Transmission parameters of vector-borne infections.

Vector-borne infections are those for which the agent (virus, bacteria, or parasite) is transmitted from an infected host (animal or human) to another by a hematophagous arthropod (mosquito, tick, lice, and flea). Two parameters quantify the dynamics of a vector-borne infection: (1) the basic reproductive number (R(0)) that is the mean number of secondary infections transmitted from an infectious host by the bite of the vector and (2) the generation interval that explores the speed of occurrence of secondary cases transmitted by the vector from an infectious case. In a population in which some individuals are immune, the parameter of interest is the net reproduction number (R) function of R(0) and the proportion of those immune. For vector-borne infectious agents, R(0) is determined by the number of vectors in contact with a given individual (m), the number of a given vector bites/day on individuals (a), the daily survival rate of the vector (p), the duration of the pathogenic agent's development cycle in the vector (n), the proportion of infected vectors that are really infectious (vector competence) (b), the probability of agent transmission from a viremic individual to the vector for one bite (c) and the host's infectiousness clearance rate (r) with R(0)=(m. a(2). p(n)/-lnp). b. c/r. These parameters are related to geographic and climatic conditions and cannot, therefore, be extrapolated from one situation to another.