anaplasmosis

• anaplasmosis (also known as human granulocytic anaplasmosis, HGA) is a tick-borne bacterial infection caused by Anaplasma phagocytophilum
• transmission and vectors: spread via bites from Ixodes ticks, which are widespread across Europe, the UK, and North America:
  • in Europe and the UK, the vector is Ixodes ricinus (which can also transmit tick-borne encephalitis and other pathogens, though simultaneous co-infection with multiple tick-borne pathogens in Europe is very rare)
  • in North America, I. scapularis and I. pacificus transmit A. phagocytophilum and are also vectors for Lyme disease
• animal reservoirs and epidemiology: infects wild and domestic ruminants, horses, birds, rodents, and domesticated dogs and cats (causing tick-borne fever in sheep and pasture fever in cattle); the vast majority of human cases occur in North America, with cases rarely reported in Europe and only one documented human case acquired in the UK (in Scotland, where the pathogen is endemic in animals)

Clinical features

• most human infections are asymptomatic and go unnoticed
• symptomatic presentation typically includes fever, muscle aches and pains, chills, and joint pains
• severe manifestations: a small proportion of patients develop more serious complications including breathing difficulties and kidney failure, requiring urgent treatment

Diagnosis

• blood film: a peripheral blood film taken early after symptom onset may reveal characteristic clusters of bacteria (morulae) within granulocytic white blood cells
• definitive testing: PCR testing is rapid and sensitive during the early stage of illness, whereas IgG antibody testing allows presumptive diagnosis in later stages or after recovery
• differential diagnosis: consider other tick-borne infections such as Lyme disease, which is seen much more frequently in humans in Europe
• specialist testing in the UK: PCR and IgG antibody testing are available through the UK Health Security Agency (UKHSA) Rare and Imported Pathogens Laboratory (RIPL); infection specialists should discuss suspected cases with RIPL clinicians prior to testing for advice on differential diagnoses and management

Management and prognosis

• seek expert advice
• antibiotic therapy: standard treatment is doxycycline for 7 to 14 days (alternative agents may be considered if contraindicated)
• clinical response: fever typically resolves within 48 hours of starting treatment, and neither drug resistance nor relapse after therapy has been reported
• prognosis: most cases resolve asymptomatically or follow a benign course; more severe illness requiring hospitalisation may occur in older or immunocompromised patients; the overall case fatality rate reported in the USA is under 1%

Reference:

1. UK Health Security Agency (UKHSA). Guidance: Anaplasmosis and Ehrlichiosis. Published 9 November 2026.