Introduction

Feline granulocytic anaplasmosis is a vector-borne disease caused by the Gram-negative, obligate intracellular bacterium Anaplasma phagocytophilum (encompassing the former Ehrlichia phagocytophila, Ehrlichia equi and the human granulocytic ehrlichiosis agent).^1,2 A phagocyto-philum infections have often been described in dogs and horses, but reports of natural infections in cats are less common.¹ The clinical signs are non-specific, but may include fever, lethargy and inappetence.²

A phagocytophilum causes granulocytic anaplasmosis in humans and animals and is transmitted by ticks of the Ixodes scapularis complex.¹ Common reservoirs for transmission are rodents and wild ruminants.¹ The New England region is a highly endemic area for A phagocyto-philum and Borrelia burgdorferi complex, as well as their shared tick vector, I scapularis.³

In Sweden, Bjoersdorff et al were first to report PCR amplification of A phagocytophilum from a cat.⁴ The DNA sequence in a 14-month-old shorthair cat with lethargy and fever was 100% identical to canine and equine A phagocytophilum strains in the same region.⁴ A tentative diagnosis of feline anaplasmosis can be made by detecting intracytoplasmic morulae within neutrophils and eosinophils.² The diagnosis is confirmed by PCR and serology in paired serum samples taken before initiating treatment.²

In the USA, the antibody prevalence for feline A phagocytophilum has been described in the range of 4.3–37.6%.¹ Direct pathogen detection via PCR or identification of neutrophilic morulae in a blood smear was observed in 0–6.9% of cats in the USA.¹ Wide ranges in the prevalence of A phagocytophilum infection in dogs and cats could be explained by large geographical areas studied, with varying environments, tick populations and reservoir host populations.¹ Stray cats have an increased risk of vector contact due to living outdoors and lack of ectoparasite prophylaxis.¹

Case description

Diagnostics

A point-of-care ultrasound showed no free fluid or obvious abnormalities in either the thorax or the abdomen. Venous blood samples were obtained in EDTA and heparin tubes. A packed cell volume was performed on heparinized blood and was within the reference interval (RI) used at this institution. A point-of-care biochemical ana-lysis performed on a NOVA Biomedical pHOx Ultra showed a decreased pCO₂ (26.8 mmHg; RI 33.2–47.3) (Table 1). A complete blood count (CBC) performed on EDTA anticoagulated blood using a Sysmex XN-V hematology analyzer showed a decreased automated platelet count (Table 2). A manual slide review indicated marked platelet clumping and variation in platelet size, causing interference with the machine count. The platelet numbers were considered adequate on manual review of the blood smear. Large platelets may indicate increased thrombopoiesis.⁵ A Fecal Dx Profile with Giardia performed by Idexx Laboratories was negative for antigens of Giardia, hookworm, whipworm and roundworm; however, a number of Toxocara cati ova (3–10) were present. A blood smear, examined microscopically by a board-certified pathologist, detected morulae that were morphologically consistent with A phagocytophilum in approximately 10% of the neutrophils present and at least one eosinophil (Figure 1). A PCR panel for feline vector-borne diseases was performed by Antech Diagnostics (Feline Flea and Tick Borne PCR T965) using EDTA anticoagulated blood and found to be positive for only A phagocytophilum (see  supplementary material). An upper respiratory PCR panel performed by Antech Diagnostics (Feline Upper Respiratory FastPanel PCR T990) using sterile swabs of the oropharynx detected no sign of upper respiratory infection.

Table 1

Point-of-care biochemical analysis in domestic shorthair cat aged 4–6 weeks infected with Anaplasma phagocytophilum

Table 2

Complete blood count in a domestic shorthair cat aged 4–6 weeks infected with Anaplasma phagocytophilum

Figure 1

Feline eosinophil containing Anaplasma phagocytophilum morula (black arrow). Wright’s stain. Image courtesy of P Ewing, Angell Animal Medical Center

Discussion

The present report describes a clinical case of feline anaplasmosis in a pediatric patient with a history of potential tick exposure due to living outdoors as a stray. To the authors’ knowledge, there have been no reports of A phagocytophilum infection in cats aged under 16 weeks in the USA. In addition, there have not been any reported cases in cats with naturally occurring A phagocytophilum morulae identified in eosinophils. In an experimental setting, morulae of A phagocytophilum have been identified in eosinophils of cats within 1 week of inoculation.⁶

The first clinical report of granulocytic anaplasmosis in cats was published in 1999; however, confirmed cases are becoming more frequent.³ Increasing prevalence of anaplasmosis in both animals and humans may be a result of several factors, including increasing awareness of vector-borne diseases and how global climate change affects the distribution and prevalence of reservoir animals and vectors.² In a recent study by Galemore et al, the prevalence of exposure to A phagocytophilum in feral cats in Massachusetts was 9.7%, whereas the prevalence of acute infection was 6.9%.³ Feral cats have a higher prevalence of A phagocytophilum than the previously reported national average of 4.3% in the USA.³ The acute infection in this cat was confirmed by the presence of morula and the positive PCR test.

