Spotlight on Ehrlichia ewingii (Sponsored by IDEXX)

In many parts of the United States, infection of dogs with the pathogen Ehrlichia ewingii is far more common than infection with the better-known Ehrlichia canis. Although these two ehrlichial pathogens share a phylogenetic similarity, there are many important differences, including tick vectors, host-cell tropisms, disease manifestations, geographic restrictions, and zoonotic potential. In fact, E. ewingii produces morulae in granulocytes rather than monocytes, is transmitted by a highly prevalent environmental tick (Amblyomma americanum, the lone star tick) as compared with the localized kennel tick (Rhipicephalus sanguineus, the brown dog tick), and often causes acute lameness or polyarthritis as a primary component of the dog's clinical presentation. It is important for veterinarians practicing in E. ewingii-endemic regions of the south central and southeastern United States to be familiar with tick transmission, disease consequences, diagnosis, treatment, and the zoonotic implications that are particular to E. ewingii.

Transmission and geographic distribution

The only proven competent vector for the trans mission of canine granulocytic ehrlichiosis is Amblyomma americanum, the lone star tick. Although E. ewingii infection has also been documented in dogs from both South America and Africa, the tick species responsible for natural transmission in those regions remains to be determined.1,2 Transstadial transmission in the lone star tick assures that ticks can be infected during all three stages of the life cycle (larva, nymph, and adult) and remain competent to transmit E. ewingii at their next blood meal.3,4

White-tailed deer, the major host species for A. americanum, apparently serve as the major reservoir for E. ewingii. Not surprisingly, E. ewingii infections occur most often in the southeastern and south central United States where both whitetailed deer and lone star ticks are plentiful (Figure 1).5-8 In fact, dogs are more likely to be seropositive for E. ewingii than for either E. canis or Ehrlichia chaffeensis in Missouri, Oklahoma, Arkansas, Kansas, Georgia, Alabama, Mississippi, Tennessee, Florida, Virginia, and New Jersey.9 The average seroprevalence for E. ewingii is 8.5% in these states.9 Lone star ticks are aggressive ticks that commonly feed on dogs, people, and numerous wildlife species.10,11 As a result, serologic evidence supporting exposure to E. ewingii is very common in A. americanum-endemic states. Antibodies against E. ewingii have been detected in 26% to 44.8% of randomly sampled dogs in Oklahoma, Missouri, and Arkansas, with 9% to 20% testing positive for E. ewingii DNA by polymerase chain reaction (PCR).9,12,13

Figure 1. This map shows the distribution of Amblyomma americanum (the lone star tick; inset) in the United States.

Pathogenesis and clinical disease

Ehrlichia ewingii is a small, obligate intracellular bacterium, and, following the bite of an infected tick, it invades granulocytes forming membrane-bound, intracytoplasmic colonies of organisms known as morulae.14 The time required from tick attachment to pathogen transmission is unknown. Morulae can be observed within granulocytes in as little as 12 days after experimental inoculation.3,15,16

Clinical illness associated with canine granulocytic ehrlichiosis is most often an acute febrile condition associated with musculoskeletal signs. Reluctance to stand or walk, lameness, a stiff or stilted gait, and joint effusion are common findings in E. ewingii-infected dogs and may be quite severe.12,17-19 Lethargy, anorexia, and central nervous system signs (e.g., head tilt, tremors, and anisocoria) may also be present.19,20 Rarely reported findings have included hemorrhage, weight loss, organomegaly, uveitis, pruritus, vomiting, and diarrhea.12,19-21 Onset of clinical signs generally occurs within 7 to 14 days following infection.

