Canine babesiosis continues to create challenges for practitioners

Canine babesiosis continues to create challenges for practitioners

Photo 3: Babesiosis can cause autoagglutination and a Coombs' positive anemia.
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.

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.

Photo 4: Clinically affected dogs with severe hemolysis may be icteric.
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.

Pathogenesis 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).

Photo 5: Babesiosis can cause a severe hemolytic crisis with intra-vascular hemolysis and hemoglobinuria.
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.

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.

Transmission 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.