Snails, Slugs, and Semi-slugs: A Parasitic Disease in Paradise

Posted on by Alex da Silva

Parmarion martensi: a semi-slug commonly found in Hawaii.

CDC plays a vital role supporting state health departments, particularly with management of rare or lesser-known pathogens. Recently, CDC’s Division of Parasitic Diseases (DPD) was contacted by the Hawaii Department of Health (HI DOH) for advice regarding three cases of presumed Angiostrongylus cantonensis (AC) infection. AC, commonly called the rat lungworm, is a parasitic worm and the most common infectious cause of eosinophilic (a type of white cell) meningitis in humans worldwide.

AC has an interesting life cycle. Infected rodents carry the adult worm and pass immature worms in their feces. Mollusks (i.e., snails, slugs, or semi-slugs) become infected by ingesting immature worms in the rat feces. Humans become infected by ingesting raw or undercooked mollusks (these guys can be tiny enough to hide on a nickel; – check it out! infected with the worms or contaminated raw produce. Transmission might also occur through ingestion of raw or undercooked freshwater shrimps/prawns, crabs or frogs. In humans, AC causes eosinophilic meningitis, the symptoms of which can include headache, stiff neck, nausea, vomiting, low-grade fever, fatigue, and abnormal skin sensations (e.g., tingling or pain). In most cases, the symptoms disappear in weeks to months and most patients recover completely, although rare cases of blindness, paralysis, and death have been reported.

A picture from differential interference contrast microscopy showing A. cantonensis. This larva was obtained from a P. martensi slug collected in Hawaii. Infective, third-stage larvae measure 0.425 mm – 0.523 mm in length
A picture from differential interference contrast microscopy showing A. cantonensis. This larva was obtained from a P. martensi slug collected in Hawaii. Infective, third-stage larvae measure 0.425 mm – 0.523 mm in length

Armed with this information, a team of scientists from USDA (HI), HI DOH and DPD began testing mollusks collected near the case-patients’ homes in Hilo Puna. We used morphologic and molecular techniques to test and document infection in slug samples sent from Hawaii. HI DOH has determined that many patients live in communities outside of the municipal water/sewage system and consume home-grown produce. The DPD molecular diagnostic parasitology lab, which I lead, continues to collaborate on testing of environmental samples and the development of methods to evaluate the effectiveness of interventions to eliminate infected rodents and mollusks. In this new phase of the collaboration, a real-time PCR assay developed in-house at DPD to detect A. cantonensis is being used to analyze mollusk samples collected in Hawaii. Future efforts will involve the transferring of this real-time PCR test and other molecular methods to Hawaii so testing can be performed in-state.

Since initial contact with Hawaii, four more cases of presumed AC infection have been reported. In response, we are helping to educate the public in Hawaii on how to prevent the infection. Information has also been developed for the healthcare community in Hawaii because many physicians may not consider AC when evaluating patients with eosinophilic meningitis. AC can be prevented by avoiding the consumption of raw/undercooked snails, slugs, freshwater shrimp/prawns, crabs and frogs; by washing raw produce thoroughly prior to eating; and by wearing gloves and washing hands after handling mollusks.

Posted on by Alex da Silva

23 comments on “Snails, Slugs, and Semi-slugs: A Parasitic Disease in Paradise”

Comments listed below are posted by individuals not associated with CDC, unless otherwise stated. These comments do not represent the official views of CDC, and CDC does not guarantee that any information posted by individuals on this site is correct, and disclaims any liability for any loss or damage resulting from reliance on any such information. Read more about our comment policy ».

    Interesting. Thanks for the information regarding the certain foods. I’ll remember this next visit to Hawaii.

    05 October 09 My wife purchased organic romaine hearts from the military on post commissary on Ft Benning Ga. I often have it for lunch, this time (the First spotting) I found a (I believe) semi-slug the heart was loaded with what I assumed to be feces from the slug… the slug was similar in appearance to the one in the picture of this site… though its back end mangled or decaying. so I removed the slug rinsed the leaves removed any browned or obviously eaten areas and consumed the lettuce as I was i a rush to eat and get back to work. Am I in any danger from eating the lettuce?

