegetation was sampled, 97% of ticks were lone-star ticks. Of 197 ticks removed by the camp staff and residents, 95% were lone-star ticks. None of the field collected ticks were tested for B. burgdorferi or any other pathogen (Kirkland, personal communication).

A more definitive investigation was just published by Masters and colleagues where they identified 17 patients in Missouri, all of which were bitten by lone-star ticks. All 17 patients experienced EM or EM-like rashes. In all cases the ticks were identified as female or nymphal Lone-star ticks.

Summary

Ticks are obligate blood-feeding parasites and by nature, can transmit infectious agents as they engage in this lifestyle. In the United States, the Lyme disease spirochete is primarily transmitted to humans by ticks such as I. scapularis and I. pacificus. The incidence of Lyme disease is linked to the seasonal appearance of nymphal black-legged ticks, which usually occurs in May, June and July. Daily activity of nymphal black-legged ticks is dictated by ambient moisture and temperature. In general. high humidity is preferred. Although other species of arthropods may harbor spirochetes, they may not be competent enough to maintain the spirochete from one stage to the next. Their role as competent vectors is presently undefined.

Literature cited

* Halouzka, J et al. 1998. Isolation of the spirochete Borrelia a~elli from the mosquito Aedes vexans in the Czech Republic. Med Vet Entomol 12: 103-105.
* Kirkland, KB, et al. 1997. Erythema migrans-like rash illness at a camp in North Carolina: A new tick-borne disease? Arch Intern Med l997 Dec8; 157(22): 2635-2641.
* Luger, S. W 1990. Lyme disease transmitted by a biting fly. New Eng. J. Med. 322:1752.
* Magnarelli, L. J. Anderson. 1988. Tick and biting insects infected with the etiologic agent of Lyme disease, B. burgdorferi Journ. Clin. Micro. 26:1482-1486.
* Magnarelli, L. J. Anderson and A. Barbour. 1986. The etiological agent of Lyme disease in deer flies, horse flies and mosquitoes. Journ Infect Dis 154:355-358.
* Magnarelli, L., J. Frier and J Anderson 1987 Experimental infections of mosquitoes with B burgdorferi, the etiologic agent of Lyme disease. Journ Infect. Dis. 156:694-695.
* Masters E, et al. 1998 Physician-diagnosed erythema migrans and erythema migrans-like rashes following Lone Star tick bites. Arch Dermatol. Aug; 134(8): 95 5-960.
* Mather, Tand M. Mather, 1990. Intrinsic competence of three Ixodid ticks as vectors of the Lyme disease spirochete. J. Med. Entomol. 27:646-650.
* Mukolwe, et. al, 1992. Attempted transmission of Borrelia burgdorferi by Ixodes scapularis, Dennacentor varablis and Amblyomma americanum. J. Med. Entomol. 29:673-677.

Answers to common (and not so common) questions about ticks

Q. 'My wife found several embedded ticks on her over the summer while I, engaged in the same outdoor activities with her, didn't find any. How can this happen?"

A. As a scientist I would ask the following: In this scenario, is the husband out on the same day that the wife finds ticks on her? If so. is he in the same exact vegetation? What I'm getting at is that ticks, such as the black-legged tick, have some basic requirements for where they set-up a home base. A prime example is that they prefer high humidity. Tick activity is much greater on a moist spring day as compared to a dry day. Temperature plays a role as well. A warm day in May with high humidity, for example, would allow nymphal black-legged ticks to leave the leaf litter and quest off of low-lying vegetation. Nymphs would be less likely to leave the forest floor on a cool, dry day.

Q. "Is it possible that some people have a greater ability to 'attract' ticks?"

A. To my knowledge, no one has done this experiment, but I find that hard to believe that a host seeking arthropod could be more attracted to one person over another. In a sense, ticks lead a life of chance and must take what comes their way.

Q. How long do ticks live?

A. It depends on the species. Soil ticks can survive for years under the proper conditions, but let's keep our discussion restricted to the hard ticks. In one example, I. scapularis nymphs survived 14 months in the unfed state. Interestingly, B. burgdorferi survived in these nymphs as well. I have kept adult I. scapularis alive for three to four months in glass vials kept in a cool, moist area. Other species of hard tick, such as Dennacentor variablis (American dog tick) can survive for many months waiting for a blood meal.

Q. By what mechanism does I. scapularis transmit the agent of Lyme disease?

A. In an unfed infected tick, the Lyme spirochete hides out in an semi-quiescent state in the midgut of the tick's digestive system. As the tick imbibes blood, the spirochete is stimulated to divide by the increased temperature and other chemical factors in the blood. The spirochetes are released from the midgut and migrate to the salivary glands. As the feeding tick continues to salivate, the spirochetes are introduced into the host. Based on laboratory experiments, this process takes approximately 48 hours. Most medical entomologists have come to the agreement that it takes this long for a victim to become infected because the tick must first ingest some blood. Some tick biologists believe that infection occurs by salivation of spirochetes, others believe it occurs by regurgitation. Regardless of how the bacterium is introduced, the feeding tick must salivate in order to successfully feed for this extended period of time because the saliva prevents the blood from clotting. This 48 hour time period is hotly debated by many people. Some feel transmission can occur in a much shorter time frame. Unfortunately there is no data to support shorter transmission times.

Q. Do tick populations in a given area sometimes disappear so areas of high Lyme incidence become areas of low incidence?

A. There are certainly normal fluctuations in tick populations. Whether these fluctuations are enough to translate into reduced Lyme cases is a complex question. In order to systematically answer this question studies must be conducted-tick surveys as well as epidemiological ("serosurveys") surveys. Tick surveying involves collecting ticks from the field from host animals and testing them for B. burgdorferi infection. Serosurveys involve testing a given human population for anti-Borrelia antibodies. A correlation between Lyme incidence and tick population infection rates must be made.