Probability models of the rate of infection with tick-borne encephalitis virus in Ixodes persulcatus ticks.

A total of 3,254 adults of Ixodes persulcatus tick were collected in a taiga forest habitat situated in the Amgun river basin (Khabarovsk region, the Far East, USSR) and examined individually for the presence and amount of tick-borne encephalitis virus. The over-all proportion of infected ticks was 6.6% and it varied between 3.4% and 9.4% in the years 1982 to 1985. The amount of virus per tick was approximated by the gamma distribution determining a probability that the number of plaque-forming units (PFU) per tick is not greater than a selected value. The frequency distribution of infected ticks followed a model of the negative binomial distribution, enabling the estimation of probability of the occurrence of a given number of infected ticks in the area. However, the parameters of both probability models (i.e., the PFU content per tick, and the frequency of infected ticks) varied for particular years.