Tuesday, 1 August 2017

Amber Reveals Prehistoric Parasites Preserved in Blood

Perhaps 20 million years ago, a pair of monkeys in what is today the Dominican Republic sat grooming each other. Somewhere nearby, possibly in the tree where they sat, there was a sticky and probably aromatic collection of plant resin. This wasn’t an unusual jungle scene at the time, but what happened next was.

One of the monkeys discovered a tick feeding and already bloated with its blood meal. The monkey pulled it free. Something punctured the body of the tick – claws perhaps, or teeth - and monkey blood leaked out. Discarded, or dropped by accident (would the monkey not have preferred to eat this tasty morsel?), the tick fell into the resin and stuck there, still oozing blood.

The tick wasn’t the only organism to die in the resin that day. Within the tiny monkey red blood cells, there were piroplasms – parasites that multiply asexually inside blood cells, burst out, and invade new blood cells. Eventually, the process produces male and female forms that can reproduce only after being ingested by a tick, in a blood meal.*

​Fossil Red Blood Cells


The piroplasms ingested by the tick in this story never reproduced. Instead, they became fossilized in amber, along with the blood cells and the tick. All are still identifiable millions of years later. This piece of amber holds the only fossilized red blood cells from a mammal yet found; the parasites within them are an added fascinating treasure.

Like the tick in this article, this mosquito was discovered in amber in the Dominican Republic.
Image by Didier Desouens CC BY-SA 4.0 (not licensed for upload to Facebook)

 

Infection with Piroplasms: Babesia, Theileria, Cytauxzoon


Was the monkey sick? Possibly. The destruction of red blood cells caused by the multiplication of the parasites can lead to severe anemia and even death. Babesia bigemina is a notorious killer of cattle in the United States.  Theileria parva causes East Coast fever and death in African cattle. Cytauxzoon felis kills domestic cats in the United States. Some infections go unnoticed, however, and animals can develop immunity. It’s possible the monkey wasn’t all that bothered by its infection.

Throughout the ensuing millennia, ticks continued to bite mammals and infect them with piroplasms, and no one even knew it was happening until the early 1880s when Theobald Smith and Frank Kilbourne proved that a tick transmitted Babesia bigemina to cattle. Not only did they solve the mystery of what was causing Texas cattle fever, it was the first proof that creatures such as insects and arachnids (spiders, ticks, mites etc.) could spread diseases with their bites.

​It took about another 75 years before anyone documented a case of piroplasms infecting humans, and then only in humans who lacked a spleen. Caused by Babesia divergens, there were very few such cases, and the disease remained a medical rarity until recently. In 1969, Babesia microti began turning up in the United States in the same places where Lyme disease is emerging. Today, it’s a spreading infection in the United States and Canada, striking thousands of people every year, and killing some.

​Evolution of Babesia


Lots of people still haven’t heard of Babesia, much less piroplasms, and for the rest of us, they still seem quite new. So the discovery of organisms resembling Babesia sp. fossilized in amber that might well be more than 20 million years old is delightful.
Amber is fossilized tree resin.
It has unique properties for preserving
organisms that become embedded in it.
Image by Wibowo Djatmiko CC BY-SA 3.0
For all of that time, they’ve been evolving and adapting to their hosts until we finally became aware of them. We might never have known Babesia has been around that long if those two monkeys hadn’t stopped to pick off a few ectoparasites in the jungle so long ago.

​Photographs published in the report of this finding are tantalizing, though not as clear as a modern-day blood film. What they mostly do is make me want to sit down at a microscope and look at that magnificent piece of amber myself!

​*Researcher George Poinar Jr. can’t be absolutely certain that the animal the tick was feeding on was a monkey, or that the parasites in the blood are piroplasms; however, after considering other evidence – red blood cell and parasite characteristics, the fossil record, knowledge of primates, piroplasms, and other organisms alive today etc. – this is most likely the correct interpretation.


