There isn't any one direct path to a definitive diagnosis of canine ehrlichiosis. The accurate diagnosis of this disease is
dependent on the veterinarian's ability to combine a heightened index of suspicion, one or more of the typical clinical signs,
appropriate testing and accurate interpretation of test results.
Photo 4: Adult females, Ixodes scapularis and Ammbylomma americanium.
Cytology is usually not reliable but is more likely to be diagnostic during the acute stage of the disease. Low numbers of
lymphocytes containing morulae may be seen on blood or buffy coat smears or lymph node aspirates 12 to 14 days after infection.
Because thrombocytopenia is a relatively consistent finding, an EDTA-preserved blood sample along with a good quality blood
smear should be sent to the laboratory as rapidly as possible. Platelet concentrations should be stable for five hours at
room temperature and 24 hours when refrigerated (4oC). The chemical profile is usually normal or near normal with mild to
moderate elevations in ALT and/or the BUN, and other non-specific changes possible.
Ehrlichia titers indicate the patient's ability to generate antibodies specific for the intracellular parasite. Seroconversion
normally occurs 13 to 19 days after infection but the antibody response may be delayed for up to 28 days. Positive titers
denote exposure; high titers are not an indication of severe infection. Antigenic diversity may exist among strains of E.
canis and cross-reactivity may occur between E. canis, E. chaffeensis and E. ewingii.
The indirect immunofluoresence antibody test (IFA) has been the gold standard for many years. There has been difficulty in
the interpretation of results and titers from one laboratory may not be the same at other facilities.
The in-clinic ELISA based SNAP® 3DX test has actually redefined the canine ehrlichia seroprevalence data. It has also caused
a great deal of confusion and diagnostic dilemmas. This convenient and easy-to-use format allows veterinarians to evaluate
a dog for Dirofilaria immitis antigen, Borrelia burgdorferi antibodies and Ehrlichia canis (just this species) antibodies
in less than 10 minutes. Positive results for any of these tests become problematic when the patient is clinically normal.
This test is positive for E. canis titers of 1:100; other test formats may be positive at 1:10, 1:20 or 1:40. A negative ELISA
test does not rule out ehrlichia exposure.
Polymerase chain reaction (PCR) testing detects the genetic footprints of the organism. It is highly specific and is becoming
more available, affordable and more reliable. A positive ehrlichia PCR test means the organism is present. PCR will be positive
during the acute phase of the disease. It may also be useful in dogs that are chronically infected with persistent antibody
titers to determine if re-exposure had occured.
Kansas State University has recently developed a molecular test that has the ability to diagnose three Ehrlichia species (E.
canis, E. chaffeensis and E. ewingii) and two Anaplasma species (A. phagocytophila and A. platys) all five of which are known
to cause disease in dogs. This rapid test uses the novel molecular biology methods and is complete in four hours. Ehrlichia
/Anaplasma species RNA is captured from the test sample and used to detect the presence of specific-pathogen RNA by a real
time RT-PCR assay.
Doxycycline has been used clinically with good success for ehrlichia infection at 5 mg/kg PO BID for three to four weeks.
Clinical response is usually noticed within 48 to 72 hours. However, complete clinical recovery may take several weeks. A
30 percent rise in the platelet count is indicative of a good response to treatment (10 percent is not adequate). Injections
of imidocarb dipropronate, 5 mg/kg IM, twice two weeks apart, has been used successfully for treatment of ehrlichiosis but
is substantially more expensive than doxycycline.
The decision to treat clinically normal antibody positive dogs is difficult. It may be helpful to check a CBC and platelet
count and affirm the possibility of tick exposure. Patients testing negative that create a high degree of suspicion for ehrlichia
should undergo a complete CBC and be rechecked for antibodies in two weeks.
Adequate supportive care may be necessary in most cases, but the new alternative treatments (granulocyte colony stimulating
factors or human recombinant erythropoiten) are expensive and may not be beneficial to most canine patients. The effects of
these new compounds are short lived and may only be useful in acutely ill patients.
The parasite/host interface is a dynamic interaction of tick, pathogen and host. Ticks are not just the mechanical vectors
of ehrlichia organisms. There is both a cellular and a humoral immune response generated to the parasite. Tick saliva contains
bioactive components that affect the host's hemostatic, inflammatory and immunologic systems.
Ticks can obtain E. canis organisms when they attach to an infected host and engorge during all phases of the disease (Photo
2, p. 3). E. canis is not transmitted transovarially in the tick. Therefore, tick larva will not transmit ehrlichia to dogs.
Larvae or nymphs that feed on acutely infected dogs or other infected hosts will drop off the host, molt and transmit infective
organisms to their next host (Photo 3, p. 5). In the tick, ehrlichia organisms are thought to multiply in the gut and salivary
glands. When infected nymphs and adult ticks feed, ehrlichia organisms are injected into the host along with tick salivary
secretions. The longer the tick is attached and feeding, the more bacteria and bioactive components will be injected. The
time needed for enough organisms to be injected to cause disease transmission is assumed to be about 24 to 48 hours for most
tick-borne pathogens. However, precise transmission data in dogs is lacking.