Salmonella typhimurium infection in broilers and its effects on gastrointestinal health and performance

Salmonella typhimurium (ST) infection not only causes salmonellosis in humans, but also can result in great economic losses in the typically narrow-margin, high-volume broiler business due to reduced growth rates and mortalities. Over the last decade, the use of antibiotics and attenuated vaccines to restrain or prevent bacterial infections in domestic animals has been criticised because of the possible development of antibiotic resistance and the potential dangers of residual antibiotics and vaccines in animal-derived food products for human consumption. For these reasons, many countries have begun phasing out growth promoting antibiotics in broiler diets. It is therefore essential for the poultry production industry to develop feed additives and processing techniques as alternatives for sub-therapeutic dietary supplementation of antibiotics. However, innovative research is needed to evaluate the efficacy of new and existing alternative products. The general aim of this trial was to determine the effects of Salmonella typhimurium colonisation of the gastrointestinal tract of broiler chicks on gastrointestinal health and production performance. The effect of Zinc-Bacitracin (Zn-BC), a commonly used antibiotic growth promoter in the poultry industry, on Salmonella colonisation was also measured. A pilot trial was first conducted to determine the level of Salmonella typhimurium required to infect broiler chicks, and the necessity of administering an immunosuppressive agent in order to obtain infection. The main trial followed to determine the effects of Salmonella typhimurium on gastrointestinal health and function. The ultimate aim of the study was to obtain baseline values of various parameters that could be used in future trials for the evaluation of antibiotic alternative products. The results obtained from the pilot trial showed that it was not necessary to administer cyclophosphamide as the Salmonella typhimurium proved to be highly virulent. The cloacal swabs taken in the second trial showed that the use of Zn-BC as an antibiotic did not inhibit Salmonella colonisation in the challenged birds. The inclusion of Zn-BC in this trial inhibited the growth of the gut microflora allowing the Salmonella to proliferate in the body of the chicken, which lead to the conclusion that the routine inclusion of Zn-BC at sub-clinical levels as a growth promoter may be detrimental when the bird gets exposed to Gram(-) bacteria, such as Salmonella. In both of the trials, Salmonella challenge resulted in enlargement of the organs with a consequent increase in the organ weights. In the pilot trial there was a significant difference (P<0.0033) of the control weights for the duodenum, ileum, caeca and liver and those of the Salmonella infected birds. Control birds that did not receive CY had duodenum weights of 1.00 (±0.236) while the birds infected with 1 x 108 CFU/mL had weights of 1.99 (±0.310), while the control birds that did receive CY had duodenum weights of 0.98 (±0.244) with the Salmonella infected birds having weights of between 1.79 (±0.299) and 2.13 (±0.006). Significant results (P<0.016) in the main trial were found to occur predominantly at 7 days of age for the duodenum, jejenum, ileum and caeca weights. Control birds in the group that did receive antibiotics had 7 day duodenum weights of 1.80 (±0.301) compared to the Salmonella infected bird which had weights of between 2.33 (±0.376) and 2.51 (±0.424). In general Salmonella did not affect the growth and performance of the challenged birds. Birds challenged with Salmonella showed a tendency to have enlarged livers, possibly due to hepatic damage. In the main trial there was a significant difference (P<0.016) in liver weights at 28 days of age between the control and Salmonella infected groups regardless of whether the birds received antibiotics or not. The control birds that received antibiotics had liver weights of 3.24 (±0.234) while the birds infected with the higher level of Salmonella had weights of 3.86 (±0.542). This finding, together with the noticeable, although mainly insignificant, trend of decreased serum albumin levels and increased serum globulin and total serum protein levels noted in infected birds can be used in conjunction to measure the effect of ST on liver damage. Salmonella colonisation resulted in an increase in the severity of lesions seen in the gastrointestinal tract (P<0.0016). Histopathology results proved to be inconsistent and did not provide any conclusive evidence on the effect of Salmonella on the organs. Villi measurements taken in the second trial showed that Salmonella significantly (P<0.016) shortened the length of the villi in the duodenum and jejenum of challenged birds when measured at 28 days of age. Control birds had duodenum villi length of 662.5 (±56.79) while those birds infected with Salmonella had lengths of between 558.9 (±77.74) and 537.0 (±51.66). There was a significant difference in the duodenum villi length regardless of antibiotic inclusion into the diet. In the birds that did receive antibiotics, there was a significant difference (P<0.016) in the jejenum villi length with the control birds having the longest villi 725.7 (±90.92) while the birds infected with the higher level of Salmonella having the shortest villi 557.2 (±124.5). It would appear that using all of the information and results obtained for liver weights, broiler performance, serum biochemical level, lesion scoring, histopathology and villous morphological measurements should be used in conjunction with one another to measure the effect of Salmonella on the broiler chicken. The results obtained in this trial clearly show just how significant a problem Salmonella infection can be in the poultry industry due to seemingly healthy adult birds displaying little or no systemic disease being non-symptomatic carriers. Many of the Para-typhoid salmonellae do not always produce clinical signs in chicks, and their presence in the poultry industry may go unrecognised for this reason.