Angiostrongylus cantonensis is a metastrongyloid nematode that is one of the most common pathogens to cause eosinophilic meningitis. Commonly referred to as the “rat lungworm,” it is transmitted by both rats (definite hosts), snails, and slugs (intermediate hosts) amongst some other transport hosts (see below). The parasite is most prevalent in the Asia-Pacific regions; however, outbreaks and sporadic cases have been reported outside of the endemic areas. To date, nine species have been identified, but A. cantonensis and A. costaricensis remain the most important human pathogens.
Rats are definitive hosts for the nematode. The adult nematode enters the rat bloodstream and matures there. Fertilization occurs, followed by the production of eggs, which are released into the blood. The eggs are lodged in the smaller pulmonary vessels and hatch. The first-stage nematodes break into the airways, then migrate to the upper airways and pharynx. First-stage larvae in the pharynx are swallowed and passed down the gastrointestinal tract and finally in the feces. Snails and slugs (intermediate hosts) ingest these larvae, which mature into the third-stage larvae (infective form) that are passed on to the definite hosts again when the definite hosts consume infected intermediate hosts. Third-stage larvae are neurotropic and develop into young adults in the brain of definite hosts, then migrate to the venous system and finally reach the pulmonary vasculature where they sexually mature and female worm lays eggs. Some other transport (paratenic) hosts such as snails, frogs, lizards, prawns, crabs, freshwater shrimps pick up the third-stage larvae by ingesting the infected intermediate hosts which are then passed on to the definite host that ingest these paratenic hosts. Vegetables covered with snail slime can also be a source of infection.
Humans become the incidental hosts by consuming the infected intermediate or paratenic hosts (or ingestion of snail slime). As these are lung flukes, their method of infection is similar in humans as they are in rats. It occurs mainly through the fecal-oral route. When larvae are ingested, they travel from the intestinal mucosa into the bloodstream, where they mature, and adult female flukes lay eggs in the terminal branches of the pulmonary arteries. The eggs hatch, and the larvae move into the pharynx, and they are coughed and subsequently swallowed and travel to the intestinal mucosa where they are expelled through feces. The intermediate hosts (snails) ingest these eggs and first-stage larvae. They then mature into third-stage larvae. At this point, the definitive hosts: rats/humans will ingest the intermediate hosts that contain the third stage larvae that enter the bloodstream and finally travel to the brain. This is the site of maturation into the adult phase. They travel via the venous drainage of the brain to the heart, where they mature into mature adults in the terminal branches of the pulmonary arteries and lay eggs. The worm inside the central nervous system, especially in the subarachnoid space, elicits an intense eosinophilic inflammation and causes acute eosinophilic meningitis. When the infestation is more severe, it will result in other inflammatory manifestations in the human hosts, including radiculitis, cranial neuropathy, myelitis, encephalopathy, coma, and even death. The nematode eventually dies in the human nervous system, and therefore humans do not transmit the worms. The incubation period usually ranges from 1 to 3 weeks but can be as long as six weeks.
Angiostrongylus spp. are classically endemic to the Asia-Pacific regions with the highest incidence in Thailand and Malaysia. Additional areas of inhabitance include the Pacific basin, including China, Vietnam, Phillippines, Taiwan, and Hawaii. Dissemination of infected rats has contributed to the spreading of the parasite to non-endemic areas such as Africa, S. America, Australia, Caribbean islands, New Orleans, and Louisiana. Travel-related exposures have attributed to sporadic cases detected in Europe and the United States.
Angiostronglyiasis is known as the rat lungworm. It is a nematode. In its natural transmission lifecycle, it is passed between rodents and slugs/snails. However, it can still infect sea animals as crabs, shrimp frogs. In these organisms, there is no reproduction of the parasite, but transmission to humans is possible by the consumption of the affected organisms. Humans are dead-end hosts.
