What Is Rat Lungworm?

Rat lungworm (also known as Angiostrongylus cantonensis or neuroangiostrongyliasis) is a parasitic worm of rats and causes a disease that affects the brain and spinal cord.  The adult pathogen is found in rodents, and larvae are passed on through infected rat faeces. Invertebrates such as slugs and snails can ingest the larvae. However, their lifecycle is incomplete until a rat eats the infected invertebrate.¹ 

The life cycle of rat lungworms

Rat lungworms have a life cycle of approximately 45 days, in which snails or slugs eat larvae and mature within these invertebrates. Rats then eat the infected invertebrates, and the larvae penetrate the intestine and enter the bloodstream. Here, larvae can move to the CNS and mature to the sub-adult stage of development. Sub-adult lungworms move back to the bloodstream to mature in the right ventricle and pulmonary artery. The females lay eggs here, and these eggs travel through the bloodstream into the lungs.²

Most cases of infection in humans from rat lungworm are diagnosed in Southeast Asia and the Pacific Basin; however, the pathogen has been identified in areas such as Australia, Africa and the Caribbean. The parasite is likely spread by rats that get onto ships and, therefore, travel to other countries, as well as the introduction of the giant African land snail to regions that now have outbreaks of the disease.³ The semi-slug has also spread in regions of Hawaii, leading to around 150 documented cases of rat lungworm within humans.⁴ Whilst human cases of rat lungworm remain rare, the increasing rodent population in infected countries could cause the number to rise.

How rat lungworm spreads to humans

Rat lungworms can be transferred to humans through the ingestion of raw or undercooked snails and slugs, as well as the consumption of raw vegetables, as these may contain small slugs. Certain other animals, such as shrimp and frogs, have previously been found to be infected with the larvae of rat lungworm. However, it should be noted that fish have not been found to carry the parasite. The passage of live larvae between humans has not been documented, so it is believed that humans are accidental hosts and infected individuals cannot pass the parasite on to another person. Infants and children who may eat slugs and snails that they come across are most at risk of infection, as well as overseas travellers who may eat undercooked fresh produce such as lettuce.⁵ There are documented cases of children becoming infected with rat lungworm due to consuming slugs and snails for ‘a dare’. Infiltration of the worm into lymphocytes, eosinophils, and plasma cells, then into the meninges (membranes that protect the brain and spinal cord) is a pathogenic pain event.

Symptoms and complications of rat lungworm infection

Most people who are infected with rat lungworm don’t have symptoms and are largely unaffected by the infection. However, the pathogen can lead to a rare type of meningitis called eosinophilic meningitis. As the majority of worms are not able to re-enter the circulatory system once in the brain, they remain in the brain tissue until they die. This can cause neurological damage due to physical damage from the worm's movements, as well as inflammation caused by immune reactions. For unknown reasons, a greater immune reaction is initiated in response to the dead worms compared to the live worms. Symptoms often start non-specific, such as nausea, vomiting and abdominal pain. These symptoms then may progress into headaches, neck stiffness, burning skin, double vision or bowel difficulties. Younger children may experience more fever, convulsion and gastrointestinal symptoms. Complications of the infection can include hearing loss, memory issues and seizures, so it is important to seek medical attention if you believe you may be infected with the parasite. In the worst cases, rat lungworm can cause paralysis, coma, permanent disability and even death.⁶

Challenges in diagnosing rat lungworm infection

Diagnosing rat lungworm can be difficult as there are no readily available blood tests that screen for the infection. Doctors may make a presumptive diagnosis based on information such as travel history, e.g. recent travel to an endemic area or ingestion of raw snails and slugs. A lumbar puncture can be performed, in which spinal fluid is collected for analysis, and eosinophils (a type of white blood cell that defends against fungal and parasitic infections) will be screened for, as well as the presence of parasites, although these are difficult to find. 

