Molecular evidence for persistence of Anaplasma phagocytophilum in the absence of clinical abnormalities in horses after recovery from acute experimental infection.

The research investigated the persistence of Anaplasma phagocytophilum, a bacterium that infects mammals, in horses. It found that the infection can persist for at least 129 days without showing clinical symptoms or causing detectable pathological changes.

Objective

• The main purpose of the study was to determine if Anaplasma phagocytophilum could persist in horses, even after they had fully recovered from the acute phase of the infection. This organism is known to persist in several other mammalian species, but there wasn’t clear evidence regarding its persistence in horses. The researchers were also keen to document any clinical findings in the three months following recovery from the acute disease.

Methodology

• Five healthy adult horses that had spontaneously recovered from experimentally induced acute disease caused by A. phagocytophilum were chosen for the study.
• These horses were closely monitored for a period of up to 129 days after infection. This involved daily clinical examinations and blood sampling at least every second day. The researchers tested the blood samples for A. phagocytophilum using polymerase chain reaction (PCR) and by checking blood smears. After the monitoring period, the horses were euthanized for postmortem examination.

Key Findings

• Throughout the monitoring period, all the horses tested positive for A. phagocytophilum at certain times, showing that the infection could persist even after the acute phase of the disease.
• In the first 66 days post infection, two horses continuously tested negative for A. phagocytophilum while the other three tested positive intermittently. After this period, four out of the five horses were intermittently testing positive, particularly following stressful interventions. One horse even tested positive immediately before being euthanized.
• Despite the persistence of A. phagocytophilum in the horses, the researchers did not observe any clinical abnormalities associated with this. The postmortem examination also did not reveal any specific changes and all the tissues tested negative for A. phagocytophilum.

Conclusion

• The study concluded that an A. phagocytophilum infection can persist in horses for at least 129 days. However, the persistence of the bacterium does not come with detectable clinical symptoms or pathological abnormalities.