Canine babesiosis (Proceedings)

Canine babesiosis is a tick-borne disease caused by a hemoprotozan parasite belonging to the order Piroplasmida within the phylum Apicomplexa. Historically, two Babesia spp. were known to infect dogs, and these were morphologically differentiated into "large" and "small" Babesia. With the advent of molecular diagnostics, we now recognize that there are at least 8 species of piroplasms that can infect dogs.

The large babesia, B. Canis, has 3 subtypes that are most likely separate, but closely related species: B. canis canis, B. Canis vogeli, and B. Canis rossi. A new species of large Babesia was recently isolated from a dog in North Carolina and it is currently under investigation as the fourth known species of large Babesia that can infect dogs. Large babesia are endemic in Europe; Southern Africa; Asia; and North, Central and South America. The most virulent type is B. canis rossi, which is known to cause severe hemolytic anemia, thrombocytopenia, fever, and shock in affected dogs, primarily in South Africa. B. canis canis is endemic in Europe and the Middle East and is intermediate in pathogenicity, causing severe disease in some dogs and subclinical carrier states in others. The least pathogenic type is B. canis vogeli, the species most commonly found in dogs in the United States. Although the different types of large babesia differ in virulence, geographical location, and tick vector, they are identical in appearance as bi-lobed, pear shaped organisms in the red blood cell.

In the United States, babesiosis is endemic in the greyhound population. In one study, 46% of 393 racing greyhounds in Florida tested positive, whereas 0% of 50 pet dogs were positive. In another study of dogs in California animal shelters, 13% were positive for Babesia canis. Every year thousands of retired racing greyhounds are adopted into households throughout the USA. These dogs, as well as stray animals, provide a potential reservoir for the disease.

Historically, the only "small" babesia known to infect dogs was Babesia gibsoni, a piroplasm that was first reported in India a century ago and is now endemic in dogs in the Far East. In 1991, a small babesia was reported in dogs from California, and was assumed to be B. gibsoni. Genetic sequencing through polymerase chain reaction has shown this pathogen to be separate and distinct from B. gibsoni. Initially, it was called "the California strain" of B. gibsoni, but it is now a new species called Babesia conradae. This parasite causes significant morbidity in infected dogs and is often more resistant to treatment than other forms. The parasite is most closely related to small piroplasms affecting humans in the Western United States. To date, molecular diagnostics have identified at least 4 distinct piroplasms affecting dogs: 1) Babesia gibsoni (formerly called "the Asian strain") that affects dogs in the Far East and is an emerging pathogen in the USA and other countries worldwide. 2) Babesia conradae - "the California strain" that has only been diagnosed in dogs from California. 3) Theileria-annae, a parasite similar to Babesia microti that causes renal disease in dogs from Northwestern Spain, and 4) Babesia equi, a parasite found in horses and other livestock but also reported in dogs from Spain.

Molecular characterization of these parasites continues to be elucidated, and further reclassification may still be needed. In general, the small canine piroplasms are more closely related to Theileria spp than to Babesia spp. based on study of the 185 rRNA gene locus. Historically, Theileria spp. have been differentiated from Babesia spp because Theileria has an extra-erythrocytia multiplication stage (schizogony), whereas Babesia does not. Other differences include transovarial transmission in Babesia vectors and only transstadial in Theileria. Also, Theileria spp commonly form 4 daughter cells within the RBC, resulting in a "Maltese Cross", while Babesia spp. Commonly produce two daughter cells (merozoites) through budding. The only piroplasm affecting companion animals that is known to have an extra-erythrocytic phase (schizogony) is Cytauxoon felis, which is closely related to Theileria spp.

Pathogenesis

Babesia spp. sporozoites are present in the salivary glands of the infected tick vector. They are transmitted to the dog during feeding. This transmission requires 2-3 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 FLP (fibrinogen like proteases) that cause the red blood cells to become sticky, resulting in capillary sludging. Parasitized cells are sequestered in the spleen, and extravascular and intavascular hemolysis occurs. 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, leismaniasis).

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 hemoglobinuria. 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 US that are seropositive for Babesia spp. 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. Naturally occurring and experimentally infected cases of B gibsoni have routinely exhibited signs of acute hemolytic anemia and thrombocytopenia. B canis has also been implicated as an underlying factor in cases of acute hemolytic anemia in dogs.

