Rat Lungworm Disease: Has Arrival of a New-to-Hawai`i Mollusk Made It Worse?

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“When I talk about rat lungworm, I immediately get people’s attention,” said Jon Martell, a Hilo physician and one of the speakers at a recent community symposium on the subject.

“It touches on their fundamental fears and ‘ick’ factors,” he continued. “It’s got rats, worms, snails, and slugs — really repulsive things. People aren’t supposed to be in the picture.”

Yet they are, and in growing numbers. The result is an increase in the incidence of rat lungworm disease – a.k.a. angiostrongyliasis – in Hawai`i, especially on the Big Island. Not only are reports of the disease rising, but the severity of the cases seems to be growing as well. And the changes may be linked to the introduction of the Asian semi-slug to the Big Island in 2004.

The Big Island’s experience with the disease may be just a taste of things to come. According to Robert Cowie, a snail expert at the University of Hawai`i, “with the increasing spread of invasive alien species, including rats, and slugs and snails, to all parts of the world, and with global warming increasing the potential latitudinal range of the parasite, it is seen as an important emerging infectious disease.”

At last month’s community symposium, sponsored by the University of Hawai`i-Hilo College of Pharmacy, Global HOPE, and the U.S. Department of Agriculture, Martell talked about the way in which the parasite, Angiostrongylus cantonensis, behaves when, in its microscopic third stage of development, it is ingested by humans instead of its primary host, rats. It burrows through the intestinal wall, ending up in the nervous system and brain, where it dies. When the body’s immune system mobilizes to fight it, the result can be serious inflammation and damage to the nervous system. The intensity of the symptoms is thought to be a function of dose: the more worms you ingest, the sicker you will be.

In the worst cases Martell has treated, the patients develop eosinophilic meningitis, or inflammation of the brain caused by eosinophils (a particular type of white blood cell) that respond to parasitic infection. The symptoms include “persistent severe headache, migrating nerve pain, sensory disturbances, urinary difficulty, paralysis, weakness, and coma,” he noted. In the most severe cases, death can occur – “scary stuff,” he said.

The disease is self-limiting, he said. In time, “the worms will die and symptoms will improve,” but, he added, the patient may be left “with a lot of damage.” 

The Accidental Host

The path to humans is not one that the rat lungworm is supposed to take. Its primary host is the rat, which becomes infected when it eats snails. The worm develops further in the rat’s brain, then moves through its bloodstream into the heart and pulmonary artery. There, the female produces eggs, which travel through the bloodstream into the lungs, where the eggs hatch into first-stage worms. At that point, the worms break through the lungs into the rat’s trachea. The rat swallows them and the worms end up in the rat’s feces, which are, in turn, eaten by snails and slugs. Inside this intermediate host (the snail or slug), the worms develop to the third stage. When a rat eats the snails, the cycle, which takes at least 45 days to complete, begins anew. It’s probably safe to say that the worm is not a good thing for the rat, but according to one Puna resident who has been dissecting trapped rats for the last year, the nematode is present in every rat he has checked.

Humans are not the intended hosts for the lungworm’s development, but they can become infected when they ingest the worm in its third stage. This can happen intentionally – when someone eats an infected snail or slug on a dare, for example (it has happened). More frequently, it is accidental – when someone eats uncooked fruits or vegetables that are contaminated with snails or slugs. Other animals – flatworms and prawns – can also contain the third-stage larvae. If these so-called paratenic hosts are eaten raw, they can provide another pathway for human infection.

Until recently, the disease caused by the presence of the worms was generally thought to be non-life threatening. The Centers for Disease Control website still reports that “most patients recover fully.”


Like a case presented to Dr. Gregory House, rat lungworm disease is hard to diagnose. The worms do not usually show up in spinal fluid or blood tests. Most diagnoses are for “probable” cases, since confirmation can be difficult. Initial symptoms are general enough – severe headaches, nausea, diarrhea – that they can often be (and have often been) dismissed as less serious infections.

But in 2004, after three people came down with illnesses that looked suspiciously like rat lungworm disease, Robert Hollingsworth of the U.S. Department of Agriculture’s research center in Hilo was invited to investigate the presence of a new slug species that one of the three had noticed on her property in Koa`e, a remote area of Puna. The resident and two of her dinner guests, Hollingsworth later wrote, had become ill “after consuming home-grown lettuce reportedly contaminated with immature semi-slugs.”

“Our initial survey in Koa`e indicated that [the Asian semi-slug, Parmarion cf. martensi] was extremely common; it was found in trash cans, in a composting toilet, in an outdoor shower area, in a planting of spider lilies …, under plastic sheeting, and in a vegetable compost pile where egg masses of P. cf. martensi were also found.”

