The next morning, whilst nursing a hangover, the patient thought back to what he had done and started to wonder if this was such as sensible idea after all. A quick search on “Dr Google” and he was in a panic.
“You see Doc, I think I’ve got lung worm!” he exclaimed.
After recovering from the mental picture of the Harry Potter film where Ron Weasley’s curse “Slugulus Eructo” backfires and he starts coughing up giant slugs, the GP thought I know nothing about this. He tapped “lung worm” into his own computer and everything that came up was either to do with dogs… or Ron’s slug curse, apparently Ron’s plastic slugs were in chocolate sauce to make them more palatable! Anyway, not very helpful, time to call a Microbiologist he thought….
Lung worm, or to give it it’s proper medical name “angiostrongyliasis”, is a parasitic infection caused by the rat lung worm Angiostrongylus cantonensis. As its name suggests, the parasite is primarily a disease of rats. Adult lung worms excrete eggs in the pulmonary arteries of the rat. The rat coughs up the larvae and swallows them, where they pass in the rat faeces. Slugs and snails then eat the rat poo and the larvae; the larvae then develop in the slug or snail into the infective third stage larva. To complete the cycle, when rats then eat the slug or snail the infective larvae migrate to the rats brain where they mature into adults before going back to the pulmonary arteries to lay their eggs… gross huh?!
Humans can become infected by eating slugs and snails containing the third stage larvae in the same way that rats are. This can either happen because a normally edible snail is eaten uncooked or a slug is eaten accidently in salad crops… or it can happen when someone is crazy enough to eat a slug as a dare. Toddlers also seem to find them a tasty garden treat much to the distress of their parents!
Where in the World do you find lung worm?
Okay, if you look on the Internet you will see that animals in the UK are at risk of lung worm, especially dogs. However, the organism that infects dogs is Angiostrongylus vasorum. The good news for us in the UK is that A. vasorum does not appear able to cause infection in humans and so it is incredibly unlikely that someone in the UK will get lung worm. We also do not have Angiostrongylus cantonensis in the UK. The main areas of the World affected by Angiostrongylus cantonensis lung worm are: Southeast Asia, especially Thailand, Malaysia and Southern Vietnam. It can also be found in Indonesia, the Philippines, Taiwan, China, Japan, Papua New Guinea, Hawaii, and many other Pacific islands. Lung worm has also been found more rarely in Africa, Australia, Cuba, Puerto Rico, and Jamaica as well as Louisiana due to ship borne spread of Pacific rats and their worms!
How does lung worm present in humans?
Despite its name, lung worm in humans actually presents with neurological symptoms and signs. It causes eosinophillic meningitis; the CSF contains lots of eosinophils as opposed to the neutrophils or lymphocytes we normally see with bacterial or viral meningitis.
Symptoms usually occur within 1-3 weeks of eating the slug or snail infected with Angiostrongylus cantonensis. Symptoms in humans relate to the worm migrating to the brain but humans are a dead-end host for the parasite. The worms don’t mature to the point of being able to lay eggs so they don’t then migrate to the human lung, the worms die in the brain which causes a localised inflammatory reaction.
Infection can be completely asymptomatic, however when symptoms do occur, the most common are:
- Severe headache
- Facial nerve paralysis
- Neck and back stiffness
- Nausea and vomiting
- Paraesthesia (sensory disturbance, often pins and needles, due to nerve damage or inflammation)
- Rarely visual disturbance due to larvae entering the eye
How is lung worm diagnosed?
The parasite itself has occasionally been found in CSF or vitreous from the eye but this is rare. In the majority of cases the diagnosis is based upon a history of eating raw slugs or snails and the presence of eosinophils in the CSF. There is often a high eosinophil count in peripheral blood as well. There is a serology test that looks for the immune response to the parasite which can confirm the diagnosis but in the UK this involves sending the serum sample to Mahidol University in Thailand, which is both expensive and slow.
How is lung worm treated?
Okay, so here is the thing about infection with lung worm… it is a self-limiting infection which gets better with supportive care. Yep, really! No antimicrobial treatment is required. The patient may need significant pain relief for the headaches and require repeated lumbar punctures to remove CSF to help with relief from pain, but they will get better on their own.
It is unlikely that antiparasitic drugs like Albendazole or Mebendazole will have any effect on lung worm because the symptoms are due to the presence of dead worms not live ones so there is nothing for the drugs to actually kill or “treat”. Indeed, these drugs can make the symptoms worse as they might kill remaining worms thereby worsening the inflammatory response. Steroids can help relieve symptoms by reducing the amount of inflammation and a common regimen is to use PO Prednisolone 60mg OD for 2 weeks.
The GP was reassured, “OK so, this particular parasite Angiostrongylus cantonensis, which infects humans, is not found in the UK…OK, and good to know it’s a self-limiting infection too, so I know what to do for those pesky returned travellers!”
The GP called their patient back in. “Okay, so the good news is you are not going to get lung worm…” they explained, “however we do have to talk about your alcohol consumption…”