What are common signs of flea infestations in dogs?

Excessive scratching and hair loss are indeed hallmark signs of flea infestations in dogs. When fleas bite, they can cause intense itching due to allergic reactions to flea saliva, leading to excessive scratching, biting, and licking at the skin. This behavior can result in hair loss, skin inflammation, and even secondary infections if the skin is damaged from the scratching.

The other options do not accurately represent the typical signs associated with flea infestations. Weight gain and lethargy are more general signs that could indicate a variety of health issues but are not specifically linked to fleas. Increased thirst and vomiting can point to numerous medical problems, such as kidney issues or gastrointestinal disorders, but they are not common indicators of fleas. Similarly, discoloration of the eyes and ears does not correlate with flea infestations and may suggest other health concerns that are unrelated to flea presence. Thus, the primary signs to watch for in the case of flea infestations in dogs are indeed excessive scratching and hair loss.