Ehrlichia - VetLearn.com

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Vol. 24, No. 4 April 2002 277 

Article #1 (1.5 contact hours) 

Refereed Peer Review 

KEY FACTS 

■ Ehrlichial infection can result in 

a wide range of manifestations 

(from inapparent infection to 

fatal illness). 

■ Diagnosis can be problematic 

because not all species of 

Ehrlichia produce cross-reactive 

antibody titers and positive titers 

may reflect either inactive 

infection or past exposure. 

■ Dogs and other domestic species 

have the potential to harbor ticks 

that can transmit certain types of 

ehrlichiosis to humans. 

Comments? Questions? 

Email: compendium@medimedia.com 

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The Increasingly 

Complicated Story 

of Ehrlichia 

University of Missouri-Columbia 

Diane E. Preziosi, DVM, DABVP* 

Leah A. Cohn, DVM, PhD, DACVIM 

ABSTRACT: Ehrlichiosis, once viewed as a rare disease in the United States, has become an 

increasingly common diagnosis in many regions of the country. Although Ehrlichia canis was 

the first species described and remains the best understood, many other species of Ehrlichia 

are now known to infect dogs. The recognition of new types of ehrlichial infections has added 

complexity to the diagnosis of ehrlichiosis. The importance of these pathogens has increased 

with the recognition of human ehrlichiosis. 

Ehrlichial infection was first recognized as a cause of canine disease in Algeria 

circa 1935. 1 Veterinarians in the United States did not become familiar 

with this disease until the Vietnam War era when military working dogs, 

including many that had never left the country, developed what was then known 

as tropical pancytopenia caused by Ehrlichia canis. 2,3 Since then, other species of 

Ehrlichia that cause disease in dogs have been identified. Classification of these 

ehrlichial species has been reorganized, diagnostic testing options have increased, 

and additional manifestations of ehrlichiosis have been recognized in a wider 

geographic area. In the mid-1980s, an ehrlichial species was recognized as a 

cause of disease in humans in the United States; since then, various other species 

have been found to infect humans as well. Some ehrlichial species that infect 

dogs can also infect humans (through tick bites), leading to concerns about the 

role of dogs in harboring a disease that may be transmitted to humans. This article 

addresses these developments and their importance in the understanding, 

diagnosis, and treatment of ehrlichiosis primarily in dogs. 

CLASSIFICATION 

Ehrlichia are gram-negative obligate intracellular bacteria that lack 

lipopolysaccharide endotoxins and rely on arthropod vectors for transmission. 

For many years, E. canis was the only ehrlichial species known to cause disease in 

dogs, and it is by far the best described veterinary ehrlichial pathogen. Several 

additional Ehrlichia species that infect dogs as either primary or incidental hosts 

are now recognized. Previous classification schemes have made use of the cell 

*Dr. Preziosi is currently affiliated with the University of Pennsylvania.

278 Small Animal/Exotics Compendium April 2002 

Table 1. Ehrlichial Agents Known to Infect Dogs 

Type Target Cells Major Host Known Vector(s) 

Genogroup I 

Ehrlichia canis Mononuclear cells Dogs Rhipicephalus sanguineus 

Ehrlichia chaffeensis Mononuclear cells Humans Amblyomma americanum, Dermacentor variabilis 

Ehrlichia ewingii Granulocytes Dogs R. sanguineus, A. americanum, D. variabilis 

Genogroup II 

Ehrlichia phagocytophila Granulocytes Ruminants Ixodes ricinus 

Ehrlichia equi Granulocytes Equidae I. ricinus, Ixodes pacificus 

Agent of human Granulocytes Humans Ixodes scapularis, I. ricinus, I. pacificus 

granulocytic ehrlichiosis 

Ehrlichia platys Platelets Dogs R. sanguineus 

Genogroup III 

Ehrlichia risticii Monocytes, enterocytes Equidae Arthropods 

Neorickettsia helminthoeca Monocytes Dogs Flukes 

type infected by the ehrlichial organism. Although this 

has proven useful, several species, such as Ehrlichia 

ewingii, Ehrlichia chaffeensis, Ehrlichia risticii, and 

Ehrlichia phagocytophila, may infect more than one cell 

type. 4,5 Current classification schemes are based on 

genetic analysis of the organism’s 16S rRNA gene 

sequence. This method produces three species 

genogroups, with the members of any one genogroup 

commonly producing cross-reactive antibody titers. 5–8 

Despite close genetic relatedness, the individual species 

within a genogroup may infect different cells within 

different hosts, use different vectors, and have different 

geographic distributions (Table 1). 