Given that the concentration of this infection is mostly in east Asia, the likely method of transfer to the western countries would be by the travel of contaminated rats and mollusks by ship. Typical mollusks include the giant African land snail and Achatina fulica and Parmoarion martensi (from southeast Asia), which have become prominent in Hawaii. In Hawaii, infection with A. cantonensis is a reportable disease, and there have been several outbreaks reported. To support this, there is also retrograde movement noted from South America during ship transport to Taiwan and China with subsequent disease outbreaks of A. cantonensis.
Clinical pathology caused by Angiostrongylus cantonensis is localized to CNS. In the CNS, these flukes produce an intense inflammatory response with eosinophilic reaction, resulting in the classic presentation of eosinophilic meningitis. When the inflammation is more severe or affects other areas of the CNS, it will result in cranial nerve deficits, radiculitis, encephalitis, and death. The eye may occasionally be the site of infection.
History and Physical
The clinical syndrome seen with A. cantonensis is due to the intense inflammatory reaction of the dying parasites. Eosinophilic meningitis (EoM) remains the most common presentation of angiostrongyliasis. Headache is the most common symptom that can be associated with neck stiffness, nausea, and vomiting. Paresthesias and hyperesthesias can be associated with symptoms that can last for several weeks. Eosinophilic encephalitis (EoE) is rare but usually fatal due to late diagnosis. Rarely, ocular involvement can manifest with foreign body sensation and blurred vision from the anterior chamber and intravitreal invasion of larvae, respectively.
CNS features of infection include new-onset severe headache resistant to NSAID therapy and paraesthesias (such as touch hypersensitivity, tingling, burning sensation). Shifting paresthesias are a notable characteristic along with migratory pain in the trunk and limbs (not in a dermatological distribution). CN VI or VII nerve palsy may occur. It is not clear why patients may also present with urinary retention. Before CNS symptoms become prominent, changes usually start with the GI tract. The incubation period for infection occurs over 1to 3 weeks (but can extend up to 6 weeks), and during this time, patients may develop nonspecific abdominal pain, nausea, and vomiting. This occurs due to the infiltration of the intestinal wall by the larva. They then circulate through the blood and ultimately penetrate the brain through the blood-brain barrier when the neurological symptoms start to occur. This period takes one to a few weeks to occur, and so patients may have a resolution of the initial GI symptoms and be asymptomatic during this period, or there may be the persistence of the GI symptoms. Emesis can also occur due to meningeal involvement. More nonspecific symptoms include lethargy, low-grade fever, difficulty sleeping, confusion. Children tend to have less prominent neurological findings and more gastrointestinal involvement. Physical exam should focus on a complete neurological and abdominal exam.
Diagnosis of neuro-angiostrongyliasis is made of clinical history, suggestive signs, and symptoms and supporting laboratory findings. Clinical history should include a focus on travel to or from an endemic area, consumption of poorly cooked foods or raw vegetables contaminated with snail/slug components, crab, shrimp, etc. Although not confirmatory, CSF eosinophilia (usually exceeds 10%, can be as high as 70%) is required for the diagnosis in the right clinical setting. Eosinophilia in the CSF is defined as 10 or more eosinophils per microliter of CSF fluid or greater than 10% of total CSF leukocytes (with at least 6 in absolute count per microliter of fluid). CSF opening pressure and protein are usually elevated, with CSF leucocytosis typically ranging from 150-2000 cells/mm. Peripheral eosinophilia is often reported; however, it does not correlate well with CSF eosinophilia or the clinical course. The article editor has personally encountered and made the diagnosis by observing a larva coming out of the spinal needle with the CSF during lumbar puncture, quite a memorable experience. Brain imaging with CT or MRI is generally not too helpful in making the etiological diagnosis.
