CAPC Recommendations:
Vector Borne Diseases:
Canine Babesiosis
Species
Canine
Babesia canis
Babesia gibsoni
Babesia conradae
Feline
Babesia spp. infection has not been reported in domestic cats in the United States.
Overview of Life Cycle
- Dogs become infected with Babesia spp. when feeding ticks inoculate infective sporozoites. After inoculation, Babesia sporozoites invade erythrocytes where they multiply asexually by binary fission. The resultant merozoites rupture the red blood cell and go on to invade additional erythrocytes.
- Following ingestion by the tick, gamogony and sporogony occur, resulting in production of sporozoites.
- Babesia canis is thought to cycle primarily between domestic dogs and Rhipicephalus sanguineus, the brown dog tick. However, Dermacentor variabilis, the American dog tick, also has been shown to be a competent vector.
- Babesia gibsoni is also transmitted by R. sanguineus although this cycle has not yet been confirmed for strains of B. gibsoni in the United States.
- Fighting between dogs is considered a more likely mode of transmission of B. gibsoni between dogs and may account, in part, for the high prevalence of infection in American Staffordshire and American pit bull terriers.
Stages (see image on right)
- Piroplasms of Babesia spp. can be found within erythrocytes on stained blood smears.
- Babesia spp. in dogs may be differentiated by their size and general morphology. Babesia canis trophozoites are larger and often found in pairs, whereas those of B. gibsoni and B. conradae are small, pleomorphic, ring-shaped organisms.
Disease
- Dogs with babesiosis often present with moderate to severe hemolytic anemia and clinical illness characterized by fever, anorexia, depression, pallor, splenomegaly, and a bounding pulse. However, disease is considered uncommon in healthy, spleen-intact adult dogs.
- In general, disease is less severe with B. canis infection than with B. gibsoni infection.
- Coinfection with Babesia spp. and other tick-borne pathogens has been documented and when present, may complicate interpretation of the clinical presentation and diagnostic assays.
Incidence and Prevalence
- Canine infection with Babesia spp.is common throughout the world. Infection is more common in areas where tick infestation pressure is high and when routine acaricide use is not practiced. Although B. canis is perhaps the most well known, infection with B. gibsoni occurs more commonly in some areas and in some breeds of dogs.
- Babesia spp. are reported from throughout the United States, a distribution that follows that of the vector tick R. sanguineus. However, infections are more commonly seen in southern states.
- Infection with B. gibsoni occurs more commonly in certain breeds, such as American Staffordshire and American pit bull terriers, a phenomenon that is thought to be attributable to direct dog-to-dog transmission via bite wounds.
Host Associations and Transmission Between Hosts
- Babesia spp. are transmitted to dogs via tick feeding. Ticks are not able to transmit infection immediately upon first attachment to a host; they require a period of approximately 24 to 48 hours of initial feeding before organisms are able to pass across the salivary glands and into the vertebrate host.
- Rhipicephalus sanguineus and D. variabilis are the most common arthropod vectors of babesiosis.
- Direct transmission of Babesia spp. from an infected to a naïve animal can also occur following blood transfusion or iatrogenic inoculation with contaminated needles or surgical instruments. Direct dog-to-dog transmission via dog bites is thought to be an important means of transmission for B. gibsoni and may account for the apparently high prevalence of this organism in American Staffordshire and American pit bull terriers.
Prepatent Period and Environmental Factors
- Sporozoites enter the blood during tick feeding. Merozoites become evident within erythrocytes on blood smears in approximately 1 to 3 weeks.
- Although clinical disease may resolve, infections with Babesia spp. organisms often become chronic in dogs. Even after appropriate therapy, infection can persist for the life of the dog.
- Infection with additional strains and/or species of Babesia spp. also may occur when tick exposure or bite wound transmission continues.
Site of Infection and Pathogenesis
- Ticks inoculate sporozoites of Babesia spp. directly into the bite wound. Organisms invade circulating erythrocytes and begin multiplying asexually.