Read the paper in the Journal of Medical Entomology:

​Poinar, George, Jr. (2017) “Fossilized Mammalian Erythrocytes Associated with a Tick Reveal Ancient Piroplasms.” J Med Entomol tjw247. doi: 10.1093/jme/tjw247

​Further Reading About Babesia and Other Piroplasms


CDC. Surveillance for Babesiosis - United States, 2014: Annual Summary

 Drisdelle, Rosemary. 2017 "Babesiosis Cases Likely on the Rise." Outbreak News Today

Schmidt, Gerald D. and Larry S. Roberts. Foundations of Parasitology 8th Ed. New York: McGraw Hill, 2009.












Thursday, 27 July 2017

Trypanosoma cruzi - A Parasite in the Blood Supply

Blood donation is now infamous for transmitting infectious diseases: HIV/AIDS and hepatitis B and C are the best-known offenders and the blood supply is screened for these viruses. American trypanosomiasis, or Chagas disease, is caused by a parasite, and it, too, can be transmitted through organ or blood donation, so we can add Trypanosoma cruzi, the cause of Chagas disease, to our list of concerns.
How do You Get American trypanosomiasis?
Triatoma infestans is
the best known of the
kissing bugs.
Image by Bärbel Stock;
CC BY-SA 3.0
Chagas disease usually starts with an insect bite: the reduviid bug, also known as the triatomid bug, or kissing bug, bites a human and takes a blood meal. As blood goes in at the mouth end, the bug defecates at the opposite end, leaving a fecal droplet full of infectious T. cruzi parasites on the skin. Later, when the bite itches and the victim scratches, the tiny parasites are rubbed into the wound and gain access to the tissues under the surface of the skin.
The parasites enter fat and muscle cells and multiply, eventually being released into the blood and thus distributed throughout the body. In time, they become quite rare in the blood but continue to live and multiply in organs indefinitely. Should another hungry reduviid bug feed on the host, there are enough T. cruzi trypanosomes circulating to pass the parasite to the bug, and subsequently to another human. Parasites present in donor blood and donated organs will also thrive in their new host.​
Symptoms of American Trypanosomiasis
Early symptoms of Chagas disease include a painful swelling at the site of the bite, swollen lymph nodes, high fever, aching muscles, enlarged liver, rash, inflammation of the heart and swelling of the face and extremities. This stage of the disease is often severe and dangerous in small children, while symptoms in victims over five years of age are generally milder and progress to a more prolonged, often asymptomatic though still destructive, form of the disease. 
In chronic Chagas disease, there is organ damage, particularly enlargement of the heart with gradual loss of function, and enlargement of the esophagus (megaesophagus) and colon (megacolon) leading to loss of function in the digestive tract. The severity of disease varies from person to person, and from one geographic area to another – when the heart and/or digestive system damage are severe enough, the patient dies.
Chagas Disease Distribution
The WHO estimates that between 6 and 7 million people are infected with T. cruzi. Most of them are in South and Central America, with about 12,000 deaths attributed to the parasite annually. 


Global distribution of Chagas disease. The red area shows where the disease is frequently transmitted by the kissing bug. In the blue area, occasional transmission occurs. In red, blue, and green areas, the disease is also transmitted from mother to fetus, and through blood or organ donation. Derived from a United Nations map.
 

With increasing migration of people from endemic areas, incidences of Chagas disease acquired through blood transfusion or organ transplant are increasing, and there is evidence that reduviid bugs in North America are carrying and transmitting the parasite. Thus, Chagas disease has become a matter for concern – an emerging disease – in North America, and both blood and organ donors are now being selectively screened for T.cruzi in both the United States and Canada.

Further reading on Chagas disease

Trypanosoma cruzi may owe its success in humans to the domestication of the guinea pig. I wrote about this in my book Parasites: Tales of Humanity's Most Unwelcome Guests.

PAHO/WHO. “ChagasDisease.” Accessed March 29, 2017. 
Schmidt, Gerald D. and Larry S. Roberts. Foundations of Parasitology 8th Ed. New York: McGraw Hill, 2009.

Wednesday, 26 April 2017

Parasite-Stress: An Unrecognized Obstacle to Social and Cultural Progress?