Serological immunoassays such as dot blot ELISA or western blot can aid in the diagnosis but are not widely available. PCR and real-time PCR based tests are reported to have good sensitivities, but again not as readily available.. The important differential diagnosis to consider is gnathostomiasis, which is another cause of eosinophilic meningitis. Gnathostomiasis usually presents with radicular pain, non-traumatic bloody or xanthochromic CSF, with brain imaging revealing SAH or intracerebral hemorrhage. Imaging may sometimes reveal myelitis, but this is not found in the case of A. cantonensis. The diagnostic immunoblot band is usually 29 kDa.
Other infections that can cause eosinophilic meningitis include gnathostomiasis and baylisascariasis. Gnathostomiasis is associated with consuming undercooked fish, poultry, or snake meat. Baylisascariasis is caused by the consumption of parasite (Baylisascaris procyonis) eggs (usually in children by ingesting soil) usually transmitted in raccoon feces. Cystercicosis caused by Tinea solium or other diseases caused by lung flukes such as schistosomiasis, and paragonimiasis are other important differentials. Coccidioidomycosis is also associated with eosinophilic meningitis. Non-infectious causes of eosinophilic leptomeningeal enhancement include non-Hodgkin lymphoma, myeloproliferative disorders, disseminated glioblastoma, and paraneoplastic manifestation from bronchogenic carcinoma. Drugs associated with eosinophilic meningitis include TMP-SMX, ciprofloxacin, ibuprofen, and intraventricular vancomycin and gentamicin. Encephalopathy and neurological deficits have been reported to be associated with idiopathic hypereosinophilic syndrome. The most important point in the diagnosis is a high index of suspicion in patients who have likely been exposed to the nematode during the previous few weeks, which can extend up to 6 weeks prior.
Prognosis is dependent on the timing of correct diagnosis as eosinophilic meningitis is rare and requires good clinical suspicion to suspect A. cantonensis as the causes as well as appropriate CSF findings. Sometimes the course of infection can be self-limiting, and other times depending on the disease burden may be fatal. There is an overall lack of national surveillance of this disease, and due to the limited diagnostic testing as well as poor general awareness among health care providers, it is difficult to quantify the outcomes.
Complications include death from eosinophilic meningitis or permanent blindness.
Ocular angiostrongyliasis has been reported in about 1% of all reported case reports. It occurs when the larva enters the anterior or posterior chamber. The exact mechanism of entry into the eye is unclear but has been thought to be secondary to travel along the retinal artery along between the optic nerve and sheath. Usually, only 1 larva is present, and symptoms include blurred vision, blindness, pain. Usually, eosinophilic meningitis is also present in these cases.
Deterrence and Patient Education
Angiostrongyliasis has been associated with the consumption of the intermediate hosts of the Angiostrongylus cantonensis species. Local cases should be reported to public health to decrease further consumption of infected intermediate hosts and to contain the source. It is essential to raise public awareness of the undercooked consumption crabs, snails, slugs, prawns.
Since the risk of infection occurs from the ingestion of the vectors, then the ingestion of raw snails/slugs, crabs, shrimp should be avoided. Additionally, there can be accidental inclusion of the raw snail slug in uncooked foods such as salads. Care should be taken in food preparation and careful handwashing.
Pearls and Other Issues
A high index of clinical suspicion is most important in making the diagnosis of Angiostrongylus meningitis. The possible clinical exposure especially travels to endemic areas, followed by acute or subacute meningitis should raise suspicion. The presence of blood or CSF eosinophilia should point to the diagnosis.
Enhancing Healthcare Team Outcomes
To improve patient safety and outcome, integrated care by the healthcare team is very important. In order to make the correct diagnosis early to improve the prognosis, it requires a high index of suspicion by the managing physician and the neurologist. The infectious disease specialist may be able to provide a good list of differential diagnoses giving the clinical, blood, CSF, and imaging findings. They are also able to recommend the best diagnostic approaches and treatments. Finally, the nurses and therapists are the ones to help improve the outcome of the patients. This team approach is especially important, given the fact that Angiostrongylus meningitis is rare among developed nations.