Tick Fever: Ehrlichia Infection in Dogs
What are Ehrlichia?
Ehrlichia, named for the Dr. Ehrlich who first described them, are a type of bacteria that infect and live within the white blood cells of their hosts. Different types of Ehrlichia live in different types of white blood cells. Hosts can be human, pet, or wild animals. Ehrlichia are spread from host to host by tick bites and their intracellular location makes them difficult to remove as most antibiotics do not penetrate to the inside of cells.
- Ehrlichia canis (mostly a dog infection)
- Ehrlichia lewinii (mostly a dog infection)
- Ehrlichia chaffeensis (mostly a human infection)
- Ehrlichia ruminantium
- (Ehrlichia risticii is now reclassified as Neorickettsia risticii and Ehrlichia platys are now reclassified as Anaplasma platys. Ehrlichia equi, Ehrlichia phagocytophila, and Human Granulocytic Ehrlichial Agent have been deemed to all be the same species and have been reclassified as Anaplasma phagocytophila).
What is Tropical Pancytopenia?
Although it was known that dogs could be infected with Ehrlichia, the scope of the illness created became evident during the Viet Nam war as the military German Shepherd Dogs became ill with an infection that seemed to wipe out their blood cells. White blood cells as well as red blood cells were affected and the disease was named tropical canine pancytopenia. This condition was studied and found not to be limited to Southeast Asia but was in fact in the good old U.S.A. as well. In the 1980s it became clear that people could be infected by Ehrlichia, though not the same Ehrlichia as for dogs, and research intensified.
What Ticks are Involved with Ehrlichia?
Different Ehrlichia species are spread by different types of ticks. (What ticks are located in a particular area limits what types of Ehrlichia infections are seen regionally.)
The Brown Dog Tick (Rhipicephalus sanguineus) usually spreads Ehrlichia canis, Ehrlichia lewinii, and other diseases.
It is important to note, where there are ticks there are many tick-borne diseases: Lyme disease, Babesia infection, Anaplasma, Ehrlichia, and probably some agents we do not even know of yet. It is not surprising for a given patient living in a tick area to be infected with multiple blood parasites.
When Dogs get Sick
There are three phases of illness with Ehrlichiosis: acute, subclinical, and chronic.
- ACUTE PHASE: This is generally a mild phase and occurs 1 to 3 weeks after the host is bitten by the tick. The Ehrlichia organism is replicated in this time period and attaching to white blood cell membranes. During this time the platelet count will drop and an immune-mediated platelet destruction will occur. The dog may be listless, off food, limp or have painful joints, and may have enlarged lymph nodes. There may be fever as well but rarely does this phase kill a dog. Most dogs will have one or two of the above symptoms, they don’t usually have all of them. Most dogs clear the organism if they are treated in this stage but those that do not receive adequate treatment will go on to the next phase.
- SUBCLINICAL PHASE: In this phase, the dog appears normal. The organism has sequestered in the spleen and is essentially hiding out there. Dogs can stay in this phase for months or even years. The only hint that Ehrlichia is hiding is a somewhat reduced platelet count and/or elevated globulin level on a blood test. The blood protein level on a lab report is divided into albumin (an important carrier protein) and globulins (every other blood protein including antibodies.) Long-term stimulation of the immune system will elevate globulins.
- CHRONIC PHASE: In this phase the dog gets sick again. Up to 60% of dogs infected with Ehrlichia canis will have abnormal bleeding due to reduced platelets numbers, but not all dogs do. Deep inflammation in the eyes called uveitis may occur as a result of the long-term immune stimulation. Neurologic effects may also be seen. Glomerulonephritis, which results in serious urinary protein loss, can also occur. Increased globulin levels are almost always seen in this stage, and albumin is often low. Most dogs in the U.S. do not show the full pancytopenia (literally reduction in all blood cell lines), however many dogs will show signs of painful joints.
How the Diagnosis is Made
Diagnosis does not rest on a single test but instead on a collection of results. The first step is to find a constellation of typical findings:
- A dog with fever, enlarged lymph nodes, bleeding, or arthritis in multiple joints.
- Low platelet numbers, high globulin levels, and mild anemia on blood testing.
When Ehrlichiosis is suspected, a blood test for antibodies against Ehrlichia organisms can be ordered. A negative titer does not fully rule out Ehrlichia, as a very sick patient will be too sick to produce antibodies and an early case may not yet have started to produce antibodies.
Recently PCR testing for the presence of Ehrlichia organisms has become available. The disadvantage of this test is that it gives either a positive or negative result rather than a quantitative number that can be tracked. Using a combination of PCR testing and antibody titers is currently recommended by the American College of Veterinary Internal Medicine. PCR testing remains positive for several weeks after infection has cleared as PCR testing does not distinguish between live and dead organisms. It takes time to clear dead organisms from the body.
Despite being one of the oldest antibiotics in use, tetracycline is probably the most effective against Ehrlichia (and any other intracellular blood parasite for that matter). Doxycycline, a more modern derivative, has a more convenient dosing schedule and has become more popular. Expect at least a month of treatment to be needed. Response is initially rapid (improvement is notable in the first few days).
After infection, it is possible to become re-infected; immunity is not lasting after a previous infection. To guard against re-infection, a tick control product such as Frontline Plus is highly recommended.