What are the clinical signs of a flea infestation in pets?

The clinical signs of a flea infestation in pets primarily include itching, hair loss, and skin infections. When fleas bite, they can cause significant irritation and discomfort to the pet, leading to intense scratching or biting at their skin. This excessive scratching can result in hair loss due to trauma and can also create open wounds, making the pet more susceptible to secondary skin infections.

Fleas can provoke allergic reactions in some animals, a condition known as flea allergic dermatitis, which exacerbates the itching and can lead to a cycle of further skin damage. In severe infestations, pets may develop even more serious dermatological issues, such as hot spots, which require medical intervention.

The other options presented do not typically correlate directly with flea infestations. While vomiting and diarrhea can occur in some situations due to various causes, they are not hallmark signs of a flea problem. Increased thirst and urination may suggest other health issues related to conditions like diabetes or kidney problems, not fleas. Additionally, weight gain and lethargy are not associated with flea infestations and may indicate different health concerns unrelated to external parasites. Therefore, itching, hair loss, and skin infections clearly represent the most relevant and direct signs of flea infestations in pets.