On Anaplasma phagocytophilum in Horses

Equine Granulocytic Anaplasmosis (EGA) is an acute tick-borne infection caused by Anaplasma phagocytophilum. The bacterium can infect various animal species and humans. Persistence of infection is documented in ruminants and is shown experimentally in dogs, but it is unknown whether it occurs in horses or if EGA induces longer-term clinical changes in horses. This thesis includes results and analyses of a serosurvey of the exposure level and association with clinical disease (excluding acute EGA) in over 2000 horses throughout Sweden. It also includes an experimental infection study of six horses using a Swedish isolate of the bacterium. Overall 17% of the horses were seropositive with large geographical variations. However, seropositivity was not statistically associated with any increase in clinical sign or disease in the animals. In the experimental study acute disease was readily induced and the clinical disease was similar to EGA described from USA. Laboratory data showed that the PCR test had the widest diagnostic window for EGA with positive signals days before onset of clinical signs. Clinically, diagnostic inclusions (morulae) in leukocytes in blood smears were visible first after a few days into clinical disease and lasted shorter time than did the PCR signal. One horse died suddenly 2 days into the acute disease, with post mortem changes of general vasculitis and hyaline thrombi in the kidneys suggesting disseminative intravascular coagulation (DIC). This was the first documented case of death attributed probably soley to EGA. The remaining horses were closely monitored for more than 3 months after spontaneous recovery from EGA. Some of the horses became transiently PCR positive during this period, mainly close in time after selected interventions to mimic stress. However no detectable clinical abnormalities were found during this follow-up period. At post mortem there were no macroscopic or microscopic changes in any of the tissues that could be associated with persistence of infection. In conclusion this thesis shows that for EGA; seropositive horses in Sweden are common but do not have increased levels of clinical disease; PCR provides the earliest laboratory diagnosis for acute EGA-infection; sudden death due to EGA can occur; and the organism can persist in some horses for up to 3 months post recovery but was clinically and pathologically silent.