Unbeknownst to most doctors, humans and horses may be falling victim to the same tick-borne ailment, until now believed to have been two separate diseases that mimic Lyme disease. Researchers at the University of California, Davis, School of Veterinary Medicine have demonstrated that the bacterium that causes the human illness is almost genetically identical to the bacterium associated with the horse disease and triggers the same flu-like symptoms in horses and humans. The findings, reported in the October issue of the Journal of Infectious Diseases, could lead to quicker and more accurate diagnosis and treatment in humans. The twin diseases -- human granulocytic ehrlichiosis (HGE) and equine granulocytic ehrlichiosis (EGE) -- are a mouthful to pronounce and equally tricky to diagnose. Symptoms include a fever, headache, shaking, chills and nausea. A blood test would reveal a significant drop in white blood cell and platelet counts in both people and horses. Although the horse disease, particularly common in Northern California, has been recognized by veterinarians for more than 30 years, the human disease was only first reported by physicians in 1994 and has since appeared primarily in the Upper Midwest and Northeast. Several deaths have been attributed to HGE. If HGE and EGE truly are the same ailment, the disease joins just a handful of other illnesses, including rabies, viral encephalitis and vesicular stomatitis, that afflict both humans and horses. Madigan and co-workers discovered the astounding similarity between the two diseases after experimentally injecting a healthy Thoroughbred stallion with blood from a Minnesota woman known to have HGE. Five days after inoculation, the horse developed a fever and other symptoms identical to those seen in the naturally occurring horse infection. "Humans and horses appear to be getting the same disease, but they aren't catching it from each other," stressed Dr. John Madigan, a professor of equine medicine and lead author of the new study. "Both are just in the wrong place at the wrong time and are being infected with the disease by ticks," he said. HGE and EGE are easily treatable in the early stages with tetracycline antibiotics. It is particularly critical, however, that the illness not be misdiagnosed as Lyme disease, which is also tick-borne and produces similar flu-like symptoms. HGE and EGE will not respond to other antibiotics often used to treat Lyme disease. If not treated quickly and with the appropriate antibiotics, both diseases can be fatal because vital organs are damaged. Typical blood tests are of no use for early diagnosis of HGE and EGE because antibodies don't appear in the blood until two weeks after the infection begins, and the bacteria that cause the infections grow only in living blood cells and can't be cultured in the laboratory, according to Dr. Jeffrey Barlough, a UC Davis veterinary researcher and study co-author. The bacteria that cause HGE and EGE belong to the bacterial group responsible for Rocky Mountain spotted fever, typhus and "Q" fever, Barlough said. "Usually people are diagnosed as having HGE when the physician learns that, in addition to the flu-like symptoms, the patient was recently bitten by a tick," he said. At present, diagnosis of the human disease is made in the same way that the horse disease is diagnosed -- by microscopic examination of a blood smear for the presence of the bacteria in certain white blood cells. The Davis researchers have shown that both diseases can be more conclusively diagnosed using a genetic method known as the polymerase chain reaction or PCR. Unfortunately, the PCR method is currently expensive, labor-intensive and not readily available in many medical laboratories. Dr. J. Stephen Dumler of the University of Maryland medical school in Baltimore, a collaborator with Madigan and a study co-author, has compared the genetic makeup of the bacterium from the woman with that of the bacterium found in horses. His group found that of the 1,433 chemical "pairs" that compose a particular gene in the HGE bacterium's DNA, all but three pairs are identical in the same gene of the horse bacterium -- constituting a 99.8 percent similarity. "This suggests that some cases of human illness that have been attributed to HGE may in fact be caused by one or more strains of the EGE bacterium, which has been thought to be almost exclusively an agent of horse disease," said Madigan. "More than likely, both the human and horse bacteria are just different strains of the same agent." In a related development, UC Davis researchers Robert Kimsey and Philip Richter have recently shown that the western black-legged tick, known to transmit Lyme disease in the western United States, is also able to transmit the EGE bacterium from one horse to another. This suggests that this tick may be responsible for transmitting EGE, HGE and Lyme disease. Kimsey and Richter will report their findings in the October issue of the Journal of Medical Entomology. Madigan and Barlough are confident that future research will be able to identify the reservoir of infection of the bacteria in nature and the types of ticks that transmit the bacteria, and will lead to faster and more practical diagnostic techniques. Their research has been funded by the California Center for Equine Health and Performance, located at UC Davis.