Why are some people tolerant of others and some not, and what does it have to do with parasites? The prevailing mood in Canada at the moment is one of racial and cultural equality, inclusiveness, and equal opportunity. Immigrants are welcome. We are all one and diverse cultures are celebrated. It’s refreshing and a source of pride for many of us even if we fail, at times, to live up to the ideal. One would think that having reached this point, we won’t backslide.

But what if there are forces at work that we’re unaware of? What if subconscious – and possibly changing - perceptions of health, or lack thereof, could sabotage our treasured cultural mosaic? If our health care system faltered and infectious diseases gained ground, would our attitudes as a nation change?

The Behavioral Immune System


Long-abandoned hospital building, used to quarantine
sick arriving immigrants, on Ellis Island immigration
station. Immigrants have long been viewed as a
potential source of contagious disease.
Carol M. Highsmith - Library of Congress
We all know our friend the immune system. It’s what fights off infection, and responds to vaccines that provide immunity to diseases we want to avoid. It’s what acts up in allergic reactions and runs amok in autoimmune diseases such as Chron’s disease and multiple sclerosis.

We don’t give much thought, however, to our behavioral immune system. That’s the system that makes us keep a bigger distance from someone who is obviously ill, reach for the alcohol hand sanitizer on the way out of the restroom and grip the door handle with a piece of paper towel, or refrain from petting a dog with visible mange.

This behavioral immune system is closely linked to the physiological immune system: research has shown that just seeing pictures of people who look ill causes an actual physical immune response. And research has also shown that being exposed to information about infectious disease makes people less willing to welcome strangers. One can see that the more we suspect there’s something contagious out there, the more wary of others our behavioral immune system will be.

Parasite-Stress Theory


Parasite-stress theory, explained in the book The Parasite-Stress Theory of Values and Sociality: Infectious Disease, History and Human Values Worldwide by Randy Thornhill and Corey L. Fincher, suggests that our attitudes towards others are shaped, at least in part, by the perceived threat of catching something infectious. The level of health risk in a group or culture might literally influence everything from individual relationships to the political philosophies of governments.

Seeing someone who looked like this (chicken pox)
would make most of us think twice about close contact.
Image by Mad Max CC BY-SA 3.0
In his review of the book, Michael Gurven summarizes the theory: “in high pathogen environments, people are expected to display traditional, collectivist/conservative values… whereas those under low pathogen stress should display progressive, individualistic/liberal values, including outgroup tolerance and generosity.” The “traditional, collectivist/conservative values” theoretically protect individuals from exposure to infectious diseases.

The authors draw lines from those high pathogen environments to many of humanity’s ills, including domestic violence, warfare, and homicide, not to mention authoritarian government. Low parasite stress corresponds to innovation, individual freedom, multiculturalism, and democracy.

Parasite Puppet Masters


In my book Parasites, Tales of Humanity’s Most Unwelcome Guests, I explored how parasites have influenced the outcome of several wars, and how fear of parasites can influence people to behave in incomprehensible and even unacceptable ways. I wasn’t tuned in, however, to the possibility that parasites might have caused wars, genocides, and homicides in the first place.

There’s disagreement about whether Thornhill and Fincher have demonstrated cause and effect, or a relationship that might be explained by something else, but for those of us already convinced that parasites rule the world, this is an area of social science to watch.

(In Thornhill and Fincher’s work, parasite refers not only to the worms, protozoa, and things that hop and crawl, but also bacteria, fungi, viruses, and any other infectious entity.)

Further reading


Gurven, Michael. 2015 "The Parasite-Stress Theory of Values and Sociality: Infectious Disease, History and Human Values Worldwide (Book Review)." American Journal of Human Biology; Wiley Online Library

Schaller, Mark, Gregory E. Miller, Wi"Mere Visual Perception of Other People’s Disease Symptoms Facilitates a More Aggressive Immune Response." Psychological Science 21(5)
ll M. Gervais, Sarah Yager, and Edith Chen. 2010

Thornhill, Randy, and Corey L. Fincher. 2014 The Parasite-Stress Theory of Values and Sociality: Infectious Disease, History and Human Values Worldwide. New York: Springer

Watters, Ethan. 2014 "The Germ Theory of Democracy, Dictatorship, and All Your Most Cherished Beliefs." Pacific Standard


Wednesday, 29 March 2017

Pinworm – A Common Intestinal Worm



Have you ever had a pinworm infection? Probably. Infecting 400 million people, pinworm is the most common nematode parasite of humans. It owes its success largely to the simplicity of its life cycle.