Clinical disease owing to anaplasmosis in cats can present with the following signs: fever, thrombocytopenia, hyperglobulinemia, lethargy, anorexia, weight loss, vomiting, polyarthritis, lameness and ocular discharge.⁷ In a recent study of A phagocytophilum infection in 16 domestic cats in the northeastern USA, all cats were lethargic, 14/16 were anorexic and 15/16 were pyrexic (>39.4°C, >103°F) on presentation.⁸ This case exhibited common clinical signs on presentation, including fever, anorexia and lethargy. Savidge et al⁸ reported that all cats with morulae identified were hyperthermic. The cat presented here was febrile at 40.8°C (105.6°F), which is consistent with previous studies.

With the exception of the intracytoplasmic morulae identified within neutrophils, reported clinicopathologic abnormalities are non-specific for infection with A phagocytophilum.⁸ Previous studies have diagnosed thrombocytopenia in over half of the cats evaluated (n = 7/11), but all samples had clumped platelets, which falsely decreases the automated platelet counts.⁸ Low platelet counts on mechanical measurements are influenced by platelet clumping, giant platelets and inadequate separation of erythrocytes and platelets, which can lead to falsely low platelet counts.¹ For this reason, a manual platelet estimate was obtained to more accurately reflect adequacy of platelet number. The minimum platelet estimate excluded thrombocytopenia in this patient. The actual incidence of thrombocytopenia in cats is unknown. The cat in this report had a venous hypocarbia, which has not been previously reported as an association with A phagocytophilum infection. This may be a true hypocarbia or artefact secondary to user error, such as exposure of the sample to air or misuse of a clot catcher, which could artificially lower the pCO₂ of the sample.⁹ The fever in this patient may have led to hyperventilation and subsequent hypocarbia.¹⁰ Further research is needed to determine if this is clinically significant.

A feline vector-borne PCR test was performed to evaluate for A phagocytophilum, Bartonella clarridgeaie, Bartonella henselae, Bartonella quintana, Ehrlichia species, Mycoplasma haemofelis, Candidatus Mycoplasma haemominutum, Candidatus Mycoplasma turicensis, Rickettsia felis and Rickettsia rickettsia (see  supplementary material).¹¹ A respiratory PCR test utilizing swab samples (nasal discharge, oropharynx and/or conjunctiva) evaluated for feline calicivirus, influenza H1N1, Bordetella bronchiseptica, Chalmydophila felis, feline herpesvirus 1 and Mycoplasma felis to rule out concurrent respiratory disease due to presence of ocular and nasal crusting.¹¹ A limitation in this case is that feline leukemia virus-antigen and feline immunodeficiency virus-antibody testing were not performed.

A phagocytophilum bacteria infect myeloid cells, primarily neutrophils (occasionally eosinophils), forming intracytoplasmic inclusions called ‘morula’.¹² A manual blood smear from this case confirmed morulae in approximately 10% of the neutrophils examined. The pathologist also confirmed morulae in at least one feline eosinophil. The detection of neutrophilic morulae is seemingly easier with a reported percentage of infected circulating neutrophils in the range of 1–24%.⁶ A diagnosis of A phagocytophilum is made based on clinical suspicions, history of exposure to Ixodes species, identification of morulae within neutrophils, positive serologic and/or PCR results and treatment response.^8,13 The cat in this report met all of the above criteria, confirming an acute infection with A phagocytophilum.

Infections are highest in the late spring and autumn, when both nymph and adult ticks are most mobile.⁸ Transmission to mammals occurs within 24–48 h of tick attachment.⁸ This case presented in the late spring/early summer, consistent with the increased activity of nymph and adult ticks.⁸ Cats, including pediatric cats, living in an Ixodes species area with outdoor exposure may benefit from monthly treatment with an effective and licensed antiparasitic agent.

Previous reports in cats have recommended treatment with 5 mg/kg doxycycline PO q12h for 28 days, but the duration of treatment is variable and clinician-dependent.⁸ Lappin et al reported that with the exception of severe lethargy, clinical signs and hematologic and biochemical abnormalities associated with A phagocytophilum infection were mild and resolved quickly after initiation of tetracycline treatment.¹⁴ The cat in this report received 5 mg/kg doxycycline PO q24h for 21 days, which is consistent with the recommendation in Plumb’s formulary¹⁵ for anaplasmosis in cats. Clinical signs resolved within 24 h and were not noted to recur. Based on this report, pediatric cats with A phagocytophilum infection may present similarly to older cats and response to treatment is rapid.

Conclusions

A phagocytophilum should be included on a differential diagnoses list for any cat that lives in an Ixodes species endemic area with potential tick exposure and presents with acute or intermittent unspecific clinical signs.⁸ Humans are also susceptible to A phagocytophilum infections, making this pathogen relevant to both veterinary and human medicine.¹ Further research is needed to determine the prevalence of A phagocytophilum infection in pediatric cats living in endemic regions.