Ehrlichiosis and Related Infections in Dogs
Canine monocytic ehrlichiosis is usually caused by the rickettsia Ehrlichia canis, although other types of Ehrlichia are sometimes involved. (Rickettsiae are a specialized type of bacteria that live only inside other cells.) Carried by ticks, the organism infects a certain type of white blood cell and causes fever and other signs. A related organism, Ehrlichia ewingi, targets other types of white blood cells called granulocytes and has been isolated from dogs and people in the southern, western, and midwestern United States. Anaplasma platys, another rickettsia, causes infectious cyclic thrombocytopenia in dogs. This infection leads to periodic losses of platelets, which causes problems with blood clotting.
The Ehrlichia and Anaplasma rickettsiae are present in many parts of the world, including the United States. They are transmitted by ticks (including the brown dog tick, lone star tick, and black-legged tick) that become infected after feeding on infected animals. People, dogs, cats, and other domestic animals are accidental hosts of these disease-causing organisms.
In infections caused by Ehrlichia canis, signs commonly progress from short to longterm, depending on the strain of the organism and the immune status of the host. In short-term cases, there is fever, widespread inflammation of the lymph nodes, enlargement of the spleen, and a decrease in the number of platelets in the bloodstream. In addition, there may be loss of appetite, depression, loss of stamina, stiffness and reluctance to walk, swelling of the limbs or scrotum, and coughing or difficulty in breathing. Most short-term cases are seen in the warmer months, when ticks are active. During this phase of infection, death is rare and the infected animal may recover spontaneously. The recovered dog may remain free of signs thereafter, or longterm disease may develop.
Longterm ehrlichiosis caused by Ehrlichia canis may develop in any breed of dog, but certain breeds (such as German Shepherds) may be predisposed. Longterm infection does not vary with the seasons. Signs depend on which organs are affected and may include enlargement of the spleen, kidney failure, and inflammation of the lungs, eye, brain and spinal cord. If the brain and spinal cord are involved, there may be problems with the nervous system, such as lack of coordination, depression, partial paralysis, and increased sensitivity to a normally painless touch. Severe weight loss is common.
Dogs infected with Anaplasma platys generally show minimal to no signs of infection, although the organism is present in the platelets. Infection with other rickettsiae causes signs similar to short-term Ehrlichia canis infection, but the disease is usually more self-limiting. Fever and a lameness that shifts from one leg to another may be present. Longterm disease (such as that seen with Ehrlichia canis infection) is not typically seen in other ehrlichial infections.
For treatment of all forms of infection caused by these organisms, your veterinarian will prescribe an antibiotic. The medication is usually given for a period of 10 to 21 days. Fever usually ends within 1 to 2 days after treatment begins. In longterm cases, the blood abnormalities may persist for 3 to 6 months, although relief from signs often occurs much sooner. Some dogs may also need supportive care. If the dog has widespread or severe bleeding, then transfusion with blood platelets or whole blood may be prescribed.
The most important preventive steps are those that control ticks, the most common source of the disease. Keeping your dog away from areas known to harbor ticks is a step you can take. Preventive medications that will keep your dog from being infested with ticks are also available from your veterinarian. Any ticks found on your dog should be promptly and properly removed to prevent the spread of disease. Remove any ticks by using fine-pointed tweezers to grasp the head of the tick (right where it enters the skin). Pull the tick straight off, making sure not to grasp or squeeze its body. If there are multiple ticks, it may be best to have your veterinarian remove them and examine your dog.
Some types of Ehrlichia bacteria can be transmitted to people. Despite the occurrence of disease in both animals and people, a tick is required for transmission so dogs and other infected animals do not pose a direct transmission risk in normal circumstances. Infection in dogs may indicate a heightened risk of human infections related to tick exposure in a given area.