Molecular, clinical, and pathologic comparison of two distinct strains of Haemobartonella felis in domestic cats

Objective-To characterize 2 strains of Haemobartonella felis by use of mole cular techniques. Animals-35 specific-pathogen-free cats, 6 months to 4 years old. Procedure-Intraperitoneal or IV inoculation with blood containing H felis s mall form (Hfsm, 18 cats) or H felis large form (Hflg, 11 cats); 6 cats wer e uninfected controls. Hfsm was evaluated for capability to cross-protect a gainst the more virulent Hflg. Morphology of both strains was compared by l ight microscopy of Wright-Giemsa-stained blood smears, and the 16S rRNA gen es were sequenced. Results-Infection with Hflg induced depression, fever, and severe macrocyti c normochromic anemia with nucleated erythrocytes. More than 95% of erythro cytes were parasitized. Inoculation with Hfsm and uninfected control blood induced mild or no clinical signs and no hematologic abnormalities. Anti-H felis IgG was first detected on postinoculation day (PID) 21, and increased to maxima( titer of 400 by PID 28. Reactivated infection was observed in 8 of 29 cats (4 Hfsm and 4 Hflg), with 5% parasitized erythrocytes during th e later attack. On PID 8. Hflg-inoculated cats had positive results of poly merase chain reaction analysis (PCR) that persisted until cats were treated with doxycycline or oxytetracycline; Hfsm-inoculated cats had positive PCR results that persisted for duration of observation (3 months). Conclusions-Genetically and morphologically distinct strains of H felis inf ect cats in the field. The level of genetic difference suggested that these strains may be different species or genera. Clinical Relevance-PCR is a critical diagnostic aid to detect occult Haemob artonella spp infection, as well as response to treatment and clearance of the organism.