FACTORS INFLUENCING THE POPULATION OF I. SCAPULARIS
Factors influencing the population of I. scapularis
The black-leggedticks, deer ticks, are external blood sucking arthropods of mammalsthat are widely distributed in the Northeastern, Southeastern andCentral USA. The scientific name of black-legged tick is Ixodesscapularis. I. scapullaris is known as a parasite of thewhite-tailed deer. White-tailed deer are grazers whose habitat arewooded areas, but can also be found in suburban ecosystems that arealmost similar to woodlands (Cortinas, M. R. 2007). The existence ofblack-legged ticks has been linked with presence and abundance oftheir primary host, white-tailed deer. I. scapularis is themain vector of spirochete Borrelia burgdorferi, and causesLyme borreliosis. This paper focuses on the study carried out byDavid et al. (1994) to determine the relationship between thepopulation of I. scopularis with the presence of deer, size ofnatural area, and geographical position.
Previous researchstudies have reported that there is correlation between the tickpopulation and presence of white-tailed deer. The studies reportedthat an increase in the deer population would lead to increase in theblack-legged tick population, especially for the larva. Some of theprevious studies also reported the existence of a strong correlationbetween the existence of deer population and the Lyme diseaseprevalence. This shows that, areas with no or few white-tailed deeris likely to have no or low cases of Lyme disease.
Ticks belong tothe class Arachnida and subclass Acari. There are two main familiesof ticks, which belong to order Ixodida: families Ixodidae andargasidae. Argasid (soft) ticks have four main genera and lack ascutum. I. scapularis belongs to genera Ixodes of thefamily Ixodida, hard ticks. Hard ticks possess a tough, sclerotizedplate on their dorsal surface known as the scutum. Like other Ixodesticks, I.scapularis has four distinct life stages: egg, larva,nymph, and adult (Cortinas, M. R., 2007). The black-legged ticks aresecondary consumers where the immature free-living stages require ablood meal in order to molt to the next stage. The female adult alsorequires a blood meal to produce eggs. The larvae of Ixodeshave three pairs of legs while nymph and adult ticks have four pairsof legs. Ixodes are adapted to the changes in theenvironmental humidity and temperature by possessing waxy, tough,dense cuticle that prevents desiccation. The cuticle covers most ofthe body except the scutum. Both scutum and the cuticle provideprotect against physical injury to the body. They also depend ontheir hosts for protect against desiccation and physical damage(Guerra et al., 2002).
I. scapularis ticks have a 3-host life-cycle that is completedafter 2 years. After mating, female adults drop off to the ground tolay eggs during spring. The eggs emerge into larva during summer andlook for a host which they feed on until they are engorged and dropoff to molt into nymphs. Nymph then looks for another host to feed onand then drop-off to the ground to molt in either female or maleadult. Adult males do not feed on blood, but utilize the food reservethey got during immature stage. Males mate with females on the hostor vegetation. Females die after laying eggs (Cortinas, M. R. 2007).
The black-leggedticks can adapt to various environments, but the tick population isdominant in deciduous forests and woodlands. Their main habitat is onthe white-tailed deer. However, the forest litter layer provides amicro-habitat in which cover is available for eggs during winter andthe molting ticks (Guerra et al., 2002). The litter layer hasa minimized fluctuation of relative humidity and temperature. Thetick spends most of the time on the ground or vegetation. I.scapularis are hardly found in exposed habitats such as lawns,grasslands or agricultural areas. The information on the habitat ofthe black-legged tick is important in order to understand itsdistribution and habitat. Dense wooded vegetation provided goodground cover that has favorable humidity and temperature. The ticksdo not only interact with the white-tailed deer for food and shelter,but they also interact with organisms such as the grey squirrels, redfox and bear. The white-footed mouse is an important host for thetick during its larval stages (Guerra et al., 2002).
David et al.(1994) carried out their study in the eastern and central LongIsland, New York. 22 natural areas in deciduous or mixed woodlandswere sampled. The researchers determined area of the samples usinginformation from the park administration. The study concentrated onthe areas with continuous or nearly closed canopies, edges of thewooded vegetation and the areas reported to have highest proportionsof ticks. Information about the incidence of Lyme disease wasobtained from the department of health services from each township.Data about deer abundance was collected through direct observation ofthe footprints, droppings, sightings and also from the parkofficials.
The population ofnymph at various samples sites was determined by flagging using 1m2flannels. The survey was conducted during peak nymphal numbers, 15-30June, 1992. Stereoscopic microscope was used to distinguish nymphalfrom larval I. scapularis. In order to determine whether thetick required deer o reproduce, larval survey was conducted duringthe larval peak periods using 0.5 m2 flags. The mean ofthe participants’ flagging efforts were used determine thecorrelation. Nonparametric Spearman rank correlation was used tocompare the means of with size of the sites, east-west geographicposition, and incidence of the Lyme disease.
According to the results of the study, there is no correlationbetween deer movement within each park and the tick population. Thefrequency of tick per 30-s flagging was highest (44.5%) in areas withdeer and lowest (8.4%) in areas without deer. From the Table 1, it isevident that natural area and nymphal tick population weresignificantly correlated. East-west geographic position did notinfluence population of ticks in the Long Island. There was asignificant relationship between the number of nymphal ticks andnumber of cases of Lyme disease increase in nymphal populationresult in increase in the incidence of Lyme disease (David et al.,1994).
Due to thesignificant correlation between the tick population and incidence ofLyme disease, the number of ticks can be used in prediction of thedisease around the study area and elsewhere. The results could be asa result of human exploitation of natural areas or movement of deerinto residential areas. The population of nymph is lower in absenceof deer. However, the presence of I. scapularis does notnecessarily required white-tailed deer. Therefore, controlling theLyme disease will not be effective by eradicating deer population.Since large natural area had relatively high population of ticks,there is higher risk of contracting the disease while using theparks. Deer should be contained within the parks only. Controlmechanisms should be more concentrated on the tick parasitism.
Cortinas, M. R. (2007). Investigations of white-tailed deerinfestation by Ixodes scapularis on the Illinois River, USA.(Dissertation Abstracts International, 69-2.)
David et al. (1994), Ixodes scapularis (Acari: Ixodiadae),Deer Tick Mesoscale Populations in Natural Areas: Effects of Deer,Area, Location, J. Med. Entomol. 31(1): 152-158
Guerra, M., Walker, E., Jones, C., Paskewitz, S., Cortinas, M. R.,Stancil, A., … & Kitron, U. (2002). Predicting the risk of Lymedisease: habitat suitability for Ixodes scapularis in the northcentral United States.