EPIDEMIOLOGY 

Most ehrlichial species rely on arthropod vectors for 

transmission, although the specific vectors are not well 

described for every species of Ehrlichia. The geographic 

distribution pattern of various ehrlichial species is 

related to the distribution of the relevant vector(s). The 

predominant vector for E. canis is the brown dog tick 

(Rhipicephalus sanguineus), which is found worldwide. 9,10 

Accordingly, E. canis has been reported in dogs from 

Africa, Europe, Asia, the Middle East, and the United 

States. In contrast to E. canis, E. ewingii is known to use 

at least three different vectors. In addition to R. sanguineus, 

both Dermacentor variabilis (American dog 

tick) and Amblyomma americanum (lone star tick) are 

capable of transmitting E. ewingii infection. 10 The primary 

distribution of the lone star tick in the midwestern 

and southeastern United States may account for the 

increased incidence of E. ewingii infection in these 

regions. 5,6,10 Ehrlichia equi infection, which is transmitted 

by Ixodes ticks, is most often reported in the upper 

midwestern and northeastern United States. 7,11,12 

In addition to ticks, other known vectors of ehrlichial 

infection include snails and flukes. E. risticii, the 

causative agent of Potomac horse fever, is transmitted 

through the ingestion of snails and can cause infection 

in dogs as well as horses. 13 Although not often thought 

of as an ehrlichial species, Neorickettsia helminthoeca, 

the causative agent of salmon poisoning disease, 

belongs to the same genogroup as E. risticii. During a 

complex life cycle, it is transmitted to dogs through the 

ingestion of flukes harbored in certain types of fish. 14 

The vectors of some ehrlichial species that affect dogs 

remain incompletely defined. 

Concurrent infection with multiple arthropod-borne 

pathogens is possible and may contribute to illnesses 

described in some Ehrlichia-positive dogs. Dogs that 

harbor one tick are likely to harbor several (of either the 

same or different species). In addition, the same arthropod 

may serve as transmission vector for several genera 

and species of infectious agents. For instance, R. sanguineus 

is capable of transmitting not only E. canis and 

E. ewingii, but Babesia canis and Babesia gibsoni as 

well. 2,10,15–17 Ixodes ticks, competent vectors for transmission 

of E. equi and the agent of human granulocytic 

ehrlichiosis (HGE), can also transmit infection with 

Borrelia burgdorferi and Babesia microti. 18–21 In addition 

to many case reports in the literature describing such 

concurrent infections, serologic surveys have documented 

these cases on a larger scale. In one such study, 6 

almost 50% of dogs diagnosed with Ehrlichia also harbored 

Bartonella vinsonii. Concurrent seropositivity has 

also been documented for B. burgdorferi and E. canis. 18 

Such simultaneous infections may worsen the severity of 

observed disease and impact the outcome of treatment.

Compendium April 2002 Ehrlichiosis 279 

TRANSMISSION AND PATHOGENESIS 

The transmission and pathogenesis of canine ehrlichiosis 

are best understood for E. canis infection. E. 

canis organisms gain entry via salivary secretions during 

the bite of an infected tick. 4 Ticks acquire the organism 

during their larval or nymph stage when they feed on 

an infected canid. Infection is spread when the tick 

feeds again on a new host. Because transovarial transmission 

does not occur, the tick vector cannot serve as a 

reservoir of disease. 4 Ticks can remain infected for long 

periods, however, allowing for disease transmission in 

early spring after the infected tick has overwintered. 

The course of clinical infection with E. canis is traditionally 

divided into three stages, which vary in 

length. These stages may be difficult to differentiate 

in natural infections. 4,22,23 

Acute Phase 

The acute phase occurs 1 to 3 weeks after the 

infected tick bites a dog. 9 During this phase, the organism 

invades leukocytes and divides to form morulae, 

which are colonies bound by a vacuolar membrane. 

Usually a specific ehrlichial species preferentially 

invades either mononuclear or granulocytic leukocytes. 

E. canis, E. chaffeensis, and E. risticii invade mononuclear 

cells, whereas E. ewingii, E. phagocytophila, and E. 

equi invade granulocytic neutrophils or eosinophils 

(Table 1). During acute infection, the most consistent 

hematologic change is the development of thrombocytopenia. 

