Patients with Alzheimer's disease who have common bacterial infections suffer greater memory loss, claims a recent study published in Neurology. The effect is said to be linked to increased levels of the pro-inflammatory cytokine tumour necrosis factor (TNF) caused by systemic inflammation.

Previous research has suggested that acute systemic inflammation might exacerbate neurodegeneration, so the researchers, based at the University of Southampton, UK, measured the level of TNF in the blood of 222 elderly people with Alzheimer's disease and assessed their ability to perform cognitive tests over a 6 month period. Any illnesses or injuries were also recorded. Patients with higher TNF levels at the start of the study had four times the rate of cognitive decline as those with low levels of TNF, and patients who experienced at least one infection or injury over the course of the study also performed worse in cognitive tests. “The worse the infection the worse the affect on the memory”, said Professor Clive Holmes, the lead author of the study ( BBC News , 7 Sep 2009).

Although they cannot be sure that the illnesses are the direct cause of the cognitive deterioration, Holmes said, “we found no evidence to suggest that people with more severe dementia were more likely to have infections or injuries.” ( , 7 Sep 2009.) And although the role of TNF in the brain remains unclear, “it is possible that finding a way to reduce those levels could be beneficial for people with Alzheimer's disease.” ( TimesOnline , 8 Sep 2009.)

The findings are coincident with two independent studies ( Nature Genetics , 6 Sep 2009) that link three genetic variants to the development of Alzheimer's disease. One such variant is in Clusterin — a gene involved in reducing inflammation in the brain — which fits with the suspected link between Alzheimer's disease and inflammation ( New Scientist , 9 Sep 2009).