If an animal has been diagnosed with polycythemia, which symptom is commonly observed?

In cases of polycythemia, a condition characterized by an increased number of red blood cells in the bloodstream, one of the common symptoms observed is excessive thirst. This occurs as the body attempts to regulate blood volume and viscosity due to the elevated red blood cell count, prompting the animal to drink more water to help maintain proper hydration and circulation.

Polycythemia can lead to thicker blood, which might result in complications such as decreased blood flow and oxygen delivery to tissues. The body's response includes increased thirst to counteract the potential dehydration and to support the heart and circulatory system in managing the increased workload.

The other options, such as vomiting, diarrhea, and weight gain, are not typical signs directly associated with polycythemia and do not help in identifying or managing the condition. Instead, excessive thirst stands out as a significant symptom for animals affected by this particular hematological disorder.