Ehrlichiosis is another infectious disease spread by ticks. Infection with Ehrlichia can cause chronic debilitating disease and can be contracted by both dogs and humans. This parasite is a rickettsia, a microorganism similar to bacteria. The organisms that cause ehrlichiosis have been reclassified into ehrlichias, anaplasma and neorickettsia. The term ehrlichiosis will probably continue to be used, but is no longer technically correct.
Canine ehrlichiosis first gained attention as a significant disease during the Vietnam War, causing the death of hundreds of military dogs overseas and chronically infecting many that returned to the U.S. (For this reason, among its many names, it is sometimes called "tracker dog disease.") It has now been reported to occur on four continents: Asia, Africa, Europe, and North America. Serologically confirmed cases have been recorded in most states of the United States.
Ehrlichia canis is the causative organism in most clinical cases of canine disease (but occasionally other strains of the organism have been found). The primary vector responsible is the brown dog tick, Rhipicephalus sanguineus. Larvae, nymphs and adults are all able to transmit the disease. It has been shown that ticks can survive up to 568 days as unfed adults, and can transmit infection for 155 days after becoming infected. This phenomenon allows ticks to over-winter and infect definitive hosts the following spring.
There are three phases to the disease in companion animals (most commonly the dog). In the acute stage, the organisms multiply and spread throughout the body. Common clinical signs include fever, ocular (eye) and nasal discharge, loss of appetite, weight loss and swollen lymph nodes. In some dogs, the clinical signs may be mild and non-specific. In other cases, they may be severe and life-threatening, including hemorrhages and convulsions. This stage may last two to four weeks.
A subclinical stage follows the acute stage. Now the organism is present but not causing any sign of disease. Animals improve clinically and antibody levels begin to rise. (If the animal passed through the acute phase without its owner being aware of the infection, the dog may enter this phase showing laboratory changes but no apparent signs of illness.) During this stage, the dog or cat may recover. However, it remains susceptible to reinfection.
If the animal cannot eliminate the Ehrlichia organism, the chronic stage ensues. Some animals may become carriers of the disease for months or years. As such, they can be sources of infection through ticks that feed upon them.
The chronic phase of this disease may be characterized by mild signs, but more often involves severe illness. Blood disorders are common, including decreased bone marrow production of blood cells, leading to anemia, increased susceptibility to infection, and bleeding tendencies. In cases of severe hemorrhage or anemia, transfusions of whole blood or plasma may be needed.
The prognosis for an animal's recovery mostly depends on how early in the course of the disease the treatment is begun. Antibiotics such as tetracycline, doxcycline and chloramphenicol are frequently effective in the treatment of ehrlichiosis in dogs and cats if given during the acute phase, but sometimes not effective at all during the chronic phase. Some newer research indicates that certain dogs may need to be treated for up to 4 months.
People are also susceptible to ehrlichiosis, but because tick bites are required for transmission, the disease cannot be spread directly from dogs to humans. Rather, infected dogs serve as sentinels, indicating the presence of infected ticks in the area. They also may be a source of the organism for either ticks that may infect humans or other dogs.
Clearly, preventing ehrlichiosis is a better course than simply trying to treat it. Controlling tick infestation is the most effective means available to us.