by George Batten.
It's so small you can't see it. It's unlikely that you will know it if it affects you or a friend. You may know
that toxoplasmosis seriously afffects unborn children and persons with weak immune systems (including
people with AIDS and those being treated for cancer or organ transplants). You probably do not know about
some other negative affects of Tosoplasma gondi (T.gondii) on society.
T. gondiiis a parasitic protozoan that infects "warm blooded" animals, including about one third of
humans in developed countries, causing the disease Toxoplasmosis (sometimes shortened to "toxo"). It has
an interesting trait: mice and rats that it infects tend to lose their fear of cats! Moreover, it reproduces sexually
only in the intestines of felids (cats, large and small; tests show this for at least 17 felid species). The sexual
reproduction produces oocysts ("eggs") which are shed in the cat's feces. Oocysts can survive for a long
time (up to two years) in the soil. Infection of tissues, including the brain, is from asexual reproduction, which
occurs in all infected animals, and produces tissue cysts, but does not produce shedding of infectious
material. Once established, the infection is for the lifetime of the animal or human.
Infection occurs by:
- consumption of raw or undercooked meat containing T. gondii tissue cysts,
- ingestion of water, soil, vegetables; or anything contaminated with oocysts shed in the feces of an
infected cat; or;
- transmission from mother to fetus, particularly when T. gondii is contracted during pregnancy.
Correspondingly, risk factors for T. gondii infection have been found to be:
- exposure to or consumption of raw or undercooked meat,
- drinking unpasteurized goat milk,
- contact with soil,
- eating unwashed raw vegetables or fruits, and
- cleaning cat litter boxes.
Pregnant women are advised to avoid cleaning cat litter boxes. Some fish off the U. S. Pacific coast have
been infected with T. gondii, presumably because of runoff from cat litter disposal (so, enjoy sushi if you want
to, but keep the risk in mind - by the way, T. gondii in runoff from cat litter has been implicated in the death
of California sea otters). Numerous studies have found that living in a household with an indoor cat is not a
significant risk factor for T. gondii infection.
What is the effect of T. gondii infection on humans? Accumulating evidence suggests that latent infection
may influence a range of behaviors and tendencies. It has been associated with impaired psychomotor
performance, enhanced risk-taking personality profiles, and higher incidence of automobile accidents. There
are correlations with OCD, Parkinson's disease, Alzheimer's disease, suicide in patients with mood
disorders, and bipolar disorder, but not with major depression or chronic depression lasting at least two
years (dysthymia). There are numerous studies linking the infection to schizophrenia.
Most of the studies have been retrospective: T.gondi seropositivity was measured after the onset
of the disease or behaviorial trait, so the results are suggestive, but cause and effect has not been
demonstrated. Some serological tests made before the onset of schizophrenia have been positive, however.
The same is true of the large cohort study mentioned below (reference 2).
As you can see, there is accumulating evidence that T. gondii has a negative effect on human society.
Just the treatment of correlated mental disorders is costly, but who can say that some antisocial activity
is not caused by T. gondii? Certainly, not everyone infected will have antisocial tendencies, but one can
wonder about some extreme cases. I would like to know if there is a correlation between T. gondii
seropositivity and road rage; the same for corporate and political decisions that affect millions of people; the same for
mass shootings (fortunately, the sample size for the latter is so small that it may not be possible to draw
any conclusions). Are infected people more inclined to support large colonies of feral cats?
What can be done to limit this disease? The first step, obviously, is education: people need to know the
risk factors and how to avoid them. Then, the reservoir of T. Gondii needs to be reduced as much as possible.
This means that we must reduce the population of outdoor cats as much as possible. In particular, we
must stop supporting feral cat colonies (colonies which BARC - Bureau of Animal Regulation and Care
- Houston's animal control organization, advocates). The approximately 10% of households that feed stray
cats should stop doing that.
Much of the information reported here appears on the Wikipedia web site Toxoplasma gondii , some parts
of which have been copied verbatim. A summary of the effects of T. gondi
is in reference 1 below, the abstract for which is available on line at the
PubMed web site.
Reference 2, below, is a complete scientific paper on a large-cohort study of the effects of T. gondii
on traffic accidents, avaialble online here.
- Keep your cats indoors.
- Do not feed stray cats.
Wear good gloves when gardening where cats defecate (most places where cats have lived; sanitize the gloves and wash your hands afterward).
- J. Flegr, "Influence of latent Toxoplasma infection on human...," Journel of Experimental Biology, 2013 Jan 1; 216(Pt 1):127-33.
- J. Flegr, et al, "Increased incidence of traffic accidents...," BMC Infectious Diseases, 2009, v. 9:72
Acknowledgement: The author greatly appreciates the aid of Page
Williams, who mad many useful comments, and suggestions, and
provided reference material.