Looking for Leucocytozoon: hemosporidia species in California great horned owls (Bubo virginianus)

Leucocytozoon species are avian blood parasites transmitted by black flies, causing primarily subclinical infections but capable of leading to acute disease in young or compromised birds. In the spring of 2023, a novel leucocytozoonosis event was observed in great horned owls (Bubo virginianus) in San Diego county, CA , with 27 of 34 individuals (79.4%) testing positive. Clinical presentations ranged from asymptomatic to severe debilitation. Blood work revealed a consistent pattern of increased buffy coat and, in some cases, absolute anemia. Three treatment regimens were evaluated: supportive care alone, sulfamethoxazole–trimethoprim with iron supplementation, and sulfamethoxazole–trimethoprim combined with pyrimethamine. Survival outcomes, however, were likely influenced by additional factors beyond drug therapy, including anemia, emaciation, hypothermia, heavy parasite burdens, concurrent trauma, or possible lead exposure—all of which may have compromised the ability of some owls to recover despite treatment. This study highlights the need for further investigation into leucocytozoonosis detection and treatment in wildlife.