The Parasite Heard 'Round the World

A few weeks back, shock and awe gripped the media after a report in Emerging Infectious Diseases (EID) described a 3-inch nematode removed from the brain of a long-suffering woman in Australia.

Honestly? was the initial reaction of Jan Slapeta, PhD, a veterinary parasitologist and co-author of the report who helped identify the red "string-like" creature as a larval roundworm of carpet pythons. "[T]he world is very strange if this is the hit story for the week," he said in his email to me. However, at the same time Slapeta found the media frenzy "astonishing," the global buzz also confirmed his long-held view that most people know precious little about parasites, much less how they enter animals and humans.

A colorful account of Ophidascaris robertsi's natural life cycle can be found in Slapeta's recent post on a University of Sydney Facebook page. Long story short: adult "mum" and "dad" worms meet and mate in pythons' esophagi and stomachs. Then, infected snakes shed hardy eggs eaten by rodents and small marsupials who, in turn, house newly hatched larvae in their liver or peritoneal cavity.

The cycle begins anew after pythons dine on parasitized prey and Ophidascaris larvae once again morph into full-grown worms inside the snakes.

But how could a human get infected? Let's explore what else we can learn from this once-in-a-lifetime case.

Parsing the Patient's Story

For starters, finding Ophidascaris in a human was surprising, but finding it in someone's brain was a true "rarity," Slapeta said, because the central nervous system (CNS) is not where the worm naturally goes. In this case, did immunosuppression play a role in its erratic journey? That's what the EID paper's senior author, infectious diseases specialist Sanjaya Senanayake, MBBS, suspects. Here's why.

Fairly early on, the 64-year-old woman was diagnosed with hypereosinophilic syndrome because her blood contained many eosinophils but standard tests failed to detect a parasite. As a result, she received mycophenolate, steroids, and finally, an immune-modulating monoclonal antibody. Over time, these agents likely weakened her blood-brain barrier.

Meanwhile, her illness progressed from abdominal pain and diarrhea to a dry cough and night sweats to unexplained opacities in her lungs and lesions in her liver and spleen. More than a year later, forgetfulness and depression prompted a brain MRI. After an enhancing lesion was found in her right frontal lobe, an open biopsy revealed the patient's true foe.

In hindsight, her route of exposure was less mysterious. In Australia, python feces containing Ophidascaris eggs can easily contaminate plants. The woman whose story circled the globe lived near a swamp rife with pythons and foraged local greens.

Neurotropic Nematodes

Worldwide, many animals harbor roundworms, and some of their larval offspring can breach human brains. Here are some examples.

Picture a familiar sight: a little pile of outdoor poop. Toxocara canis and Toxocara cati are intestinal worms of dogs and cats whose feces litter playgrounds and sandboxes, gardens, you name it. From there, Toxocara ova can contaminate children's fingers and produce. Once inside us, the eggs' newly hatched larvae may then migrate to our eyes and brains, triggering chorioretinitis, meningitis, encephalitis, or cerebral vasculitis.

In addition, because mice experimentally infected with T. canis larvae develop injury-related biomarkers also seen in Alzheimer's disease, some scientists are exploring whether neurotoxocariasis might contribute to long-term cognitive or neurodegenerative disorders. Given the infection's prevalence (remarkably, seropositivity in humans is as high as 80 to 90% in the tropics, while in industrialized countries, it ranges between 2 and 40%), this research bears watching.

Even more notorious for acute and violent CNS harm is Baylisascaris procyonis, an ascarid of raccoons. Its life cycle is similar to Toxocara, except that ingesting Baylisascaris eggs leads to meningoencephalitis, which is often fatal, especially in children. Risk factors? Pica tops the list. Maybe think twice before befriending local raccoons.

Finally, Angiostrongylus cantonensis is a globalizing, foodborne nematode native to Asia that now threatens humans in six continents. Although rats are the parasite's primary hosts, humans contract A. cantonensis after ingesting third-stage larvae in slugs and snails or other larvae-containing foods such as raw or undercooked shrimp. In affected areas, a poorly washed salad or a blended drink like a fruit or green smoothie can also carry dangerous, invisible cargo.

Migration to brain and spinal cord -- both in rats and humans -- is a biological hallmark of the parasite's life cycle. In humans, neuroinvasive A. cantonensis larvae often trigger eosinophilic meningitis and, on occasion, far worse sequelae including life-changing disability -- even death. In the U.S., this parasite now thrives in multiple southeastern states (it was most recently discovered in rats in Atlanta's zoo) and is highly prevalent in Hawaii. Travelers take heed!

A Final Takeaway

I've always loved "This Wormy World," Norman Stoll's 1946 presidential address to the American Society of Parasitologists. Stoll's brilliant talk following World War II presented a sweeping perspective and systematic measurement of the global toll of human helminthic infection. To this day, it remains a widely quoted publication.

Since then, we've seen enormous progress: better surveillance, diagnostics, anti-helminthic treatments, and control programs for almost all of our common, wormy foe. But what really disturbs me is today's greatly diminished knowledge of parasites in general and helminths in particular.

In the words of my UCLA colleague Lawrence Ash, PhD, co-author of a comprehensive atlas of human parasitic diseases now in its sixth edition: "What has happened to the teaching of parasitology today as opposed to, say, 40 or 50 years ago, is that medical school curricula have changed. At the same time, there's a general feeling that parasites are not a very big problem in this country, which I would question. As a physician who's never taken a class, how do you develop a differential diagnosis if you've never heard of a parasite that might be involved in someone's illness? It's a big problem."

Can the parasite heard 'round the world help revive interest? One can only hope.

Comment