EHRLICHIA INFECTION (CANINE)
WHAT ARE EHRLICHIA?
Ehrlichia, named for the Dr. Ehrlich who first described them, are a special type of bacteria which 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.
THE CRAZY EHRLICHIA NAME GAME
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 very 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 for which type of Ehrlichia one was seeing. Or so we foolishly thought.
With the advent of more specific molecular biology it was found that the rules above are not absolute. Today, RNA sequencing is used to identify the Ehrlichia at hand. In fact, some of the organisms previously believed to be Ehrlichia have been completely reclassified into other groups. We mention this as older terminology dies hard and many people still use the older names of organisms. Here is a current list of the organisms currently classified as Ehrlichia as of a massive renaming/reclassification that occurred in 2001:
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 ewingii, and other diseases.
The Brown Dog Tick
The Lone Star Tick (Amblyomma americanum) is the main vector of Ehrlichia ewingii. (Note the “star” on the tick’s main body.)
The Lone Star Tick
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.
Infections with Ehrlichia ewingii tend to produce arthritis in addition to the above scenario.
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:
When Ehrlichiosis is suspected, a blood test for antibodies against Ehrlichia organisms can be ordered. A positive test indicates that the dog has been exposed to Ehrlichia and does not imply active current infection necessarily. A negative titer does not fully rule out Ehrlichia, either, as a very sick patient will be too sick to produce antibodies and an early case may not yet have started to produce antibodies.
The antibody level or “titer”can be done with an IFA (immunofluorescent antibody) or by ELISA (Enzyme-linked Immunosorbant assay). The IFA test is the traditional test and can be positive 7-28 days after the dog has become infected. The ELISA test is included in a new in-house test kit from IDEXX labs (the “snap 4DX” test) that also includes a Lyme Disease test and a Heartworm test. The 4DX test is geared specifically to detect antibodies against Ehrlichia canis. It is not entirely clear if antibodies against other types of Ehrlichia will be similar enough to be detected by either the IFA or ELISA tests reliably. It takes 6-9 months after infection for titers to begin to drop.
Recently PCR testing for the actual presence of Ehrlichia organisms has become available. The disadvantage of this test is that it gives either a positive or negative results rather than an actual quantitative number that can be tracked. A combination of PCR testing as well as antibody titers are 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.
If one is lucky one will actually see the organisms on a blood smear, and this, of course, clinches the diagnosis.
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).
If immune-mediated secondary reactions to the 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.
After infection, it is possible to become re-infected; immunity is not lasting after a previous infection.
Page last updated: 10/12/2011