What is one of the signs that a pet may have a serious parasitic infection?

Sudden behavioral changes in a pet can indicate a serious parasitic infection because parasites often affect the host's overall health and well-being. When a pet is experiencing discomfort, pain, or systemic illness due to a parasite, it can lead to changes in their normal behavior. For instance, a pet may become more withdrawn, irritable, or exhibit changes in appetite or activity levels.

Behavioral changes can serve as a critical warning sign for pet owners, prompting them to seek veterinary assistance. This can lead to timely diagnosis and treatment, preventing further complications associated with serious parasitic infections.

The other options, while they can sometimes be associated with underlying health issues, are not as direct indicators of a serious parasitic infection. Increased appetite alone may occur with certain parasites but is not a definitive sign. Hair loss can result from various skin conditions, including allergies or infections, which are not exclusively linked to parasitic diseases. Consistent weight gain could indicate an underlying metabolic or dietary issue rather than a parasitic infection.