MORPHINE can make it harder to fight off gut infections, microbiologists said last week. This might explain why intravenous drug users with AIDS seem to suffer frequent and severe bouts of Salmonella poisoning. Until now, doctors have tended to blame the infections on immune system suppression by HIV.
For over a century scientists have known that people who use heroin and morphine seem more prone to infections, though no one was quite sure why. To study the drug's effect on gut infections, Toby Eisenstein and her colleagues at Temple University in Philadelphia fed mice with a virulent strain of Salmonella typhimurium. The mice were also given a surgical implant that released either morphine or a placebo.
"Morphine dramatically sensitised these mice to infection with Salmonella," Eisenstein reports. The animals on morphine all died of Salmonella poisoning within five days, while only half of the control mice died. And the mice on the placebo that died took twice as long to do so as those on morphine.
The most impressive demonstration of morphine's effect came by accident, when a lab worker fed each mouse around 210 Salmonella organisms, a hundredth of the planned dose. All the mice receiving morphine that were given this tiny dose of Salmonella died within five days, while all the control mice survived.
The observations, which will appear in the April issue of The Journal of Infectious Diseases, are intriguing because sharing needles for intravenous drug use is a common cause of HIV transmission. The incidence of Salmonella infections is 20 times higher in AIDS patients than people with normal immune systems, and people with HIV can have severe cases of Salmonella that also affects the brain and lungs.
Doctors have assumed that HIV patients have an increased risk because their immune systems are suppressed. But Eisenstein thinks morphine may also be implicated if people with HIV take the drug to control pain or because they are addicts. Morphine reduces movement of the gut, which may prevent bacteria being washed out quickly. She also suspects that the drug reduces the ability of infected cells along the gut wall to produce cytokines, chemicals that summon help from the immune system.
And it isn't only people with HIV who are at risk, Eisenstein adds. She says it is possible that morphine could also increase the risks for hospital patients taking it for pain if they become infected with an antibiotic-resistant "superbug".
Author: Nell Boyce
New Scientist issue 26th February 2000
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