Diagnosis, treatment of tick-borne diseases
Q: Please discuss diagnosis and treatment of tick-transmitted diseases in the United States.
A: Dr. Adam J. Birkenheuer gave an excellent lecture on "Hematological and Biochemical Changes in Tick-Transmitted Diseases" at the 2008 American College of Veterinary Internal Medicine Forum in San Antonio. Here are some relevant points from that lecture:
While many veterinarians diagnose dogs and cats with classic signs and presentations, those with atypical presentations often are missed. A missed diagnosis often results in therapies that can lead to persistent illness or, in some cases, actually worsen the outcome.Many factors contribute to the emergence of tick-transmitted diseases, including climate change, urbanization, vector epidemiology, alternative forms of disease transmission, ease of animal transport and advanced diagnostic techniques.
Veterinarians need to be vigilant for tick-transmitted diseases, especially cases that don't respond to treatment and those in which "things just don't add up."
Canine babesiosis is an emerging disease in North America and is typically caused either by Babesia gibsoni or Babesia canis vogeli. Babesia gibsoni is most commonly detected in American Pit Bull Terriers. Babesia canis is most commonly diagnosed in Greyhounds. In addition to tick transmission, infection via blood transfusions, dog fights and perinatal routes also occur.
Babesiosis can be acute or chronic in nature. There are variations in the clinical presentation depending on the species of piroplasm, age and breed of the host and presence of concurrent disease.
The most common hematologic findings are thrombocytopenia and anemia. Despite the fact that most veterinarians associate babesiosis with anemia, most studies demonstrate that thrombocytopenia is the most common hematologic abnormality in dogs. Thrombocytopenia is suspected to be immune-mediated (ITP).
The thrombocytopenia can be severe (< 50,000 plt/ul), but evidence of bleeding is rare. Some cases have had ITP without anemia. The anemia is primarily due to immune-mediated destruction and is often (> 85 percent) Coombs' positive. The degree of anemia is variable and can be severe (PCV < 10 percent), but some infected dogs have normal hematocrit. But no matter what the hematocrit, some degree of RBC regeneration is usually detected.
The effects on the leukon are variable and inconsistent. Some cases have a profound leukocytosis with a left shift that often accompanies a strong regenerative response. The most common abnormalities detected on a serum chemistry profile include mild increases in liver enzymes and hyperglobulinemia.
Other clinical signs include fever, lymphadenopathy, splenomegaly, pigmenturia and jaundice. A common misconception exists that all cases of babesiosis exhibit intravascular hemolysis. For B. gibsoni, the most commonly diagnosed form of Babesia in the United States, this seems to be a rare finding.
Microscopy or PCR can easily rule in babesiosis, but it is difficult to rule out babesiosis completely. Currently microscopy, PCR and serology are all considered to maximize your chances of identifying the infection. The organisms stain well with a modified Wright's stain.
Evaluation of capillary blood (ear or toenail) may improve parasite recovery. There is variable seroreactivity, so serology against both B. canis and B. gibsoni is warranted. Convalescent (three to four weeks) titers may be helpful in cases with acute onset of illness and low or negative acute titers. Seroreactivity of > 1:64 is suspicious for exposure in most laboratories.
PCR tests should be able to identify and differentiate all common canine Babesia species. PCR is the most accurate way to identify which species of Babesia is present. In one study, a single PCR test identified 85 percent of B. gibsoni and two consecutive PCR tests identified 100 percent.
Currently imidocarb dipropionate (6.6 mg/kg IM, repeat in two weeks) is the nation's only approved treatment for canine babesiosis. Atovaquone 13.5 mg/kg PO TID (with a fatty meal) and azithromycin 10 mg/kg PO Q24 in combination for 10 days has been shown to reduce or eliminate B. gibsoni (Asian) parasitemia as determined by PCR. A combination therapy of doxycycline, clindamycin and metronidazole has shown some promise in a small experimental study.
Babesia canis is likely to be cured by imidocarb dipropionate. Babesia gibsoni may be cured by atovaquone and azithromycin combination therapy. Other treatments may result in a clinical remission with persistent parasitemia. These dogs are at risk for recrudescence and may act as a reservoir. Two consecutive blood-smear evaluations and PCR six to eight weeks post-treatment confirms a cure.
Serology is unlikely to be helpful for short-term follow-up, since antibody titers may persist for months following treatment. Feline babesiosis has not been reported in the United States, but cats can be infected by Cytauxzoon felis.