Which of the following best describes ectoparasites?

The correct description of ectoparasites is that they are parasites that live on the exterior surface of the host. Ectoparasites, such as fleas, ticks, and mites, attach themselves to the outside of the animal's body and feed off their host's skin, blood, or other tissues. This external location can lead to irritation, allergic reactions, and the transmission of diseases to the host animal. Understanding the characteristics of ectoparasites is crucial in veterinary medicine for diagnosing and treating infestations effectively.

The other options describe different types of relationships or characteristics that do not apply to ectoparasites. For instance, parasites that live within an animal's body are referred to as endoparasites. Beneficial parasites do not exist in a context where they harm their host, as ectoparasites generally cause some level of detriment. Lastly, while some parasites can be challenging to treat, ectoparasites can indeed be treated with appropriate medications and preventive measures, which contradicts the notion that they cannot be treated.