Clinical disease
Anemia and thrombocytopenia are the most consistent clinicopathologic abnormalities detected in dogs with babesiosis independent
of which Babesia species causes the infection.45 Both the anemia and thrombocytopenia are associated with immune-mediated destruction and can be nearly indistinguishable
from idiopathic immune-mediated cytopenias. However, variations in the presentations among Babesia species infections do occur. Babesia canis vogeli infection is most commonly associated with anemia and thrombocytopenia, and severe clinical disease is more frequently detected
in puppies than in adults.23,24 Adults can have subclinical infections. Babesia canis canis is associated with disease in both puppies and adult dogs, and signs associated with anemia are the predominant presenting
complaint.29,31,32 However, many adult carriers of B. canis canis are relatively asymptomatic.46
Babesia canis rossi is also associated with clinical disease in both puppies and young adults.33,47 Besides the classic signs associated with hemolytic anemia, B. canis rossi is the subspecies of B. canis that seems to be associated with the most diverse range of clinical diseases and presentations. Some dogs have neurologic
signs that can either be associated with hypoglycemia or possibly cerebral ischemia. Another subset of dogs have severe metabolic
acidosis and azotemia.34,48 Atypical babesiosis associated with B. canis
rossi is usually associated with a poor prognosis.
As with the Babesia canis subspecies, anemia and thrombocytopenia are the most consistent hematologic abnormalities detected in dogs infected with
small piroplasms. Babesia gibsoni infection can have a diverse range of clinical manifestations and does not appear to be associated with age.49 Some dogs with B. gibsoni infection suffer from fatal anemia while other dogs that have presumably recovered from the acute stages of infection have
normal hematocrit results. In some studies, thrombocytopenia was a more common hematologic abnormality than anemia was.4 Infection with the western piroplasm detected in Southern California was associated with a mortality rate of nearly 60%.41,50
Theileria annae infection is associated with azotemia in up to 40% of the cases reported, and azotemia was associated with mortality in those
cases.42,43
Diagnosis
Figure 1A. Canine red blood cells infected with Babesia canis (Wright's-Giemsa; 500X).
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No one test is clearly the best for definitively diagnosing babesiosis as none of the tests are 100% sensitive or 100% specific.
Three basic types of tests are available to help diagnose babesiosis: light microscopy, serology, and molecular testing.51 Light microscopy (Figures 1A & 1B) is widely available and has a high specificity when the blood smears are evaluated by properly trained individuals, but
it probably has the lowest sensitivity of the available tests.52 Because of the high degree of morphologic variation of the piroplasms during natural infections, microscopy cannot always
allow accurate identification of the Babesia species or subspecies present.26,36
Serologic testing detects antibodies that react with Babesia species antigens and includes immunofluorescence assays (IFAs) and enzyme-linked immunosorbent assays (ELISAs). Although
many ELISA-based tests for canine babesiosis have been described, an IFA against whole parasite preparations is the primary
serologic test available to practitioners. Since an IFA relies on whole parasite preparations and each laboratory uses different
strains for antigen preparation, inherent variability can exist among laboratories. Serologic testing appears to be sensitive,
but the sensitivity is not 100% as evidenced by the recognition of babesiosis in dogs that do not have detectable antibodies
against Babesia species antigens.22,23,35 Individual laboratories establish cutoff titers indicative of exposure to Babesia species, so do not compare the results of different laboratories.53
Figure 1B. Canine red blood cells infected with Babesia gibsoni (Wright's-Giemsa; 750X).
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Testing for Babesia species DNA in clinical samples has improved our ability to confirm Babesia species infections and is the only way to definitively determine the species or strain of Babesia present.36 Most of these tests are based on the polymerase chain reaction (PCR), which exponentially amplifies a specific fragment
of DNA.54-56 A positive PCR test result is indicative of current infection, when the appropriate controls have been used. As with IFAs,
individual laboratories perform different PCR tests that have different sensitivities and specificities. It is imperative
that the laboratory performing the PCR is strictly adhering to the practices used to avoid contamination and is using appropriate
positive and negative controls. Unfortunately, not many studies have been done to determine the clinical sensitivity of PCR
testing for diagnosing canine babesiosis. One PCR test had an overall sensitivity of 87% when used to detect B.
gibsoni in asymptomatic carrier dogs and detected 100% of the carriers when two consecutive tests were performed 30 days apart.5
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