Eosinophilic meningitis is an infection of the brain occurring in association with an increase in the number of eosinophils. The organism most commonly causing eosinophilic meningitis is a rat lung worm called angiostrongylus cantonensis. Other worm infections that can cause a similar picture include paragonimiasis, gnathostomiasis and schistosomiasis.
Angiostrongylus is a worm of around 20 mm in length, which lives in the pulmonary artery in the rat, hence its name rat lung worm. Adult worms lay eggs in the arteries which hatch into first stage larvae. These larvae penetrate the artery wall to enter the lungs and migrate up the windpipe and into the gut and are expelled in the feces of the rodent. Once outside the body, they survive in damp conditions and penetrate the skin of snails and slugs. Snails and slugs can also become infected by eating rodent feces containing the larvae. There follows a further development stage within the snails and slugs. Infection is spread back to the rat when rats eat snails and slugs, and larvae in the tissues of these mollusks penetrate through the gut wall into the circulation and reach the rat brain. In the rat brain the larvae develop further, and when mature migrate through the rat body to the pulmonary artery where they mature to an adult worm.
In this complicated life cycle, humans become infected by accident when they eat an infected snail, or food that has been contaminated by slugs or snails, particularly SALADS. In the intestines the larvae penetrate the lining and migrate to the brain, spinal cord and eye. There they attempt to develop into final stage larvae but are generally unable to do this because development can only occur in rats. The larvae gather on the surface of the brain and spinal cord, causing intense inflammation. Very occasionally a larva will reach maturity and become an adult worm in the lung.
Angiostrongylus is found predominantly in south-east Asia, particularly Thailand and the Pacific Islands. It is a rare infection in travelers. It turns out to be more common than thought even in Hawaii.
Mollusks intended for human consumption should be cooked thoroughly, or frozen for 12 hours to kill the larvae. Mollusk numbers should be controlled in food growing areas. Measures should be taken to eradicate rodents near human habitations and farming areas.
The severity of eosinophilic meningitis varies widely depending on the number of larvae present. The infection is generally benign and self-limiting, but can be severe with neurological complications or even death. Symptoms reach their peak around 2 weeks after onset, but tend to settle after another 2 weeks at the most.
The diagnosis is made based on clinical features, a knowledge of whether the area is high risk for this infection, abnormalities on examination of cerebrospinal fluid from a lumbar puncture and scanning of the head. Larvae can sometimes be found in cerebrospinal fluid or in the eye. Both blood and cerebrospinal fluid have elevated levels of eosinophils. Antibody tests may be of benefit.
In many patients the infection is mild and as mentioned above tends to resolve within a month. The mortality rate is low, around 2 to 3%. However patients who develop weakness or blindness as a result of eosinophilic meningitis are unlikely to regain power or their sight.