Angiostrongylus cantonensis

Synopsis

CAPC Recommends

• Dogs, as well as birds, horses and other mammals, serve as incidental hosts for Angiostrongylus cantonensis.
• There is no effective treatment. Prevention is key and dogs should not be allowed to ingest raw or undercooked intermediate hosts (mollusks) or paratenic hosts including crustaceans, fish or frogs.
• Care should be taken around handling snails or slugs. Angiostrongylus cantonensis is a zoonotic parasite causing eosinophilic meningitis in humans.

Species

Canine (incidental host)

Angiostrongylus cantonensis

Overview of Life Cycle and Stages

• Adults are found in the pulmonary arteries of rats and other rodent species. Males are 15-22mm and females 18-33mm in length. First stage larvae (L1) are found in rat feces and have a characteristic notched posterior, measuring approximately 270-300 µm.
• The L1s are ingested by mollusks, the intermediate host. Both terrestrial and aquatic mollusks have been reported as intermediate hosts. Inside the snail the larvae develop to the third stage (L3), the infective stage.
• Rats become infected when they ingest infective mollusks or paratenic hosts, which include crabs, shrimp, fish and frogs. The L3 penetrate the intestine and travel passively in the blood stream making their way to the brain where they develop to an immature adult. They then re-enter the circulation and mature in the pulmonary arteries.
• Reported incidental hosts include dogs, birds, horses, humans and non-human primates, opossums and other related animals.
• Incidental hosts become infected through ingestion of the mollusk or paratenic host.

Disease

• Clinical signs in dogs include: hind limb weakness, hypereasthesia, urinary incontinence, ataxia, tail paresis, ascending paralysis, muscle atrophy, severe pain, coma and death.
• In other incidental hosts, including humans and non-human primates, infections result in eosinophilic meningitis leading to headaches, stiff neck, fever, vomiting and paralysis of the face and limbs.
• Ocular angiostrongyliasis may also develop.
• The level of disease is likely dependent on the number of infective larvae ingested.
• A report from Australia indicated 22 dogs infected with A. cantonensis with a history of ingesting the mollusk intermediate hosts. All developed progressive neurologic signs. Clinical signs included hyperasethesia, loss of balance, bilateral hind limb and tail paresis, muscle wasting, incontinence, facial twitching, vomiting and diarrhea (Lunn et al., 2012).

Host Association and Transmission Between Hosts

• Ingestion (purposeful or accidental) of an infected mollusk (raw, undercooked) is the most common route of transmission to a vertebrate host.
• Reports have shown that dead snails in water sources can emit live, infective larvae.
• Ingestion of a paratenic host (crustaceans, fish, frogs) raw or undercooked can also serve as a source of infection for the definitive host (rat) or incidental hosts.

Prevalence

• Infections in dogs have been reported in Australia and are common in Hawaii.
• Human infections have been reported in over 30 countries, mainly in parts of Asia, the Pacific Islands, and Caribbean.
• In the US, it is considered endemic in Hawaii and has been reported in Florida, Alabama, Georgia, Louisiana, Oklahoma, Tennessee, Texas and California.
• Infections may be underdiagnosed due to difficulty in diagnosis and symptoms that mimic other neurologic diseases.

Site of Infection and Pathogenesis

• In incidental hosts (including dogs), infective larvae penetrate the intestine and make their way to the central nervous system. Larvae arrest their development and subsequently die.
• Eosinophilic meningitis develops due to dead and dying larvae in the CNS.

Diagnosis

• Diagnosis is difficult in incidental hosts and is primarily based on clinical signs and history (travel to or living in endemic areas, ingestion or interaction with intermediate or paratenic hosts).
• Neurologic signs typically occur ~11 days post infection.
• Detection of eosinophils in CSF is often suggestive of parasitic infiltration and is useful in narrowing the differential diagnoses.
• ELISAs are available to detect mAbs of A. cantonensis and are often used in human medicine.
• Fecal exams are not useful in dogs or other incidental hosts as only the rat definitive host will have larvae in feces.
• PCR of CSF fluid or, less often, blood has been successfully used for diagnosis in incidental hosts, but false negatives are common.
• Treatment based on a presumptive diagnosis may be necessary.

Control and Prevention

• Preventing potential incidental hosts (dogs, birds, horses, etc.) from ingestion of mollusks and/or paratenic hosts.
• Prevent incidental hosts from drinking water where dead snails have been found.
• Washing and inspecting produce thoroughly with clean water to avoid accidental ingestion of mollusks.
• Cooking meat thoroughly (mollusks, paratenic hosts) to avoid ingestion of infective larvae.

Treatment

• Supportive care is often the only or best option. Pain management is essential.
• Corticosteriods have been effective in relieving inflammation caused by the presence and migration of worms. This can be used in conjunction with gradual, increasing doses of anthelmintics to limit the inflammatory reactions from the die-off of an unknown number of worms.
• Fenbendazole is often used for dogs and non-human primates, although efficacy is questionable. Moxidectin and milbemycin have also been used.
• Dogs should be monitored closely during the treatment protocol for adverse events associated with worm die-off.
• Ocular angiostrongyliasis requires surgery to remove larvae.

Public Health Considerations

• Humans are incidental hosts, becoming infected through ingestion of infective larvae in mollusks or paratenic hosts. Infections result in eosinophilic meningitis and infections can be severe.

Selected References

• Walker AG, Spielman D, Malik R, Graham K, Ralph E, Linton M, Ward MP. 2015. Canine neural angiostrongylosis: a case-control study in Sydney dogs. Aust Vet J. 93: 195-199.
• Lunn JA, Lee R, Smaller J, MacKay BM, King T, Hunt GB, Martin P, Krockenberger MB, Spielman D, Malik R. Twenty two cases of canine neural angiostrongylosis in eastern Australia (2002-2005) and a review of the literature. Parasites Vectors. 2012; 5:70.
• Wang QP, Wu ZD, Wei J, Owen RL, Lun ZR. Human Angiostrongylus cantonensis: an update. Eur J Clin Microbiol Infect Dis. 2012; 31:389-395.
• Walden HDS, Slapcinsky JD, Roff S, Mendieta Calle J, Goodwin ZD, Stern J, Corlett R, Conway J, McIntosh A. Geographic distribution of Angiostrongylus cantonensis in wild rats (Rattus rattus) and terrestrial snails in Florida, USA. PLoS ONE. 2017; 12(5):e0177910.
• Slom TJ, Cortese MM, Gerber SI, Jones RC, Holtz TH, Lopez AS, Zambrano CH, Sufit RL, Sakolvaree Y, Chaicumpa W, Herwaldt BL, Johnson S. An outbreak of eosinophilic meningitis caused by Angiostrongylus cantonensis in travelers returning from the Caribbean. N Eng J Med. 2002; 346:668-675.
• Cowie RH. Biology, systematics, life cycle and distribution of Angiostrongylus cantonensis, the cause of rat lungworm disease. Hawaii J Med Pub Health. 2013; 72(S2):6-9.
• Ansdell V, Kramer KJ, McMillan JK, et al. Guidelines for the diagnosis andtreatment of neuroangiostrongyliasis: updated recommendations. Parasitology. 2021;148(2):227-233.
• Odani J, Sox E, Coleman W, Jha R, Malik R. First documented cases of canine neuroangiostrongyliasis due to Angiostrongylus cantonensis in Hawaii. J Am Anim Hosp Assoc. 2021;57(1):42-46.