Study Case: Canine Granulocytic Ehrlichiosis with Joint Infection

Meli Hanson, DVM; Heather L. Tarpley, DVM; Kenneth S. Latimer, DVM, PhD; Holly Moore, DVM.

Class of 2005 (Hanson) and Department of Pathology (Tarpley, Latimer, Moore) College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388

Signalment - Canine, Labrador retriever, male, 2-year-old.

Presenting problem - Rear limb lameness; the dog would not voluntarily rise after staying in the yard overnight.

History - The dog had a history of jumping out of a truck three days prior to admission to the University of Georgia Veterinary Medical Teaching Hospital, but lameness was not observed by the owner prior to admission. The dog was an indoor / outdoor pet that was confined by a fence when outside.

Physical Examination - Pain, effusion, and crepitus were present bilaterally upon flexion of the stifle joints. Normal reflexes and voluntary motor function were present in all four limbs. Temperature = 101.1ºF, pulse = 102 beats / minute, and respiration = panting. The patient had a body condition score of 4 / 5. All other physical examination findings were unremarkable.

Diagnostic findings -

Laboratory data -

1. Few shift platelets noted.

2. Rare basophilic inclusions in neutrophils consistent with Ehrlichia sp. morulae (Figure 1).

3. Few reactive lymphocytes.

Figure 1. Buffy coat smear from a dog with granulocytic ehrlichiosis. Three segmented neutrophils are present, one of which contains a gray, mulberry-like ehrlichial morula. Also present are two reactive lymphocytes and scattered erythrocytes.

Left stifle joint arthrocentesis -

Color: red

Transparency: cloudy

Total nucleated cells: 6.7 x 10³/ µl

Differential: 50% macrophages, 45% neutrophils, 5% small lymphocytes

Other findings: Neutrophils were mostly nondegenerate and rare cells contained Ehrlichia sp. morulae.

Diagnosis: Suppurative inflammation with evidence of Ehrlichia sp. infection.

Right hock joint arthrocentesis -

Color: colorless

Transparency: cloudy

Total nucleated cells: 41.1 x 10³/ µl

Differential: 49% neutrophils, 38% macrophages, 13% lymphocytes.

Other findings: Neutrophils were predominantly nondegenerate and rare cells contained Ehrlichia sp. morulae.

Diagnosis: Marked, chronic purulent inflammation with Ehrlichia sp. morulae.

Figure 2. Synovial fluid from a dog with granulocytic ehrlichiosis. Six macrophages, two small lymphocytes, and two neutrophils are present. One neutrophil contains a gray, mulberry-like ehrlichial morula.

Radiographic findings - Radiographs revealed effusion of the left stifle and right tarsus. The hip joint, right stifle, and left tarsal joints were unremarkable.

Ehrlichia canis Antibody Titer -

Reciprocal IgG antibody titer: <64

Titers = 64 are considered negative for active infection with Ehrlichia canis. A convalescent Ehrlichia canis antibody titer was unavailable for this patient.

Lyme Disease Antibody Titer -

Reciprocal IgM antibody titer: 64

Reciprocal IgG antibody titer: 128

IgM titers ≤1,024 and IgG titers ≤256 are considered seronegative.

Rocky Mountain Spotted Fever (RMSF) Antibody Titer -

Reciprocal IgM antibody titer: <8

Reciprocal IgG antibody titer: <64

These titer results are considered seronegative for active RMSF infection.

Thrombocytopenia with granulocytic Ehrlichia sp. morulae in peripheral blood.

Suppurative inflammation with intracytoplasmic Ehrlichia sp. morulae within neutrophils, left stifle and right hock.

Problem 1 - Thrombocytopenia with granulocytic Ehrlichia sp. morulae. With respect to granulocytic ehrlichiosis, thrombocytopenia is often a clinicopathologic finding. There are several mechanisms by which this occurs. In the acute phase of the disease, immunologic and inflammatory mechanisms may lead to increased platelet consumption due to platelet-associated IgG and other antibodies. Also, a serum cytokine, platelet migration-inhibition factor (PMIF), has been found in dogs with ehrlichiosis. This cytokine is produced by lymphocytes that encounter infected monocytes and/or neutrophils. Additionally, dogs infected with Ehrlichia sp. have been found to have a reduced platelet half-life due to splenic sequestration.

In the chronic form of the disease, hematologic changes such as thrombocytopenia are generally less severe. Hematopoiesis is suppressed, resulting in pancytopenia.

Problem 2 - Suppurative inflammation with intracytoplasmic Ehrlichia sp. morulae within neurophils, left stifle and right hock. Granulocytic infections are more likely to cause polyarthritis than monocytic or thrombocytic infections. This patient’s reluctance to walk and pain in the left stifle and right hock were most likely due to polyarthropathy. This is supported by physical examination and radiographic findings. With respect to ehrlichiosis, joint disease can result from hemarthrosis or immune complex deposition in the joint, leading to arthritis and neutrophilic migration into the joint. This finding is most commonly associated with granulocytic strains of Ehrlichia sp.

