Our Little Friend of the Week: Parasites Edition (3/3)
Sorry for the delay…I’ve been both busy and lazy, forgive me 😛 Now. The first two were the ‘good’ and ‘bad’ of parasites. I wanted to end this “Trilogy” with the most interesting bits of information about our little parasitic buddies. Granted, these would still count as the “bad”, but I find these to be more interesting. I hope you will too. Now, allons-y!
Mind-altering Parasitism is mostly what I will focus on today. I find it to be really fascinating how parasites can alter the brain chemistry of its host. Here’s an example of such an occasion…from an article I had read awhile back when I took Parasitology 1 a year ago.
Toxoplasma gondii: the “brain-hijacker”
This single-celled pathogen infects over half the world’s population, including an estimated 50 million Americans. Each of Toxoplasma’s victims carries thousands of the parasites, many of these residing in the brain. Scientists began to study this little fellow to try to find the reason for its impeccable success.
Researchers in Sweden report that the parasite spreads through the body by manipulating mobile cells that are part of the immune system. Toxoplasma hijacks these dendritic cells and makes them race around the body and ignore commands from other immune cells to commit suicide. Thus, a problem. The dendritic cells sneak the parasites into the brain and other organs, acting much like a Trojan horse. Very Ninja like, I’d say. But this can cause SERIOUS issues for the victim, as you can imagine.
Strategies like this one have made Toxoplasma incredibly widespread yet incredibly obscure at the same time. Mention the parasite to most people and chances are you will draw a blank (unless they are enthusiasts like myself!). Pathogens that infect far fewer people, like the Ebola and West Nile Virus, are far more known.
For most people, Toxoplasma causes no serious effects. It manages this by hijacking our cells and immune system, and establishing a careful harmony between parasite and host. Moral of the story: Once you get infected…you’re infected for life. More or less. However, for most people it wouldn’t be much of a problem. Toxoplasma can, however, cause serious brain damage in those with weak immune systems, like fetuses and adults with AIDS (or those taking immunosuppressants).
Cats play a major role in the parasite’s success. Our lovely felines can carry it in their intestines, where they can produce egglike cysts called oocysts. A single infected cat can shed 100 million oocysts in its droppings. The oocysts can survive in the soil for over a year and can contaminate drinking water. Oocysts can infect humans, by the way. Undercooked pork, chicken and other meat is another mode of being infected. So be careful when you cook. Once Toxoplasma enters a host, it spreads quickly. Within hours it can be detected in the heart and other organs. It is even able to infect the brain, which is protected from most pathogens by a tight barrier.
Scientists believe that the parasite is directing cells to move and to disseminate through the body. Antonio Barragan and his colleagues at the Karolinska Institute in Stockholm had put dendritic cells in a dish and injected them with Toxoplasma. They noticed that the parasites triggered a peculiar change: the dendritic cells became hyperactive, crawling for an entire day. Injecting dendritic cells carrying Toxoplasma spread the parasites to the brain and other organs MUCH faster than injecting Toxoplasma alone. The researchers concluded that Toxoplasma was taking charge of the dendritic cells and riding along with them. Their results are published online in the journal Cellular Microbiology if you want to learn more. Or google it, my friends.
Furthermore, As Toxoplasma spreads through the body, it invades cells. Unlike other pathogens, Toxoplasma can enter almost every type of cell in the bodies of thousands of host species. The parasite slips into a cell by latching onto its surface and pulling the membrane over itself. It just sits there, and the host doesn’t recognize it as a foreign body it should destroy. Thus, no autoimmune reaction happens. But have no fear, there is a bit of relief in this. If Toxoplasma simply spread from cell to cell, it could cause serious harm. BUT killing its host is not in the parasite’s best interests: its goal is to get into its final host, cats, the only creature in which Toxoplasma can reproduce by making oocysts that are shed in feces. Toxoplasma has evolved to be extremely contagious, but not very harmful.
However, there are certain individuals who should be careful. Toxoplasma becomes a menace when it does not have a healthy immune system to control. Pregnant women infected for the first time by Toxoplasma may pass it to their unborn children. Without a strong immune system to keep the parasite in check, a fetus can suffer massive brain damage. Up to 4,000 children are estimated to suffer toxoplasmosis in the United States each year. Toxoplasma is also dangerous to adults with weakened immune systems. The cause may be AIDS or immune-suppressing drugs given to people who receive organ transplants. A quiet Toxoplasma infection can suddenly “explode”, if you will.
Most scientists believe that people with healthy immune systems had no effects from Toxoplasma. But some studies in recent years have hinted that the parasite can exert surprising effects on behavior, at least in animals. In 2000, British scientists demonstrated that rats infected with Toxoplasma lost their fear of cats. They proposed that this strategy increased the parasite’s chances of getting into its final host. Clever, isn’t it?
In fact, it went so far as that the mice held a mild ATTRACTION to the cats odor. So, it is not just a loss of an old behavior…but a development of a new (odd) one. The mice were still afraid of other animals such as dogs or rabbits…but not of cats (which they are hard-wired to be frightened of…necessary for survival). It is possible that the parasite alters the production of certain neurotransmitters that are responsible for these instinctive fears and lack of attraction. More studies are being done to get a more accurate sense of what is really going on. So we don’t have all the answers right now.
Last note: Recent studies suggest that Toxoplasma is rare in meat sold in stores in the United States. However, experts still recommend cooking meat thoroughly to kill any parasites…which you should be doing ANYWAY. If you didn’t have any reasons to before, you surely do now. Also, as for the cats…you can keep your darling fluffy…just keep her inside (if you have a weak immune system at least). She will thank you, and you’ll be a bit healthier yourself!
I hope that this didn’t terrify anyone..lol I hope to at least have peaked your interest, even in the slightest bit. Comments are welcome and encouraged. Not sure what will come next week…I’ll be busy with band camp so mentally I’ll be a bit drained…but I’m sure it’ll be something interesting. Perhaps it’s time for me to blog about my FAVORITE disease…Flesh Eating Bacteria! We’ll see. Anyway, see you next time.