Salmonella in broilers under heat stress

Given that Brazil is world's main poultry meat exporter, importing countries are becoming more and more demanding about sanitary control (Back, 2002). The presence of pathogenic microorganisms in broilers and the probable contamination of carcasses during processing, do present relevant aspects to be considered in terms of animal and public health (Moreno et al., 2006). Salmonella is considered one of the most important food-borne pathogens, mainly in chicken food products and byproducts, due to the fact that the germ is broadly distributed in nature, a great number of reservoirs, presenting non specific serum types to the host and because it presents several antimicrobial multi-resistant strains (Bersot, 2006). Quinteiro-Filho et al. (2010) verified that heat stress was capable of increasing the colony forming units of Salmonella spp. (CFU/g) in the spleen of birds causing a reduction in weight gain, as well as reduced feed intake, and increased mortality rate. Hence, the objective of this study to evaluate the interaction of acute heat stress on the incidence of Salmonella spp. throughout the productive chain of broilers.

The trial was performed in experimental climate chambers at the School of Agricultural and Veterinarian Science - FCAV - UNESP, Jaboticabal Campus, SP, Brazil. Five hundred Cobb birds were used, of which, 100, were raised in a thermoneutral climate chamber during the entire experimental phase, acting as the control group for this study. Thermoneutral temperature was adjusted according to the ideal one for each growing stage: 0-21 days 28°C; 22-35 days 24°C; and 36-42 days 21°C. The remaining 400 animals, subjected to stress, were kept in another chamber, initially adjusted to a thermoneutral temperature ideal for each growing phase, and during the stress phases, the temperature went up to 32ºC. Birds were subjected to heat stress during different time periods (0, 24, 48, and 72 hours), starting at 21, 35 and 42 days of age and after each heat stress period 12 of them were killed per each treatment:

T1 -    birds of 21 days raised at a thermoneutral temperature of 28°C of 0-21 days not subjected to heat stress;

T2 -    birds of 21 days raised at a thermoneutral temperature of 28°C of 0-21 days and subjected to heat stress of 32°C during 24 hours;

T3 -    birds of 21 days raised at a thermoneutral temperature of 28°C of 0-21 days and subjected to heat stress of 32°C during 48 hours;

T4 -    birds of 21 days raised at a thermoneutral temperature of 28°C of 0-21 days and subjected to heat stress of 32°C during 72 hours;

T5 -    birds of 35 days raised at a thermoneutral temperature of 24°C of 22-35 days not subjected to heat stress;

T6 -    birds of 35 days raised at a thermoneutral temperature of 24°C of 22-35 days and subjected to heat stress of 32°C during 24 hours;

T7 -    birds of 35 days raised at a thermoneutral temperature of 24°C of 22-35 days and subjected to heat stress of 32°C during 48 hours;

T8 -    birds of 35 days raised at a thermoneutral temperature of 24°C of 22-35 days and subjected to heat stress during 72 hours;

T9 -    birds of 42 days raised at a thermoneutral temperature of 21°C of 36-42 days not subject to stress;

T10 -  birds of 42 days raised at a thermoneutral temperature of 21°C of 36-42 days subjected to heat stress during 24 hours;

T11 -  birds of 42 days raised at a thermoneutral temperature of 21°C of 36-42 days and subjected to heat stress during 48 hours;

T12 -  birds of 42 days raised at a thermoneutral temperature of 21°C of 36-42 days subjected to heat stress during 72 hours.

The laboratory analysis were performed at the Microbiology Laboratory, Meat Research and Development Center (CTC), Institute of Food Technology (ITAL). In order to detect the existence of Salmonella spp., the Polymerase Chain Reaction (PCR) methodology was used; this was proposed by the manufacturer "Biocontrol". After evisceration, 2 samples from each treatment group were collected, made up by 3 chicken and 2 control samples, a total amount of 48 samples. Neck skin samples were taken after plucking: at the exit of the plucking machine, at the entrance to the chiller, as well as at chiller exit, for a total of 48 samples. Cloacal swabs were collected before and after placing the birds in their compartments. Environmental samples were collected before and after bird placement, for a total of 66 samples. These samples were collected wearing disposable shoe covers and walking parallel to the feeders throughout their length.

Tables 1, 2 and 3 show that (Table 1) both the environment as well as the chicken were not contaminated with Salmonella spp. at the beginning of the study.

Table 1. Presence of Salmonella in chickens and the environment at the beginning of the test.

According to Silva & Duarte (2002), salmonella is widely distributed in nature and the importance of its dissemination has been broadly studied, mainly in poultry production. Such natural dissemination can be one of the possible explanations for the initial absence of Salmonella in the environment and in the animals before placement in their pens, due to the fact that at the beginning of the trial there were no differences in the handling of the birds, nor in temperature between those in the treated groups or the control group.

Table 2. Presence of Salmonella in the environment and in the cloacal swabs on days 2, 3 and 15

Table 3. Presence of Salmonella spp. In the different treatment groups

When analyzing the results from the pens, we can see that all samples from the control group, at 21 days were positive for Salmonella spp. This did not happen with samples from the treatment groups, which presented a low initial contamination level, lasting throughout the experiment. In order to keep the temperature of the control group, some measures had to be adopted, which could have probably made it more susceptible to the outside environment, favoring the entrance of some vectors, hence, contamination. Some neck skin samples from the control group at day 21 days and from those treated on day 35, were contaminated with Salmonella at the entrance to the chiller, but at the exit, they were negative, showing no carcass contamination during the chilling process.

Of all the analyzed samples on days 21, 35 and 42 (n=192), 39 samples (2031%) were Salmonella spp. positive to PCR. Nonetheless, 3 of such samples (7.69%) proved negative with traditional tests. This fact may be explained by the presence of Salmonella DNA, which is detected by PCR; however, the dead bacteria cannot reproduce or create the typical colonies observed in traditional tests, which require a live organism. Contrary to Quinteiro-Filho et al. (2010), Table 3 shows no correlation between changes in the heat stress period and the increase of contamination. On the other hand, population reduction seems to be related to the reduction of Salmonella positive samples. Animals processed on day 21 had 25% infected samples, on day 35 15%; and on day 42, 7.8%.

The different heat stress periods had no influence on Salmonella spp. contamination.

Back A. 2002. Manual of Doenças of Aves. Casacavel, Brasil.

Bersot LS. 2006. Salmonella no Brasil: sua importância no abate of aves. In: V Simpósio of Sanidaof Avícola of UFSM, Santa Maria.

Moreno MRF, Sarantinopoulos P, Tsakalidou E. 2006. The role and application of enterococci in food and health. International J. Food Microbiol. 106:1-24.

Silva EN & Duarte A. 2002. Salmonella Enteritidis em aves: Retrospectiva no Brasil. Rev. Brás. Ciênc. Avíc. 4:85-100.

Quinteiro-Filho WM, Ribeiro A, Ferraz-of-Paula V, Pinheiro LM, Sakai M, Sá LRM, Ferreira AJP, Palermo-Neto J. 2010. Heat stress impairs performance parameters, induces intestinal injury, and ofcreases macrophage activity in broiler chickens. Poult. Sci. 89(9):1905-1914.