Hollingsworth shipped 26 of the semi-slugs to the Centers for Disease Control in Atlanta, where all of them tested positive for the presence of rat lungworm. 

After that first visit, Hollingsworth made a more thorough survey of the Puna area, soliciting information from residents on the distribution of the semi-slug as well as another mollusk, the Cuban slug (Veronicella cubensis), which was also suspected to be an important rat lungworm host. 

The spread of the semi-slug on the Big Island has been rapid. When Hollingsworth and his colleagues reported their findings in Pacific Science two years later, they wrote that the semi-slug “has essentially a continuous distribution in lower elevations of the Puna district” and that “such a widespread distribution is surprising for a species whose presence was confirmed only in 2004.”

One of the features that sets the semi-slug apart from other snails and slugs in Hawai`i, Hollingsworth and colleagues wrote, is its “propensity … to climb and locate rich food sources, including bird food, dog food, cat food, fish entrails, and papayas.” That climbing behavior, combined with its high population densities, “apparent attraction to rich food sources, and a naturally high rate of infection by A. cantonensis, increases the likelihood that people will come into contact with semi-slugs and the parasitic nematodes they carry.”

Additional facts about the semi-slug suggest it may be the mollusk equivalent of a perfect storm when it comes to transmission of the rat lungworm to humans. The juveniles are extremely small (about 2 millimeters long) and almost transparent, making them difficult to spot when cleaning leafy vegetables. Still, they can carry enough nematodes to cause disease. The likelihood that juveniles have nematodes is increased by the fact that the adult semi-slugs die after laying their eggs. “The nematodes can exit the dead adults,” Hollingsworth reported when speaking at the Hilo meeting last month, “and the worms can then be picked up by the small juveniles.”

That’s not to say that other snail and slug species should be ignored. Robert Cowie said that of the 16 snail and slug species examined, rat lungworm was found in 13. (The three where the nematode was not found may still be carriers; studies by others have detected it in at least one of these species.) Cowie also noted that
the lungworm was found in snails and slugs from every island except Lana`i. (It may be there as well; the Lana`i survey was very limited, he noted.)

A Difficult Diagnosis

According to Marlena Dixon, the East Hawai`i disease investigator for the Department of Health, rat lungworm disease was first reported in Hawai`i in the 1950s, but throughout the second half of the 20th century, there were few suspected cases. That changed in the mid-2000s, when the number of cases began to rise. In 2010, nine cases were reported, 90 percent of them from the east side of the Big Island.

The reported cases are either confirmed or suspected. Because confirmation is difficult, most of the reported cases fall into the “suspected” categories. Still, there are probably far more unreported cases. Among the 100 or so people attending the Hilo symposium, more than a dozen raised their hands when a speaker asked how many had first-hand experience of the disease.

Ann Kobsa, a research biologist in Puna, said she had surveyed residents in her neighborhood of Puna. Of the 137 individuals she polled, 21 (15 percent) said they had had rat lungworm disease. And three of those individuals said they had had it more than once. Of those who said they had the disease, 43 percent still had symptoms years later, Kobsa said.

Because the disease was relatively rare and initial symptoms mimicked other, less serious conditions, until recently, doctors were slow to diagnose angiostrongyliasis. In late 2009, two young people who were repeatedly turned away from the Hilo Medical Center emergency room eventually came down with life-altering cases of the disease. One of them, Silka Strauch, was in a coma for months before being flown back to her native Germany, where she now requires around-the-clock care. Graham McCumber, the other, defied the doctors’ poor prognosis for recovery and is now able to do many things for himself, although he is still suffering from brain damage.

Even if diagnosis is prompt, treatment options are fairly limited. McCumber’s mother, Kay Howe, spoke of how she augmented the conventional medications administered in the hospital – antihelminthics (worm-killing drugs) steroids (anti-inflammatory drugs to suppress the immune reaction to the worms), and painkillers, for the most part – with alternative nutritional supplements, vitamins, and acupuncture. (She has written up her experiences on the Malama O Puna website: http://www.malamaopuna.org/ratlung/graham.php.)

Aside from humans, other animals can become sick from rat lungworm. The disease has been reported in dogs, horses, and primates, and may affect other pets and livestock as well.

In August, the University of Hawai`i hosted a three-day workshop on rat lungworm, bringing together experts from all over the world to discuss the worm, its life cycle, and the disease it causes. A summary of the presentations and a list of priorities for future research have been posted on the website of Robert Cowie’s lab: http://www.hawaii.edu/cowielab/Angio%20website%20home.htm

Patricia Tummons

Volume 22, Number 6 — December 2011

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