On pages 105 to 109, Dr. Moalem goes into detail about the parasite toxoplasma gondii. T. gondii is a parasite which affects mainly rodents and cats, although it is known to have effects in humans as well. There have been links to schizophrenia, as well as attractiveness in women and aggression in men. Outside research has seen links between T. gondii and higher rates of suicide as well (http://healthland.time.com/2012/07/03/are-cat-ladies-more-likely-to-attempt-suicide/).
As we have read, T. gondii's effect on rodents are due to evolution. Read the passage about T. gondii and explain how how this parasite relates to our understanding of evolution. Connect this to Big Idea #1 (The process of evolution drives the diversity and unity of life).
Keeping the process of evolution in mind, explain why T. gondii might have the aforementioned effects on humans. Although Dr. Moalem mentions that these might be random coincidences, we never know. Dr. Moalem previously gave examples of how humans once thought there were no connections between ulcers and infections, although now we know that there is a strong relationship. Become forward thinking scientists! Come up with some theories on why T. gondii affects humans in this way. Be sure to approach this from an evolutionary standpoint and bring Big Idea #1 into your answer. Feel free to do outside research to supplement your research as well.
Carolyn Fan, cfan3@students.d125.org
T. Gondii is a parasitic protozoan that causes the disease toxoplasmosis. Toxoplasmosis is linked to slowed reaction time, 2.5 times increased chance of auto accidents, anti-social behavior in men, promiscuity in women, schizophrenia and bipolar disorder. Although T. gondii can infect, be transmitted by, and asexually reproduce within humans and virtually all other warm-blooded animals, the parasite can sexually produce only within the intestines of members of the cat family.
ReplyDeleteT. gondii has been shown to alter the behavior of infected rodents in ways thought to increase the rodents' chances of being preyed upon by cats. Because cats are the only hosts within which T. gondii can sexually reproduce to complete and begin its lifecycle, such behavioral manipulations are thought to be evolutionary adaptations to increase the parasite's reproductive success. When a healthy rat smells a cat, it flees. But rats infected with the Toxoplasma brain parasite actually follow cat odors, often presumably to their doom. Toxoplasma has long been an example of a parasite manipulating the behavior of its mammalian host. The protozoan needs to be in a cat’s intestine to reproduce. It is excreted in a cat’s feces and has to find its way into another cat to survive and reproduce. When a Toxo-infected rat smells cat urine, it has increased activity in brain regions associated with sexual attraction. This attraction overwhelms the innate fear response by causing, in its stead, a type of sexual attraction to the normally aversive cat odor, causing rats to touch the infected feces and then get eaten by a cat to complete the circuit. This relates to Big Idea #1, The process of evolution drives the diversity and unity of life. When certain rats were attracted to cats and then eaten by the cat, they allowed T gondii to survive and reproduce because the protozoan living on the rat would sexually reproduce inside the cat. This was a selective advantage for T gondii, and over many generations the protozoan adapted so that rats hosting T gondii would ultimately become attracted to cats, allowing the protozoan to survive and sexually reproduce within the cat. (http://www.scientificamerican.com/podcast/episode.cfm?id=toxoplasma-infected-rats-love-their-11-08-17)
Toxoplasma also affects humans. T. gondii may have subtle effects on personality and psychomotor performance. If so, this would be consistent with the effects of T. gondii on rodent behavior. In the rodent model, the effects of T. gondii are best explained in evolutionary terms by the manipulation hypothesis; the parasite changes the behavior of the rodent in such a way as to increase the chances of the parasite's getting into a feline and completing its life cycle of survival and reproduction. Humans are dead-end hosts for T. gondii, because the chances that a human being will be eaten by a feline are infinitesimally small. Among our primate ancestors, however, this was not always the case, as suggested also by contemporary studies of the frequency with which monkeys and apes are eaten by large felines in Africa. For example, a study performed in the Ivory Coast confirmed that primates account for a large proportion of leopards' diet and revealed the predation pressure exerted by large felines on 8 different monkey and 1 chimpanzee species. In addition, parasites are not aware that they have entered dead-end hosts, so they are likely to exert whatever effects they do in any host. In this regard, it is interesting to consider the increase in traffic victims among T. gondii–infected humans as a contemporary example of manipulation activity of a parasite. Since primates were normal victims for felines in Africa, T. gondii lived in these primates, and caused them to be attracted to larger feline cats, so that the T. gondii could be eaten in order to survive and reproduce. Since humans are a type of primate, T. gondii incidences are extremely frequent in human populations, because the T. gondii want humans to become attracted to felines in order to sexually reproduce inside of them. However, the T. gondii do not know that cats do not typically eat humans. This is the same evolutionary relationship that rats have with the T. gondii protozoan that relates to Big Idea #1, The process of evolution drives the diversity and unity of life: over time, the T. gondii adapted causing its hosts to become attracted to felines, so that they would be eaten and sexually reproduce inside the cat, allowing them to survive and reproduce. (http://schizophreniabulletin.oxfordjournals.org/content/33/3/757.full)
ReplyDeleteDopamine is a neurotransmitter that helps control the brain's reward and pleasure centers. Dopamine also helps regulate movement and emotional responses, and it enables us not only to see rewards, but to take action to move toward them. It is known that T. gondii increases dopamine in rodents and also that treating the rodents with a selective dopamine uptake inhibitor differentially alters the behavior of the infected and uninfected rodents. Also the observed low level of novelty seeking in humans infected with Toxoplasma is supposedly associated with high dopamine levels in the ventral midbrain. The mechanism of the dopamine increase by T. gondii is not known but may involve the inflammatory release of dopamine by increasing cytokines such as interleukin. The dopamine imbalance between the mesolimbic and mesocortical regions in the brain is suspected to play a role in the development of schizophrenia and other personality disorders in affected humans, which could explain the observed association between schizophrenia and toxoplasmosis.
(http://www.psychologytoday.com/basics/dopamine)
(Taylor Young, tayloryoung1995@yahoo.com)