If they carry the parasite lungworm or have consumed slug bait, then land slugs can be harmful to dogs. Sea slugs, which are found in areas like New Zealand, contain the neurotoxin tetrodotoxin, which can also be toxic to dogs.
Dry land slugs are actually harmless and nontoxic animals. Nevertheless, when a dog ingests slugs with lungworm, it can lead to a major infestation. If not treated promptly, an infestation can be fatal. Symptoms of a lungworm infection include respiratory problems, such as breathing difficulty and coughing. Other symptoms are weight loss, anemia, vomiting, seizures and vision loss. When diagnosed in its early stages, it is treatable. Severe infestations are more difficult to treat.
Likewise, slug bait can contain the ingredient metaldehyde, which is toxic for dogs. Eating slugs treated with this product can cause a dog to experience symptoms like drooling, vomiting, tremors, convulsions and agitation. Metaldehyde poisoning in dogs requires immediate treatment by a veterinarian, according to petmd.com.