    Dear Mr. Stevens,
    Thank you for your inquiry about the risk of acquiring Angiostrongylus infection after eating lettuce on which you found a semi-slug. Angiostrongylus cantonensis infection occurs mostly in Asia, the Pacific (including Hawaii), and the Caribbean, and can cause eosinophilic meningitis. Sporadic cases have also occurred in South America and Africa. Angiostrongylus costaricensis occurs mostly in Central and South America and causes eosinophilic enteritis, a gastrointestinal disorder. A handful of cases of Angiostrongylus cantonensis infection have occurred in the continental United States, one in Louisiana from an ingested snail found locally, but other cases may have been due to imported snails, slugs, semi-slugs, or rats harboring Angiostrongylus larvae. Imported produce may be contaminated with snails, slugs, semi-slugs, or their slime containing Angiostrongylus larvae. Imported snails, slugs, and rats harboring larvae may also result in contamination of local produce. While there is likely less risk of infection from produce grown in the continental United States, the occurrence of Angiostrongylus species in the United States is not well defined, and the risk from imported produce is unknown. Washing produce thoroughly should theoretically reduce the risk of Angiostrongylus infection.

    For more information, please call 1-800-CDC-INFO or 1-770-488-7775, or refer to the CDC website regarding Angiostrongylus:

    I noticed slugs around my lettuce plants. What precautions should I take before eating my lettuce. I first of all dipped my lettuce a couple of times in hot water. I then rinsed it several times is cool water, and also as a precaution. I put a little dish washing liquid in my water and dipped the lettuce in it, then I dipped my lettuce several times in clean water. Have I taken enough precautions in order to eat my lettuce. I know there have been a few cases of people getting seriously sick from eating home grown lettuce, and I don’t want to get sick. I am trying to grow my own basil, lettuce, rosemary, and tomatoes since produce is so expensive.

    I had problems with snails getting into the cat food ! My car has been sick for two months. The vet says it’s ear infection but,after three weeks of antibiotics,I am being to wonder. Thank you.

    I have slugs that come on my porch to get to my cat’s food…..can the slugs carry bacteria that will make my cats sick
    I would really like it if anyone who knows to please give me an email at:
    Thank u
    Jenny and her cats

    So an infected slug’s slime will potentially or most likely contain larvae. These larvae are not visible with naked eyes (microns?)
    If a leafy vegetable is contaminated with larvae (not slug) , how can one ascertain that rinsing with water alone was adequate to rinse off all larvae?

    I live on the north east shore of oahu hawaii. I have a covered outdoor kitchen and bathroom area. I often see these semi slugs around the shower area. After noticing the strange slugs I googled why they had weird looking growth on the back and came to find they are the semi slug. Anyway I don’t grow any vegetables but I am very worried about there slime trails they leave. Do I need to be worried? Can stepping on the trails be hazardous?

    We know little about how much or how often parasite larvae can be found in the slime left behind by infected slugs and snails, or if you can become infected by accidentally eating slime. Washing fruits and vegetables thoroughly under running water before eating, cutting, or cooking is the recommended way to remove potentially infected slugs and snails and their slime. In areas where the parasite is present, cooking food properly also is recommended. More information is available at the following link:

    People get rat lungworm disease by eating infected snails, slugs, prawns, freshwater shrimp or frogs. There are no reports of humans getting rat lungworm disease through skin contact with slime, for example, by stepping on a slime trail barefoot. The larvae of the parasite that causes rat lungworm disease are not able to penetrate intact skin. It may be possible to get infected if you have an open wound that comes into contact with slime, but it is important to note that most slime does not contain the infective form of the parasite. More information is available at

    We have a coffee farm and have seen semislugs crawling up a tree. Does the Corry slug bait kill semi slugs? Is there an easier way to get rid of these slugs on a 2 acre farm such as spraying to get rid of slug eggs?

    I recently bought a water snail from a pet store and spilled the pet store aquairium water on my floor, i cleaned it up with paper towel and clorox wipes. I also have a near heald scratch on my hand. My question is can aquarium snails from a pet store become infected and if so can handling thier waste water with a scratch cause infection? I woke up in the middle of the night with a sore neck and slightly concerned.

    Today in a park on Skyline Blvd south of SF I saw a banana slug curled into a circle with what looked like a small pile of dark worms. The worms had invaded the stomata where air enters the lung, and under the mantle, even apparently trying to penetrate the skin of the banana slug. I straightened out the slug and pulled a worm-like animal out of the breathing hole, and from under the mantle, then another piece of worm appeared at the same site, and I pulled several more pieces out of what appeared to be a worm. As I removed these worms the slug seemingly became more active and began twisting itself. I placed the slug away from where the pile of worms were. Could they have been worms? They were not at all active but kept coming out of the breathing hole.

    I live in Southern Cali, Los Angeles. My son,4yr old , plays with snails,Rollie polies and slugs. Should I have him stop playing with them . Or is it ok as long as he doesn’t eat them and washes his hands.?

