phagocytophila

Phagocytophila is a historical term for a group of small, obligate intracellular bacteria now classified as Anaplasma phagocytophilum, the etiologic agent of granulocytic anaplasmosis. Modern taxonomy places this organism in the family Anaplasmataceae. The name phagocytophila appears in older literature, but the species is typically referred to as Anaplasma phagocytophilum in current use.

Biology and lifecycle: Anaplasma phagocytophilum is a gram-negative, pleomorphic bacterium that infects neutrophils in humans and other mammals. Within host cells, it replicates in membrane-bound compartments and forms morulae, which are dense clusters of bacteria visible in blood cells. The organism is an obligate intracellular pathogen and requires a mammalian host and a tick vector to complete its lifecycle.

Transmission and reservoirs: The primary transmission route is through tick bites, especially by Ixodes species such as Ixodes scapularis in North America and Ixodes ricinus in parts of Europe. Reservoir hosts include small mammals and deer, which help maintain the pathogen in nature. Ticks acquire the bacterium during feeding and can transmit it during subsequent feedings.

Clinical features: In humans, infection is known as granulocytic anaplasmosis and presents with fever, headache, myalgia, malaise, and often leukopenia and thrombocytopenia. Disease ranges from mild to severe, and can be dangerous in elderly or immunocompromised individuals. In ruminants, the infection can cause tick-borne fever, abortion, and reduced productivity.

Diagnosis, treatment, and prevention: Diagnosis relies on serology (such as paired sera) and molecular methods (PCR) to detect Anaplasma phagocytophilum DNA, with microscopy showing morulae in neutrophils in some cases. Doxycycline is the first-line treatment for humans and animals. Preventive measures focus on tick control and avoidance of tick habitats, especially during peak tick activity.