On pages 105-109 in chapter "Of microbes and men", Dr. Moalem discusses the T. gondii virus and how it has adapted to affect cats. He explains how there is a cycle of how the virus is transferred from cat to mouse and spread to other cats. This relates to Big Idea 4 (Biological systems interact, and these systems and their interactions possess complex properties). Although this virus has very visible effects on mice, the effect on humans is not as obvious.
Describe the life cycle of the T. gondii virus in terms of its relationship to cats and mice. How is this advantageous for the virus? What effect does the virus have on mice? What effect does it have on humans? Explain the reasons for the difference in how the virus is expressed in each. Do the same for another animal that can be affected by T. gondii and explain the difference in the effect on this animal and the effect on mice.
-Mikhail Iouchkov (mikhailiouchkov@gmail.com)
Describe the life cycle of the T. gondii virus in terms of its relationship to cats and mice. How is this advantageous for the virus? What effect does the virus have on mice? What effect does it have on humans? Explain the reasons for the difference in how the virus is expressed in each. Do the same for another animal that can be affected by T. gondii and explain the difference in the effect on this animal and the effect on mice.
-Mikhail Iouchkov (mikhailiouchkov@gmail.com)
Like most parasites, T. gondii is always looking for hosts. It can “infect just about every warm-blooded animal but can reproduce in a way that guarantees its survival only in cats” (105). Even though this parasite can asexually reproduce in humans and most warm-blooded animals, it can only sexually reproduce in cats which are definitive hosts. This is why it’s crucial for the parasite to find a host that can serve as cat food. Dr. Moalem gives a brief description of the relationship between cats, T. gondii, and mice/rats. He describes that the virus induces a behavioral change within infected mice/rats leading them to become “fat and lethargic” (106). The mice/rats also lose their natural fear of predators and as a result are easy prey for hungry felines. Its ability to manipulate its host is seen to be an evolutionary adaptation that increases its chance for reproductive success and thus survival (Big Idea 1). However, it is not completely understood how the parasite is able to affect the behavior of its host. Once in the cat, the parasite sexually reproduces and produces oocysts or “spore cells” (106). An oocyst is a cyst containing a zygote formed by parasitic protozoan. This is advantageous for T. gondii because it can produce millions of oocysts increasing the chance of infection and survival. Once the oocysts are distributed in the cat’s feces, intermediate hosts (hosts in which only asexual reproduction can occur) may ingest the oocysts and become infected. If the infected host happens to become cat food, the cycle begins again.
ReplyDeleteThe effects of T. gondii on its host varies. According to published studies by Dr. Webster, it’s been observed that rodents infected with T. gondii are more active and are less fearful of cat urine. In fact, the parasite manipulates the rodent to become attracted to the pheromones in the urine. Pheromones are chemicals that are produced that can affect the behavior of another animal. There is also evidence that the effects on the brain are very specific since cognitive skills of rodents remain intact although it manipulates the brain circuits responsible for fear. Cognitive skills refer to the ability to understand ideas from experiences and information. Basically the parasite is able to brainwash the rodents into converting their innate fear to that of a fatal attraction. On the other hand, for most human hosts the effects of T. gondii are often minimal. It has been found that infected humans show differences in their behaviors. However, studies have shown that human hosts who are either pregnant or diagnosed with HIV are at a larger risk for more serious complications such as “blindness, damage to the heart and liver, and inflammation of the brain” (107). According to research conducted at Charles University in Prague, it’s been found that infected men are more suspicious and jealous while infected females had more warmth. Possible differences with the expression of the virus is due to the fact that rodents are more beneficial hosts for the virus because they can serve as cat food. Therefore the effects of T. gondii are more profound to make the rodents easy prey. Dr. Moalem also describes that the effect of T.gondii in humans is not “host manipulation in the evolutionary sense, because it doesn’t do anything for the parasite” (109). According to a journal reviewing T. gondii in wild birds, it’s been reported that affected pigeons became ill and weak and had high fevers. T. gondii was found in many tissues of the birds, especially in the lungs and spleen. Some were also dull and anorexic contrasting the rodents that became fat due to the infection. However there is the similarity that the infected rodents and birds both acted sluggishly.
Faith Teodoro (fteodor4@students.d125.org)
Sources:
(http://news.nationalgeographic.com/news/2007/04/070403-cats-rats.html)
(http://www.stanleyresearch.org/dnn/Default.aspx?tabid=204)
(http://naldc.nal.usda.gov/download/15113/PDF)