Association between transfer of passive immunity and health disorders in multisource commingled dairy calves raised for veal or other purposes: Systematic review and meta-analysis.

Inadequate transfer of passive immunity (ITPI) in newborn dairy calves remains an important risk factor for mortality and morbidity. Most available studies are focused on calves delivered and raised on the same farms. This setting is far different from calves transported and commingled from different farms to be raised as veal or for other purposes. The aim of this systematic review and meta-analysis was to describe the association between ITPI and important health outcomes (mortality, bovine respiratory disease, and diarrhea) in multisource commingled dairy calves raised for veal or other purposes. We searched studies through CAB abstracts (via CAB direct), PubMed, and Web of Science (via ISI) databases until September 2, 2021. Observational studies and randomized trials written in English or French assessing ITPI association with any of the selected outcomes were included. Young dairy calves transported to commercial facilities and explicitly stated as being raised for veal production or not (then considered as "other") were our populations of interest. If raw or adjusted data were available for ≥5 studies for a given outcome of interest, then random effect meta-analysis models were used to investigate ITPI effects on this outcome. Nineteen studies were selected from 6,221 abstracts retrieved in the initial search. We observed significantly higher odds of mortality in calves with ITPI compared with those with successful transfer of passive immunity [odds ratio (OR) = 2.46; 95% confidence interval (CI): 1.43-4.22, n = 8 studies]. Calves with ITPI had higher odds of diarrhea (OR = 3.03; 95% CI: 1.2-7.62, n = 7 studies). A significant publication bias toward publishing studies with positive results was found in studies reporting on bovine respiratory disease (n = 5 studies), which revealed nonsignificant associations after correction of publication bias (OR = 1.40; 95% CI: 0.77-2.6). Heterogeneity could not be thoroughly investigated for mortality and diarrhea due to the limited number of studies. Therefore, the pooled estimates of the random models should be interpreted with caution despite their robustness to sensitivity analyses. In this study, we also observed that multiple definitions for transfer of passive immunity and outcomes were used in the literature. Moreover, the raising system definition was often limited. There seems, therefore, to be a need for standardized definitions of these parameters, as well as a better description of systems used for multisource commingled dairy calves raised for veal production or other production purposes.