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Depression should be re-defined as an infectious disease rather than an emotional disorder, argues one scientist.

The condition could result from a parasitic, bacterial or viral infection and future research into the condition should search for these micro-organisms, argues Dr. Turhan Canli, of Stony Brook University, U.S., the Daily Mail reports.

If his theory is true, he hopes a vaccination to protect against depression could be developed in future.

Writing in the journal Biology of Mood and Anxiety Disorders, Dr Canli said: 'It is time for an entirely different approach.

'Instead of conceptualising major depression as an emotional disorder, I suggest to re-conceptualise it as some form of an infectious disease.

'I propose that future research should conduct a concerted search for parasites, bacteria, or viruses that may play a causal role in the etiology of major depression.'

He divides his theory into three arguments.

He said patients with depression exhibit sickness-like behaviours.

They lose their energy, have difficulty getting out of bed and lose interest in the world around them, displaying the same symptoms as people who have an infectious disease.

Studies of the patients with depression also show the presence of inflammatory markers in their brains, he said.

These inflammatory markers might indicate the immune system has been activated in response to some kind of pathogen, which could be a parasite, bacterium or virus - this being what is causing the symptoms of depression, he argued.

His second argument is that there are examples of parasites, bacteria or viruses affecting emotional behaviour throughout nature.

The best known example is Toxomaplasma gondii (T.gondii), a parasite which lives in the intestinal tract of a cat.

There, it lays eggs which are dispersed into the environment when the cat excretes.

When a rat comes in contact with these eggs and becomes infected, it becomes attracted to the scent of cat urine.

The rat’s behaviour is through parasitic cysts being deposited across its brain, activating the same pathways involved in sexual arousal.

He says one third of the world’s population is thought to be infected with T.gondii. The infection is associated with markers of inflammation, as observed in depressed patients.

Of patients with a diagnosed major depression or bipolar disorder, those with a history of attempting suicide had higher levels of antibodies created to fight T.gondii bacteria.

‘Yet, large-scale studies of major depression and T. gondii or systematic searches to discover other potential parasitic infections have not yet been conducted,’ Dr Canli argued in his paper.

Similarly, he cites a number of studies in which rodents’ emotional behaviour changes when they are exposed to bacteria.

Large studies looking at associations between viruses and depressions exist already too, he said.

He cites an analysis of 28 studies, which found a link between viruses and depression. These included the Borna disease virus (BDV), the herpes virus responsible for cold sores, varicella zoster virus, which causes chicken pox and and Epstein-Barr virus, which causes glandular fever.

One study found BVD virus was 3.25 more likely to be found in depressed people than in non-depressed people, he said. Another found BVD in the brain’s of two out of 30 depressed patients.

His final argument is that re-conceptualising depression as an infectious disease is useful when thinking about the genetics of the disease.

The search for specific genes linked to depression has come up empty, he said, adding: ‘Perhaps, we have been looking at the wrong organism.’

While studies have looked for internal changes in human genes, which might explain depression, 8 per cent of the human genome is based on external changes, such as from viruses.

Bacteria, viruses and parasistes could transfer genes into our cells, leading to changes in our genetic structure.

They could also be passed during birth or through contact between parents and their children, he said.

He concluded that large scale studies involving patients with depression and people without the condition should be carried out to see if parasites, viruses or bacteria could be the cause.

He added: ‘Such efforts, if successful, would represent the ‘end of the beginning’ , as any such discovery would represent the first step toward developing a vaccination for major depression.’