It is known that some diseases and parasitic infections can change what animals do, but now it seems that human mood might also be affected.
Parasites and Animal Behaviour
Some parasites cause obvious animal behaviours – dogs and cats scratch at fleas – but a few parasites and diseases have truly bizarre effects:
- Grasshoppers and crickets parasitised by 'Horsehair Snakes' will seek out water and commit suicide by drowning to allow the parasite to continue its life-cycle.
- 'Zombie Ants' infected with parasitic fungus will climb to specific locations in bushes and die there just to help the fungus survive.
- Animals infected with rabies are likely to become unusually aggressive, and their bite will pass on the infection.
These are just a few examples of the way in which some parasites and infections are known to control animal behaviour. Here the behaviour changes to help the parasite survive, but new evidence suggests that human infections can have effects on mood!
Human Mood and Infection
It used to be thought that the human brain was in some way isolated from the effects that infection has on the immune system, and that little was able to pass from the blood system into brain tissue, but it now appears that some chemicals can pass through the blood-brain barrier.
The New Scientist has reported that: "it seems the immune system, and infections that stimulate it, can influence our moods, memory and ability to learn", and: "the immune system may even shape our basic personalities, such as how anxious or impulsive we are."
Some infections lead to the production of chemicals that can enter the brain and alter behaviour of some individuals:
- Sammy Maloney developed obsessive compulsive disorder after being infected with a bacteria that is normally associated with sore throat.
- Bacteria injected into cancer patients (in an attempt to destroy tumours) had the unexpected effect of inducing a feeling of well-being. It did nothing for the cancer, but led to: "an improvement in mood and quality of life".
- Inflammation causes the release of cytokines that can cause depression. The New Scientist article quotes an experiment where injecting interferon-alpha (that causes a release of inflammatory cytokines) led to patients showing symptoms of depression.
Studying the way these chemicals affect the brain (in animals as well as humans) could be of enormous benefit. It might be possible to change the way disease vectors (such as the malarial mosquito) behave, and to find cures for some human problems.
Reference: 'Happiness is catching' Linda Geddes, New Scientist, 15 January 2011,
(and online at: 'Infectious moods: How bugs control your mind' 12 January 2011).