Pinworm eggs can only be seen through a microscope.
Adapted from Susan A Secretariat Catalina Maya Rendón

Pinworm (sometimes called seatworm) infection is infestation with the tiny roundworm Enterobius vermicularis. The infection, called enterobiasis or oxyuriasis, is common in children because they tend to be careless about hand washing and often put fingers in their mouths. It is also more common in women than in men, probably because women spend more time caring for children and therefore come in contact with the worm more often.

The female pinworm is 8mm to 13mm long (five-sixteenths to half an inch), while the male is so tiny that he would be very difficult to see without a microscope.

The Life Cycle of Pinworm


Human infection with E. vermicularis usually begins when an infective egg is swallowed:
  • Eggs hatch in the duodenum (small intestine), releasing larvae.
  • Larvae mature to adult male and female worms, which are found in the greatest numbers in the cecum (at the beginning of the large intestine).
  • Male and female worms mate and each female produces five thousand to fifteen thousand microscopic eggs.
  • Females travel along the length of the large intestine, ultimately exiting the body via the anus.
  • The majority of eggs are deposited on the perianal skin, though eggs are also laid internally and are passed in stool.
  • Eggs are distributed to clothing, bedding, and towels, and spread via the hands to household objects such as doorknobs, handles and taps. They also become airborne and settle almost anywhere with dust.
  • At normal body temperature, a pinworm egg matures within about six hours and an active larva can be found within.
  • Infective eggs are swallowed or inhaled and the life cycle begins again.

Life cycle of Enterobius vermicularis CDC
Some human parasites, such as Ascaris lumbricoides, the large intestinal roundworm, are infective only after the eggs have spent time maturing in warm moist soil. Others, such as Taenia saginata, the beef tapeworm, must pass through another host before they can infect humans again. These requirements limit parasites to places where the climate is warm or the other host is available. Because E. vermicularis passes directly from person to person, however, it spreads unchecked through human populations all over the world.

 

 

 

Symptoms of Pinworm Infection


We hear anecdotal reports of pinworm infections that sound positively agonizing. But many infections (as many as a third) cause no symptoms or very mild discomfort. In rare cases enterobiasis has more serious consequences.

The most common and revealing symptom is itching in the perianal area, possibly a result of the movements of female worms. Sensitivity to the eggs is also thought to develop with repeated infection. Scratching in response to the itching leads to lesions, increased irritation, and possibly bacterial infection.

Many people with symptoms complain of abdominal pain, and small lesions may develop in the intestine causing inflammation and sometimes infection. Other general symptoms include insomnia, irritability, grinding the teeth during sleep, loss of appetite, nausea, and even vomiting. Female worms sometimes migrate away from the anus and invade the vulva or vagina. Very rarely, worms penetrate intestinal tissue with serious consequences.

Diagnosis and Treatment of Pinworm Infection


Enterobiasis is usually confirmed when adult female worms and/or eggs are recovered from the perianal skin. Stool samples for parasitology sometimes yield worms and eggs as well, but this is not the best specimen choice.

Antiparasitic drugs clear up enterobiasis. Consult a physician or pharmacist, however, before self-treating. Unfortunately, reinfection typically occurs because other family members are also infected and infective eggs remain in the house. Physicians generally recommend that all family members take the drug, with a repeat treatment two weeks later. Because eggs in the environment dry out fairly quickly and it takes about a month for a swallowed larvae to mature and produce eggs, the second treatment should prevent a recurrence.

Further reading about pinworm

 


Roberts, Larry S., and John Janovy Jr. 2009. Gerald D. Schmidt & Larry S. Roberts’ Foundations of Parasitology. Boston, McGraw Hill.