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 organism. This small Babesia organism
appears to be identical to the original pathogen from Okinawa, Japan, that is endemic in Northern Africa, the Middle East
and Southern Asia. The parasite appears to be a rapidly emerging pathogen in the United States that is currently endemic in
the Pit Bull population in diverse areas of the country east of the Mississippi River.
Photo 3: Babesiosis can cause autoagglutination and a Coombs' positive anemia.
Researchers at Oklahoma State University experimentally infected dogs with blood from two naturally infected Pit Bulls from
Oklahoma. One of the source dogs had been treated twice with imidocarb but still had detectable organisms in the blood. Parasitemia
was detected in all dogs within one to five weeks after innoculation and peaked at four to six weeks before declining. The
degree of parasitemia was 1.9-6 percent, except in the splenectomized dog which reached 16.4 percent before euthanasia. All
dogs developed regenerative anemia and marked thrombocytopenia within one to three weeks post innoculation. Clinical signs
included lethargy, fever and pallor, but were mild or inapparent in some dogs (Photo 4). Parasitemia persisted for three to
four weeks and then became undetectable as the dogs apparently entered a carrier state.
Initial reports of the Babesia gibsoni organism isolated from dogs in the Midwest and Southeast 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
dogs tested in Alabama were subclinically infected. Dogs with subclinical infections had lower hematocrits and platelet counts
and increased mean platelet volume 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 Southeast/Midwest United States isolate.
A third species of small babesia has been isolated from dogs in Northwest Spain. 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. This
organism was first isolated in 2000, and 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.
Photo 4: Clinically affected dogs with severe hemolysis may be icteric.
Babesia spp. sporozoites are present in the salivary glands of the infected tick vector and are transmitted to the dog during
feeding. This transmission requires two or three days. The sporozoites enter the red blood cells and multiply by binary fission.
Although dogs usually mount a good humoral immune response to infection, they are unable to clear the parasitemia and become
chronic carriers. The parasites induce fibrinogen like proteases (FLP) that cause the red blood cells to become sticky, resulting
in capillary sludging. Parasitized cells are sequestered in the spleen, and extravascular and intravascular hemolysis occurs
(Photo 5, p. 10). The incubation period following tick transmission is 10-21 days.
Infected dogs may exhibit either peracute, acute or subclinical signs of disease. Pathogenicity is increased in young dogs,
immunosuppressed dogs, heavily parasitized dogs, and when there is exposure to a virulent strain or concurrent infection with
other tick-borne pathogens (ehrlichiosis, hepatozoonosis, leishmaniasis).
Peracute signs include acute onset of hypotensive shock, vasculitis, extensive tissue damage, hypoxia and death. Signs of
acute disease include fever, lethargy, hemolytic anemia, thrombocytopenia, splenomegaly, lymphadenopathy, icterus and hemo-globinuria.
Less common signs include ascites, peripheral edema, ulcerations, stomatitis, gastroenteritis, CNS signs, acute renal failure
and rhabdomyolysis. Acute infections of virulent strains of Babesia canis have been associated with induction of the systemic
inflammatory response syndrome (SIRS) and multiple organ dysfunction syndrome (MODS) secondary to massive immunostimulation
and cytokine release. Signs of MODS can include coagulopathies (DIC), adult respiratory distress syndrome (ARDS), cerebral
dysfunction and acute renal failure.
Photo 5: Babesiosis can cause a severe hemolytic crisis with intra-vascular hemolysis and hemoglobinuria.
Most dogs in the Uinted States that are seropositive for Babesia canis or B. gibsoni have subclinical infections. However,
severe disease has been seen in puppies born to seropositive dams, and has been reported in a dog that received a blood transfusion
from an asymptomatic dog with a positive babesia titer. Babesiosis has also been implicated as an underlying factor in cases
of acute hemolytic anemia in dogs.
Babesia organisms are transmitted to dogs by ticks during feeding. Known vectors outside the United States include Haemophysalis
bispinosa and H. longicornis. In the United States, Rhipicephalus sanguineus is the suspected vector for both species. 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. It is possible that biting flies or other blood-sucking insects
may be capable of transmission as well.