Predicting the effect of vaccination on the transmission dynamics of heartwater (Cowdria ruminantium infection)

We used a mathematical description of the transmission dynamics of the tick -borne infection Cowdria ruminantium in commercial beef enterprises in Zimb abwe to consider the potential impact of a candidate vaccine to prevent hea rtwater. The important characteristics of the vaccine were (1) a delay in d evelopment of full protection, (2) prevention of clinical disease but not o f infection and (3) a waning period of protection in the absence of challen ge. We considered three different scenarios in which the vaccine might be u sed: prophylactically in susceptible cattle prior to the introduction of in fection into a herd; in susceptible cattle in the face of an epidemic (i.e. , when the infection is introduced and disease is first noticed); and at eq uilibrium (i.e., when parasite, vector and host have been co-existing for s ome time). The epidemic rise in infection was modelled assuming two differe nt patterns (i.e., resulting from slow and fast increases in tick challenge ). Vaccination (administered both in the face of an epidemic and prophylactica lly) reduced and delayed the peak of the epidemic. With insufficiently freq uent revaccination, this can result in the epidemic occurring during a peri od of susceptibility, so that the benefit derived from a more-efficacious v accine is lower than that from a less-efficacious vaccine. A vaccine of onl y 30% or 50% efficacy (if given to the whole herd) can have important effec ts on both morbidity and mortality if administered with sufficient frequenc y. However, a highly efficacious vaccine (e.g., 90%) can have only minimal effect if revaccination occurs too infrequently - especially if the epidemi c of disease occurs when tick challenge is high and vaccination-related imm unity has waned. There was a fairly consistent pattern of decreasing return s on increasing protection, although this was reversed in the situation of annual vaccination undertaken prophylactically combined with an epidemic of infection that occurred when the tick challenge was relatively low. Vaccination in equilibrium situations was most beneficial at low and interm ediate tick challenges. There was very little effect of vaccination in high -transmission areas regardless of vaccine efficacy and/or frequency of reva ccination because most animals were infected during periods of innate or ma ternally derived immunity (i.e., under endemic stability). Our results suggest that where relatively high tick challenge can be achiev ed and consistently maintained, vaccination may be used in susceptible herd s to minimise losses in a policy of transition to endemic stability. (C) 19 99 Elsevier Science B.V. All rights reserved.