The common garden slug often prompts concern from gardeners and pet owners about its potential toxicity. The direct answer is no; slugs do not produce toxins that are harmful upon contact or ingestion. However, this simple answer does not mean slugs are safe or harmless, as they represent a significant biological hazard due to what they carry internally. The actual danger posed by slugs is not a toxin they generate, but rather a serious parasite they can host.
Understanding the Difference Between Poisonous and Venomous
To accurately assess the risk from a slug, it is helpful to understand the scientific distinction between “poisonous” and “venomous.” An organism is deemed poisonous if it contains toxins harmful when consumed, inhaled, or absorbed through the skin, meaning the delivery is passive. For example, a poison dart frog is poisonous because its toxins are on its skin.
In contrast, a venomous organism actively injects its toxin, known as venom, into another creature, typically through a specialized mechanism like a bite, sting, or barb. Slugs possess neither a specialized mechanism to inject a toxin nor do they produce toxins harmful upon ingestion, meaning they are neither poisonous nor venomous.
The Primary Health Risk: Internal Parasites
The most significant danger associated with slugs comes from their role as intermediate hosts for parasitic nematodes, or roundworms. The primary concern is Angiostrongylus cantonensis, commonly known as the rat lungworm. This parasite requires slugs and snails to complete its life cycle, as the larvae develop into their infectious third stage within the mollusk’s tissues.
Humans and pets become infected accidentally by consuming the slug or snail, or fragments of them, often unintentionally in contaminated produce. The infectious larvae travel to the central nervous system, where they cannot complete their life cycle because humans are a dead-end host. This migration causes angiostrongyliasis, which most frequently manifests as eosinophilic meningitis.
Symptoms of this neurological infection can include severe headaches, neck stiffness, fever, and nausea. In rare cases, the condition can lead to permanent brain or nerve damage. The presence of this pathogen reinforces that the slug itself is not toxic, but the infectious agent it carries presents a serious health threat. Over 160 species of gastropods have been identified as intermediate hosts for the Angiostrongylus genus.
Slime Secretions and Safe Handling
Slugs secrete mucus, or slime, which is a complex substance composed primarily of water, salts, and proteins. This slime serves several functions, including facilitating locomotion, adhering to surfaces, and protecting the soft body from injury. The mucus trail itself is generally non-toxic to human skin and is not a poison.
However, the slime can potentially be a vector for the internal parasites the slug carries. Angiostrongylus cantonensis larvae can be shed in the mucus trail left on surfaces, including garden vegetables. While the risk of infection from mucus alone is minimal, it is a plausible route for transmission, especially if contaminated slime is transferred from hands to the mouth.
Therefore, any direct interaction with slugs requires strict hygiene practices to mitigate the parasitic risk. Thorough hand washing with soap and water is mandatory after touching a slug, its trail, or any surfaces they have crawled on. It is also important to wash all garden produce completely before consumption to remove any slime trails or slug fragments that might harbor infectious larvae.