Ehrlichia, are a type of bacteria that infects and lives within the white blood cells of their hosts. Different types of Ehrlichia live in different types of white blood cells. Hosts can be humans, pets, 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 the inside of cells. At the Manchester Animal Hospital, we recommend testing for tick-borne disease annually. For pets exposed to ticks frequently, every six months is a good idea as Manchester is a Lyme disease endemic location, Erlichia is routinely included with Lyme Disease Tests.
Ehrlichia canis Infection
Ehrlichia canis infection is a worldwide disease. In the U.S., it is spread by the bite of the brown dog tick. It is unknown how long the tick needs to remain attached in order to transmit the Ehrlichia organism but the previously accepted idea of 48 hours is probably outdated and a much shorter time is more likely. It is important to realize that this tick can transmit other tick-borne infections so infection with multiple organisms is not unusual. It can be difficult to separate which symptoms go with which tick-borne infection.
Ehrlichia canis was not well scrutinized until the Vietnam War when military German Shepherd dogs in the war zone began dying in large numbers from an infection that appeared to wipe out all their blood cells. This condition, called “Tropical Canine Pancytopenia,” was studied and found not to be limited to Southeast Asia. It had been in the U.S.A. all along. It turned out that German Shepherd dogs get a much more severe form of the disease, so it wasn’t until large numbers of German Shepherd dogs were infected that the disease could be defined.
In the 1980s it became clear that people could also be infected by Ehrlichia, though not the same Ehrlichia as for dogs. When the human disease was recognized, research intensified.
There are three phases of illness with ehrlichiosis: acute, subclinical, and chronic.
This phase occurs one to three weeks after the tick bites the host. The Ehrlichia organism is replicating in this period and attaching to white blood cell membranes. The platelet count will drop during the acute infection, and immune-mediated platelet destruction will occur. The dog will be listless, off food, and may have enlarged lymph nodes and/or spleen. There may be fever and even neurologic symptoms, but although the dog may seem sick, this phase of infection is rarely life-threatening. 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 after 1 to 4 weeks.
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 hints that Ehrlichia might be hiding are a somewhat reduced platelet count and/or elevated blood globulin level (the protein portion that includes miscellaneous antibodies.) Long-term stimulation of the immune system will elevate globulins, sometimes dramatically. Not all dogs ever progress to the chronic stage, but when they do, the prognosis is worse.
In this phase, the dog gets sick again. Up to 60% of dogs infected chronically with Ehrlichia canis will have abnormal bleeding due to reduced platelet numbers. Deep inflammation in the eyes, called uveitis, may occur as a result of long-term immune stimulation. Neurologic effects may also be seen. Glomerulonephritis, resulting in serious urinary protein loss, can also result. Increased globulin levels are almost always seen in this stage, albumin is often low. Most dogs in the U.S. do not show full pancytopenia (literally reduction in all blood cell lines) but severe cell deficiencies are associated with high mortality rates.
Ehrlichia ewingii Infection (also called Canine Granulocytic Ehrlichiosis)
Ehrlichia ewingii is a different species of the Ehrlichia genus. It infects the white blood cells known as granulocytes and is not generally as serious as Ehrlichia canis.
Dogs are usually only mildly ill or may appear uninfected, but classically, symptoms include swollen, stiff joints, arthritis, and fever.
Ehrlichia ewingii is spread by the bite of the lone star tick.
How the Diagnosis is Made
There are two main tests for Ehrlichia: PCR testing for Ehrlichia DNA or blood testing for Ehrlichia antibodies. Antibody testing has been the main diagnostic for many years
Traditionally, when ehrlichiosis is suspected, a blood test for antibodies against Ehrlichia organisms can be ordered or performed in minutes using an in-house test kit. There are some limitations to antibody testing. A positive test indicates that the dog has been exposed to Ehrlichia but does not necessarily imply an active current infection. A negative titer does not fully rule out Ehrlichia, either, as a very sick patient may be too ill to produce antibodies, and an early case may not yet have started to produce them. Antibody titers can be measured in the laboratory, which makes it much easier to track progression; the in-house test kits are simply either positive or negative, and a numeric value is not provided.
It takes six to nine months after infection for antibody levels to begin to drop. Antibodies against Ehrlichia canis and Ehrlichia ewingii will cross-react, so it is not easy to determine which organism a dog has been exposed to.
Recently PCR testing for the presence of Ehrlichia DNA has become available. Commonly a laboratory will offer a tick panel, which uses PCR testing to screen for a group of classic tick-borne diseases. PCR testing remains positive for several weeks after the infection has cleared, as it does not distinguish between live and dead organisms. It takes time to clear dead organisms from the body.
These two forms of testing are complementary, which means an antibody test can be used to screen dogs to identify those that have been infected. Treatment can be prescribed, and after the treatment has been completed and a couple of weeks have passed, the PCR test can check to see if the infection has cleared by testing for any residual Ehrlichia DNA.
Doxycycline is probably the most effective against Ehrlichia (and any other intracellular blood parasite, for that matter). Expect at least a month of treatment to be needed. The response is initially rapid (improvement is notable in the first few days). Dogs in the acute or subclinical stage at the time of treatment can expect to be cured. A common protocol involves 28 days on doxycycline and a PCR test two weeks post-treatment. If the test is negative, then another final PCR test is done in two months. If the post-treatment test is positive, another 28 days of doxycycline is used, and a two-week post-treatment PCR is repeated as before. If this second post-treatment PCR test is still positive, another medication (such as chloramphenicol or imidocarb) should be used.
If immune-mediated secondary reactions to Ehrlichia are a problem (such as immune-mediated arthritis or immune-mediated platelet loss), corticosteroids such as prednisone can be used to palliate the situation while the antibiotics are starting to work.
Dogs in the chronic stage are more difficult to treat as they are sicker and more debilitated. A blood transfusion may be needed, and the mortality rate is much higher.
Surviving dogs can become re-infected as Ehrlichia immunity is not life-long
Dr. Lamb is the Veterinarian at the Manchester Animal Hospital.