Potential risk of pathogen transmission by acaricide-poisoned ticks

Many aspects of tick poisoning with acaricides have yet to be elucidated. One of them is the influence of acaricide poisoning on tick infectivity to their hosts. To clarify this problem, we should know how tick poisoning develops after acaricide application. Data obtained during more than three decades of work with ticks of various species and with acaricides of different groups are presented in the paper. The first important phenomenon found was the gradual and progressive development of toxic symptoms after acaricide application, with death of the treated ticks delayed for days or even weeks ('slow-death syndrome'). The development of symptoms was much faster after the application of fenthion, an organophosphorous acaricide, when compared to DDT, a chlorinated hydrocarbon. The larger the adult ticks of a particular species, the more refractory they were to acaricide action. The duration of the development of toxic symptoms directly correlates with the degree of species-specific refractoriness. A special index T(LD(50)) (T(LC(50)) was introduced for comparing the duration of the poisoning development between different tick species, populations etc. The index determines the time when the final mortality is reached after using the LD(50) of a particular acaricide. Another index , T(LD(90)), was used for practical purposes. The values of these indices decreased with increasing age of tick populations. The prolonged duration of poisoning was also observed in nymphal ticks of species with a prolonged life-span and the ability to over-winter (Ixodes, Haemaphysalis) but not in ticks characterized by a short life-span (Dermacentor). During the entire period of poisoning, from acaricide application until tick death, the individual tick passes through six stages defined by its locomotor capabilities. The stages are the same for nymphs and adults of both hard and soft ticks after treatment with various acaricides. When ticks are at the initial stages of poisoning (1st to 3rd stage for DDT or 1st to 2nd for fenthion), they can attach to hosts and imbibe blood. The average body weight of such ticks after repletion corresponded to that of control ticks. Engorged nymphal ticks normally molted to adults, engorged females normally oviposited, and their progeny did not differ from the progeny of control females. This second important phenomenon, called 'overcoming the poisoning', was observed in all studied species of ticks from several genera. Thus, the slow development of tick poisoning creates a potential for ticks to attach to hosts and to gorge blood, and for infected ticks to transmit pathogens to those hosts, while the ability to overcome the poisoning allows the ticks to survive and makes possible the subsequent trans-stadial and transovarial passage of pathogens. These data can be considered as strong circumstantial evidence of the risk that ticks can present to humans and animals at the initial stages of poisoning after acaricide treatment.