- Rupture of red blood cells during asexual replication leads to intravascular hemolysis and hemolytic anemia. Immune-mediated clearance of parasitized erythrocytes contributes to the anemia.
- Thrombocytopenia, hypoalbuminemia, and hyperglobulinemia also occur.
- Splenectomy and immunosuppression exacerbate both parasitemia and clinical disease from infection with Babesia spp.
Diagnosis
- Definitive diagnosis of Babesia spp. infection relies on identification of piroplasms in erythrocytes on stained blood smears. Although organisms may be numerous in blood smears from acutely infected animals, parasitemias are often low, and organisms may rarely be found in samples collected from chronically infected dogs or asymptomatic carriers.
- Serology may be helpful in identifying the presence of antibodies to B. canis, which cross-react with those to B. gibsoni, thereby allowing indirect detection of previous infection with either organism.Indirect fluorescent antibody (IFA) assays to determine IgM and IgG titers to B. canis are available through diagnostic laboratories.
- Particularly in acute infections, dogs with evidence of Babesia spp. may be serologically negative, necessitating repeat testing using convalescent sera.
- In recent years, molecular diagnosis of Babesia spp. infection in dogs and cats via polymerase chain reaction (PCR) of whole blood has become readily available. However, results should be interpreted with caution because the techniques used in different diagnostic laboratories vary. Amplification of related organisms by nonspecific primers can result in false-positive reactions. Conversely, apparent false-negatives may occur if extraction procedures fail to remove PCR inhibitors present in a blood sample or if the level of circulating parasitemia falls below the level of assay detection due to a normal decrease in circulating organisms or temporary suppression of infection following treatment. To maximize the utility of molecular diagnostics, blood samples should be collected early in the course of clinical disease and before the initiation of chemotherapy and should be submitted to experienced diagnostic laboratories with stringent quality control measures in place.
Treatment
- Dogs infected with B. canis usually respond to treatment with imidocarb diproprionate at a dose of 6 mg/kg IM administered twice at 14-day intervals. A higher dose administered once is recommended occasionally but can result in neurotoxicity.
- Babesia gibsoni is considered more difficult to treat than B. canis. Recommended treatment protocols for B. gibsoni involve combining atovaquone (13 mg/kg PO q8h for 10 days) with azithromycin (10 mg/kg PO q 24 h for 10 days); imidocarb diproprionate is not considered effective for treating B. gibsoni.
- Babesia spp. infections can also be treated with pentamidine isethionate (16 mg/kg IM q24h for 2 doses).
- Treatment for babesiosis reduces parasitemia and supports resolution of clinical signs, although the infection itself may not be eliminated. Dogs diagnosed with Babesia spp. should be considered to be permanent carriers of the infection.
Control and Prevention
- Vaccines are not available to prevent Babesia spp. infection of pets in North America. Stringent adherence to routine application of effective acaricides is essential for preventing infection and disease caused by these organisms.
- Babesia spp. are primarily transmitted by R. sanguineus, a tick that can survive indoors and establish long-term infestations in homes and kennels. Although more common in warmer months and milder climates, this tick can occur anywhere there are dogs. CAPC recommends that all pets be maintained on tick-control products year-round.
- Attached ticks found on pets should be removed promptly to prevent transmission of any pathogens they may harbor. To avoid both zoonotic infection and accidental inoculation of agents into the pet during the removal process, ticks should be retracted using forceps or a commercial tick-removal device, and care should be taken to avoid contact with tick contents, ideally by wearing gloves. Careful attention to handwashing following tick removal is also recommended.
- Blood donors should be screened for infection with Babesia spp. by serology, PCR, and blood smear, and any dogs testing positive excluded.
- Discourage dog fighting to prevent direct dog-to-dog transmission of B. gibsoni.
Public Health Considerations
- Babesia canis and B. gibsoni are not known to infect people.
- People can become infected with Babesia microti and other related Babesia spp. that normally infect rodents. The source of infection to people is an Ixodes spp. tick; dogs are not involved in the maintenance or transmission of this zoonotic infection.