It is quite likely that many dogs infected with E. ewingii either remain clinically healthy or develop a brief, self-limiting illness, as has been reported in experimentally infected dogs.15,16 The often mild pathogenicity following infection is further supported by the high seropositive rates among dogs lacking historical evidence of clinical disease attributable to canine granulocytic ehrlichiosis, and the lack of reported mortality due to E. ewingii infection.3,9,15 Immunosuppression may exacerbate or potentiate disease manifestations in infected dogs, as it apparently does in E. ewingii-infected people.22-24 Experimental infection with E. ewingii has been more successfully attained in dogs receiving either cyclophosphamide or glucocorticoids.3,16,25 Similarly, co-infection with other tick-transmitted pathogens may worsen disease manifestations in infected dogs.26

Evidence to date suggests that canine and human granulocytic ehrlichiosis causes only an acute illness in dogs and people. This association is in contrast to the pathogenesis of canine monocytic ehrlichiosis, caused by E. canis, where the acute illness typically resolves spontaneously, after which the dog enters a period of subclinical infection, followed in some cases by the development of chronic disease manifestations that can be quite challenging to treat.27-29 Due to the acute nature of canine granulocytic ehrlichiosis, disease manifestations have only been identified when vector lone star ticks are active, predominantly during the late spring and summer.19,30 At the University of Missouri Veterinary Medical Teaching Hospital, from 32 dogs with granulocytic morulae and clinical illness compatible with E. ewingii infection, only two were identified outside of the months May to August (in September and November) (Cohn, L: Unpublished data). Despite the fact that illness documented to result from E. ewingii infection is acute in nature, actual infections can be persistent.15,19 It is possible that chronic E. ewingii infections may contribute to as-of-yet unrecognized disease manifestations or pathophysiologic consequences.

Clinicopathologic findings and diagnosis

As with most ehrlichial infections, thrombocytopenia is the most consistent clinicopathologic abnormality associated with E. ewingii infection.3,15,18-20 Nevertheless, a normal platelet count does not rule out the possibility of active infection, and thrombocytopenia has been found to be cyclical in some experimentally infected dogs over a period of weeks.15 Occasionally, mild anemia and reactive lymphocytes are identified.12,19 Similar to other rickettsial infections, short-lived leukopenia was recognized during the early acute phase of experimental infection in dogs.15 Serum biochemical abnormalities are mild and nonspecific but may include increases in hepatic enzyme concentrations, hyperglobulinemia, hypokalemia, and hyperphosphatemia or hypophosphatemia.15,20,21 Proteinuria, which has been associated with both E. canis and Anaplasma phagocytophilum infections, has thus far not been identified as a component of canine granulocytic ehrlichiosis.31,32 Arthrocentesis from joints of affected dogs with polyarthritis reveals neutrophilic inflammation.19 On occasion, morulae are identified in granulocytes from peripheral blood (Figure 2), cerebrospinal fluid, joint fluid, prostatic fluid (Cohn L.: Personal observation), or potentially other bodily fluids.18-20 Unfortunately, observation of morulae is neither sensitive nor specific; microscopic differentiation of A. phagocytophilum from E. ewingii morulae is impossible, therefore PCR confirmation of the infecting organism is recommended, particularly when morulae are found in dogs in non-endemic regions.

Figure 2. A morula in a neutrophil. These morulae caused by infection with either Ehrlichia ewingii or Anaplasma phagocytophilum appear identical. While microscopic recognition of morulae is extremely useful, it is an insensitive diagnostic tool for disease confirmation. (Courtesy of Dr. Linda Berent, university of Missouri Veterinary Medical Diagnostic Laboratory.)

A presumptive clinical diagnosis can be made based on compatible signs in dogs from endemic regions during the spring or summer. However, there is substantial overlap in the clinical presentation for dogs infected with other tick-borne pathogens, including Rickettsia rickettsii and Bartonella species. Examination of a peripheral blood smear to identify thrombocytopenia and allow visual inspection of granulocytes for morulae is often the only diagnostic test performed before treatment is initiated. Culture of the pathogen from the blood has thus far been unsuccessful in a research setting, so disease diagnosis via pathogen isolation is not possible. Instead, identification of E. ewingii nucleic acid via PCR has been used. PCR testing not only detects E. ewingii, but also can be used to distinguish this pathogen from related Ehrlichia species and from A. phagocytophilum. In addition, PCR testing may be helpful in identifying those dogs with subclinical, persistent E. ewingii infections.12,19,22,33

