Causes of Ehrlichiosis
There are several different organisms that cause ehrlichiosis in domestic dogs. One of the most common is Ehrlichia canis, which is a species of bacteria that is transmitted to dogs through the saliva of brown dog ticks. Brown dog ticks are found in most tropical and subtropical areas world-wide. They are fairly common throughout the United States. Some species of Anaplasma bacteria also cause ehrlichiosis in dogs. These organisms are carried by hard-shelled Ixodes ticks, which are found mainly in California and in the Northeastern and upper Midwestern states. They are primarily a parasite of platelets, which are the cells in blood that are primarily responsible for its ability to clot. Finally, some Neorickettsia bacteria can cause ehrlichiosis in dogs. They also can cause Potomac Fever in horses and salmon poisoning disease in dogs, which are completely different disorders.
Immature ticks become infected during their early larval or nymph stages, when they feed on an infected animal. Dogs become infected when they are bitten by adult ticks that carry the bacteria. When an infected tick bites a dog, the tiny bacterial parasites are transferred through the tick’s saliva into the dog’s bloodstream at the site of the bite.
Ehrlichiosis usually proceeds through three distinct phases. The first, or acute phase starts somewhere between 10 and 20 days after the tick bite and typically lasts from 2 to 4 weeks. During that time, the bacteria multiply inside certain white blood cells called “monocytes.” The infected cells circulate in the dog’s bloodstream for about 24 hours. Then, they start sticking to the inner lining of blood vessels, causing varying degrees of inflammation. The bacteria also migrate out of the blood into tissues, especially those of the spleen, liver, lung and lymph nodes.
The second, or subclinical phase happens between 6 and 9 weeks after the tick bite and can last for years. During this phase, white blood cells and tissues continue to be damaged by the infective organisms. In addition, the parasites begin to invade red blood cells, platelets and bone marrow. Many dogs show no signs of sickness during these first two phases. Sometimes, they even recover spontaneously. If they don’t, they enter the chronic phase of ehrlichiosis, with variable signs and severity of disease.
Prevention of Ehrlichiosis
The best way to prevent ehrlichiosis is to keep dogs from being bitten by ticks. A number of good topical tick preventatives, including collars, dusts, dips, drops and sprays, are widely available. A veterinarian is the best person to recommend a good parasite prevention protocol.
Ehrlichiosis cannot be transferred directly between dogs or from dogs to people. The infective parasites can only be transferred through saliva from the bite of an infected tick. However, people can become infected if they are bitten by an infected tick. People also can develop ehrlichiosis by coming into contact with the blood of infected ticks. Gloves should be worn whenever anyone removes a tick from a dog to reduce the risk of cross-contamination. Ticks should be disposed of in tightly sealed containers.