Shawn Kearns, DVM, DACVIM
Babesia gibsoni is one of several emerging infectious diseases in the United States. There are several genotypes documented, but the most common in the United States is the Asian genotype and will be the focus of this article. The organism is a small pleomorphic, intra-erythrocytic parasite mostly associated with red blood cell destruction. Although not as commonly seen in the New England area, clinicians should be aware of clinical signs and treatment due to the increasing prevalence in previously non-endemic areas as well as due to documentation of presence in fighting dogs, in particular American pit bull terriers.
There is a high prevalence of Babesia gibsoni in American pit bulls, possibly because transmission can occur through direct blood contamination including sharing needles for vaccinations or re-using surgical instruments for tail docking or ear cropping.
The primary mode of transmission of Babesia organisms to dogs is by ticks during feeding. This transmission requires 2-3 days and, in the United States, Rhipicephalus sanguineus is the suspected vector. The normal incubation period is 7-21 days. Transplacental transmission is also thought to occur because B. gibsoni has been detected in puppies as young as 3 days old. Transmission can also occur through direct blood contamination which includes sharing needles for vaccinations or re-using surgical instruments for tail docking or ear cropping. The organism can also be transmitted through dog fighting. All of these potential methods of transmission may help explain the high prevalence in the pit bull breed. In a survey looking at non-American pit bull terriers that were diagnosed via PCR with B. gibsoni, the dogs were of varying breeds and had similar clinical findings. However, most important to note is that 6/10 dogs had a history of fighting with an American pit bull terrier. Because of potential of transmission through blood, all patients being used for blood donation should be tested for Babesia spp.
B. gibsoni is traditionally differentiated from the other common Babesia organism seen in the U.S., Babesia canis, through the appearance on blood smears. The B. canis organisms are typically larger (4-5μm) and appear as bilobed piriform organisms that occur in pairs. B. gibsoni are only 1-2.5um in diameter and appear as single round to oval or ring-shaped organisms in red blood cells.
Acute clinical signs of canine babesiosis include lethargy, fever, hemolytic anemia, thrombocytopenia, and splenomegaly. These are similar findings to what we see with immune-mediate hemolytic anemia and thrombocytopenia so it is important to keep this disease in mind when faced with this presentation. Although acute infection is associated with severe anemia and thrombocytopenia, many dogs survive the acute phase and become chronic carriers. Chronic infections are very common and may be asymptomatic in nature, however, these pets may serve as reservoirs of disease and are therefore still important to identify. One study reported that 55% of American pit bull dogs tested in Alabama were sub-clinically infected and despite being asymptomatic had lower hematocrits and platelet counts compared to dogs that were negative. Although an atypical presentation, there is also one report of immune-complex mediated glomerulonephropathy in a B. gibsoni positive dog (via PCR) so babesiosis should be added to the differential list for protein-losing nephropathy cases.
Definitive diagnosis is made based on the medical history, clinical signs and identification of Babesia within infected erythrocytes, positive serologic results, and detection of amplification of nucleic acid extract from blood or tissues (polymerase chain reaction; PCR). PCR analysis is currently the most sensitive assay for detecting subclinical infections. The cross reactivity between B. canis and B. gibsoni is variable, however. The PCR test for B. canis is specific and will not detect other species while the PCR test for B. gibsoni may detect a variety of species.
Treatment is aimed at the supportive care needed in an acute presentation for the anemia and at trying to clear the organism. Many drugs and drug combinations have been used in the treatment and management of canine babesiosis including babesiacidal drugs (diminazene aceturate, imidocarb diproprionate), antibiotics and anti-protozoal agents. To date, though, no single drug has effectively and consistently eliminated B. gibsoni from infected dogs and the side effects, as well as availability, of some of the medications limit their use. Many infectious disease specialists currently recommend the combination of atovaquone and azithromycin, however, there is evidence for point mutations within the cytochrome b gene of B. gibsoni that may confer resistance to atovaquone. Atovaquone can also be cost limiting for many clients and while compounded forms are available, they may not be as effective. Other drug combinations used with variable success include: clindamycin (alone), doxycycline/metronidazole/enrofloxacin, atovaquone/proguanil/doxycycline, clindamycin/dimenazene/imidocarb
The risk of infection can be reduced for dogs living in endemic areas by providing aggressive tick control with a topical and oral acaricide and flea/tick collar, as well as inspecting them daily for ticks given transmission generally takes 2-3 days of feeding.
In summary, Babesia gibsoni’s prevalence throughout the United States is increasing which mandates an increased awareness among all veterinarians. Although not commonly seen in the New England area, patients with compatible clinicopathologic findings, a history of dog fighting, or those with travel history should be considered for testing.
For more information, please contact Angell’s Internal Medicine service at 617-541-5186 or firstname.lastname@example.org.