24 This multifactorial change results from vascular 

endothelial inflammation with resulting platelet 

consumption, immunologically mediated destruction 

of platelets, and splenic sequestration of platelets. 9,24,25 

Interestingly, thrombocytopenia seems to be a common 

finding during infection with all species of Ehrlichia 

described thus far. In addition to thrombocytopenia, 

thrombocytopathia may accompany E. canis infection. 

26 Hyperglobulinemia during acute E. canis infection 

is usually the result of a polyclonal gammopathy, 

but the gammopathy does not seem to be caused by 

ehrlichial-specific antibody production. 9,27 Few dogs 

succumb to the acute disease; most either clear the 

organism (likely via cell-mediated immunity) or enter 

the subclinical stage of infection. 4,9 

Subclinical Phase 

The subclinical phase of E. canis infection is characterized 

by persistence of the organism in the host in the 

absence of clinical illness. It appears likely that organisms 

are retained at low numbers in splenic mononuclear 

cells during subclinical infection. 28 The length of 

the subclinical phase may range from weeks to years; in 

fact, it is not known what percentage of subclinically 

infected dogs will ever develop clinical illness. In a single 

study of naturally infected dogs, 53% demonstrated 

positive E. canis antibody titers 4 years after the presumed 

time of infection but remained clinically asymptomatic. 

Many of these dogs did, however, demonstrate 

abnormalities suggestive of ehrlichial disease on complete 

blood cell counts (e.g., hyperglobulinemia, 

thrombocytopenia). 29 Although subclinical infection is 

well documented for E. canis, it is less clear whether 

other Ehrlichia species also induce persistent but subclinical 

infections. In at least one case, 12 E. ewingii 

morulae were observed in an asymptomatic dog. 

Chronic Stage 

The pathogenesis of the chronic stage of E. canis infection 

is poorly understood because adequate models are 

not available. Not all infected animals progress to the 

chronic stage, and the factors that influence progression 

are still unknown. Persistent E. canis infection results in 

persistent antibody formation (both nonspecific and 

ehrlichial directed). 9,30,31 Unfortunately, humoral immunity 

to E. canis provides no protection. 4,32 In fact, many of 

the manifestations of chronic ehrlichial disease may result 

from an exuberant but nonprotective humoral immune 

response. 4,9 As in the acute stage, hemorrhagic tendencies 

may be related to either thrombocytopenia or thrombocytopathia. 

In addition to the mechanisms mentioned 

previously, hyperglobulinemia associated with chronic 

infection is often more pronounced, and hyperviscosity 

syndrome with resultant thrombocytopathy may result. 26 

Glomerulonephritis may result from the deposit of antigen–antibody 

complexes with resultant inflammatory 

damage. 23 Bone marrow hypoplasia has been a classic 

finding associated with chronic E. canis infection and 

may result in pancytopenia. 3,22,23 Nonregenerative anemia 

commonly associated with chronic E. canis infection may 

be caused by either anemia of inflammatory disease or by 

pancytopenia due to bone marrow hypoplasia. 22,23,33 Concurrent 

infectious diseases documented in dogs with E. 

canis have been attributed to immunosuppressive effects 

of chronic infection. 23 

CLINICAL FINDINGS 

Ehrlichia canis 

Clinical findings associated with E. canis infection 

vary tremendously and are likely influenced by the 

strain of organism, host immune status, and breed of 

animal. 4,34 German shepherds are classically thought to 

be more susceptible to infection and to have a more fulminant 

course of infection than dogs of other 

breeds. 34–36 By definition, no clinical signs are apparent 

during the subclinical phase of infection, although 

hematologic abnormalities may be identified. 29 Often,


either the acute stage of infection goes unnoticed or 

signs are mild enough that owners do not seek veterinary 

care for their pets. When dogs are evaluated during 

the acute stage of infection, signs are largely nonspecific 

and include lethargy, fever, anorexia, weight loss, 

splenomegaly, and generalized lymphadenopathy. 4,37 

Most diagnoses of E. canis infection occur during the 

chronic stage of infection. As occurs with dogs presented 

during the acute stage, the owners of dogs with 

chronic ehrlichiosis most commonly report nonspecific 

signs, such as lethargy, anorexia, and weight loss. 23,34 

Signs attributable to bleeding tendencies, including 

epistaxis, melena, petechial and/or ecchymotic hemorrhages, 

hyphema, retinal hemorrhage, and hematuria, 

occur in 25% to 60% of cases. 22,23,33,34 Additional physical 

examination findings include lymphadenopathy, 

fever, pale mucous membranes, and splenomegaly. 22,23,33,34 

Anterior uveitis, retinal changes, or neurologic abnormalities 

are noted occasionally. 23,38 Ataxia, paraparesis, 

conscious proprioceptive deficits, head tilt, nystagmus, 

and seizures have all been reported as neurologic manifestations 

of infection. 23 The importance of lameness as 

a clinical sign of E. canis infection is debatable. 

