Systemic toxoplasmosis manifested as protozoal arthritis and secondary immune mediated thrombocytopenia in an apparently immunocompetent adult cat.

Toxoplasma gondii infection in cats is mostly subclinical. Systemic disease usually occurs in immunocompromised individuals. This report describes an unusual systemic toxoplasmosis, presenting as parasitic arthritis in an adult 14-year-old neutered male indoor domestic shorthair cat, with no identifiable cause of immunosuppression, and referred for lethargy, anorexia, and fever. Laboratory abnormalities included non-regenerative anemia, thrombocytopenia, increased serum amyloid A concentration, and polyclonal hypergammaglobulinemia. With persistent discomfort on palpation of the pelvic region and hind limb joints, synovial fluid was aspirated from the left carpus, elbow, and tarsus, and cytological analysis supported inflammatory polyarthritis. Additionally, the elbow joint fluid showed free crescent-shaped organisms, morphologically resemblingT. gondii tachyzoites. Serum anti-T. gondii IgG titer was initially 1:256 (cut-off titer, 1:50), although this was insufficient to confirm active infection. Whole-blood T. gondii polymerase chain reaction (PCR) was negative. The cat markedly improved with amoxicillin-clavulanate, enrofloxacin, clindamycin and supportive treatment. Two weeks later, serum anti-T. gondii IgG titer increased fourfold (1:1024), confirming seroconversion and active systemic toxoplasmosis. The infection likely triggered secondary immune-mediated thrombocytopenia (sIMT) and hemophagocytic syndrome, which improved following antibiotic therapy. T. gondii-associated immune thrombocytopenic purpura is reported in humans, but to our knowledge, never in cats. This case highlights the diagnostic value of synovial fluid cytology and seroconversion in feline systemic toxoplasmosis. Negative blood PCR test should not exclude active infection. Toxoplasmosis should be considered in cats presenting with fever of unknown origin, polyarthritis, or unexplained hyperglobulinemia, regardless of the immune status or lifestyle.