Rat Lungworm and Meningitis
Rat Lungworm. No, that’s not the name of my lawyer. Officially known as Angiostrongylus cantonensis it’s a nematode – a microscopic worm parasite – that can cause meningitis in humans and now its vector of choice, the giant African land snail, has been spotted in Texas.
A. cantonensis begins life as an egg laid in the pulmonary arteries of infected rats. After hatching, the first stage larva travel to the rat’s pharynx and are swallowed and exit the rat by way of its stool. These larva are then eaten by a slug or snail where they develop into a second stage and then finally into a third stage larva. The infected snail or slug is then eaten by a rat and the third stage larva travel to the rat’s brain and mature into adult nematodes. The adults then travel back to the rat pulmonary arteries to lay eggs and complete the circle of life.
Other animals including humans can become “incidental hosts” to A. cantonensis by accidental ingestion of the third stage larva. With the exception of the French, most people don’t eat snails or slugs however, the third stage A. cantonensis larva can be found in snail slime trails, small pieces of snails or slugs, and even the neonate forms of sails and slugs. These can be very difficult to see and easy to accidentally ingest. Infected sails and slugs can even contaminate water sources with larva for up to 72 hours after crawling into the water and drowning. Because contaminated food or water sources are impossible to detect, most people infected by A. cantonensis don’t know how it happened but it’s almost always a case of ingesting improperly prepared food such as fresh fruits and salads. Children who play in the dirt in tropical climates are also at risk because young children invariably will ingest some of what they are playing in.
Just as with the rat host, A. cantonensis will travel to the central nervous system in infected incidental human hosts and cause an eosinophilic meningitis (the immune system’s response to a parasitic infection is the proliferation of white blood cell known as an eosinophil which can be detected on a lumbar puncture or spinal tap). Symptoms start anywhere from a few days up to a month after exposure most commonly with a severe frontal headache in 90% of patients. Additional symptoms include stiff neck, nausea, vomiting, fever, and in 75% of patients increased sensitivity and tingling (hyperesthesias and paraesthesias) of the skin of the trunk or extremities. Occasionally, infection can lead to partial paralysis of facial or other muscles but this usually resolves with time and in unusual cases, the larva can infect the eye and cause blurred vision without meningitis symptoms. Thankfully, mortality from A. cantonensis infection is as low as 0.5% of all infections. The larva rarely travel to the pulmonary arteries in humans since we are not the usual hosts and so transmission from human to human is not known to occur (with the theoretical exception of cannibals).
The diagnosis of “Rat Lungworm headache” (cerebral angiostrongyliasis) is almost always made by the clinical presentation and the finding of large numbers of eosinophils in the cerebrospinal fluid from a lumbar puncture (spinal tap). The actual larva are rarely detected and CAT scans and MRIs of the brain are usually non-specific or difficult to interpret. However, the body is almost always able to clear the infection and the treatment is mainly supportive and limited to pain control, occasional lumbar puncture for CSF removal to reduce pressure, and steroids. The vast majority of patients recover completely though some can have hypersensitivity and/or tingling of parts of the skin on the trunk or extremities for several weeks.
So the good news is that despite the fact that this infection involves brain invading microscopic worms, rats, gigantic snails, infected slime trails, and bad fruit, it’s rare to see fatalities and cases of permanent neurological damage though an infection can really ruin your vacation. The take home point (THP) on this is that prevention is the best medicine. If you find yourself in a tropical area, make sure that your food, especially fresh fruits and vegetables are properly prepared by completely washing and cleaning prior to serving . . don’t play with giant African land snails . . and don’t eat the dirt.