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Being infected with the stomach bug Giardia may increase your risk of having irritable bowel syndrome or chronic fatigue years later, say researchers. But we can’t say that the infection actually causes these conditions.

By Sophie Ramsey

BMJ Group News

What do we know already?

Giardia lamblia is a common parasite (a protozoan) that’s found in water and can infect the small bowel. It’s a frequent cause of travellers’ diarrhoea among people visiting developing countries, where the drinking water may not be purified. Outbreaks of Giardia infection (called giardiasis) can happen in developed countries as well, if the parasite gets into the water supply. You can also be infected if you drink unpurified water from a stream or lake.

Symptoms of giardiasis include stomach pain, diarrhoea, wind or bloating, headache, low-grade fever, and nausea and vomiting. Most people recover within two to four weeks.

Giardiasis has not been thought to lead to any lasting problems. But infections with other microorganisms have been linked to a higher risk of later illness. Notably, several bacteria have been linked to irritable bowel syndrome (IBS), and several viruses to chronic fatigue syndrome.

If you have IBS, your bowels stop working properly, causing pain in your abdomen, cramps, bloating, constipation, or diarrhoea. If you have chronic fatigue, you feel exhausted and weak a lot of the time for no obvious reason.

Researchers have now looked at whether Giardia might also raise the risk of these problems. They followed up with 817 people who’d developed giardiasis in 2004 after a water reservoir in Bergen, Norway, became contaminated with the parasite. They then compared them with a similar group of people living in Bergen who hadn’t been infected.

What does the new study say?

Both IBS and chronic fatigue were much more common in people who’d had giardiasis.

Around 46 percent of people who’d had the infection developed IBS symptoms within three years, compared with 14 percent of people who hadn’t been infected. Chronic fatigue was also reported by around 46 percent of people who’d had giardiasis, compared with 12 percent of those who hadn’t.

After taking into account people’s ages, sex, and other factors, the researchers estimated that people who’d had giardiasis had more than three times the risk of IBS and four times the risk of chronic fatigue.

People in the giardiasis group were also nearly seven times more likely to have both IBS and chronic fatigue, compared with those who hadn’t had the infection.

How reliable is the research?

This was a large study that was carefully done. However, we need to be a bit cautious about these findings for a couple of reasons.

First, the researchers relied on mailed questionnaires to determine whether people had IBS or chronic fatigue. Although the questionnaires asked detailed questions about people’s symptoms, we still can’t be certain whether they had these conditions.

Also, this type of study can’t prove that people were more likely to develop IBS or chronic fatigue because they’d had giardiasis. There might have been something else about the people who’d had giardiasis that made them more likely to get these conditions. But it’s hard to know, as the researchers didn’t look closely at other potential risk factors relating to people’s health, lifestyle, or family history.

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