Morulae in neutrophils with a negative rapid test: anaplasmosis
4-year-old English Setter hunting dog from northern Spain with a temperature of 40.3 °C, lethargy and shifting lameness for 4 days. Several ticks were removed 10 days ago. The smear shows basophilic inclusions in the cytoplasm of some neutrophils.
| Parameter | Result | Reference |
|---|---|---|
| Platelets | 48,000/µL | 200,000–500,000 |
| White blood cells | 5,200/µL | 6,000–17,000 |
| Rapid antibody test (Anaplasma, Ehrlichia, Borrelia, Dirofilaria antigen) | All negative | — |
What is the most appropriate interpretation?
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A. The negative test rules out tick-borne diseases
Rapid tests detect antibodies, which take 1–3 weeks to appear. In the acute phase they can be negative even though infection is active.
B. They are Ehrlichia canis morulae, which typically infects neutrophils
Ehrlichia canis infects monocytes and its morulae are very rarely seen on smears. Neutrophil morulae in Europe correspond to Anaplasma phagocytophilum.
C. Acute granulocytic anaplasmosis before seroconversion; treat with doxycycline
Correct. Neutrophil morulae with fever, thrombocytopenia and lameness after tick exposure (Ixodes) indicate acute Anaplasma phagocytophilum infection. Antibodies have not yet appeared; blood PCR or paired serology 2–4 weeks later confirms it. Doxycycline is the treatment of choice.
D. They are toxic granules from sepsis; look for a bacterial focus
Toxic change (cytoplasmic basophilia, Döhle bodies) does not form mulberry-like inclusions. The context of ticks and thrombocytopenia also points to a vector-borne infection.
lightbulb Key takeaway
A positive rapid test indicates exposure, not necessarily disease; a negative one does not rule out acute infection. Thrombocytopenia is the most consistent abnormality in ehrlichiosis and anaplasmosis; if suspicion is high, run PCR, repeat serology and do not delay doxycycline.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.