Diagnosis

Babesiosis is usually diagnosed by finding parasitized red blood cells on a blood smear stained with Wright's, Giemsa, or Diff-Quick® stain. Because parasitized cells may be more prevalent in capillary blood, the smear should be made from the pinna of the ear or the nail bed to increase the chances of finding them. Parasitized cells can also be found on buffy coat smears, as the parasitized cells are just below the white blood cells in a centrifuged capillary tube.

Serology can be used to detect dogs with occult parasitemia. B canis and B gibsoni cross react on the IFA test and must be differentiated based on parasite recognition on the blood smear unless PCR testing is available (research labs only). IFA titers > 1:80 are considered positive for B canis infection, and titers > 1:320 are generally seen with B gibsoni infection.

Treatment

There is only one drug approved for definitive treatment of babesiosis in the USA - imidocarb dipropionate (Imizol, Schering-Plough, Union, New Jersey). The recommended dosage is 6.6 mg/kg IM or SC, repeated in 2 weeks. This drug is also effective against Ehrlichia canis and Hepatozoon canis, and is the drug of choice in multiple parasite infections. Parasitemia and clinical signs of disease are usually eliminated within 24-48 hours after administration, but many dogs will develop a subclinical carrier state. Side effects of imidocarb include pain at the injection site, salivation, lacrimation, gastrointestinal signs, and tremors. Pre-treatment with atropine (0.04 mg/kg SQ) may prevent these cholinergic side effects. Imidocarb is very effective against B canis but less effective against B gibsoni.

The most effective treatment to date is actually clearing parasitemia and resulting in a negative PCR test in 83% of the dogs tested is a combination of atovaquone (13.5 mg/kg PO q 8 h with a fatty meal) and azithromycin (10 mg/kg PO q 24 hours) for 10 days. Dogs should be retested by PCR in 60 days and retreated if still positive. Although this treatment is effective, it is also expensive ($300 - 900/dog).

Another babesiacidal drug is diminizene aceturate. This drug is not available in the USA, but is available in other countries. A single injection of 3.5 mg/kg IM will clear parasitemia and improve clinical signs in dogs with B canis. A higher dosage (5 - 7.5 mg/kg IM, repeated 2-3 times at 2 week intervals has been recommended for dogs with B gibsoni.

Other drugs that may be effective against babesiosis include clindamycin and metronidazole. Clindamycin has been used to treat B microti in people, and metronidazole (25-65 mg/kg q 24 h for 10 days) resulted in clinical improvement in one group of dogs with B gibsoni. Doxycycline has also shown some benefit in preventing or reducing parasitemia. All 3 drugs given together were effective in clearing parasitemia in a group of dogs with B. gibsoni.

Supportive care is required in dogs with acute or peracute disease. Treatment includes intravenous fluid therapy, blood transfusion, correction of acidosis, and management of concurrent disease. In one study, 85% of babesia infected dogs with hemolytic anemia had a positive direct antiglobulin (Coomb's) test. In addition, 21% of these dogs exhibited autoagglutination. The babesia parasite induces an immune mediated response against the red blood cell membrane resulting in hemolytic anemia. Glucocorticoids (Prednisone 2 mg/kg q 12 h) are usually required in these cases to stop the hemolysis. Once the parasitemia has been cleared, the glucocorticoids can usually be tapered off over 2 - 3 months.

Prevention

The best method of prevention of disease is by aggressive control of the tick vector. In endemic areas, dogs should receive a topical acaricide (Fipronil) as well as a flea/tick collar. Owners should inspect their dogs daily for ticks, as disease transmission requires tick attachment for 2-3 days, and prompt removal of ticks within 24 hours will prevent disease.

If dogs must travel in endemic areas, imidocarb (6 mg/kg SQ) has been shown to prevent infection for 2 weeks. Doxycycline (10 mg/kg q 12 hours) also prevented infection in experimentally inoculated dogs, and a lower dose (5 mg/kg q 24 h) resulted in reduced signs of disease.

A vaccine is available in France (Pirodog), that effectively prevents against infection by the homologous strain for 5-8 months. Unfortunately, this vaccine affords no protection against other strains.

Kennels are at risk for transmission of disease, and young puppies are at risk for developing acute anemia. To prevent exposure, aggressive tick control programs should be combined with serologic testing, quarantine, and treatment of all new animals introduced to the kennel.

Dogs with a positive babesia titer should never be used as blood donors. In endemic areas, blood donors can be splenectomized to reveal occult hemotogenous parasitic infections.

Continued research is needed to determine the best methods of prevention and treatment of this emerging tick-borne disease in the USA.