Serologic tests are often used in epidemiologic studies of ehrlichial prevalence, and can also be used as a tool when diagnosing canine ehrlichiosis. Depending on the route of exposure, experimentally infected dogs develop E. ewingii antibodies between 7 and 21 days after infection and remain seroreactive for at least 10 months.15 As with any acute infection, the absence of an antibody response during the acute illness does not rule out infection. If an initial serologic test finding is negative, convalescent titers acquired 2 to 3 weeks later are required to confirm a clinical diagnosis through seroconversion or to rule out exposure to E. ewingii. Seronegative infection has not been reported. Just as importantly, because antibody titers persist for months after an acute, tick ticktransmitted infection, positive documentation of antibody titers against E. ewingii can only confirm prior exposure, but does not confirm disease causation or active infection at the time of sample collection. Based upon experimental studies, some dogs may be persistently infected, yet PCR negative, potentially because the organism is sequestered outside of the vasculature or persists in the blood at levels undetectable by PCR.

Ehrlichia ewingii infection in dogs

Multiple serologic methodologies are available for antibody detection, including E. canis immunofl uorescence assay (IFA) and ELISA (SNAP® 4Dx® Plus Test —IDEXX Laboratories, Inc.). Detection of cross-reactive antibodies from dogs infected with E. ewingii on E. canis IFA can be variable; on the other hand, antibodies to E. ewingii are not serologically crossreactive with A. phagocytophilum by any serologic methodology.18,19,21,33-35 Th e E. canis peptides in the commercially available, in-clinic SNAP® 3Dx® Test do not detect E. ewingii antibodies in most infected dogs, whereas these peptides do crossreact with E. chaff eensis antibodies.34,36 For the SNAP® 4Dx® Plus Test kit, addition of an E. ewingii peptide allows, for the first time, serologic detection of E. ewingii specific antibodies. Thus, a single "positive blue dot" on a SNAP 4Dx Plus Test will reflect serologic evidence of exposure to E. canis and/or E. ewingii. In addition, antibodies to E. chaffeensis may also crossreact on the Ehrlichia spot.34 Importantly, serologic testing can be used not only to diagnose a specific infection, but can also be used to rule out infections with other pathogens that might cause similar clinical signs or pathologic abnormalities (e.g., anaplasmosis, bartonellosis, Lyme disease, canine monocytic ehrlichiosis, Rocky Mountain spotted fever) or to characterize atypical or more severe disease manifestation that occurs during co-infections.37,38

Treatment and prevention

Treatment should not be withheld pending disease confirmation from dogs with clinical signs compatible with canine granulocytic ehrlichiosis in endemic regions. Although mortality has not been reported, dogs with illness attributable to canine granulocytic ehrlichiosis often seem to be in marked pain. Fortunately, the clinical and hematologic manifestations of illness usually respond rapidly, within 24 to 48 hours after treatment with tetracycline or doxycycline begins. Although a 14-day course of oral doxycycline

Acute onset of canine granulocytic ehrlichiosis in a young dog

(5 mg/kg every 12 hours or 10 mg/kg once daily) is thought to clear E. ewingii, E. canis is generally treated for 28 days.19,20 Th erefore, unless granulocytic morulae are observed or serologic or PCR evidence confirms the ehrlichial species responsible for infection is E. ewingii, the longer course of therapy is justified. Supportive care, including analgesia for polyarthropathy, may be necessary. Clinical, hematologic, and PCR evidence suggests that dogs may clear E. ewingii spontaneously within weeks to several months, even if treatment is withheld.15,16

