Everything about slugs and snails in your garden isn’t bad.
In the garden folklore, slugs and snails are weather detectors. “When the black snail does cross your path, black cloud much moisture hath.” Snails climbing trees are a harbinger of hot weather. Slugs burrowing deep into the ground in summer are a sign of impending drought, and slugs burrowing deep into the ground in autumn are a sign winter is coming soon.
There’s no doubt that slugs and snails help to clean up garden debris. Almost all common garden snails and slugs (except the uniquely destructive Field Slug Deroceras reticulatum), prefer dead garden detritus to living plants. Their feces make a nitrogen-rich, mineral-laden fertilizer that enhances plant nutrition.
Of course, there are many ways snails and slugs are bad for your lawn and garden. Snails don’t do a lot of damage at ground level, but they are strong climbers. They find their way up flowering plants to eat flower buds. They climb fruit trees and feast on fruit just as it is turning ripe. They strip bark off young trees and chew smooth, irregular holes through leaves. Or they just live underground and eat the roots like the Keel Slug.
Slugs can be even more destructive. They may devour bulbs while they are still in the ground. They mow down seedlings as soon as they emerge from the ground. They leave a slimy trail everywhere they go that attracts other slugs.
While snail and slug damage to garden plants is bad, some things about slugs and snails in your garden are very bad. There is one very good reason every gardener needs to minimize contact with slugs and snails: They spread disease.
Both garden snails and garden slugs are potential hosts of the rat lungworm, Angiostongylus cantonensis. This parasitic nematode primarily infects rats, with adult worms infesting the blood vessels that transport blood from the lungs to the heart. Eggs hatch inside the rat, crawl out of the lungs and into its mouth, and are eventually discharged in feces. Snails eat the feces and a new batch of lungworms hatches inside them.
If other rats eat the infected snails, the cycle continues, but if humans eat the infected snails, the parasite takes a different path. In humans, rat lungworms travel to the brain. The worms initially lodge themselves in the meninges covering the brain. They may burrow into the brain itself, causing high pressure in the fluid of the brain and spinal column and damaging brain tissue. The immune system fights the infection with high fever. How much damage is done to the brain depends on how many worms are consumed with raw snails, their feces, or their mucus. It is not necessary to eat “snail sushi” to get the infection. You can also get it from unwashed vegetables that are contaminated by slime from infected snails. This infection was first reported in the southern United States in 1987, and occasionally occurs in Britain.
A closely related parasite called Angiostrongylus costaricensis is also spread by rats and snails to humans. It infects the intestines rather than the brain. Once it is inside a human host, it never leaves. It is not expelled with human feces. This parasite can cause severe intestinal problems. Most of the cases of this infection in human hosts have been recorded in Central and South America. In both Britain and the United States, yet another parasite in the same genus, Angiostrongylus vasorum, causes lungworm infections in dogs, and a similar species called Aerlurostrongylus abstrusus infests the lungs cats. Neither dogs nor cats commonly eat snails, but puppies and kittens experiment and can get infected. Cats and dogs can also become infected if they eat other animals that have eaten infected slugs or snails.
A liver fluke called Dicrocoelium can be transmitted from snails to cattle, sheep, and people. These tiny liver worms make a temporary home in snails until they are expelled in slime balls. Ants eat the slime, and people (as well as grazing animals) get the flukes when they accidentally (or intentionally) eat uncooked ants.
Several thousand people every year contract parasites from contact with garden slugs and snails. Uncounted hundreds of millions of plants, of course, are devoured by these slimy invaders to the garden.
Snails and slugs alike can transmit Salmonella, which causes severe food poisoning.
If you would like to learn more about which are the best methods for trapping and killing these slimy animals the read these two articles The Top Three Slug and Snail Traps and Poisons and the Top Three Snail and Slug Repellents and Barriers.