Although lameness is often listed as a clinical sign of 

infection in the older literature, many of the animals 

displaying lameness were found to have granulocytic 

morulae, suggesting that the infection may have been 

caused by E. ewingii rather than E. canis. 39–41 Other clinical 

signs of chronic E. canis infection relate to complications, 

including glomerulonephritis with nephrotic 

syndrome, or pancytopenia resulting in secondary infections 

and severe anemia. 

Clinicopathologic abnormalities are also nonspecific. 

Although normal platelet concentrations do not rule out 

chronic ehrlichiosis, most cases display some degree of 

thrombocytopenia. 22,23,33,34 Nonregenerative anemia 

(often mild to moderate) is also identified in most 

cases. 22,23 White blood cell counts may fall below, within, 

or above reference ranges in dogs with ehrlichiosis. 22,23,33 

Although hyperglobulinemia has not been reported consistently, 

it is observed in most cases. 22,23,34 Albeit usually 

due to polyclonal gammopathy, hyperglobulinemia may 

present as a monoclonal gammopathy that can be easily 

mistaken for multiple myeloma. 27,42 Other serum biochemistry 

abnormalities noted with some regularity 

include hypoalbuminemia, elevated alkaline phosphatase, 

and elevated alanine transaminase concentrations. 

22,23,33 Proteinuria may occur independently or concurrently 

with glomerulonephritis. 22,23,33,43,44 Although 

early descriptions of E. canis infection were of a tropical 

pancytopenia, the pancytopenic manifestations now 

appear to account for a small minority of cases in the 

United States. Cytologic evaluation of bone marrow 

aspirates often displays increased numbers of plasma 

cells, with either hypoplasia (suggesting chronic infection) 

or hyperplasia (suggesting acute or chronic infection) 

of the other marrow elements. 3,22,23,37 

Ehrlichia ewingii 

E. ewingii is one of two ehrlichial agents known to 

result in granulocytic infection in dogs, with the other 

agent being E. equi. 12 Unfortunately, identification of 

granulocytic morulae does not differentiate E. equi infection 

from E. ewingii infection, which likely predominates 

in the southern and lower midwestern United 

States. Because E. ewingii belongs to the same genogroup 

as E. canis, E. canis titers should be positive during infection. 

12 Many descriptions of granulocytic ehrlichial infection 

are of dogs with acute-onset polyarthritis, and these 

cases have more often than not been ascribed to E. 

ewingii infection. 5,39–41,45 The lameness may involve more 

than one leg or appear to shift from limb to limb. Joint 

stiffness and occasional joint swelling due to effusion 

may be noted, and dogs are often febrile. Splenomegaly 

and hepatomegaly have been reported. Bleeding tendencies 

may be noted, and many infected dogs have mild to 

moderate thrombocytopenia. 12 Central nervous system 

involvement, particularly meningitis, has been 

reported. 46 Dual infection with both E. canis and E. 

ewingii has also been reported in association with profound 

ataxia and epistaxis. 47 Polyarthritis resolves quickly 

with appropriate therapy. Fatal granulocytic ehrlichial 

infections seem to be extremely rare in dogs. 

Ehrlichia equi 

As with E. ewingii, the true incidence of E. equi infections 

in dogs is unknown, but E. equi may account for a 

significant proportion of granulocytic ehrlichiosis in the 

northeastern and upper midwestern United States and 

California, where equine infections are endemic. 7,11,12 

Experimental infection with E. equi in dogs produced 

only mild to inapparent clinical signs, but naturally 

infected dogs have presented with nonspecific illness, 

including fever, lethargy, and thrombocytopenia. 11,48 

There are no unique clinical findings attributed to infection 

with E. equi, but polyarthritis is described less frequently 

than for E. ewingii. 11 Without an index of suspicion, 

veterinarians may not request the specific diagnostic 

testing required to differentiate this ehrlichial infection 

from others, and E. canis titers may be negative. 11,12 

Ehrlichia risticii 

E. risticii, the causative agent of Potomac horse fever, can 

infect dogs and cats as well as horses. This agent is transmitted 

not by a tick bite but rather by ingestion of snails, 

perhaps explaining why canine infection is not commonly


described. When dogs are infected, lethargy, vomiting, 

bleeding disorders, and arthralgia have been reported. 13 

Because E. risticii belongs to a different genogroup than E. 