Discussion - Ehrlichiosis is a tick-borne disease caused by obligate intracellular rickettsial parasites. There are three general classes of host cells which various Ehrlichia species infect: mononuclear (E. canis, E. chaffiensis) granulocytic, (E. ewingi, E. equi) and thrombocytic (Anaplasma platys, formerly known as E. platys) cells.

Typical clinicopathologic abnormalities related to ehrlichiosis include thrombocytopenia, leukopenia, low-grade nonregenerative anemia, intracellular morulae, hyperglobulinemia, hypoalbuminemia, increased ALT and ALP activities, and prolonged bleeding times. This patient exhibited morulae and thrombocytopenia. Morulae in granulocytic ehrlichiosis are more likely to be visualized on peripheral blood smears or tissue aspirates than in monocytic or thrombocytic infections.

Other clinical findings common of ehrlichiosis include depression, lethargy, mild weight loss, anorexia, petechiae, anterior uveitis and retinal disease, neurologic dysfunction including seizures, stupor, ataxia with upper or lower motor neuron dysfunction, muscle atrophy, hyperesthesia, and polyarthritis. There are both acute and chronic stages of ehrlichiosis, and each can be asymptomatic, mild, or severe. The pattern of clinical findings depends upon the species and strain of Ehrlichia or Anaplasma sp. that is responsible for infection. Acute infection is associated with fever, depression, lymphadenopathy, thrombocytopenia, and a nonerosive polyarthritis. Though less common, arthritis is also seen in chronically infected animals. In general, the severity of the disease will be increased in young animals, with more virulent strains of Ehrlichia, and if concurrent disease is present.

While this dog did not demonstrate many findings typical of ehrlichiosis, the presence of morulae in neutrophils (in both peripheral blood and joint fluid), thrombocytopenia, and polyarthropathy confirmed a diagnosis of granulocytic ehrlichiosis. Based on these findings, the most likely etiologic agent is Ehrlichia ewingi. This species of Ehrlichia occurs in the southern and mideastern United States, while E. equi, which also causes granulocytic ehrlichiosis, polyarthropathy, and thrombocytopenia, has been reported in California and Oklahoma.

Though only Ehrlichia canis antigen is used in serologic testing to measure serum antibody titers for ehrlichiosis, there is cross-reactivity between E. canis and E. ewingi. This dog theoretically should have had an elevated antibody titer if he were infected with E. ewingi. The lack of a significant titer in this patient may be due to early infection in this dog with either E. ewingi or E. equi, both of which may be associated with morulae in neutrophils. E. ewingi cannot be adequately cultured in vitro. Measurement of a convalescent serum antibody titer is recommended approximately 3 weeks after the first titer is determined. A 4-fold change in titer would confirm infection. Unfortunately, convalescent antibody titers were not available for this patient.

Interestingly, persistent rickettsial infection and recrudescent disease have been demonstrated in dogs experimentally infected with Ehrlichia canis. Ehrlichial organisms were shown to remain in cellular phagosomes, and fusion of phagosomes containing ehrlichial organisms with lysosomes did not occur.

Attenuated and killed subunit vaccines have been developed and have been shown experimentally to prevent lethal infection and to reduce morbidity. Availability of the vaccines is severely restricted due to excessive cost and health risks of producing these agents. Furthermore, the vaccines do not meet modern expectations in their stability and standardization. Development of rickettsial vaccines is also difficult due to the multiple species of Ehrlichia and the low level of cross-protection between species.

Therapy and Patient Outcome - The patient was placed on doxycycline to eliminate the rickettsial infection and on carprofen to control joint pain. Monthly tick prevention was also recommended. Further serologic testing on this patient was not performed and the patient’s response to therapy is unknown.

References

1. Dasch GA: Rickettsial Diseases Program, Department of the Navy Naval Medical Research Institute, National Naval Medical Center, Maryland. Rickettsia, Infection and Immunity: DOI: 10.1006/rwei.1999.0536.

2. Ettinger SJ, Feldman EC: Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat, vol. 1, 5th ed. W.B. Saunders Co., Philadelphia, 2000, pp. 402-406.

3. Foley JE, Foley P, Madigan JE: Spatial distribution of seropositivity to the causative agent of granulocytic Ehrlichiosis in dogs in California. Am J Vet Res 62:1599-1605, 2001.

4. Goldman EE, Breitschwerdt EB, Grindem CB, et al: Granulocytic ehrlichiosis in dogs from North Carolina and Virginia. J Vet Intern Med 12:61-70, 1998.

 5. Neer TM: Canine monocytic and granulocytic ehrlichiosis. In: Greene CE (ed): Infectious Diseases of the Dog and Cat, 2nd ed. W.B. Saunders Co., Philadelphia, 1998, pp.139-147.

6. Skotarczak B: Canine ehrlichiosis. Ann Agric Environ Med 10:137-141, 2003.

Acknowledgment

"Chester" by Hollie Moggridge is from the Artline Etc. website and is used with permission.

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