Antibiotics "may help Salmonella spread"
Researchers say their findings may have wider implications for human health.
Findings from a US study have revealed that treating salmonella-infected mice with antibiotics may make them sicker and more contagious.
Scientists from the Stanford University School of Medicine gave oral antibiotics to mice infected with Salmonella typhimurium.
In the US, S. typhimurium is responsible for around 1 million cases of food poisoning, 19,000 hospitalisations and 400 deaths every year. Around 70 to 90 per cent of those infected shed relatively small numbers of bacteria, meaning they are not very contagious, while 10 to 30 per cent are asymptomatic, but shed huge amounts of bacteria. These are known as the "superspreaders".
A minority of the infected mice (one in five) were asymptomatic, but were shedding large amounts of bacteria in their faeces. According to findings published online in the Proceedings of the National Academy of Sciences, treating these superspreaders with antibiotics made no difference to their condition or the amount of bacteria shed.
The rest of the infected mice, who were showing symptoms, were shedding only light amounts of bacteria. After being treated with the antibiotic Streptomycin, however, these mice became sicker and started shedding bacteria on a similar level to the superspreaders. Another antibiotic, neomycin, produced the same results.
Senior author Denise Monack said: "They lost weight, had ruffled fur and hunched up in the corners of their cages. They also began to shed much larger quantities of bacteria."
Stanford researchers say they are concerned about the public and animal health implications if the same is true in livestock animals.
However, they say their research offers clues for identifying superspreaders, which could help curtail or prevent epidemics. The superspreaders had a smaller immune response than the other mice. Professor Denise Monack says this explains their absence of symptoms.
"Instead of jousting with the germ, they tolerate it. Their immune cells have been rewired and aren't responding to the inflammatory signals in the intestines in the same way."
Being able to identify the superspreaders could help deal with or even prevent epidemics, researchers say, as those who are asymptomatic evade treatment, yet are more contagious. Stanford scientists hope their findings indicate the possibility of a blood test to identify superspreaders.
Professor Monack said the findings may have wider implications for human health: "If we can figure out what leads to this immune dampening in superspreaders, it could potentially be helpful in suppressing symptoms of people with chronic inflammatory intestinal disorders, such as Crohn's disease or inflammatory bowel disease."
Veterinary practices are being urged to help owners safely dispose of any unused and expired pet medicines - not just antibiotics - during November's Animal Medicines Amnesty. 
