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Ehrlichiosis (tropical canine pancytopenia) was diagnosed in Illinois in a 4 1/2-year-old mixed breed dog that had spent the previous 2 years in India. The dog had periodic epistaxis, anemia, leukopenia, hypoalbuminemia, and hypergammaglobulinemia. A titer of 1:160 for Ehrlichia canis was detected by the indirect fluorescent antibody test (IFAT). Subinoculation of blood into a clinically normal dgo resulted in typical signs of canine ehrlichiosis, and E canis was identified in circulating mononuclear cells. The recipient died 23 days after inoculation, and typical gross and microscopic lesions of ehrlichiosis were found at necropsy. Ecchymoses and petechiae were found on the gallbladder, kidneys, small intestinal mucosa, cerebrum, and cerebellum. Plasmacytosis and perivascular cuffing was evident in various organs, especially the cerebrum, cerebellum, and meninges.

Dogs (German Shepherd Dogs and Beagles), cates, rhesus macaques (Macaca mulatta), and baboons (Papio anubis) were inoculated with Whrlichia equi, the etiologic agent of equine ehrlichiosis. Within 3 to 7 days after inoculation, morulae were observed in the eosinophils of cats, neurtrophils of macaques and baboons, and in both neutrophils and eosinophils of dogs. The severe disease produced in horses by this agent was not a feature of E equi infection in dogs, cats, macaques, and baboons. However, a susceptible horse, inoculated with the pooled blood of 2 infected macaques, developed severe clinical signs of equine ehrlichiosis. Infection with E equi did not alter the susceptibiltiy of dogs to infection with Ehrlichia canis and did not prevent development of signs of disease resulting from this infection. The broad experimental host range from this infection. The broad experimental host range of E equi distinguishes it from other ehrlichial agents which are characterized by a rather narrow host range. The susceptibiltiy of nonhuman primates to infection with E equi provides a basis for consideration of the potential transmission of ehrlichial agents to man.

Certain aspects of the development of Ehrlichia canis, causative agent of canine ehrlichiosis (tropical canine pancytopenia) in Rhipicephalus sanguineus ticks were studied. It was found that partial feeding of nymphs infected as larvae with E canis was a desirable, if not necessary, preliminary treatment for successful infection of dogs with ground-up ticks. It remains unclear whether feeding increased the number or altered the virulence of ehrlichiae within tick tissues. Ehrlichia canis organisms were detected by immunofluorescent microscopy in the midgut and hemocytes and by electron microscopy in the midgut and salivary glands of partially engorged adult ticks which had been infected as larvae and nymphs. Organisms were not observed in the ovary. Intracytoplasmic inclusions contained 1 to 80 elementary bodies, each provided with 2 distinct membranes. Infection of the midgut and salivary gland was confirmed by injecting homogenates of these tissues into susceptible dogs. Staining of gut smears of partially engorged adult ticks by fluorescein-conjugated anti-E canis antibody was found to be a reliable indicator of the infection.

Two strains of Rhipicephalus sanguineus acquired Ehrlichia canis by feeding as either larvae or nymphs on acutely infected dogs and, in subsequent instars, transmitted the agent to normal dogs. Three strains of R sanguineus transmitted E canis as adults after their larval and nymphal stages fed on infected dogs. More than 400 adult female ticks were fed on infected dogs as larvae or nymphs or both, but none transmitted E canis transovarially.

The kinetics of antibody production response to experimentally induced infection of dogs with Ehrlichia canis was determined by ion-exchange and molecularsieve chromatography and by indirect fluorescent antibody (IFA) test. The first IFA antibody at 7 days after inoculation resided in immunoglobulin M (IgM) and immunoglobulin A (IgA) classes. At approximately 21 days after inoculation, the antibody was in IgM, IgA, and immunoglobulin G (IgG) classes. Thereafter, antibody concentrations continued to increase in the IgG class; those in the other 2 immunoglobulin classes had a variable pattern. In 2 dogs which died 60 and 114 days after inoculation, a decrease of antibody concentration in the 3 immunoglobulin classes was evident at the time of death. In the carrier dog, however, which was killed 147 days after inoculation, antibody concentrations sustained increasing titers in the 3 immunoglobulin classes.