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Parasites and Infections

Swedish researchers have discovered that malaria parasites secrete a substance that makes their hosts smell attractive to mosquitos.
- THE ASSOCIATED PRESS, Malaria Parasite May Trigger Human Odor to Lure Mosquitoes, NYT, FEB. 13, 2017


It has been suggested that kissing, nearly a human universal, may serve to share viruses before conception since catching them during pregnancy may damage the fetus:

Kissing is nearly ubiquitous in human cultures (although in some it’s more like sniffing). The practice puzzles infectious disease types, because swapping saliva clearly increases the risk of contagion. But maybe that’s the point. Humans carry various chronic viral infections. Acquiring these viruses during pregnancy can harm the fetus. So romantic kissing, some scientists speculate, may allow women to acquire potentially dangerous infections from their babies’ fathers before pregnancy, increasing the odds of healthy gestation. Making out may be a crude form of self-vaccination.

- Moises Velasquez-Manoff, Microbes, a Love Story, NYT, FEB. 10, 2017


Millions of American children have been exposed to a parasite that could interfere with their breathing, liver function, eyesight and even intelligence. Yet few scientists have studied the infection in the United States, and most doctors are unaware of it.

The parasites, roundworms of the genus Toxocara, live in the intestines of cats and dogs, especially strays. Microscopic eggs from Toxocara are shed in the animals’ feces, contaminating yards, playgrounds and sandboxes.

These infectious particles cling to the hands of children playing outside. Once swallowed, the eggs soon hatch, releasing larvae that wriggle through the body and, evidence suggests, may even reach the brain, compromising learning and cognition. ...

In some cases, larvae from Toxocara enter the eyes and cause blindness. They can also infect the liver and lungs, leading to a potentially damaging inflammatory reaction.
Usually, however, signs of infection are more subtle: a slight fever, fatigue, abdominal pain and cough — symptoms that describe any number of illnesses. Few pediatricians think to test for Toxocara. ...

“We know that larvae go to the brain in humans,” said Celia Holland, a parasitology professor at Trinity College Dublin in Ireland. But beyond that fact, she says, “we know very little.”

Dr. Holland has conducted experiments finding that mice with Toxocara larvae in the brain showed reduced learning ability and less inclination to explore. ...
Using a cross-section of national data, they found that the mean test scores were lower among children who tested positive for exposure to Toxocara, even after accounting for other known influences such as household income and lead levels in blood. Another study by researchers at Brigham Young University, published in 2015, found a similar association in adults.

- LAURA BEIL, The Parasite on the Playground, NYT, JAN. 16, 2018


An unassuming single-celled organism called Toxoplasma gondii is one of the most successful parasites on Earth, infecting an estimated 11 percent of Americans and perhaps half of all people worldwide. It’s just as prevalent in many other species of mammals and birds. In a recent study in Ohio, scientists found the parasite in three-quarters of the white-tailed deer they studied.

One reason for Toxoplasma’s success is its ability to manipulate its hosts. The parasite can influence their behavior, so much so that hosts can put themselves at risk of death. ...
Toxoplasma manipulates its hosts to complete its life cycle. Although it can infect any mammal or bird, it can reproduce only inside of a cat. The parasites produce cysts that get passed out of the cat with its feces; once in the soil, the cysts infect new hosts.

Toxoplasma returns to cats via their prey. But a host like a rat has evolved to avoid cats as much as possible, taking evasive action from the very moment it smells feline odor.

Experiments on rats and mice have shown that Toxoplasma alters their response to cat smells. Many infected rodents lose their natural fear of the scent. Some even seem to be attracted to it.
- Carl Zimmer, Parasites Practicing Mind Control, NYT, AUG. 28, 2014


But a first infection during pregnancy will cross the placenta, Dr. Grigg said, potentially leading to fetal death, stillbirth or problems in a newborn, including an enlarged head, cognitive deficits and almost certainly eye disease. Newborns born to mothers without previous infection are also vulnerable to the parasite, he said.

