Based on morphologic appearance, this California organism was initially identified as Babesia gibsoni. However, recent evidence,
based on PCR assays and gene sequencing, has identified at least three different small Babesia organisms that infect dogs.
The organism initially identified as Babesia gibsoni has now been shown to be genetically distinct from small Babesia isolates
originating from Japan, Sri Lanka and Malaysia. The genetic sequence of the California isolate most closely resembles Theileria
species and will likely be renamed in the future.
Experimental inoculation of dogs with the California isolate causes acute parasitemia, lethargy, anemia, thrombocytopenia
and hemoglobinuria. Histopathologic lesions include diffuse nonsuppurative periportal and centrilobular hepatitis, multifocal
necrotizing arteritis, membranoproliferative glomerulonephritis, reactive lymphadenopathy, diffuse erythrophagocytosis and
In experimentally infected dogs, treatment with diminazine aceturate reduced parasitemia to undetectable levels, but parasitemia
rapidly recurred following splenectomy. Parasitemia ranged from 0.9-54.8 percent in these dogs. In naturally infected dogs,
parasitemia has been reported to be 5-40 percent.
Cases of naturally infected dogs in California were initially misdiagnosed as immune-mediated hemolytic anemia because the
parasites were not recognized on the blood smears of infected dogs and the dogs had positive direct Coombs' tests. Treatment
of the California isolate has not been rewarding. Although anti-babesial drugs may reduce circulating parasitemia, no treatment
has been found that will eliminate the carrier state. Even if recovered dogs do not succumb to chronic disease, they serve
as a potential reservoir for infection of other dogs.
In 1999, Babesia gibsoni infection was reported in nine dogs from North Carolina. Since that time, infections have been reported
in dogs from Oklahoma, Alabama, Georgia, Indiana, Missouri, Wisconsin, Michigan and Florida. Almost all of the reported cases
have occurred in American Pit Bull Terriers or American Staffordshire Terriers.
A recent study indicated that the DNA sequences of Babesia isolates from dogs in Oklahoma, North Carolina, Missouri, Indiana
and Alabama were identical to the sequences of isolates from dogs in Japan, Malaysia and Sri Lanka but distinct for the California
This small Babesia organism appears to be identical to the original pathogen from Okinawa, Japan, that is endemic in northern
Africa, Middle East and southern Asia.
The parasite appears to be a rapidly emerging pathogen in the United States that is currently endemic in the American Pit
Bull Terriers in diverse areas of the country east of the Mississippi River. Infected dogs develop regenerative anemia and
marked thrombocytopenia within one to three weeks of infection.
Clinical signs included lethargy, fever and pallor or may be mild or inapparent in some dogs. Parasitemia persisted for three
to four weeks and then became undetectable as the dogs apparently entered a carrier state.
Not as pathogenic
Initial reports of the Babesia gibsoni organism isolated from dogs in the Midwestern and Southeastern United States seem to
indicate that it is not as pathogenic as the California isolate.
Although acute infection is associated with severe anemia and thrombocytopenia, many dogs survive the acute phase and become
chronic carriers. A recent study reported that 55 percent of American Pit Bull Terriers tested in Alabama were subclinically
infected. Dogs with subclinical infections had a lower hematocrit and platelet count compared to dogs that were negative.
The tendency to relapse or exhibit signs of vasculitis, protein-losing nephropathy or hepatic failure that is seen in dogs
infected with the California isolate has not been reported in dogs chronically infected with the Midwestern or Southeastern
Another Babesia species
A third species of small Babesia has been isolated from dogs in Northwest Spain in 2000. This parasite has been provisionally
named Theileria annae, and closely resembles Babesia microti, a small Babesia pathogen affecting humans and rodents. All of
the infected dogs in Spain exhibited intense regenerative hemolytic anemia, and some also had evidence of renal failure.
Further research is needed to characterize this new species which is genetically distinct from the California isolate and
the other small Babesia organism from the United States.
Babesia organisms are transmitted to dogs by ticks during feeding. This transmission requires two to three days. Known vectors
outside the United States include Haemaphysalis bispinosa and H. longicornis. In the United States, Rhipicephalus sanguineus
is the suspected vector. The risk of infection can be reduced for dogs living in endemic areas by providing aggressive tick
control with a topical acaricide and flea/tick collar as well as inspecting them daily for ticks.
Transplacental transmission from dam to offspring is thought to occur because Babesia gibsoni has been detected in puppies
as young as 3 days old. The incubation period is seven to 21 days. Transmission can also occur through direct blood contamination.
Blood donors should be tested negative for babesiosis. PCR analysis is currently the most sensitive assay for detecting subclinical
infections. The cross reactivity between B. canis and B. gibsoni is variable. The PCR test for B. canis is specific and will
not detect other species. The PCR test for B. gibsoni, however, will detect a variety of species. Blood contamination can
also occur through practices such as sharing needles for vaccinations or reusing surgical instruments for tail docking or
ear cropping. The organism can also be transmitted through dog fighting.