What is the most likely cause of weight loss, diarrhea, and tenesmus in a four-year-old mixed breed dog?

The most likely cause of weight loss, diarrhea, and tenesmus in a four-year-old mixed breed dog is associated with an infection caused by Trichuris vulpis, commonly known as whipworm. These parasites are known to primarily affect the large intestine of dogs, leading to inflammation and subsequent gastrointestinal symptoms.

Whipworms can cause weight loss because the irritation and damage to the intestinal lining can impair nutrient absorption. This is often coupled with diarrhea, which may be intermittent and can vary in severity. Tenesmus, or the straining to defecate, is a significant symptom associated with whipworm infection due to the irritation of the rectum and large intestine.

While other parasitic infections such as hookworms, coccidia, and Giardia can also lead to diarrhea and weight loss, the specific combination of symptoms, including tenesmus, aligns more closely with a whipworm infection. Whipworm infections can result in more pronounced signs of straining due to the unique manner in which they affect the gastrointestinal tract.