Finding Rickettsiae in All the Wrong Places
Julie Rawlings, MPH
Despite our egocentric worldview, we must remember that both domesticated and wild animals can be infected with the organisms that cause Lyme disease, human granulocytic ehrlichiosis (HGE), babesiosis, and Rocky Mountain spotted fever (RMSF) in humans. Dr. Louis Magnarelli, with the Department of Entomology at The Connecticut Agricultural Experiment Station, presented recent findings on the laboratory detection of tick-borne organisms in humans, animals, and ticks at the first day of the 13th Lyme Disease Conference.
Report From Connecticut
Dr. Magnarelli quoted recent Connecticut Department of Health statistics: 3434 human Lyme disease cases, 104 HGE cases, and 45 human babesiosis cases were reported in Connecticut during 1998. Human cases of Lyme disease and HGE have occurred in all counties of the state. Babesiosis has been most prevalent in New London County, particularly in coastal areas. RMSF is the least common tick-borne disease in Connecticut--there are usually fewer than 5 cases reported each year. It is thought that many tick-borne infections go undetected and unreported.
When suspected, most clinicians rely on antibody tests to confirm tick-borne infections. Multiple assays can be conducted to determine whether persons or domesticated animals have been exposed to more than 1 pathogen.
Together Again, for the First Time
In the Northeastern United States, Ixodes scapularis is the primary vector for Borrelia burgdorferi, the causative agent of Lyme disease; for the HGE agent; and for Babesia microti, an agent of babesiosis. According to Dr. Magnarelli, studies were conducted during the early 1990s to determine whether Lyme disease patients also had antibody to the HGE agent (Ehrlichia equi or a closely related organism) and B microti. Analyses of 511 human sera in Connecticut revealed that 20 (3.9%) persons had antibody to both B burgdorferi and the HGE agent. Antibody to all 3 agents was detected in 4 (0.8%) additional serum samples.
In view of this information, Dr. Magnarelli said that the focus of research shifted to the white-footed mouse, a chief reservoir for both B burgdorferi and B microti, and to the white-tailed deer, a preferred host for I. Scapularis in adults. Using indirect fluorescent antibody (IFA) staining methods, researchers found antibody to 2 or 3 tick-borne pathogens in mice and verified coexisting antibody to B burgdorferi and the HGE agent in deer. Western blot analyses of mouse and deer sera confirmed the presence of antibody that reacted very strongly to an HGE agent outer surface protein, with a molecular mass of about 44 kilodaltons. In addition, HGE agent DNA was detected in deer blood specimens, which were collected from various locations in Connecticut.
Canis and Equus
Dr. Magnarelli said that veterinarians who have learned about the advances in laboratory diagnosis of HGE infections have begun to request assistance in diagnosing Ehrlichia infections in dogs and horses. Using DNA detection procedures, IFA staining methods, and immunoblotting techniques, researchers have documented granulocytic ehrlichial infections, caused by the same organism that infects humans, in horses from Connecticut and in the lower Hudson River Valley region of New York State. Ehrlichia equi was subsequently isolated from equine blood samples. Detection of specific antibody in dogs also verified their exposure to granulocytic ehrlichiae. Both dogs and horses developed strong immune responses to the p44 antigen of the HGE agent.
In his summary, Dr. Magnarelli said that advances made in detecting DNA of and specific antibody to tick-borne pathogens has aided the laboratory diagnosis of Lyme disease, HGE, and babesiosis. He also suggested that because there is increased evidence of coinfections in people and domesticated animals, laboratory testing for evidence of all 3 of these infections should be considered when a tick-borne disease is suspected.[/b]