Summer is in full swing – and with it comes an increased risk of granulocytic anaplasmosis!
Here’s a quick overview of what’s important:
● The pathogen behind granulocytic anaplasmosis in humans, dogs, cats, horses, and wildlife is Anaplasma phagocytophilum.
● The castor bean tick (Ixodes ricinus) is the main vector – In the UK, Ixodes ricinus activity has been recorded sporadically at all times of the year, although generally larvae begin questing in spring or early summer (April/May), peaking in activity between June and August and declining in September. Nymphs are generally active from February through to November, activity peaking in May/June, with a smaller second peak between September and November
● Infections should be considered as a differential diagnosis throughout the year, especially in summer, particularly when you see:
o Fever, lethargy, or reduced appetite
o Evidence of tick infestation
o Thrombocytopenia
● Both clinical signs (e.g. fever) and lab findings (e.g. low platelet count) are very similar across species – including in people.
● Doxycycline (or oxytetracycline/doxycycline in horses) is the treatment of choice, usually leading to rapid clinical improvement within just a few days.
● A PCR follow-up 5–8 days after treatment is recommended to confirm that the pathogen has been cleared.
● Year-round tick prevention is essential – especially given the role of this pathogen in the One Health context.
About the pathogen – Granulocytic anaplasmosis is caused by Anaplasma phagocytophilum, a gram-negative, obligate intracellular bacterium that affects humans, dogs, cats, horses, and wildlife. In the U.K, the main vector is the castor bean tick (Ixodes ricinus), part of the Ixodes ricinus/persulcatus complex, with peak activity typically occurring in spring and early summer. The spread of the disease closely follows the distribution of these ticks, with the highest prevalence among dogs, cats, and horses observed in Northern and Central Europe. In addition to transmission via tick bites, the pathogen can also be spread through blood transfusions. Due to climate change, infections with Anaplasma phagocytophilum are now possible throughout the year, with peak incidence during the summer months.
Clinical signs – In both animals and humans, the most common clinical signs include fever, lethargy, and loss of appetite. Lameness and gastrointestinal symptoms are reported less frequently. These signs are non-specific and do not clearly indicate a particular disease, so further diagnostic testing is often required. Granulocytic anaplasmosis is an acute illness, with symptoms usually appearing suddenly.
Diagnosis – A fast and cost-effective option is the microscopic examination of a peripheral blood smear. Morulae (inclusion bodies) can sometimes be detected in neutrophilic granulocytes, and more rarely in eosinophils (see Fig. 1). PCR testing from EDTA blood or synovial fluid offers a highly sensitive and specific detection method and is considered the gold standard for diagnosis. Antibody detection in serum or plasma merely confirms previous exposure and is not suitable for diagnosing acute granulocytic anaplasmosis. Antibody levels can remain elevated for years following exposure.
Treatment and prognosis – If treatment is started in time with an antibiotic effective against intracellular bacteria, the prognosis is excellent. Doxycycline is the treatment of choice for dogs (5 mg/kg body weight BID orally), cats (10 mg/kg SID orally), and humans. Although clinical improvement is often rapid, doxycycline should be given for (21–) 28 days in both dogs and cats. In cats, tablets should be administered with food or water to prevent oesophagitis or oesophageal damage. In horses, both oxytetracycline and doxycycline have been described in various treatment protocols. The goal of therapy is complete elimination of the pathogen. In rare cases, clinical signs and a positive A. phagocytophilum PCR may recur if treatment is stopped too early. A follow-up PCR test is recommended 5–8 days after completing the treatment.
Prevention – There is currently no vaccine available against A. phagocytophilum. Preventive efforts should therefore focus on year-round tick protection using products licensed for Ixodes ricinus.
Reinfection – Antibodies produced during infection with A. phagocytophilum are not considered protective. Therefore, both animals and humans can become reinfected after subsequent tick exposure.
More information and current literature can be found at: https://vbd.laboklin.com/vector-borne-diseases-english/
Dr. Elisabeth Müller
Specialist in Veterinary Microbiology (FTA), Dipl. ECVM
Chief Executive Officer (CEO)