canis, antibody cross-reactivity may be lacking. Unless 

genogroup-specific titers are requested, dogs with E. risticii 

infection may display negative ehrlichial titers. 13 

Ehrlichia chaffeensis 

Although E. chaffeensis is primarily notable as a 

human pathogen, dogs are also susceptible to infection 

with this organism. Experimentally infected dogs seem 

to have mild or inapparent disease. 49 However, a report 

of three dogs infected naturally with E. chaffeensis documented 

more serious signs, including vomiting, epistaxis, 

lymphadenopathy, and anterior uveitis. 6 Because 

E. chaffeensis shares genogrouping with E. canis, routine 

titers should prove positive in infected dogs. 6 

Ehrlichia platys 

E. platys is unique among the Ehrlichia species because 

of its predisposition for platelets rather than leukocytes. 

E. platys does not share serologic cross-reactivity with E. 

canis, but co-infections have been documented. 8,15,50,51 

Although infection with E. platys results in cyclic thrombocytopenia 

in dogs, it is seldom the cause of clinical 

bleeding unless trauma or surgery is performed during 

the thrombocytopenic cycle. 52,53 The disease manifestations 

associated with E. platys may be more severe in 

strains found outside the United States. 54–56 Veterinarians 

should consider specific testing for E. platys infection in 

dogs with recurring evidence of thrombocytopenia for 

which another cause (including the more common 

ehrlichial infections) cannot be documented. 

Feline Ehrlichiosis 

The topic of feline ehrlichiosis deserves separate mention 

from canine ehrlichiosis. 57 Cats have been experimentally 

infected with both E. risticii and E. equi, producing 

either subclinical infection or mild illness. 48,58 To 

date, experimental infection with E. canis or E. ewingii 

has not been attempted. Naturally occurring feline ehrlichiosis 

has been documented in only 31 cats worldwide. 

These cats presented with various clinical signs, including 

fever, anorexia, arthropathy, gastrointestinal signs, 

and general malaise. 57,59–62 Until a clearer picture of the 

importance and clinical presentation of feline ehrlichiosis 

is developed, ehrlichiosis should remain a consideration 

in cats with various unexplained clinical illnesses. Diagnosis 

relies on ruling out other causes for the described 

clinical illness in combination with either identification


Figure 1—A round, basophilic morula of E. ewingii is seen in 

the cytoplasm of the neutrophil. (Courtesy of Steve Stockham, 

DVM, MS, Kansas State University.) 

of morulae within the peripheral blood cells or serologic 

evidence of exposure to an ehrlichial agent as well as resolution 

of clinical signs after appropriate antibiotic therapy. 

