Can parasites manipulate thoughts?

Parasite-induced modification of animal behavior, is one of the least understood phenomenon of parasite\host relationships.
There are plenty of clinical studies, many focusing on animals and insects in the natural environment.
There have been studies on domestic animals, and on humans. Plenty of scientific data exists on the subject, but it still remains a shadowy area of research especially where human behavior is concerned.

Toxoplasma gondii has been characterized as a parasite capable of inducing behavioral modifications of intermediate hosts. The exact mechanics of this process is still being evaluated.
Many larger parasites like worms, [which are animals] have rudimentary nervous systems, and therefore can secrete hormones, neurotransmitters, and many other biologically active soluble proteins. The potential for direct, or indirect physiological and psychological affects on the host are obvious. The wastes produced by these organisms further add to the complexity of this interconnectedness.

In many cases, host behavior modification by parasites have been seen as an adaptive response to benefit the parasite by increasing it's probability for successful transmission.
It would seem an evolutionary process is active within the host, for purposes of survival.
In order to reason with the mechanics involved in host manipulation, one must first examine the biological parameters confronting the parasite.

First, the energy spent on manipulation of host, will cause a deficit of energies spent on other functions such as growth.

A theory that governs parasitic manipulation effort on host behavior goes something like this:

1) typical same species population size decreases
2) prevalence increases
3) longevity of parasitism within host decreases
4) passive transmission decreases
5) potential reproductive capacity decreases

To access some of the some of the physiological responses associated with parasitism, clinical studies have focused on some of the following areas: neurology, endocrinology, neuromodulatory and immunomodulatory components and cognitive studies.

There are many intriguing examples in nature, that highlight the incredible evolutionary sparing between parasite\host dynamics which can determine and alter destinies of species populations.
These are important observations to consider, when evaluating the circumstances of human pathology.

In my next blog, I will focus on some clinical examples of host manipulation from a cross-section of human parasites.