The economic value of R0 for selective breeding against microparasitic diseases.

ABSTRACT

BACKGROUND: Microparasitic diseases are caused by bacteria and viruses. Genetic improvement of resistance to microparasitic diseases in breeding programs is desirable and should aim at reducing the basic reproduction ratio [Formula see text]. Recently, we developed a method to derive the economic value of [Formula see text] for macroparasitic diseases. In epidemiological models for microparasitic diseases, an animal's disease status is treated as infected or not infected, resulting in a definition of [Formula see text] that differs from that for macroparasitic diseases. Here, we extend the method for the derivation of the economic value of [Formula see text] to microparasitic diseases. METHODS: When [Formula see text], the economic value of [Formula see text] is zero because the disease is very rare. When [Formula see text]. is higher than 1, genetic improvement of [Formula see text] can reduce expenditures on vaccination if vaccination induces herd immunity, or it can reduce production losses due to disease. When vaccination is used to achieve herd immunity, expenditures are proportional to the critical vaccination coverage, which decreases with [Formula see text]. The effect of [Formula see text] on losses is considered separately for epidemic and endemic disease. Losses for epidemic diseases are proportional to the probability and size of major epidemics. Losses for endemic diseases are proportional to the infected fraction of the population at the endemic equilibrium. RESULTS: When genetic improvement reduces expenditures on vaccination, expenditures decrease with [Formula see text] at an increasing rate. When genetic improvement reduces losses in epidemic or endemic diseases, losses decrease with [Formula see text] at an increasing rate. Hence, in all cases, the economic value of [Formula see text] increases as [Formula see text] decreases towards 1. DISCUSSION: [Formula see text] and its economic value are more informative for potential benefits of genetic improvement than heritability estimates for survival after a disease challenge. In livestock, the potential for genetic improvement is small for epidemic microparasitic diseases, where disease control measures limit possibilities for phenotyping. This is not an issue in aquaculture, where controlled challenge tests are performed in dedicated facilities. If genetic evaluations include infectivity, genetic gain in [Formula see text] can be accelerated but this would require different testing designs. CONCLUSIONS: When [Formula see text], its economic value is zero. The economic value of [Formula see text] is highest at low values of [Formula see text] and approaches zero at high values of [Formula see text].