Which is NOT a common internal parasite found in animals?

Flukes are indeed internal parasites, but they are less commonly recognized compared to the others listed. They belong to a group of flatworms known as trematodes and can infect various organ systems in animals, often requiring specific environmental conditions and intermediate hosts to complete their life cycle. In contrast, coccidia, heartworms, and pinworms are more commonly encountered in everyday veterinary practice.

Coccidia are single-celled organisms found in the intestines of animals and are often associated with gastrointestinal diseases. Heartworms are a type of roundworm that primarily affect dogs and cats, residing in the heart and pulmonary arteries, leading to serious cardiovascular issues. Pinworms are specific to certain species like horses and cause irritation in the gastrointestinal tract. These parasites are more prevalent and frequently seen during routine veterinary examinations or treatments, making flukes less common by comparison among the listed options.