606 Clinical Aust Vet J Vol 80, No 10, October 2002 Babesia gibsoni infection in three dogs in Victoria CJ MUHLNICKEL a , R JEFFERIES b , UM MORGAN-RYAN b and PJ IRWIN c Small intraerythrocytic parasites were observed in the blood of three related male American Pit Bull Terriers. Two of the dogs, both less than 1-year-old, were anaemic at the time of initial examination and the third, an adult and sire of the two younger dogs, had a normal haemogram and low para- sitaemia. The morphological appearance of the erythrocyte inclusions, analysis of a 450-bp region of the 18S rRNA gene and antibody titres provided evidence that this parasite was Babesia gibsoni, a species not previously reported in Australia. Aust Vet J 2002;80:606-610 AAHL Australian Animal Health Laboratory EDTA Ethylenediamine tetra-acetic acid IFA Immunofluorescent antibody PCR Polymerase chain reaction B abesiosis is a well-recognised haemoprotozoal disease that is transmitted by the bite of an infected ixodid tick. 1 Historically, two species of Babesia parasite were recog- nised in dogs, informally differentiated into ‘large’ and ‘small’ by their morphological characteristics in Romanowsky-stained blood smears. 2,3 T he larger (2.4 μm x 5.0 μm) piroplasm, referred to as Babesia canis , is typically piriform and occurs singly or in pairs within erythrocytes. T hree species of B canis are now recognised based on analysis of the small subunit ribo- somal-RNA gene. 4,5 Two pathogenic forms, B canis rossi and B canis canis are endemic to regions of southern Africa and Europe respectively and B canis vogeli , the least virulent form, has a worldwide distribution and is transmitted by the brown dog tick Rhipicephalus sanguineus . B canis vogeli is found throughout northern and central Australia. 6,7 T he small Babesia infecting dogs has been previously attrib- uted to B gibsoni. It is a pleomorphic organism measuring 1.0 μm x 2.8 μm usually observed singly within erythrocytes. 2,8 Since the first description nearly 100 years ago, B gibsoni has been reported to occur endemically in Asia, parts of Africa and sporadically in North America and Europe. 8,9 Recently, molec- ular phylogenetic analysis of small canine Babesia parasites from around the world has revealed at least three genetically distinct forms 10 and has resulted in a separation of the small piroplasms from dogs originating from Asia, Spain and California into three distinct taxonomic groups. 11 T he clinical severity of babesiosis in dogs is influenced by the species of parasite and to a lesser extent by the age and immune status of the affected individual. T he clinical picture is similar for both small and large babesial infections, in general tending to be more severe in young dogs with a subclinical carrier state common after recovery from infection. Acute babesiosis is char- acterised by the development of pallor, lethargy, pyrexia and anaemia. Host immune responses to the parasite and altered erythrocyte membrane proteins lead to development of anti- erythrocyte antibodies, immune-mediated red cell destruction and a combination of intravascular and extravascular haemol- ysis. 2,12 Changes reflecting this pathogenesis include morpho- logical evidence of immune-mediated haemolysis in the blood smear and many are Coomb’s test positive. 13 T hrombo- cytopenia is also commonly reported, with variable leukocyte changes, influenced by the effects of stress, systemic inflamma- tion and bone marrow stimulation. 2,8,14 Following infection by virulent species such as B canis rossi, dogs may die after devel- oping complications such as disseminated intravascular coagula- tion and multiple organ dysfunction, with acute renal failure, coagulopathies, hepatic failure and neurological signs. 1, 2,15,16 T he definitive diagnosis of babesiosis in dogs depends on the identification of the parasites in Romanowsky-stained blood smears. 1 It has been reported that experienced microscopists can identify babesial infections of 1 parasite per 10 5 to 10 6 erythro- cytes, 17 but this is further facilitated by a clinical suspicion for the infection. By virtue of their small size, B gibsoni strains are not readily observed. T he sensitivity of detection of erythrocytes infected with B canis species is increased when capillary blood from the ear tip, rather than venous blood, is used to make the smears. 7 Canine babesiosis in Australia is currently recognised as being caused by B canis vogeli. In the following report we describe the discovery and characterisation of a small babesial parasite in three related American Pit Bull Terriers in southeast Australia. The morphological appearance of the erythrocytic inclusions, analysis of a 450-bp region of the 18S rRNA gene and antibody titres provided evidence that this parasite was B gibsoni (Asian isolate), a species not previously reported in Australia. Case reports Case 1 A 9-month-old male American Pit Bull Terrier was presented to the veterinarian due to lethargy of 2 days duration. On exam- ination, the dog was found to have mild abdominal pain, splenomegaly and pale mucous membranes. T here was no known access to rodenticides, potential oxidant toxins and no evidence of blood loss noted. Haematological examination of a venous blood sample revealed a regenerative anaemia and thrombocytopenia (Table 1) with spherocytes observed in a peripheral blood smear. An immune mediated aetiology was suspected and immunosup- pressive therapy (prednisolone 2 mg/kg twice daily) was insti- tuted. A haemogram repeated 3 days later revealed an improved haematocrit and the presence of intra-erythrocytic parasites (Table 1). T he organisms were singular and pleomorphic, most commonly with a ring-like configuration (Figure 1). They ranged in diameter from 1 μm to 3 μm and were not typical of canine red cell parasites previously recognised in Australia. a IDEXX Laboratories Melbourne, Unit 124/45 Gilby Rd, Mt Waverley Vic. 3149 b School of Veterinary Biology and c School of Veterinary Clinical Science, Division of Veterinary and Biomedical Science, Murdoch University, South Street, Murdoch, Western Australia.