57 Both granulocytic and monocytic infections have 

been described, with the expected variations in serologic 

reactivity to different ehrlichial agents. 57,60,62 

DIAGNOSIS 

Ehrlichiosis is usually diagnosed based on clinical 

signs, consistent laboratory abnormalities, and 

Ehrlichia-specific testing. Although observation of 

intracellular morulae is diagnostic, the search for morulae 

is most often unrewarding. 35 The use of concentration 

techniques, such as buffy coat examination with a 

Romanovsky-type stain, maximizes the chance of identifying 

morulae. 3 Morulae may be observed in white 

blood cells from peripheral blood or other fluids, 

including cerebrospinal and joint fluids. 39,40,46,47 In general, 

morulae are more readily apparent during the 

acute phase of monocytic E. canis infection or during 

infection with the granulocytic species E. ewingii and 

E. equi (Figure 1). 5,11,23,35 

The most commonly employed diagnostic test for 

suspected ehrlichial infection is indirect fluorescent 

antibody (IFA) serology. This form of testing does not 

detect the actual ehrlichial organism but rather 

ehrlichial-reactive antibody in the serum. Veterinarians 

employing IFA testing must understand that a positive 

titer in a dog from an endemic area does not confirm 

that the disease under investigation is caused by 

ehrlichial infection. Rather, a positive titer confirms 

exposure to the organism but may be observed after 

exposure and clearance of the organism, during the 

subclinical stage, or after successful treatment, as well as 

during active infection. Likewise, a negative titer does 

not rule out infection. Moribund animals may cease to 

produce antibody, resulting in a diminished or negative 

titer, and acutely infected dogs require 7 to 21 days for 

seroconversion, potentially resulting in negative titers 

in infected dogs. 63 Depending on the particular IFA test 

used, serology can be more or less specific for a species 

of Ehrlichia. Antibody generated to one species of 

Ehrlichia may cross-react with other species. Species 

that fall within the same genogroup are likely to possess 

serologic cross-reactivity. Likewise, titers to one 

genogroup may be negative during infection with 

members of a different genogroup (i.e., E. canis titers 

may be negative during infection with E. equi). Currently, 

titers are commercially available for members of 

each genogroup, including E. canis, E. equi, E. risticii, 

and E. platys. a 

Polymerase chain reaction (PCR) detects actual DNA 

from the ehrlichial organism. Primers are chosen to 

amplify a portion of the bacterial DNA, allowing detection 

of that genetic material. The use of generic 

ehrlichial primers allows detection of any of several 

species of Ehrlichia using one test. Alternatively, by 

using primers directed at a specific, highly variable portion 

of the bacterial gene, PCR can identify the specific 

species of Ehrlichia in an affected animal. Such primers 

are currently available for E. canis, E. ewingii, E. equi, 

E. chaffeensis, E. risticii, and E. platys. 6 Many commercial 

laboratories use generic primers and then may or 

may not speciate positive samples. b,c Although PCR can 

be a sensitive test, the choice of sample will influence 

results. Most commercial laboratories that perform 

PCR request whole blood samples, but organisms 

might at times be sequestered in low numbers in such 

tissues as the spleen or bone marrow. 64 PCR may be 

used occasionally in conjunction with IFA to differentiate 

between exposure to Ehrlichia and true infection 

(such as after completion of treatment and in the presence 

of continued clinical signs and persistent ehrlichial 

titers). 65 The laboratory’s quality control is crucial for 

reliable PCR results. 

A commercial in-house screening kit that tests for B. 

burgdorferi antibody d has been marketed. Screening 

ELISA tests of this kind are designed to produce a positive 

response in animals with an antibody titer corresponding 

to an IFA titer of greater than 1:500. 

Reported sensitivity and specificity of these test kits are 

98.9% and greater than 99%, respectively. As with any 

test, predictive value is more important than either sensitivity 

or specificity, but it depends on both as well as 

a ProtaTek International, Inc., Chandler, AZ. 

b Antech Diagnostics, Irvine, CA. 

c Tick-borne disease testing facility, College of Veterinary Medicine, 

North Carolina State University, Raleigh, NC. 

d Snap ® 3Dx Test, IDEXX Laboratories, Inc., Westbrook, ME.


on the prevalence of the disease in question. Thus a 

positive screening test in an endemic area is more likely 

to be a true positive than is the same result in an area 

with a lesser disease prevalence. As is true of the IFA 

test, a positive titer must be interpreted with caution 

because it may represent current or resolved infection 

or merely exposure. 

TREATMENT 

Tetracycline-related antibiotics have been the treatment 

of choice for ehrlichial infections for years. 3 

Excellent absorption and an infrequent dosing interval 

make doxycycline the preferred drug. 32,66 Although a 7to 

14-day course of doxycycline at 10 mg/kg/day has 

been previously recommended, 4,6,32 this length of treatment 

may be inadequate. 66,67 In fact, in a study of subclinically 

infected dogs treated with doxycycline for 6 

weeks, ehrlichial organisms could still be identified by 

PCR in one of four dogs. 67 Although the appropriate 

length of treatment has not been clearly determined, 

treatment for dogs with chronic infection may be quite 

long in duration and titers may remain elevated for 

months to years, regardless of the treatment length. 

Even long courses of treatment (average, 210 days) did 

not produce negative titers in a significant proportion 

of naturally infected dogs. 30 Although most studies have 

been directed at E. canis infections, doxycycline has 

proven effective for E. equi and E. ewingii infections 

but less effective for E. chaffeensis. 6,12,32 

Imidocarb dipropionate, only recently available in 

the United States, has also been used successfully to 

treat canine ehrlichiosis. Although a study in 1980 

found that imidocarb produced more effective clearance 

of E. canis than did a 2-week course of tetracycline, 

recent studies using doxycycline have documented 

no difference in the clinical response of dogs 

treated with either drug alone or with both drugs 

simultaneously. 68,69 Imidocarb is administered at 5 

mg/kg IM, with a second injection 2 weeks after the 

first. 4,69 Pretreatment with atropine may lessen the 

occurrence of unpleasant anticholinergic side effects, 

including salivation, serous nasal discharge, diarrhea, 

and dyspnea. 

