JUL 24, 2015 09:12 AM PDT

How Salmonella Infects Chicken Flocks

According to the CDC, non-typhoidal Salmonella causes over 1 million illnesses, 23,000 hospitalizations, 450 deaths, and approximately $365 million in direct costs in the US annually. Of those illnesses, 100,000 are attributed to antibiotic resistant Salmonella. The rise of resistant bacteria found in food-producing animals may be attributed to the use of antibiotics in livestock production. Studies published by the US Food and Drug Administration reports that more antibiotics are sold for food-producing animals than for people nationally.
New study finds chickens can be colonized with more than one Salmonella strain at a time.
The issue of antibiotic use in food-producing animals is quite controversial. In livestock production, antibiotics are sometimes used in subtherapeutic doses in animal feed in order to promote growth and improve feed efficiency. In some countries the use of antibiotics in animals used for food is banned due to the rising concerns of antibiotic resistance in human medicine. In developing countries, antibiotics are often sold illegally or can purchased on the internet. In order to prevent antibiotic misuse, some organizations are trying to restrict the use of all drugs in livestock production. However; the World Health Organization for Animal Health states that antibiotic resistance in humans is overestimated and restricting the use of antibiotics in livestock production is detrimental to animal health as well as the ability to produce economical quality meats.

In order to help producers determine how Salmonella spreads through poultry flocks, researchers at the University of Arkansas are trying to map out the Salmonella transmission process within flocks. While their data is still preliminary, they have already collected some interesting information. In brief, they inoculated six different strains of Salmonella in varying doses into Salmonella-free flocks. The Salmonella strains were marked with a specific DNA identification code so they could follow each strain through the flock transmission process. Chickens were inoculated either orally (direct), through feed, or through their water supply. For each treatment method chickens were infected with either high or low doses of the 6 Salmonella strains. Infected chickens were then placed with Salmonella-free chickens so they could trace the spread of the bacteria through the flock for each treatment group. In birds that were infected orally, the Salmonella-free birds were infected with only one strain of Salmonella that was given at a high dose. Birds that were infected with an oral low dose were infected with multiple strains of Salmonella. The other two treatment groups (feed and water) demonstrated significantly different results. In both the feed and water inoculated groups the Salmonella strains given in high doses initially spread to the other chickens in equal proportions. As time went on, more of the Salmonella administered in low doses entered into the flock and the strains began to be found equally among all of the birds despite the dosage.

These findings are the first to demonstrate that chickens can become contaminated with more than one strain of Salmonella at a time. Previous reports have stated that once a chicken becomes colonized by one strain of Salmonella, it would prevent the other strains from colonizing the bird - also known as the colonization inhibition theory. Researchers from the Arkansas study hope to further explore how Salmonella is transmitted in chicken flocks in order how to determine how it enters the flocks so they can stop the transmission and therefore; stop transmission to humans.

Sources: CDC, Reuters, Food Safety News
About the Author
  • I am a postdoctoral researcher with interests in pre-harvest microbial food safety, nonthermal food processing technologies, zoonotic pathogens, and plant-microbe interactions. My current research projects involve the optimization of novel food processing technologies to reduce the number of foodborne pathogens on fresh produce. I am a food geek!
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