Microsporidia (Enterocytozoon, Encephalitozoon)

Microsporidia are very small, spore-forming organisms that can only survive inside cells; they are related to fungi. In the human gut, mainly Enterocytozoon bieneusi and Encephalitozoon intestinalis occur, less often other Encephalitozoon species. They are detected in stool by PCR, which picks up the genetic material of Enterocytozoon and Encephalitozoon; under the microscope, the spores, only a few thousandths of a millimetre in size, are difficult to see. The analysis is part of the “Worms, microsporidia” profile together with the worms. Other single-celled gut parasites such as Giardia or Cryptosporidium have their own pages.

How microsporidia infect gut cells

The spores are taken up via contaminated water, food or contact with people and animals. In the gut they evert a coiled polar tube, pierce the membrane of a gut cell with it and inject their contents directly into the cell. There the organism multiplies, uses the host cell's energy and forms new spores, which are released when the cell dies and are excreted in stool. Enterocytozoon bieneusi stays mainly in the cells of the small-intestinal villi and the bile ducts. Encephalitozoon intestinalis can also enter immune cells and, with weakened defences, spread to other organs such as the kidneys.

What happens when microsporidia colonise the gut?

Infected cells of the small-intestinal villi die, the villi become shorter and the surface for absorbing nutrients shrinks. Watery diarrhoea develops, often with bloating and abdominal pain; fat and sugar are absorbed less well, which can lead to weight loss. How severe this is depends almost entirely on the state of the immune defences. In people with an intact immune system, an infection usually passes without symptoms or as temporary diarrhoea, for example after travel. With markedly weakened defences – for example in advanced HIV infection or after an organ transplant – the diarrhoea can become chronic and involve the bile ducts or other organs.

How the immune system keeps microsporidia in check

Because microsporidia live inside cells, antibodies alone achieve little. T cells are decisive: cytotoxic T cells recognise and eliminate infected cells, and helper cells release interferon-γ, which activates further immune cells. If these T cells are missing, the organism can multiply unchecked – which is why microsporidia were first described as a cause of diarrhoea in people with AIDS. Antibody studies suggest that healthy people have also had contact with microsporidia without noticing. A positive result in people without weakened immunity is therefore always considered together with the symptoms.

No microsporidia detected – what does it mean?

Without detection by PCR, a gut infection with Enterocytozoon or Encephalitozoon is unlikely. Because the spores are not shed evenly, light colonisation can still go undetected in a single sample. Many other causes come into question for persistent watery diarrhoea, such as other single-celled parasites like Giardia and Cryptosporidium, worms, bacterial or viral gut pathogens and non-infectious causes such as intolerances or inflammation of the gut lining.

The interpretation is given in your report.