Healthy dogs that are seroreactive to any Ehrlichia species do not require immediate treatment. Considerations for diagnostic evaluation and the potential initiation of treatment will become even more important as veterinarians use screening tests that recognize antibodies to any of these three important ehrlichial species (E. ewingii, E. canis, and E. chaffeensis). Healthy dogs with a positive serologic response to any Ehrlichia species antigen might have been infected without evidence of obvious clinical illness, might have recovered spontaneously from a mild clinical infection, or might be chronically infected. In each case, further diagnostic investigation is warranted, beginning with a complete blood count and blood smear evaluation. For healthy dogs with a normal platelet count and the absence of leukocytic morulae, anemia, or hyperglobulinemia, no further evaluation or treatment may be required. If morulae are identified in neutrophils, monocytes, or (rarely) other circulating cell types or a dog is found to have thrombocytopenia, anemia, or hyperglobulinemia, PCR is recommended to confirm active infection and treatment is warranted. Alternatively, some veterinarians will simply opt to treat seroreactive dogs with a 28-day course of doxycycline, which is adequate to treat either an acute infection with E. ewingii or a chronic infection with E. canis.39,40

Ehrlichia infection: Treatment and prevention strategies

There are no vaccines for canine granulocytic ehrlichiosis or for any canine or human ehrlichial infection, so prevention consists of reducing exposure to tick bites. Year-round topical acaricides are recommended for all dogs in tick-endemic regions.41 (Th is is in accordance with the Companion Animal Parasite Council's recommendations.) Rapid manual removal of ticks may be helpful, but the length of time necessary for transmission of E. ewingii following vector tick attachment is unknown.

Spotlight on Ehrlichia ewingii

Public health considerations

Human infection with E. ewingii occurs most often in immunocompromised people, and reported cases are relatively uncommon.22,24,42,43 Ehrlichia chaff eensis causes a more severe and potentially life-threatening illness in humans (monocyticehrlichiosis), is carried by the same tick vector as is E. ewingii, and can also infect dogs.5,9,10,33,44-46 Although experimental infection of dogs with E. chaffeensis has produced only clinically inapparent infection or mild thrombocytopenia, coinfection with other pathogens or concurrent immunosuppression might result in more severe disease manifestations.33,45,46 Certainly, both human granulocytic and human monocytic ehrlichiosis can result in serious morbidity and, in the case of E. chaffeensis infection, mortality.47,48 While there are no reports of direct dog-to-human transmission of either E. ewingii or E. chaffeensis, identification of an infected pet would suggest that the pet's owner may also come into contact with ticks carrying these pathogens. In endemic regions, veterinarians should educate their clientele about the importance of tick control and the possibility of human ehrlichial infections. Veterinarians play a critically important role in the national public health infrastructure relative to the prevention of human tick-borne illness.

Leah A. Cohn, DVM, PhD, DACVIM

Department of Veterinary Medicine and Surgery

College of Veterinary Medicine

University of Missouri

Columbia, MO

Dr. Cohn is a professor of veterinary medicine. She is a specialist in small animal internal medicine and is a diplomate of the American College of Veterinary Internal Medicine. She holds a PhD in veterinary immunology and infectious diseases. She has been a faculty member at University of Missouri since 1995, where she serves as the director of graduate studies and associate department chair. She is currently the president of the American College of Veterinary Internal Medicine. Her research emphasis is infectious and respiratory disease. Current projects include treatment of Cytauxzoon felis infection and immunotherapy for feline asthma.

Edward B. Breitschwerdt, DVM, DACVIM

Department of Clinical Sciences

College of Veterinary Medicine

North Carolina State University

Raleigh, NC

Dr. Breitschwerdt is a professor of internal medicine at NCSU's College of Veterinary Medicine and an adjunct professor of medicine at Duke University Medical Center. He is a diplomate of the American College of Veterinary Internal Medicine. He has published more than 250 articles in scientific journals. His research areas include infectious diseases and immunology, with a particular emphasis on vector-transmitted, intracellular pathogens. Since 1984, he has supervised the Intracellular Pathogens Research Laboratory and co-supervised the NCSU Vector-Borne Disease Diagnostic Laboratory.

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