Post-traumatic infection or wound infection: This is a common complication following fractures, especially if the fracture is open or if there was a delay in treatment. The presence of a low-grade fever could indicate the onset of an infection.

Deep Vein Thrombosis (DVT): Immobility following an ankle fracture increases the risk of developing DVT, which can cause low-grade fever among other symptoms.
Reaction to immobilization or casting: Sometimes, the materials used for casting or the immobilization process itself can cause a mild allergic reaction or irritation, leading to a low-grade fever.
Atypical pneumonia or viral infection: Patients with limited mobility might be more susceptible to respiratory infections, which could present with low-grade fever.

Osteomyelitis: Although less common, osteomyelitis (infection of the bone) is a serious condition that requires prompt treatment. It can occur after a fracture, especially if the fracture was open or if there was significant soft tissue damage.
Septic arthritis: If the fracture involves a joint, there's a risk of septic arthritis, which is a serious infection within the joint space.
Fat embolism syndrome: This is a rare but potentially life-threatening condition that can occur after long bone fractures, including the femur, but it's less common with ankle fractures. However, it's crucial not to miss it due to its high mortality rate.

Complex regional pain syndrome (CRPS): While not typically associated with fever, CRPS can cause a variety of systemic symptoms and should be considered in the differential diagnosis of post-fracture complications, though fever would be an atypical presentation.
Erythromelalgia: A rare disorder characterized by burning pain, heat, and redness in the hands and feet, which could potentially cause a low-grade fever, though it's an uncommon condition and less directly related to ankle fractures.