Detection and molecular characterization of a novel large Babesia species in a dog

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Abstract

Babesia canis has generally been considered the only large Babesia to infect dogs. Here we describe the molecular characterization of a large Babesia species that was detected in the blood and bone marrow of a dog with clinical and hematological abnormalities consistent with babesiosis. Analysis of the 18S rRNA genes revealed a unique sequence that shared 93.9% sequence identity with B. bigemina and 93.5% sequence identity with B. caballi, compared to 91.2–91.6% identity with B. canis canis, B. c. vogeli, and B. c. rossi. Cross-reactive antibodies against B. canis, B. gibsoni (Asian genotype), or B. gibsoni (California genotype) antigens were not detected in acute or convalescent serum samples. The dog was treated with imidocarb diproprionate, which resulted in the resolution of clinical signs, and subsequently Babesia DNA was not detectable by PCR in post-treatment samples. The organism described in this report represents a genetically unique large Babesia sp. and is the eighth genetically distinct piroplasm capable of infecting the domestic dog.

Introduction

Babesia canis is presumed to be the only large Babesia species to infect dogs throughout the world. Historically, the microscopic identification of large (≥3–5 μm) piroplasms in canine erythrocytes was sufficient for the diagnosis of a B. canis infection. Geographic location, differences in pathogenicity, antigenic variation, and differences in vector specificity have all been used to support the existence of three subspecies of B. canis: B. c. canis, B. c. rossi, and B. c. vogeli (Hauschild and Schein, 1996, Hauschild et al., 1995, Lewis et al., 1996, Uilenberg et al., 1989). There are only a few reports that describe the molecular characterization of the large Babesia organisms infecting dogs (Caccio et al., 2002, Carret et al., 1999, Zahler et al., 1998, Criado-Fornelio et al., 2003). Molecular evidence from these reports also supports the existence of three genetically distinct subspecies, namely B. c. canis, B. c. rossi, and B. c. vogeli.

The advent of molecular biology, particularly the polymerase chain reaction (PCR), has resulted in the identification and differentiation of many genetically unique organisms that are morphologically indistinguishable. For example, it was previously accepted that B. gibsoni was the only small piroplasm that infected dogs. However, recent studies on the molecular characterization of the small piroplasms of dogs revealed that there are at least three genetically distinct organisms (Kjemtrup et al., 2000a; Zahler et al., 2000a, Zahler et al., 2000b). It is likely that these data will result in the reclassification of the small canine piroplasms into three distinct Babesia species or possibly two Babesia species and one Theileria species (T. annae). Additionally a recent report described the presence of DNA from a fourth small piroplasm, namely T. equi, amplified by PCR from the blood of dogs in Spain (Criado-Fornelio et al., 2003).

In this report, we describe the 18S rRNA gene sequence of a unique large piroplasm identified in the blood and bone marrow of a dog from North America with clinical babesiosis.

Section snippets

Patient

A 7-year-old female spayed Labrador retriever undergoing chemotherapy for lymphoma was evaluated in May 2002 for fever (T = 105.3 °F), anemia, thrombocytopenia, and leukopenia. Serial hemograms are displayed in Table 1. The dog received vincristine (oncovin) 0.7 mg/m2 IV 1 week prior to presentation. According to the owner, there was no recent history of tick exposure. The dog had not traveled outside of the state of North Carolina and had never received a blood transfusion. The dog had acquired

Results

Anemia, thrombocytopenia and leukopenia were the primary hematologic abnormalities identified (Table 1). Parasites were present within ≈0.002% of mature red blood cells (RBC). These organisms were oval to pyriform and irregular or amoeboid in shape with one or two organisms per infected RBC. Typically, only one oval or irregular/ameboid cell was noted within infected RBCs; however rare cells with two pyriform-shaped organisms joined at a 90° angle were noted. The parasites varied in size from

Discussion

In this report, the authors have described and characterized a novel large Babesia sp. detected in a dog with clinical and hematological abnormalities consistent with babesiosis. The 18S rRNA gene sequence of this organism is genetically distinct from all of the gene sequences deposited in GenBank. The clinical signs identified in this dog were consistent with those described with canine babesiosis caused by other species of Babesia. The cause of the leukopenia in this dog was difficult to

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