Genetics of disease resistance in wean-to-finish pigs

Heritability can be defined as the proportion of variation within a trait that can be attributed to inherited genetic factors (Falconer and Mackay, 1996). A heritability estimate ranges between 0 and 1 where an estimate of < 0.10 may be considered low and a heritability of > 0.30 might be considered high. Genetic correlations range from -1 to 1 with zero indicating no genetic association between two traits and a genetic correlation close to -1 or 1 meaning two traits are very similar.

Disease resistance

Contributing factors to wean-to-finish survival are numerous and reported in reviews by Gebhardt et al. (2020a) and Gebhardt et al. (2020b). Regarding infectious contributors to wean-to-finish survival, Lundeheim (1979) showed heritability estimates for pneumonia ranged from 0.04 to 0.14 and reported a heritability estimate for pleuritis of 0.13. The same study reported genetic correlation estimates between health disorders with growth rate, backfat and muscling were low (rg = -0.16 to 0.15).

Similarly, Kadowaki et al. (2012) showed a heritability estimate for mycoplasma pneumonia score of 0.07. The same authors reported genetic correlation estimates between health disorders with growth rate and backfat were low (rg = -0.13 to 0.04). Lundeheim (1988) reported heritability estimates for chronic pneumonia and chronic pleuritis of 0.23 and 0.08, respectively. In contrast, Henryon et al. (2003) found a heritability estimate for respiratory lesions of < 0.01. Taken together, these results suggest genetic selection to reduce the incidence of respiratory disorders in the finishing period may be possible.

Selecting against pigs that require antibiotics is perhaps another strategy to enhance wean-to-finish survival. Gorssen et al. (2021) estimated heritability for a range of phenotypes derived from finishing pen treatment records. The authors reported heritability estimates for all antibiotic treatments ranged from 0.18 to 0.44 and treatments specifically against respiratory diseases varied from 0.01 to 0.15. In agreement, Henryon et al. (2001) found heritability estimates for treatments of respiratory disease and diarrhea of 0.12 and 0.16, respectively. Under a natural disease challenge, Putz et al. (2019) reported heritability estimates for treatments per pig ranged from 0.13 to 0.29.

In contrast, in a high health herd, Guy et al. (2018) showed heritability estimates for pig treatments ranged from 0.04 to 0.06. The same study reported common litter effects for pig treatments varied from 0.09 to 0.18. Again, this demonstrates the relative importance of the litter a pig was reared in on its subsequent wean-to-finish health. Collectively, these findings establish the treatment of disease in the wean-to-finish phase to be a heritable trait that may be improved through genetic selection.

The studies by Putz et al. (2019) and Gorssen et al. (2021) correlated individual treatment records with wean-to-finish survival and finishing survival, respectively. Putz et al. (2019) reported a genetic correlation estimate between treatments per pig and wean-to-finish mortality of 0.93 indicating they were very similar traits. In agreement, Gorssen et al. (2021) found a genetic correlation estimate of 0.60 between used antibiotic dose with finishing mortality. Jointly, these results suggest genetic selection for phenotypes computed from treatment records could indirectly improve wean-to-finish survival or add accuracy to genetic selection for wean-to-finish survival. Hence these studies outline the importance of maintaining digitized pig treatment records, perhaps even more so within a genetic commercial test herd.