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 R 0 . Recently, we developed a method to derive the economic value of R 0 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 R 0 that differs from that for macroparasitic diseases. Here, we extend the method for the derivation of the economic value of R 0 to microparasitic diseases. Methods: When R 0 ≤ 1 , the economic value of R 0 is zero because the disease is very rare. When R 0 . is higher than 1, genetic improvement of R 0 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 R 0 . The effect of R 0 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 R 0 at an increasing rate. When genetic improvement reduces losses in epidemic or endemic diseases, losses decrease with R 0 at an increasing rate. Hence, in all cases, the economic value of R 0 increases as R 0 decreases towards 1. Discussion: R 0 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 R 0 can be accelerated but this would require different testing designs. Conclusions: When R 0 ≤ 1 , its economic value is zero. The economic value of R 0 is highest at low values of R 0 and approaches zero at high values of R 0 . © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article' s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article' Publisher's NoteSpringer Nature remains neutral with ...

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