What is Ehrlichia?
Ehrlichia, named after Dr. Ehrlich who first described them, are a type of bacteria that infects 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 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 to the inside of cells.
The two Ehrlichia of concern are Ehrlichia Canis and Ehrlichia Ewingii
The Crazy Ehrlichia Name Game (We're Making Sure You're on the Right Page)
Ehrlichia were originally grouped according to the type of white blood cell they attacked. The granulocytic line of white blood cells, used by the body in attacking more acute inflammatory invaders, has a lobulated nucleus and characteristic blood cell appearance. The monocytic line, used by the body for chronic forms of inflammation, has a more rounded nucleus. By finding Ehrlichia inside certain types of white blood cells, one could get a better idea of which type of Ehrlichia one was seeing. Or so we foolishly thought.
- Ehrlichia equi - lives in what are called granulocytes (a type of white cell).
- Ehrlichia lewinii - lives in granulocytes as well.
- Ehrlichia canis - lives in what are called monocytes (a different type of white blood cell).
- Ehrlichia risticii - is now reclassified as Neorickettsia ristici, and Ehrlichia platys is 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.
If your dog has been diagnosed with Anaplasmosis, read that article instead of this one.
Ehrlichia Canis Infection (also Called Canine Monocytic Ehrlichiosis)
Brown dog tick. Photo courtesy of CDC.
Ehrlichia canis infection is a worldwide disease. In the U.S., it is spread by the bite of the brown dog tick (Rhipicephalus sanguineus). 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to remain attached in order to transmit the Ehrlichia organism but the previously accepted idea of 48 hours is probably outdated and a much shorter time is more likely.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 (Babesiosis and Rocky Mountain Spotted Fever) 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. In fact, it had been in the good old 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 together 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.
When Dogs Get Sick: Ehrlichia Symptoms
There are three phases of illness with Ehrlichiosis: acute, subclinical, and chronic.
This phase occurs 1 to 3 weeks after the host is bitten by the tick. The Ehrlichia organism is replicating in this time period and attaching to white blood cell membranes. During the acute infection, the platelet count will drop, and immune-mediated platelet destruction will occur. The dog will be listless, off food, and may have enlarged lymph nodes and/or spleens. There may be fever and even neurologic symptoms as well, but although the dog may seem pretty 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)
Lone star tick. Photo by CDC.
Ehrlichia ewingii is a different species of the Ehrlichia genus. It infects the white blood cells known as granulocytes as described at the top of the page and is not generally as serious as Ehrlichia canis. Dogs are usually only mildly ill or even apparently infected but classically symptoms revolve around swollen stiff joints/arthritis and fever.
Ehrlichia ewingii is spread by the bite of the lone star tick (Amblyomma americanum) as pictured on the right.
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 but the recent availability of PCR testing has changed the approach.
Of course, if one is lucky the organisms will be seen on a blood smear, which clinches the diagnosis.
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.
Alternatively, a test kit for Ehrlichia can be run while you wait if your veterinarian's clinic is so equipped. These tests are accurate but only produce a positive or negative result, so it is not possible to see how an antibody level is changing.
It takes 6-9 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 completed and a couple of weeks have passed, the PCR test can check for any residual Ehrlichia DNA to indicate the infection has cleared.
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 2 weeks post-treatment. If the test is negative then another final PCR test is done in 2 months. If the post-treatment test is positive, another 28 days of doxycycline is used and a 2-week post-treatment PCR is repeated. 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.