Ticks as the main target of human tick-borne disease control: Russian practical experience and its lessons

For several decades, the emphasis in human tick-borne disease control has been on the epidemiologically-based preventive (non-specific) approach where tick vectors were the main target of control impact. A long-term, large-scale campaign for controlling the taiga tick Ixodes persulcatus, the main vector of tick-borne encephalitis, was carried out in Russia in the 1950s to 1970s. The practical experience accumulated during that campaign could be of great value for the current development of strategies of tick-borne disease control. A general scheme of human protection from tick-borne diseases is presented where the required investment into protection is proportional to the risk of human infection and particular strategies of control and protection are differentiated. The critical point in the preventive approach is the necessity of radical tick suppression in the areas with the highest risk of human infection that can be successfully achieved only through chemical treatment directed at the eradication of the entire tick population. The following aspects are considered: the tick population (or a group of populations) as a desirable target of any acaricidal impact (biological and geographic aspects, the fate of the population after treatment); the advantage of long-term planning for control campaigns; and the influence of acaricidal impact on foci of tick-borne diseases. The conception of losses of potential pesticidal impact efficacy provides much room for the improvement of conventional tick control strategies making them more efficient and safe to the environment. The current tendency to make humans the main target of control through vaccination and/or medical treatment (specific approach) does not fit the objectives of effective human protection from tick-borne diseases, especially because of a tick population's ability to carry and transmit more than one pathogen over the same area.

Human toxicosis caused by the tick Ixodes redikorzevi in Israel

The clinical symptoms of a tick toxicosis caused by Ixodes redikorzevi in northern Israel are described. The patient was a nine-year-old Arab boy from an agricultural village who was admitted to a local hospital and had been suffering for 4 days from fever and torticollis. A partially engorged female tick was removed from the nuchal area of the scalp, and the symptoms disappeared within 2 hours. Morphometrical measurements indicated that the tick was attached to the skin for 3-4 days.

Rhipicephalus sanguineus and R. turanicus (Acari:Ixodidae): closely related species with different biological characteristics

Life cycle parameters of 2 closely related tick species, Rhipicephalus sanguineus Latreille and R. turanicus Pomerantsev, were studied under laboratory conditions. Both Rhipicephalus, which have small adults, demonstrated the same adaptations as large tick species inhabiting deserts and semideserts: high reproductive rate, decrease in egg size, and an increase in interstage growth to compensate for the smaller size at birth. Pronounced quantitative differences between both species were discerned in relation to these adaptations. Female R. turanicus produced twice as many eggs as R. sanguineus which was facilitated by the greater amount of blood engorged by females and by the smaller egg weight in R. turanicus as compared with R. sanguineus. In all developmental stages, the weight increase from unfed to fed ticks was greater in R. turanicus than in R. sanguineus (23% higher in larvae, 118% in nymphs, and 26% in females). The increase in weight in R. turanicus from the unfed larva (0.013 mg) to the unfed female (3.31 mg) was 254-fold, and in R. sanguineus it was 127-fold (from 0.021 to 2.54 mg). In nymphal R. turanicus, the higher density and the greater height of the dorsal epicuticular folds, as well as procuticular indentations found inside the folds allow this tick to stretch its alloscutum during blood engorgement to a greater extent than R. sanguineus. The rates of blood ingestion (for nymphs and females), egg maturation, and metamorphosis were 1.1-1.7 times greater in R. turanicus than in R. sanguineus. A life cycle strategy with both a higher reproductive rate and faster development in R turanicus may be explained by its greater dependence on environmental factors than that in R. sanguineus.

Physiological age of ixodid ticks: aspects of its determination and application

This review analyzes 30 yr of data concerning physiological age of ixodid ticks. The level and state of nutritional reserves in unfed ticks has been proposed as an index of tick physiological age. The dynamics of the use of these substances reflect tick aging. Thus, physiological age of ixodid ticks is determined by estimating the irreversible changes caused by natural vital activity in the body of unfed ticks, which inevitably raises the probability of tick death. The 4 age groups used in most studies correspond to different periods in the life of unfed ticks: the postmolting development, the commencement of active life (= questing activity), the midperiod of active life, and the final period of active life. The aging of adult ixodid ticks has been studied in natural populations and laboratory colonies of different species belonging to several genera. The dynamics of age composition through the activity season observed in adult Ixodes persulcatus, I. ricinus, and Dermacentor reticulatus from field populations correlated well with the known data on the life history of these species. Physiological age was successfully used as an indicator of the patterns of tick activization (= beginning of questing activity). Tick susceptibility to acaricides was shown to increase with tick aging. The success of maintenance, reproduction, and transmission of pathogens was found to be strongly influenced by tick physiological age. The physiological age can provide information about the present physiological status of an individual tick or a group of ticks and, on this basis, predictions can be made (for example, life expectancy, degree of response to certain factors). The physiological age does not give reliable information about the past life of ticks, in particular their calendar age. In this respect, the meaning of the physiological age of ixodid ticks corresponds to the biological (physiological, functional) age of animals, as used in gerontology, and differs from the physiological age of mosquitoes and other bloodsucking dipterans. Balashov's histological method of tick age determination successfully used by many authors, remains the most appropriate, until now. However, there is ample room for further advancement of methodologies of tick age assessment. The future methods should be based on the quantitative estimation of a number of characters (test-battery) related to different organs or systems.

Ability of successful attack in two species of ixodid ticks (Acari: Ixodidae) as a manifestation of their aggressiveness

Patterns of attacking hosts by two closely related species of ixodid ticks (Ixodes persulcatus and I. ricinus) were compared in parallel field tests. The ability of active unfed adults of both sexes to adhere to a flannel flag dragged over grass vegetation and to remain on the flag during this process were estimated. The tests were conducted under two temperature conditions, 6-10 degrees C and 17-22 degrees C. In all test versions, adults of I. persulcatus were more successful both in adhering to the flag and in remaining on it. There were no consistent differences between males and females of the same species. The results demonstrated a differing ability of successful attack in both tick species. This ability is a complex derivative of the tick activity and aggressiveness. An attempt is made to analyse the latter phenomenon, which is considered as a general one for all bloodsucking arthropods. The aggressiveness of arthropods should be taken into account as one of the leading factors influencing the sampling results.