Inflammation is the body's defense mechanism against injury and disease, but if it occurs in the brain, it can lead to epilepsy. The infections that most commonly affect the brain are AIDS, meningitis and tuberculosis, and the inflammatory processes that they trigger there appear to be consistent.

However scientists at the National Institute of Allergy and Infectious Diseases (NIAID), Maryland, USA, have now discovered that a well-known tapeworm causes epilepsy in a slightly different way.

Taenia solium (T sol) is a parasite that infects pigs and humans. Its larvae become lodged in the brain, and here they can form dead, calcified cysts. This is known as neurocysticercosis. Neurocysticercosis affects 50,000,000 people world-wide, and is the most common cause of seizures and epilepsy in developing countries where T sol is endemic.

MRI brain scans of people infected by T sol frequently show calcified cysts, but sometimes swellings known as perilesional oedemas are seen surrounding the cysts. Perilesional oedemas are not found in most of the other diseases that can cause seizures, and their clinical significance in terms of seizure activity was, until recently, unknown.

The researchers at the NIAID performed a study between 1999-2006, whereby they examined 110 people infected with T sol, who had calcified cysts and a history of seizures. 29 of the study participants experienced a seizure in this time frame, and 24 of these were assessed using MRI within 5 days of their seizures. A control group of 23 people who were infected with T sol but who didn't have a seizure between 1999-2006 also underwent MRI.

When the MRI scans were examined, 50% (12 out of 24) of patients in the study group were seen to have perilesional oedema, compared to 9% (2 out of 23 participants) of the control group. This suggests a strong association between perilesional oedema and seizure activity.

The scientists concluded that perilesional oedema is common and is associated with episodic seizure activity in people who have calcified T sol cysts.

As perilesional oedema is not found in the most common diseases causing seizures, its presence suggests that a different inflammatory mechanism is triggered by T sol to other disease agents. This is significant because, once this mechanism has been established, drugs that counteract this inflammation could potentially be developed, and seizures in the millions infected by T sol may be prevented.

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