    Thanks for your question. CDC recommends that children younger than 5 years old should not be allowed to handle snails and slugs that might carry the parasite Angiostrongylus cantonensis. General recommendations to protect your and your child’s health include:
    • Wash your hands with soap and warm water after playing with your pets or other animals, after outdoor activities, and before handling food.
    • Teach children the importance of washing hands to prevent infection.
    • Teach children that it is dangerous to eat dirt or soil.

    For more information, please visit

    What effect will these have on a well water holding tank if they were to fall inside I’m having a problem what coliform in my well and the only only evidence I see is possibly slugs sneaking through the vent

    Thanks for your comment on this Public Health Matters post. If you have a question for CDC, please contact CDC-INFO ( Operators are available by phone and email to help you find the latest, reliable, and science-based health information on more than 750 health topics. The phone number is 800-CDC-INFO (800-232-4636).

    Hello sir,
    I’m from india, today small snail bite me on my leg and a small spot of blood apear. I extract som blood from spot by pushing. Please tel me am i in any danger?

    Dear CDC,

    Over 30 years ago, I went on a North Pacific Coast camping bike tour down the coast. While camping on Vancouver Island, BC, Canada, a large banana slug somehow got into my tent and sleeping bag while I was busy setting up my campfire to cook my dinner. When I woke the following morning to hang-dry my sleeping bag, I found a banana slug curled up & injured in my bag down where my leg would’ve been during my sleep. I soon discovered a small abrasion on my calf muscle. Later, my touring mates pointed out that my calf had a small “bullseye” of red & yellow rings around the abrasion point. By that point we’d done some hard riding up into the mountains of the Olympic Peninsula. By the time we returned to the road from Port Angeles to Forks, WA, the rings had expanded to encompass my entire calf muscle. However, the rings were not simply red & pink. From the abrasion, the skin was a sickening yellow about the size of a quarter follow by rings of various colors of angry red, purple, dark grey(just like the color of a hard bruise), clear, red and pink fading into my normal skin color. We stopped a US Ranger around Forks. He said he’d never seen anything like it before & asked whether I needed a ride to a hospital. I said no because the worst seemed to have ended already after a few days of seeing tracers from passing car lights and feeling weak & odd. I was beginning to feel better by the time I found the US Park Ranger. And as I continued riding hard every day, the rings slowly disappeared over the next two weeks. My friends riding with me guessed that I’d probably been bit by a brown recluse by we all postulated I was out of danger of any necrosis due to the day-long hard use of my legs peddling and that this increased the circulation of the blood & venom of the bite such that their was no opportunity for any flesh necrosis. I was young & had never heard of Lyme disease. But the US Ranger should’ve been familiar with the signs of Lyme disease. We later guessed that his comment about never seeing anything like it before was due to the extra colors & perhaps the large diameter of those rings to fully cover my calf muscle. To him, the extra colors & size was something other than Lyme to his knowledge. So I’m wondering about what pathogen may have been transferred to me through that “bite?” My health has been in an accelerated decline ever since, both physically and mentally. I’ve tested negative for Lyme even though the bullseye ring is considered a presumptive positive for Lyme. Can you please tell me what may have happened to me & how to treat it? Thanks for everything you do for the world!

    Dear CDC,
    I was “bitten” by a banana slug on Vancouver Island, B.C., Canada about 30 years ago. The result was a bullseye mark that covered my entire calf muscle. Unlike the red & pink rings that indicate a presumptive Lyme infection from a tick, my bullseye had several differently colored rings from yellow, angry red, purple, dark grey(similar to a severe ankle sprain), a yellowish clear ring, and a large outer pink ring. This bullseye ring caused my to see “tracer lights” from oncoming traffic, aches, weakness and strange dreams while I was riding on a vigorous bicycle tour & continuing to camp in the Olympic rainforests of WA state. I’m quite certain I was bitten by a banana slug & not a tick. I tested negative for Lyme antibodies. What infected me? Are the debilitating symptoms I have now, at age 55, due to an untreated & undiagnosed chronic infection? I imagine it’s rare to be “bitten” by banana slugs but I didn’t ingest the slug, I was bitten.

Post a Comment

Your email address will not be published. Required fields are marked *

All comments posted become a part of the public domain, and users are responsible for their comments. This is a moderated site and your comments will be reviewed before they are posted. Read more about our comment policy »

Page last reviewed: January 19, 2018
Page last updated: January 19, 2018