Diagnosis can also be confirmed by polymerase chain reaction (PCR), which detects rat lungworm DNA within the cerebral spinal fluid and other tissues.⁷ A definite history of contact or ingestion of the parasite is not required to make a diagnosis and begin treatment, a presumptive diagnosis should be seen as sufficient. 

A neurologic examination, including an eye examination, can be performed to assess the nervous system, eye muscles, papilledema (swelling of optic disks) and the presence of larvae. Eosinophilic meningitis is diagnosed when eosinophils account for more than 10% of the white blood cell count and when there are at least 6 total white blood cells per microlitre of cerebral spinal fluid. However, eosinophils may be absent within the early stages of the disease, making early diagnosis difficult. Therefore, the presence of any eosinophils in the cerebral spinal fluid should be viewed as abnormal. 

Individuals may be asked to return for a second lumbar puncture several days later if suspicion of rat lungworm remains high. In certain cases, an MRI scan of the brain may be required to view the white matter, with a 3D MRI scan providing more specific information. A chest CT scan may be considered if respiratory symptoms are present, as there have been reports of lesions within the chests of individuals due to the movement of the larvae, as well as identification of the parasite within the pulmonary arteries during autopsy.⁸

Treatment and management of rat lungworm infection

As the parasite will eventually die and cause inflammation, doctors should prescribe a high dose of corticosteroids as early as possible (when a presumptive diagnosis is made) to reduce inflammation, although this may not be necessary in mild cases. A study in Thailand found that individuals who took the corticosteroid prednisolone had a dramatic reduction in head pain compared to those who did not take the corticosteroid, suggesting that the drug improves the symptoms of eosinophilic meningitis.⁹ Anti-parasitic drugs may help kill the parasite, although there is little evidence for this within humans. Those who have lived through meningitis often have healthcare needs that require long-term medical treatments, and this can have social and educational implications. It is unclear if corticosteroids can help individuals who are critically ill, comatose or have long-term disabilities due to eosinophilic meningitis. Individuals with symptoms should always consult their healthcare provider for more information and treatment guidance.¹⁰

Rat lungworm can be prevented through many simple measures, such as not eating raw slugs or snails, thoroughly washing raw vegetables before consumption and supervising young children in environments where they can get their hands on slugs or snails. 

If you grow your vegetables, it is beneficial to plant them alongside plants such as fuchsias, geraniums or lavender, which act as natural snail repellents. Physical barriers such as copper foils can be wrapped around planting boxes as these react with snail and slug slime, disrupting their nervous system. Maintaining high levels of sanitation is important when controlling hosts such as rats. Cleaning up leftover food and cleaning gutters will help to control the population of rats.¹¹ 

In Hawaii, an area that has had an increase in serious cases of rat lungworm, youth and agriculture projects have been implemented to encourage school children to get involved in community health education and host control programmes. These children are involved in developing pest management plans for the control of invasive hosts of rat lungworms, decreasing the spread of the disease.¹²

Currently, there are potential vaccinations that can be administered to rats that kill the rat lungworm and prevent its lifecycle from progressing. However, it is debated whether these vaccines are effective, as data collected by researchers in Hawaii suggested that vaccines do not provide adequate protective immunity to guard against infection by rat lungworm. However, current studies identify possible drugs that can be used in treating rat lungworms, such as pyrantel pamoate and ivermectin.¹³

Summary

Rat lungworm is a parasitic worm that is found in slugs and snails, primarily in Asia. Rat lungworms can be ingested by humans through the consumption of slugs or snails that can be found on unwashed, raw vegetables. This parasite can cause eosinophilic meningitis, which causes symptoms such as neck stiffness, headaches, fevers and seizures. Complications of this disease can be fatal. Doctors will prescribe corticosteroids to treat rat lungworm, and a lumbar puncture can be performed to diagnose the disease. Rat lungworms can be prevented by thoroughly washing vegetables and supervising young children when they are in environments where they could consume snails or slugs. Always consult a medical professional if you feel that you may have symptoms of eosinophilic meningitis.