Episode Summary

How do clinical microbiologists diagnose parasite infections? Bobbi Pritt discusses how molecular tests can complement or supplant traditional diagnostic tests for all kinds of infectious diseases, including parasites. She also tells the story of how an atypical molecular test result led to the discovery of a new cause of Lyme disease.

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Julie's Biggest Takeaways

Parasites are incredibly varied in many characteristics, including their size! Some are microscopic, while others are macroscopic and can be seen with the naked eye. Not just small macroscopic, although some worms at 35 cm can be considered quite large. Some tapeworms can reach 50 feet!

Bobbi Pritt's blog started as an exercise to share the cases she observed while a student at the London School of Hygiene & Tropical Medicine. She wanted to share these cases with students back at the Mayo Clinic, but found the audience grew to include clinical parasitologists, microbiologists, and parasite-interested people worldwide. Part of its success relies on its succinctness: a short, digestible case study with the minimum information needed to make a diagnosis.

Pritt's research focuses on developing molecular tests to detect microorganism RNA or DNA. Molecular tests can be used as a complementary diagnostic test or as the primary test, which can give healthcare workers definitive information to make therapeutic decisions much more quickly than a test that requires culturing the microorganism.

A new bacterium that causes Lyme disease, Borrelia mayonii, was found because the molecular tests that detect Borrelia burgdorferi are flexible enough to detect multiple species but yet differentiate between the different types of organisms. It was an astute technologist working at the bench who recognized the readout was slightly different than expected, leading to the investigations that revealed B. mayonii is a distinct species from B. burgdorferi.

One of the outstanding questions in parasitology is the relationship of Blastocystis (formerly known as Blastocystis hominis but which may actually be several species) to human health. Blastocystis lives in the intestinal tract and may cause irritable bowel-like syndrome. Definitive evidence on whether Blastocystis causes intestinal disease has yet to be presented, and there is a lot of opportunity for research in this area.

Featured Quotes

"Part of why I do this is to demonstrate the beauty - that would be part of the wonderful - but also the creepy, and also the dreadful part, knowing what diseases parasites cause. I try to show the full spectrum."

"We probably see more decoys when it comes to worms, macroscopic worms, than we do true worms! In a stool specimen, you're much more likely to see mucus strands or food particles than you are to see worms, depending on where you live in the world. We have to be able to tell a piece of onion from a tapeworm."

"[Discovery of Borrelia mayonii] doesn't really change the population at risk for Lyme disease because it overlaps with the population at risk for Lyme disease by Borrelia burgdorferi. It means there is a second cause of Lyme disease that individuals can be exposed to in the upper midwest."

"It's really going to be interesting in the coming years what we're going to be detecting as we go more and more to the use of molecular testing."

"Follow what you love! If that passion happens to be parasites, that's wonderful! There are a number of different opportunities for a career in clinical parasitology; you don't have to get a doctoral level degree."

Links for This Episode

History of Micobiology Tidbit

Dracunculus, as Bobbi mentioned, has played a major role in human history, and dracunculiasis is one of the diseases with such an unusual and unambiguous presentation that ancient texts including the Bible are recognized as describing it. One of the first to recognize this was Gottlob Friedreich Heinrich Kuechenmeister, who was both a parasitologist and a theologian, who wrote about the connection in an 1855 textbook about Animal and Vegetable Parasites. The disease has been described in numerous other ancient texts, including a description of how to remove the worm in Egyptian medial Ebers Papyrus, written in 1550 BCE.

Like many parasites, Dracunculus has a complicated life cycle, so while it was long recognized to be an infectious disease, for many years scientists thought that people were infected through their skin. There are male and female Dracunculus worms, but the females are the large worms that can grow up to 80 cm (30 inches) in subcutaneous tissues; the males are much smaller, topping out at 4 cm (1.6 inches). After mating, the female worm will travel through an infected person, often down their leg, and the outline of the worm is visible. Worm movement is very painful for the infected person, as is the emergence of larvae, which occurs from a blister formed by worm movement toward the skin surface. Naturally, the painful burning sensation may cause people to want to find relief by dunking their leg in water, which releases the larvae into the water where they are eaten by water fleas or other copepods and begin the cycle again. It's actually ingestion of contaminated water that transmits the disease to humans, but it took many scientists a long time to determine this.

In the early 1800s, several scientists discovered these large female worms could contain larvae, but it wasn't until the 1870s that the life cycle of Dracunculus was understood to include intermediate stages in crustacean hosts, and that ingestion of infected water fleas that live in untreated drinking water was determined to be how people became infected. The Russian naturalist Alekej Fedchenko played a large role in determining the role of these, but it was the follow up confirmation of Fedchenko's experiments by Patrick Manson in the 1890s that helped these observations gain wide acceptance. Manson's short paper, published in the British Medical Journal, discusses his experiments on Guinea worm based on a case of Guinea worm, not commonly seen in British hospitals in Victorian England, is a great example of how naturalists and experiments were previously shared among scientists: 10 short paragraphs, citing hearsay as frequently as published results, and the descriptions purely written, no graphs, charts, figures, or images of any kind.

Dracunculitis, or Guinea worm disease, is one of the major success stories in modern public health efforts. There are no vaccines or treatments for infected individuals and yet the number of infected people has plummetted from 3.5 million cases in 1986 to 30 cases in 2017. The amazing effect of improved infrastructure has shown it doesn't always require a vaccine or new therapeutic to greatly improve the lives of people affected by infectious disease.

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