Other antibacterial agents have also been evaluated 

for the treatment of ehrlichial infections. Chloramphenicol 

may be used in puppies to avoid the discoloration 

of erupting teeth associated with tetracycline. 4 

Quinolones cannot be recommended currently as a 

treatment for ehrlichiosis. Although enrofloxacin has 

been used successfully to treat experimental Rickettsia


rickettsii infection in dogs, it does not seem an effective 

treatment for E. canis. Most dogs with experimental E. 

canis infections that were administered dosages of 10 

mg/kg PO q12h for 21 days remained thrombocytopenic 

and blood culture positive. 32 Similarly, 

ciprofloxacin was unsuccessful in eliminating infection 

with E. chaffeensis in humans. 70 

In addition to antibiotics, other supportive therapies 

may be required during ehrlichial infection. Glucocorticosteroids 

can attenuate the immune-mediated 

destruction of platelets associated with infection, and 

thus a short course (2 to 7 days) of prednisone may be 

indicated if thrombocytopenia is severe or pending 

diagnostic testing to differentiate ehrlichiosis from 

immune-mediated thrombocytopenia. 4,23 Steroids may 

also be indicated for the treatment of polyarthritis, vasculitis, 

or meningitis associated with certain types of 

ehrlichial infections. 4,46,47 Other supportive treatments 

might include the administration of parenteral crystalloid 

or colloidal fluids or blood transfusion, as indicated. 

3,4 Complications of chronic E. canis infection 

(e.g., glomerulonephritis, pancytopenia) may likewise 

require specific supportive therapy. 

EVALUATING TREATMENT 

Evaluation of treatment remains problematic. Resolution 

of clinical signs and normalization of platelet 

counts are usually noted within days of initiating 

proper treatment for acute cases and often for mild 

chronic cases as well. 4,23,69 Even after rapid clinical 

improvement, studies have found that platelet counts 

may again decrease after completion of doxycycline 

therapy, titers remain elevated, organisms can still be 

cultured, and PCR results remain positive. 64,69 Serum 

antibody titers can remain elevated for months to years 

after appropriate treatment in dogs that remain clinically 

and hematologically normal. 30,31,64,69 These persistently 

positive titers, which tend to correlate with the 

highest initial titers, may indicate continued infection 

or re-infection or may be indicative of a past infection 

only. 23,30 PCR may offer the best option for documenting 

clearance of the organisms after therapy, but even 

PCR evaluation can be problematic. PCR detects bacterial 

DNA but cannot distinguish between living and 

nonliving organisms. However, it is unlikely that killed 

ehrlichial organisms would persist in the body for 

more than several weeks, rendering a positive PCR 

result strongly suggestive of active infection. The 

greater problem is related to false-negative results when 

sampling tissues that contain low levels of organisms 

(e.g., blood, bone marrow). 64,65 Practically speaking, 

treated dogs for which the clinical and laboratory evidence 

of disease is resolved need not be further evalu- 

ated for the presence of organisms. In animals treated 

appropriately for infection but in which evidence of 

disease remains, a positive PCR would warrant continuation 

or alteration of therapy, while a negative PCR 

should prompt consideration that the observed disease 

may not be due to ehrlichiosis. 

PREVENTION AND CONTROL 

Unfortunately, exposure to E. canis does not confer 

protective immunity. Effective tick control is the most 

important preventive measure for infection or re-infection. 

Several highly efficacious products are available 

for direct application to dogs, including Frontline ® Top 

Spot (Merial Ltd, Iselin, NJ), Preventic ® collars (Virbac, 

Inc., Fort Worth, TX), and Kiltix ® Topical Tick 

Control for Dogs (Bayer Animal Health, Shawnee Mission, 

KS). In addition, many premise sprays are available 

to decrease tick populations in the dog’s local environment. 

The prophylactic use of tetracycline 

antibiotics during tick season has also been advocated 

for prevention of infection, but this option is not free 

from either the cost or risk associated with antibiotic 

use. 4 To prevent disease associated with chronic E. canis 

infection, it has been suggested that dogs in endemic 

areas be serologically monitored and positive animals 

treated regardless of the presence of clinical signs. 4 We 

suggest that a positive screening test (e.g., ELISA) be 

used as evidence that further evaluation is needed. At a 

minimum, such evaluation should include a thorough 

physical examination, quantitative platelet count, and 

measurement of serum globulin. Positive screening tests 

might also prompt confirmatory tests, including either 

IFA or PCR, prior to initiation of antibiotic therapy. 

