Antibiotics could open door to harmful viruses, scientists claim
Taking antibiotics may increase the chances of a serious viral infection, new research suggests.
By wiping out "friendly" bacteria in the gut, the drugs can impair the body's immune system, scientists believe.
Researchers issued a new warning about the dangers of unnecessary use of antibiotics in light of the early findings from a study of mice.
US lead author Professor Michael Diamond, from Washington University School of Medicine in St Louis, said: "The immune system is activated differently if the gut does not have a healthy microbiome.
"If someone is sick with a bacterial infection, they absolutely should take antibiotics. But it is important to remember that there may be collateral effects.
"You might be affecting your immune response to certain viral infections."
In laboratory experiments, the Washington University team showed that mice are more susceptible to severe West Nile disease if the make-up of their gut bacteria has been changed by antibiotics.
The discovery may help explain why some people exposed to West Nile virus show such a wide range of reactions, said the scientists. Some develop life-threatening brain infections while others show no signs of infection at all.
Many other viral infections follow a similar pattern, causing little or no harm to the majority of individuals, mild to moderate symptoms in others, and severe disease in an unlucky few.
Genetics cannot fully explain the variety of responses and the reasons for the differences are not well understood.
For the new study, the researchers gave mice either a dummy placebo drug or a cocktail of four antibiotics for two weeks before infecting them with West Nile virus.
Around 80% of the mice that received no antibiotics survived the infection, compared with only a fifth of those treated with the drugs.
Just three days taking antibiotics was enough to increase the risk of dying from West Nile infection, and the mice were vulnerable for more than a week after treatment stopped.
The antibiotics used in the study were vancomycin, neomycin, ampicillin and metronidazole.
Only ampicillin and vancomycin increased susceptibility to West Nile virus if given alone, while metronidazole amplified the effect of these drugs.
Mice treated with antibiotics were found to have low numbers of immune cells called "killer" T-cells that play a vital role in fighting viruses.
Co-author Dr Larissa Thackray, also from Washington University, said: "It is likely that antibiotics use could increase susceptibility to any virus that is controlled by T-cell immunity, and that's many of them."
The findings, published in the journal Cell Reports, need to be confirmed in humans whose gut bacteria populations are different from those of mice, said the scientists. But they advised people to avoid taking antibiotics unless they are really necessary.
Prof Diamond said: "There's a number of people who get sick, some more than others, for reasons we don't understand.
"If your immune system doesn't get activated because your microbiome is perturbed by antibiotics or anything else - diet, other infections, underlying medical conditions - you may be at higher risk of severe viral disease."
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