The most common bacterial cause of food poisoning seems to have another trick up its sleeve.
A new study looking at men who have sex with men (MSM) has shown for the first time that Campylobacter is likely being passed on as a sexually transmitted infection (STI) as well.
Campylobacter is a genus of squiggly bacteria that can cause humans quite a butt-ache. The bacterium is one of the reasons you've been told to make sure poultry is thoroughly cooked before you eat it, and why you are more likely to end up with a bout of gastroenteritis while on holidays.
But even if you're fastidious with cooking your meat and purifying your water, you've probably experienced it at some point in your life. The disease it causes - campylobacteriosis – manifests with diarrhea, stomach cramps, fever and pain, which will usually go away on its own after a few days to a week.
"Transmission to humans occurs primarily through unsafe handling or consumption of raw or undercooked chicken, consumption of raw milk, or contact with domestic animals," the research team – led by University of Oklahoma infectious disease epidemiologist Katrin Kuhn - wrote in their new paper.
"However, a large proportion of cases cannot be easily explained by these factors and it has been suggested that other infection routes (e.g., the environment) are equally important in explaining transmission of this disease."
There have been a few outbreaks in the past among MSM, but sexual contact had not been considered a possible transmission route until now. The researchers wanted to investigate whether sexual contact could be somehow involved in transmission.
They went digging into public health data in Denmark, as most infectious diseases there are reported to a national institute. To find MSM, they extracted data on 4,186 men over the age of 18 who had acquired a disease through MSM contact, and then matched each one to three to five other men at the same age and place of residence.
Once they calculated the matched odds ratios and adjusted for travel and other potentially conflating factors, the team found that MSM had 14 times higher odds of having been infected with Campylobacter, while Salmonella – another bacterial genus which causes food poisoning - saw no difference between the MSM and controls. The men in the study were also significantly less likely to have caught Campylobacter abroad.
"Our findings indicate a strong likelihood that Campylobacter can be transmitted during sexual contact," the team write.
"Given previous reports of outbreaks and high incidence of Campylobacter among MSM, this is not surprising."
This study didn't have the specifics to be able to explain what type of sexual contact was causing Campylobacter to spread – but the researchers make an educated guess that it would be through either anal-oral contact (also known as anilingus) or some other sexual activity where small amounts of fecal matter could end up near someone's mouth.
This is not without precedent. Shigella, another bacterial family which causes gastro, has been known since the 70s to be able to spread through anal sexual contact. But not all foodborne bacterial species can be passed on via oral-anal contact.
"The probability of infection with a foodborne bacterium from fecal-oral contact is directly related to the infectious dose," the team wrote.
"For Shigella spp. this dose can be as low as 10–1,000 organisms, and for Campylobacter, 500–10,000 organisms."
In comparison, Salmonella needs a lot more bacterial cells to enter your body to make you sick – somewhere in the range of a million.
As part of the study the researchers also found that Shigella infection was 74 times higher in MSM than the controls, which combined with the lack of difference in Salmonella infection leads credence to the idea that the MSM in the study weren't just eating more undercooked chicken.
Of course, it's not just MSM that enjoy sex in this way, as the researchers make clear; they note that the finding may also provide some insight into why young adults in general are more likely to end up with campylobacteriosis – they're more likely to be engaging in butt stuff.
"Combining this theory with high-quality national surveillance data, our results offer additional reasonable explanations for why surveillance statistics from some countries show that adult men are more frequently infected with Campylobacter than are women and why Campylobacter incidence peaks among young adults," they add.
The research has been published in the CDC journal Emerging Infectious Diseases.