Canine babesiosis continues to create challenges for practitioners

Parasites of the genus Babesia are hemoprotozoan organisms that can infect red blood cells of vertebrate hosts. Although more than 100 species of Babesia have been identified, only two have been reported to infect dogs: Babesia canis and Babesia gibsoni. These organisms have traditionally been differentiated based on their appearance in stained blood smears. Babesia canis organisms are larger, and appear as bilobed piriform organisms that often occur in pairs and are about 4-5 um in length (Photo 1). Babesia gibsoni organisms are smaller (1-2.5 µm in diameter) and appear as round to oval or ring-shaped organisms, usually single, in red blood cells (Photo 2). Babesia canis is endemic in Europe, Southern Africa, Asia and the Americas. Babesia gibsoni is found in Southern Asia, the Middle East and Northern Africa. Recent reports indicate that Babesia gibsoni is an emerging vector-borne disease among dogs in the United States, as well.

Photo 1: Babesia canis in the red blood cells of an infected dog. Note the paired piriform shaped organisms.

Babesia canis - "large" babesia

There are three subtypes of Babesia canis: B. canis canis, B. canis vogeli and B. canis rossi. These strains differ in virulence, geographic location and tick vector, but are identical in appearance. In the United States, the most common strain is B. canis vogeli, which is the least pathogenic form. Although severe hemolytic anemia, thrombocytopenia and life-threatening disease have been reported in young dogs, heavily parasitized dogs and dogs transfused with infected blood; most dogs infected with B. canis vogeli in the United States are subclinical carriers.

Although not highly pathogenic, the B. canis vogeli organism appears to be endemic in the Southeastern United States, particularly among Greyhounds. In 1992, Taboada found that 46 percent of Florida racing Greyhounds had positive titers for Babesia canis. In another study of dogs in California animal shelters, 13 percent were positive for Babesia canis. In 1995 alone, 16,000 Greyhounds were adopted into households throughout the United States. These dogs, as well as stray animals, provide a potential reservoir for the disease. A recent study differentiating the three subspecies of Babesia canis by polymerase chain reaction and restriction enzyme analysis suggests that they may actually be closely related but distinct and separate species. The most pathogenic type, B canis rossi, is endemic in South Africa. Babesia canis canis is found in Europe and parts of Asia and is considered intermediate in pathogenicity. A fourth type of large canine piroplasm was recently identified in a dog from North Carolina undergoing chemotherapy for lymphosarcoma. Although the organism was morphologically identical to B. canis, PCR and antibody tests for known canine species were negative and gene sequencing showed marked differences from B. canis and B. gibsoni gene fragment analysis.

Photo 2: Babesia gibsoni organisms can easily be overlooked in a blood smear because they are small, usually single and pleomorphic. Most of the dogs in the United States reported to have Babesia gibsoni infection have been American Pit Bull terriers. Subclinical disease and a carrier state are common in this breed.

"Small" babesia-gibsoni and others

Small babesia organisms infecting dogs were first identified as Babesia gibsoni in dogs and jackals from India in 1910. This parasite is now considered endemic among dogs in Northern Africa, the Middle East and Southern Asia. The first case of a small babesia organism in a dog in the United States was reported in 1968, but this dog was a Bull Terrier from Malaysia that had been infected prior to shipment to the United States. In 1991, Conrad reported 11 dogs from Southern California that were infected with a small babesia organism and developed severe hemolytic anemia and thrombocytopenia. Based on morphologic appearance, the California organism was initially identified as Babesia gibsoni. However, recent evidence, based on PCR assays and gene sequencing, has identified at least three different small babesia organisms that infect dogs.

The organism initially identified by Conrad et al as Babesia gibsoni has now been shown to be genetically distinct from small babesia isolates originating from Japan, Sri Lanka and Malaysia. The genetic sequence of the California isolate most closely resembles Thelieria spp, and will likely be renamed in the future.

Experimental inoculation of dogs with the California isolate causes acute parasitemia, lethargy, anemia, thrombo-cytopenia and hemoglobinuria. Histopathologic lesions include diffuse nonsuppurative periportal and centrilobular hepatitis, multifocal necrotizing arteritis, membranoproliferative glomerulonephritis, reactive lymphadenopathy, diffuse erythrophagocytosis and extramedullary hematopoesis. In naturally infected dogs, parasitemia has been reported to be 5-40 percent. Cases of naturally infected dogs in California were initially misdiagnosed as immune-mediated hemolytic anemia because the parasites were not recognized on the blood smears of infected dogs and the dogs had positive direct antiglobulin (Coomb's) tests (Photo 3). It has been suggested that Giemsa or Field staining of blood smears is superior to Wright's or Diff-quick stains, and may facilitate detection of parasites. Treatment of the California isolate has not been rewarding. Although anti-babesial drugs may reduce circulating parasitemia, no treatment has been found that will eliminate the carrier state. Relapses were seen in two out of five dogs treated by Conrad et al, and six out of 11 died or were euthanatized. The chronic antigenic stimulation associated with persistent infection can result in chronic glomerulonephritis, hepatic failure or vasculitis. Even if recovered dogs do not succumb to chronic disease, they serve as a potential reservoir for infection of other dogs.