“This can be a very serious infection,” said Dr. McLeod, who is also a professor of ophthalmology and pediatrics at the University of Chicago. “It can cause a devastating disease in infants, with significant harm for them at birth and also later in life. It can have consequences for them and their families, lifelong.” ...
An acutely infected cat or kitten can excrete in two weeks up to 500 million oocysts — the infectious form of the parasite — which can remain infectious in soil and water for up to a year, Dr. McLeod said. A person can get infected from even one of these oocysts. “It’s an amazingly effective dissemination system,” she said.
- KAREN WEINTRAUB, How Worried Should Cat Owners Be About Toxoplasmosis?, NYT, JUNE 8, 2017


The parasite, which is excreted by cats in their feces, is called Toxoplasma gondii (T. gondii or Toxo for short) and is the microbe that causes toxoplasmosis—the reason pregnant women are told to avoid cats’ litter boxes. Since the 1920s, doctors have recognized that a woman who becomes infected during pregnancy can transmit the disease to the fetus, in some cases resulting in severe brain damage or death. T. gondii is also a major threat to people with weakened immunity: in the early days of the AIDS epidemic, before good antiretroviral drugs were developed, it was to blame for the dementia that afflicted many patients at the disease’s end stage. Healthy children and adults, however, usually experience nothing worse than brief flu-like symptoms before quickly fighting off the protozoan, which thereafter lies dormant inside brain cells—or at least that’s the standard medical wisdom.

But if Flegr is right, the “latent” parasite may be quietly tweaking the connections between our neurons, changing our response to frightening situations, our trust in others, how outgoing we are, and even our preference for certain scents. And that’s not all. He also believes that the organism contributes to car crashes, suicides, and mental disorders such as schizophrenia. When you add up all the different ways it can harm us, says Flegr, “Toxoplasma might even kill as many people as malaria, or at least a million people a year.” ...

Familiar to most of us, of course, is the rabies virus. On the verge of killing a dog, bat, or other warm-blooded host, it stirs the animal into a rage while simultaneously migrating from the nervous system to the creature’s saliva, ensuring that when the host bites, the virus will live on in a new carrier. But aside from rabies, stories of parasites commandeering the behavior of large-brained mammals are rare. The far more common victims of parasitic mind control—at least the ones we know about—are fish, crustaceans, and legions of insects, according to Janice Moore, a behavioral biologist at Colorado State University. “Flies, ants, caterpillars, wasps, you name it—there are truckloads of them behaving weirdly as a result of parasites,” she says.

Consider Polysphincta gutfreundi, a parasitic wasp that grabs hold of an orb spider and attaches a tiny egg to its belly. A wormlike larva emerges from the egg, and then releases chemicals that prompt the spider to abandon weaving its familiar spiral web and instead spin its silk thread into a special pattern that will hold the cocoon in which the larva matures. The “possessed” spider even crochets a specific geometric design in the net, camouflaging the cocoon from the wasp’s predators.

Flegr himself traces his life’s work to another master of mind control. Almost 30 years ago, as he was reading a book by the British evolutionary biologist Richard Dawkins, Flegr was captivated by a passage describing how a flatworm turns an ant into its slave by invading the ant’s nervous system. A drop in temperature normally causes ants to head underground, but the infected insect instead climbs to the top of a blade of grass and clamps down on it, becoming easy prey for a grazing sheep. “Its mandibles actually become locked in that position, so there’s nothing the ant can do except hang there in the air,” says Flegr. The sheep grazes on the grass and eats the ant; the worm gains entrance into the ungulate’s gut, which is exactly where it needs to be in order to complete—as the Lion King song goes—the circle of life. “It was the first I learned about this kind of manipulation, so it made a big impression on me,” Flegr says. ...
Those who tested positive for the parasite, both studies showed, were about two and a half times as likely to be in a traffic accident as their uninfected peers. ...
The approach brought to light a striking talent of the parasite: it has two genes that allow it to crank up production of the neurotransmitter dopamine in the host brain. ...
In fact, he says, schizophrenia did not rise in prevalence until the latter half of the 18th century, when for the first time people in Paris and London started keeping cats as pets. ...
In a 2011 study of 20 European countries, the national suicide rate among women increased in direct proportion to the prevalence of the latent Toxo infection in each nation’s female population. ...
But T. gondii is just one of an untold number of infectious agents that prey on us. And if the rest of the animal kingdom is anything to go by, says Colorado State University’s Janice Moore, plenty of them may be capable of tinkering with our minds. For example, she and Chris Reiber, a biomedical anthropologist at Binghamton University, in New York, strongly suspected that the flu virus might boost our desire to socialize. Why? Because it spreads through close physical contact, often before symptoms emerge—meaning that it must find a new host quickly. To explore this hunch, Moore and Reiber tracked 36 subjects who received a flu vaccine, reasoning that it contains many of the same chemical components as the live virus and would thus cause the subjects’ immune systems to react as if they’d encountered the real pathogen.

The difference in the subjects’ behavior before and after vaccination was pronounced: the flu shot had the effect of nearly doubling the number of people with whom the participants came in close contact during the brief window when the live virus was maximally contagious.
- KATHLEEN MCAULIFFE, How Your Cat Is Making You Crazy, The Atlantic, MARCH 2012


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