Brain Chemistry in Rats Altered by Parasite

Bizarre bravery in rats explained . . . with serious implications for the human brain.

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Toxoplasma gondii, a common protozoan parasite prevalent in 10–20 percent of UK humans and about a quarter in the U.S. ,1 is known to provoke uncommon bravery in rats encountering cats. In fact, infected rats are literally attracted to the smell of cats. The logical result of such foolhardy behavior gets the parasite into the cat digestive system where it can complete its life cycle and escape to infect more mammals, including humans.

“A complex range of interactions exist between a pathogen [such as Toxoplasma gondii] with its host, which may include manipulation of the host for the pathogen’s own advantage,” write University of Leeds researchers led by Dr. Glenn McConkey. The aim of their study was to figure out how the parasite does it.

Dr. McConkey’s team has now demonstrated this parasite can affect dopamine levels in mammalian brains. Dopamine is a neurotransmitter. It relays information about movement, thought, behavior, and pleasure. Abnormalities in dopamine are associated with schizophrenia and Parkinson’s disease. Curiously, dopamine-affecting drugs used to treat schizophrenia will inhibit reckless rat bravery.

McConkey’s group showed that Toxoplasma gondii encodes for the production of a key enzyme involved in dopamine production. They also found infected brain cells in mice produce excess dopamine. “Based on these analyses, it was clear that T. gondii can orchestrate a significant increase in dopamine production in neural cells,” says Dr. McConkey. “Humans are accidental hosts to T. gondii and the parasite could end up anywhere in the brain, so human symptoms of toxoplasmosis infection may depend on where parasite ends up. This may explain the observed statistical link between incidences of schizophrenia and toxoplasmosis infection.” Although the relationship of dopamine regulation to schizophrenia remains unclear, “observed effects on dopamine metabolism could also be relevant in interpreting reports of psychobehavioral changes in toxoplasmosis-infected humans.”2

The dopamine link in this bizarre parasitic manifestation is a giant step forward.

So how does the parasite manipulate its hosts? Natural selection “rewards” those parasites whose effects include alteration of a neurotransmitter that happens to affect host behavior in a way that optimizes parasite spread. Of course, much more remains to be learned about how the parasite triggers dopamine production and which target cells produce behaviors advantageous to its propagation. And clearly, while infection with this parasite could be a contributing factor in some complex conditions as schizophrenia, it cannot be blamed as the cause since most people hosting the parasite in their brains are not schizophrenic. The researchers readily admit that much work remains to be done. Nevertheless, the dopamine link in this bizarre parasitic manifestation is a giant step forward.

From the Bible we know that God created a perfect world without death or disease. Therefore, the microorganisms in it originally were harmless. Since man first sinned, a complex combination of mutations, horizontally transferred genes, environmental changes, and host changes have left us with a number of harmful microorganisms in addition to those that still fulfill vital ecological roles. Parasites, particularly those that enslave their hosts, are reminders of the way natural selection has functioned in this fallen world. They are a grim reminder of the awful effects of sin on all of God’s creation.

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  1. Emese Prandovszky, Elizabeth Gaskell, Heather Martin, J. P. Dubey, Joanne P. Webster, and Glenn A. McConkey, “The Neurotropic Parasite Toxoplasma Gondii Increases Dopamine Metabolism,” PLoS One 6, no. 9 (September, 21, 2011), doi:10.1371/journal.pone.0023866.
  2. Ibid.


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