What clinical signs are associated with Marek's disease in poultry?

Marek's disease is a viral illness caused by a herpesvirus affecting chickens and other poultry. One of the hallmark clinical signs of Marek's disease is the presence of enlarged nerves, particularly the sciatic nerve. This enlargement can lead to neurological symptoms and affect the bird's ability to walk, resulting in paralysis in some cases.

In addition to neurological signs, affected birds may exhibit other symptoms related to the viruses, such as weight loss or decreased egg production, but the enlarged nerves are a defining characteristic that differentiates Marek's disease from other poultry diseases.

The other options present symptoms that are associated with different conditions in poultry. For instance, swollen joints and nasal discharge typically indicate infectious synovitis or respiratory disease, severe respiratory distress is often linked to infections like avian influenza or Newcastle disease, and diarrhea with blood suggests gastrointestinal infections or conditions like coccidiosis. Thus, the specific manifestation of enlarged nerves is a key indicator of Marek's disease, making it the correct choice in this context.