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.

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).

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.

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.

Transplacental transmission from dam to offspring is thought to occur, as Babesia gibsoni has been detected in puppies as young as 3 days old. Transmission can also occur through direct blood contamination. Blood donors should be tested negative for babesiosis. Pit Bull type dogs or dogs with positive titers should not be used as blood donors.

Blood contamination can also occur through practices such as 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.

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. IFA titers > 1:80 are considered positive for B. canis infection, and titers > 1:320 are generally seen with B. gibsoni infection. PCR analysis is currently the most sensitive assay for detecting subclinical infections. The cross-reactivity between B. canis and B. gibsoni is variable. The PCR test for B. canis is specific and will not detect other species. The PCR test for B. gibsoni, however, will detect a variety of species.

Treatment

The only drug approved for definitive treatment of babesiosis in the United States is imidocarb diproprionate (Imizol, Schering-Plough, Union, New Jersey). The recommended dosage is 6.6 mg/kg IM, repeated in two weeks. Side effects of imidocarb include pain at the injection site, salivation, lacrimation, gastrointestinal signs and tremors. Pre-treatment with atropine (0.04 mg/kg SC) may prevent these cholinergic side effects. Imidocarb is very effective against B. canis, but less effective against B. gibsoni. In many dogs, the degree of parasitemia is markedly reduced within 24-48 hours after administration, but three or four treatments may be required to clear the parasitemia. Even then, many dogs apparently develop subclinical infections and remain chronic carriers.

Another babesiacidal drug is diminizene aceturate. This drug is not available in the United States but can be obtained 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 two or three times at two-week intervals) has been recommended for dogs with Babesia 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.

Supportive care is required in dogs with acute or peracute disease. A blood transfusion or Oxyglobin® is indicated in dogs with severe hemolytic anemia. In one study, 85 percent of babesia infected dogs had a positive direct antiglobulin (Coomb's) test and 21 percent exhibited autoagglutination. The use of glucocorticoids in these cases is controversial because immunosuppression may exacerbate the parasitemia. The author prefers to treat these dogs with imidocarb and supportive care, and will only use glucocorticoids if hemolysis continues to cause severe anemia. Once the parasitemia has cleared, glucocorticoids can be tapered off fairly quickly in most cases. Other supportive measures include fluid therapy to correct dehydration and acidosis, and management of concurrent diseases, such as ehrlichiosis. Sucralfate is generally indicated to minimize gastrointestinal bleeding associated with thrombo-cytopenia or immunosuppressive doses of steroids.

Prevention

The best method of prevention in endemic areas is aggressive control of the tick vector. An effective topical acaricide combined with a flea/tick collar is usually very efficacious in preventing tick exposure.

Owners should inspect their dogs daily for ticks. Prompt removal of ticks within 24 hours should prevent disease transmission, because it has been reported that the tick must be attached for two to three days to transmit the organism. In kennels where puppies are being lost to disease, aggressive tick-control measures should be instituted including spraying the environment as well as treating animals. Although it has not been proven, it is likely that any blood-sucking insect that moves from dog to dog could directly transmit the organism through blood contamination. Therefore, an insecticide that prevents biting flies, fleas, and mosquitoes would have the best chance of preventing spread of this organism in a kennel situation.

Because babesiosis can be vertically transmitted from dam to offspring, serologic testing should be carried out to remove infected dogs from the breeding pool.

To avoid transmission through blood contamination, poor kennel practices such as sharing needles for vaccination or re-using surgical instruments for tail docking and ear cropping should be avoided. Dogs with positive babesia titers should never be used as blood donors. More stringent regulations concerning serologic testing, quarantine and treatment of dogs entering the United States from endemic areas may prevent continued spread of the disease into this country. Dog fighting should obviously be avoided, because it is also a potential method for spreading disease.

Treatment of asymptomatic dogs with subclinical infection is controversial, and it is not known how effective treatment is in eliminating the carrier state. However, treatment with imidocarb followed by a negative PCR test in two months may help eliminate the reservoir of disease. Development of new vaccines, such as the French vaccination for B. canis canis, is needed because there is no cross-protection for the different babesia organisms. As evidence mounts concerning the spread of this emerging tick-borne disease to new areas, continued research is needed to determine better methods of prevention and treatment.

Suggested Reading

Dr. Douglass Macintire received her veterinary degree from Texas A&M University in 1980. In 1981, she completed a one-year internship in small animal medicine and surgery at Louisiana State University. From 1981-84, she completed a residency in small animal medicine at Auburn University. She also received a master of science degree in veterinary medicine from Auburn University. From 1984-1990, She taught emergency medicine at the University of Pennsylvania. She became a diplomate of the American College of Veterinary Internal Medicine in 1986. In 1990, she became the first individual to pass the certification examination issued by the American College of Veterinary Emergency and Critical Care. She is a professor at Auburn University College of Veterinary Medicine where she teaches emergency medicine and is the co-director of the Auburn University Critical Care Service. Dr. Macintire has spoken extensively on subjects pertaining to emergency medicine and critical care and infectious diseases to both national and international audiences. She serves as the small animal editor for Compendium on Continuing Education for the practicing veterinarian.