Infections and the risk of dementia

The oral herpes virus (herpes simplex virus 1 or HSV1) causes cold sores and stays in the body for life. It's very common - most people will eventually get an HSV1 infection. Although it usually infects the body, it also has been found in the brain.

One of the main links between HSV1 and dementia is that HSV1 DNA has been found within the protein clumps that build up in Alzheimer’s disease. Aside from this, some researchers think that herpes could cause excess inflammation in the brain. In lab-grown brain cells, HSV1-induced inflammation changes how well they work. This could then potentially trigger or worsen Alzheimer's disease.

Some population studies suggest that there is a connection between HSV1 and dementia. However, others showed no connection with dementia, only an increased risk of mild memory and thinking problems.

Some people have a version of a gene (ApoE) that increases their risk of Alzheimer's disease (ApoE4). Herpes infection in the brain appears to be more common in people with this version of the gene. One theory is that ApoE4 increases the likelihood of the herpes virus getting into the brain. Some researchers also think that ApoE4 allows more virus particles to attach to brain cells, making it easier to infect them.

Although herpes infections are for life, most of the time the virus is dormant and symptom-free. It is thought that only the re-activation of the virus increases a person's risk of developing Alzheimer's disease. This can be prevented by an anti-viral treatment.

Currently, there is no strong evidence that herpes virus infection causes Alzheimer's disease. The increased presence of the virus in the brain may instead be related to changes to the immune system due to Alzheimer's disease.

Studies have shown that people with memory and thinking problems are more likely to develop pneumonia. Also, people who are hospitalised with pneumonia are more likely to experience memory and thinking decline.

Chlamydophila pneumoniae (C. pneumoniae) is one of the bacteria that can cause pneumonia. The bacteria have been found inside the brain cells of people with Alzheimer's disease. Its presence in the brain may cause excess inflammation. This could contribute to the underlying Alzheimer's disease mechanism. It has been shown in mice that the bacteria can get into the brain along the olfactory nerve, which transmits smell signals from the nose to the brain.

A review of several studies shows conflicting evidence of the link between C. pneumoniae infection and Alzheimer's disease. This may partially be because many of the studies have been quite small, making it harder to detect a significant relationship. Even if a connection between C. pneumoniae infection and Alzheimer's disease was confirmed, it's not clear whether the infection itself affects the progression of the disease.

Research into other bacteria that cause pneumonia has shown that they also lead to increases in the risk of developing dementia.

Syphilis and Lyme disease are both caused by the same class of bacteria, known as spirochetes.

If a syphilis infection reaches the brain it can cause a condition known as neurosyphilis. Some of the symptoms of this include memory and thinking problems. If caught early it can be treated, but some case studies have shown continued thinking problems. These cases are few and there is little evidence to connect syphilis infection with Alzheimer’s disease.

In rare cases Lyme disease can also affect the brain, causing dementia-like symptoms. With early treatment, this is reversible.

One case study describes a man who developed dementia with Lewy bodies years after a Lyme disease infection. But it is difficult to tell whether there was a connection. There is also a lack of evidence of the connection between Lyme disease and Alzheimer’s disease.

Gum disease might play a role in the development of Alzheimer's disease. This comes from the discovery of gum disease bacteria and their toxic products in the brains of people who had Alzheimer's disease. The same bacteria make Alzheimer’s disease worse in mouse models and blocking their toxic products seemed to help.

Gum disease has also been associated with a greater risk of developing Alzheimer's disease, with faster loss of memory and thinking abilities in people living with the disease.

Alzheimer’s Society funded research has shown that gum inflammation, missing teeth and tooth decay are all associated with poorer memory and thinking skills in people over 60. This might mean that poor oral health can contribute to memory and thinking decline, but it is also possible that people with thinking decline take less care of their dental health.

More research is needed to better understand the connection between gum disease and Alzheimer’s disease and other dementias.

The blood-brain barrier protects the brain by controlling what substances can pass from the blood into brain tissue. In Alzheimer's disease, the blood-brain barrier is damaged, particularly in the brain region that the disease damages most.

Evidence shows that the blood-brain barrier can be damaged by excess inflammation, protein build-up and the ApoE4 gene. These are all linked to Alzheimer's disease.

Bacteria, viruses, and other harmful substances can get into the brain more easily once it has been weakened. This may explain why certain viruses and bacteria, such as herpes and spirochetes, are more common in the brains of people with Alzheimer's disease.

If infectious bacteria, viruses or fungi reach the brain they can activate the brain’s immune cells, called microglia. When microglia are activated, they can cause inflammation in the brain. Inflammation is usually good as it deals with damage or infections.

However, if it lasts too long and doesn’t resolve it can cause problems. Excess inflammation like this is thought to be involved in the progression of dementia by causing nerve cell death.  

Infections might not even need to get into the brain themselves to cause excess inflammation there. Signals that the body is ill often travel to the brain. This is why we sometimes have ‘brain fog’ when we get ill. But with serious or long-term infections this may lead to excess inflammation in the brain.

Because the blood-brain barrier of people with Alzheimer's disease is leakier than normal, immune cells from the body and other inflammatory signals can get into the brain. This causes the brain's own immune cells to activate, leading to inflammation and possibly death of brain cells. Brain inflammation can also make the blood-brain barrier even leakier. This allows even more of the body's immune cells through and starts a vicious cycle of damage.