What symptoms indicate the presence of biting and sucking lice in dogs?

Excessive itchiness and hair loss are classic symptoms indicating the presence of biting and sucking lice in dogs. These parasites feed on the dog's skin and blood, leading to irritation that results in intense scratching and grooming behavior, which can further contribute to hair loss. The irritation caused by the louse bites may lead to inflamed skin, making the affected areas prone to secondary infections as well. Lice infestation typically causes discomfort and distress for the dog, making itchiness and hair loss prominent indicators.

In contrast, the other options listed do not correlate with the specific symptoms of lice infestations in dogs. For instance, frequent urination and vomiting may suggest a different health issue, commonly related to digestive or urinary problems, rather than a parasitic infestation. Excessive drooling and lethargy can point towards other health concerns, such as dental disease or systemic illness. Internal bleeding and fever are symptoms associated with serious medical conditions, but they are not typical signs of lice infestation. Therefore, excessive itchiness and hair loss are indeed the most relevant indicators of lice presence.