A. phagocytophilum resembles the etiologic agents of tick-borne fever and bovine petechial fever. It is present in cytoplasmic vacuoles of neutrophils and occasionally eosinophils during the acute phase.
Clinical signs & symptoms:
Lethargy (weakness / loss of energy)
Anorexia (loss of appetite leading to weight loss)
More acute signs include:
Lymphadenopathy (swollen / enlarged lymph nodes)
Haemoglobinuria (dark red haemoglobin-containing urine)
Leukopenia (reduced leukocytes)
Thrombocytopenia (reduced platelets)
Petechial to ecchymotic haemorrhages (spotted areas of blood accumulated within the tissue)
Icterus (Jaundice – yellowing of the skin and eyes that is caused by too much bilirubin in the blood)
Ataxia (loss of muscle coordination)
Inflammatory arthritis – maybe mono- or polyarthritis (single or multiple joint involvement)
Limb oedema (swelling of fluid within tissues).
Severity of signs varies with the age of the animal and the duration of the illness. Signs may be mild (pyrexia only) or more severe (more often in older horses). Fever is generally at its highest during the first 1-3 days. Lower-grade fevers persist for 6-12 days when other signs become more severe. Any concurrent infection can cause exacerbation of symptoms.
Rarely, myocardial vasculitis can cause transient ventricular arrhythmias.
Differential diagnoses include viral encephalitis, primary liver disease, Equine Infectious Anaemia (EIA), purpura haemorrhagica, and viral arteritis.
Classical microscopic diagnosis relies on identification of morulae in circulating neutrophils in Giemsa- or Wright’s-stained blood smears. However, detectable numbers of morulae are only present during acute infection. Therefore organisms may be difficult to find in blood smears.
Polymerase Chain Reaction (PCR) of whole blood has become more readily available. However, results should be interpreted with caution because the techniques used in different diagnostic laboratories vary. Amplification of related organisms by nonspecific primers has been shown to result in false-positive reactions. Conversely, false-negatives may occur if extraction procedures fail to remove PCR inhibitors present in a blood sample. They may also occur if the level of circulating rickettsaemia falls below the level of assay detection, due to normal decrease in circulating organisms or temporary suppression of infection following antibiotic treatment. To maximise the utility of molecular diagnostics, blood samples should be collected early in the course of clinical disease and before the initiation of antimicrobial therapy, and should be submitted to experienced diagnostic laboratories with stringent quality control measures in place.
Indirect Fluorescent Antibody (IFA) assays to determine IgM and IgG titres are also available.
I.V. Tetracycline @ 7mg/kg q24h for 8 days is generally affective at eliminating infection.
Penicillin, Chloramphenicol, and Streptomycin appear to have no inhibitory effect.
Horses with severe ataxia and oedema may benefit from short-term corticosteroid treatment (Dexamethasone @ 20mg/kg q24h over 2-3 days).
I.V. = intravenous
I.M. = intramuscular
P.O = by mouth
q8h = every 8 hours
q12h = every 12 hours
q24h = every 24 hours
There is currently no vaccine against A. phagocytophilum for horses in the UK.