This study aimed to investigate antibody responses to porcine reproductive and respiratory syndrome virus, influenza A virus, and Mycoplasma hyopneumoniae in pigs at different stages of production, to determine the interaction in antibody response between those pathogens, and to examine the effects of nursery diet complexity on antibody responses to those pathogens.
In general, seropositivity indicates that an animal has either absorbed maternally derived antibodies or been exposed to infectious agents through natural infection or vaccination. In this study, seropositivity proportions were high at weaning for all three pathogens, likely due to the absorption of antibodies through the sow’s colostrum and milk (17,18), low at weaning, and high again for PRRSV and IAV at the end of the grower and finisher stages. The decline in seropositivity observed from weaning to nursery in this study indicates the loss of maternal antibodies (2), which also suggests that pigs may be particularly susceptible to pathogens post-weaning. Antibodies to both PRRSV and IAV appeared to be fairly prevalent in the high seropositivity groups, and thus the risk for infection from these agents is high throughout production. Because none of the groups included in the multivariable analyses were vaccinated for any of the pathogens of interest, it is largely assumed that increases in antibody responses post-weaning were the result of natural infection. Implementation of vaccination paradigms may be beneficial to enhance the development of the immune response. Additionally, monitoring for the presence of disease using techniques such as ELISA will help to identify specific pathogens present on a farm and minimize doubt of falsely diagnosing one disease for another.
Unlike PRRS and IAV, pigs were more likely to be seropositive for M. hyopneumoniae only at weaning compared to end of nursery in the high seropositivity groups. These results either indicate the vulnerability of weanlings to infection, as antibody responses did not seem to increase significantly in later stages of production, or the lack of M. hyopneumoniae infections in these stages. Due to the nature of M. hyopneumoniae and its tendency to produce chronic infections in the host, the assumption is that after the decline of maternal antibodies, the young pigs mount a slower immune response (19). This seems to suggest that the pathogen is not being cleared from production but rather the immune response is slower to respond.
The second objective of this study was to determine if infection with one infectious agent influences the activity of another. It was found that pigs seropositive for PRRSV were more likely to be seropositive for M. hyopneumoniae. The present study did not determine if co-infection with PRRSV and M. hyopneumoniae produced more severe disease, but these results have been reported in the past (9,20). Additionally, while managerial factors, such as pig density and pig flow, would affect the spread of disease on farm, it is possible that infection with one agent would increase susceptibility to the other agent(s). This suggests that while controlling for the presence of one infectious agent is important, in order to prevent more severe disease, care should be taken to prevent co-infection as much as possible. Understanding which pathogens are a threat on a farm-specific basis using techniques such as ELISA may help in reducing the detrimental effects of co-infection.
The final objective of this study was to investigate whether nursery diet complexity had an impact on antibody responses to PRRSV, IAV, and M. hyopneumoniae. Pigs fed a low complexity nursery diet were more likely to be seropositive for PRRSV and may have been likelier to be seropositive for IAV. However, there was no significant association between nursery diet complexity and M. hyopneumoniae seropositivity. The diet complexity has also been previously found to have no effect on antibody responses to Salmonella (14). These results may suggest that the low complexity diet increased the susceptibility of pigs to PRRS and influenza viruses but had no effect on susceptibility to M. hyopneumoniae and Salmonella. Alternatively, these results may indicate that the LC diet elevated the immune response to PRRSV and IAV while having no effect on the immune response to M. hyopneumoniae and Salmonella. Interestingly, nursery diet complexity only affected seropositivity to the viral pathogens but not the bacterial pathogens. The association found between nursery diet and antibody response to pathogens tested in this study should be interpreted with caution and need to be investigated more thoroughly while evaluating innate and cell mediated immune responses in additional pigs. Further investigation into the effects of nursery diet complexity on antibody responses to other notable porcine pathogens may help shed more light on the effects of diet on immune development. Additionally, other branches of the immune system, such as cell-mediated immune responses, could also be examined to determine if additional facets of the immune system are affected.
While seropositivity at the pig level was relatively high, a proportion of pigs remained seronegative throughout all stages of production. This indicates either that these pigs were never exposed to the infectious agents; that animals were exposed but the pathogens were unable to bypass the innate immune system in order to establish infection and activate the adaptive immune system; that an immune response was generated but was not robust enough to be read as seropositive by the ELISA kits; or that pigs had not yet seroconverted at the time of sample collection. However, there may have also been some variation in results based on the ELISA kits used for antibody detection. The IDEXX ELISA kits have been found to have 100% sensitivity and 99.9% specificity for PRRSV (21); 86 and 89% for IAV (22); and relatively low sensitivity (63%) but high specificity (100%) for M. hyopneumoniae (19). However, as noted by Erlandson and colleagues (18), the low sensitivity of the M. hyopneumoniae test is likely due to the nature of the infectious agent and the slow immune response produced by M. hyopneumoniae rather than the efficacy of the ELISA kits themselves. As such, there may have been false negatives generated in this study, but the classification of groups as high and low seropositivity and considering a pig “seropositive” if it tested seropositive at least once over the course of production likely worked to counteract this issue.
The ELISA kits used in this study were unable to differentiate between antibody responses to natural infections and vaccination. However, because only one farrowing source was vaccinated for M. hyopneumoniae and the corresponding groups were not included in the analyses, the seropositivity observed in the high seropositivity groups can be largely assumed to be from maternal antibodies in the early stages of production and natural infection later in life. These results may help to encourage vaccination in post-weaning pigs, when the interaction between maternal antibodies and vaccine antigens is minimized (2).