ZOONOTIC POTENTIAL 

Since the first description of human monocytic ehrlichiosis 

(HME) in the United States in 1986, 71 the disease 

has garnered both scientific and public attention. 

The agent of HME, which shares a penchant for 

monocytic cell infection and produces antibodies crossreactive 

to E. canis, was named E. chaffeensis. 72 E. chaffeensis 

infection in dogs has been identified in both 

experimental and natural settings, with the organism 

persisting for months. 6,49 These long-lasting infections 

raise the concern that dogs may serve as a reservoir of 

infection for human disease. 72 Adding to those concerns, 

it seems that doxycycline may not easily clear the 

organism from naturally infected dogs. 6 In addition to 

HME, HGE has been recognized as well. The causative 

agent of HGE, which differs from that of HME, has 

proven to be nearly identical to E. phagocytophila and 

E. equi. Although these agents are most commonly 

associated with infection in small ruminants and/or


horses, respectively, both organisms have been isolated 

from dogs, again raising the possibility that dogs may 

serve as a reservoir for human infection. 8,11,73,74 Additionally, 

the common agents of canine ehrilichiosis, E. 

canis, and E. ewingii can occasionally cause clinical disease 

in humans. 75 The symptoms of HME and HGE 

are quite similar and include fever, headache, and myalgia. 

Left untreated, either infection may prove fatal to 

humans. 8,71,72,76 

CONCLUSION 

Ehrlichiosis is a bacterial infection transmitted largely 

through the bite of infected ticks. Dogs with ehrlichiosis 

can no longer be assumed to have an E. canis infection 

alone. Veterinarians should be aware of the similarities 

and differences between E. canis and other arthropodborne 

infections. Dogs with suggestive clinical signs and 

laboratory abnormalities may be started on doxycycline 

pending specific diagnostic testing. Veterinarians practicing 

in endemic areas are confronted with the often 

difficult task of sorting out ehrlichial infection from 

mere exposure. Once a diagnosis is established, treatment 

should continue for at least 3 weeks. Tick prevention 

for animals in endemic areas should be maintained 

throughout tick season, not only to prevent disease but 

also to lessen the potential for dogs to become carriers of 

disease for their human companions. 

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ARTICLE 

CE 

#1 CE TEST 

The article you have read qualifies for 1.5 contact hours of Continuing 

Education Credit from the Auburn University College of Veterinary 

Medicine. Choose the best answer to each of the following questions; then 

mark your answers on the postage-paid envelope inserted in Copendium. 

1. Ehrlichial organisms are best classified by the 

a. type of tick involved in transmission. 

b. type of host cell infected. 

c. genetic analysis of the 16S rRNA segment. 

d. species of host animal infected. 

2. Which is the primary transmission vector for E. canis? 

a. D. variabilis c. A. americanum 

b. R. sanguineus d. Ixodes scapularis 

3. Which statement regarding transmission of ehrlichiosis by ticks is true? 

a. Ticks can infect dogs only in the fall. 

b. Ticks serve as the reservoir for ehrlichial organisms. 

c. The ehrlichial organism is passed transovarially prior to infection of dogs. 

d. Ticks transmit the infection through salivary secretions released during a bite. 

4. What are the most common presenting complaints in ehrlichiosis? 

a. lethargy, fever, and weight loss 

b. epistaxis and fever 

c. lameness and lethargy 

d. secondary infections 

5. Which Ehrlichia species most commonly causes lameness? 

a. E. canis c. E. equi 

b. E. chaffeensis d. E. ewingii


6. A positive E. canis titer in an asymptomatic dog in an 

endemic area indicates that the dog has 

a. been exposed and has produced antibodies. 

b. entered the subclinical phase of infection. 

c. been infected in the past but has cleared the organisms. 

d. any of the above 

7. Infection with _________ is the most likely explanation 

for observed granulocytic morulae and a negative 

E. canis titer. 

a. E. ewingii c. E. equi 

b. E. platys d. E. chaffeensis 

8. Six months after treating an infected dog with doxycycline, 

a positive E. canis IFA titer can reveal 

a. persistence of the titer despite elimination of the 

organisms. 

b. persistence of the organism. 

c. re-infection with organisms. 

d. any of the above 

9. Administration of ______________ for _______ days 

is the recommended treatment protocol for chronic 

ehrlichiosis. 

a. doxycycline; 21 c. doxycycline; 7 to 10 

b. tetracycline; 7 to 10 d. enrofloxacin; 21 

10. ___________ has not been reported to infect humans. 

a. E. chaffeensis c. E. equi 

b. E. ewingii d. E. platys

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