Adam J. Birkenheuer, DVM, PhD, DACVIM
Infectious diseases are recognized with increased frequency in both general and referral practices. While many practitioners are diagnosing patients with classic signs and presentations, those with atypical presentations are frequently missed. A missed diagnosis often results in therapies that can lead to persistent illness or in some cases may actually worsen the outcome. Many factors are contributing the emergence of infectious diseases and include climate changes, urbanization, vector epidemiology, alternative forms of disease transmission, ease of animal transport and advanced diagnostic techniques. Veterinarians need to be vigilant for infectious diseases, especially in cases that don't respond to treatment and those in which things "just don't add up."
Canine babesiosis is an emerging infectious disease in North America and is typically caused by either 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 have been documented.
Babesiosis can be acute or chronic in nature. There are variations in the clinical presentation depending on which species of piroplasm, age and breed of the host and presence of concurrent disease. The most common hematological effects are thrombocytopenia and anemia. Despite the fact that most practitioners associate babesiosis with anemia, most studies have demonstrated that thrombocytopenia is the most common hematological abnormality in dogs with babesiosis. 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%) Coombs positive. The degree of anemia is variable and can be severe (PCV < 10%), but some infected dogs have normal hematocrits. No matter what the hematocrit is, 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. Experimental cases have had transient leukopenia. The most common abnormalities detected on a biochemical profile include mild increases in liver enzymes and hyperglobulinemia. Other clinical signs that may be seen 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 US, this seems to be a rare finding. Renal failure and proteinuria have been associated with B. annae (T. annae) in Spain. Hypoglycemia is identified in 20% of dogs with B. canis rossi infections.
Microscopy or PCR can easily rule babesiosis in, but it is very difficult to rule out babesiosis completely. At this point in time I recommend that 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 (i.e., Diff-quik® or Leukostat®). Evaluation of capillary blood (ear or toenail) may improve parasite recovery. There is variable seroreactivity, so serology against both B. canis and B. gibsoni and is warranted. Convalescent (3-4wks) 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 labs. PCR tests should be able to identify and differentiate all common canine Babesia spp. PCR is the most accurate way to identify which species of Babesia is present. In one study, a single PCR test identified 85% of B. gibsoni and 2 consecutive PCR tests identified 100%.
Currently imidocarb dipropionate (Imizol® 6.6mg/kg IM, repeat in 2 wk.) is the only approved treatment for canine babesiosis in the U.S. Atovaquone 13.5mg/kgPO TID (with a fatty meal) and azithromycin 10mg/kg PO Q24 in combination for 10 days has been shown to reduce or eliminate B. gibsoni (Asian) parasitemias as determined by PCR. A combination therapy of doxycycline, clindamycin and metronidazole has shown some promise in a small experimental study.1
Babesia canis is likely to be cured by imidocarb. Babesia gibsoni may be cured by atovaquone and azithromycin combination therapy.2 Other treatments may result in a clinical remission with persistent parasitemia. These dogs are at risk for recrudescence, and may act as a reservoir. We recommend two consecutive blood smear evaluation and PCR 6-8 weeks post treatment. 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 U.S. However, cats can be infected by Cytauxzoon felis (see cytauxzoonosis)
Canine ehrlichiosis/anaplasmosis is caused by several different species of Ehrlichia/Anaplasma. Those species reported in the US include E. canis, E. ewingii, E. chafeensis, A. phagocytophilum (E. equi), and A. platys (E. platys). Ehrlichiosis can either be acute or chronic. Most cases are recognized during the chronic stage.
The hematological effects of ehrlichiosis/anaplasmosis can be variable, but the most common abnormality detected is thrombocytopenia. Despite the fact that many clinicians suspect ehrlichiosis/anaplasmosis in dogs with hemolytic anemia, a non-regenerative anemia is more commonly identified. Ehrlichiosis is only occasionally associated with a secondary IMHA. Ehrlichiosis can also cause pancytopenia. The effects on the leukon are variable. Both leukocytosis and leukopenia have been reported. Lymphocytosis can be seen. The accompanying clinical signs are often vague including; fever, lethargy, anorexia, weight loss, and vomiting. Bleeding tendencies such as epistaxis, petechia, or ecchymosis may also be present. Hyperglobulinemia (polyclonal much more commonly than monoclonal), hypoalbuminemia, lymphadenopathy, proteinuria, polyarthritis (common with E. ewingii), and/or uveitis may be present. The lack of thrombocytopenia does not rule out ehrlichiosis. Anaplasma platys only appears to cause thrombocytopenia, and not systemic illness. Anaplasma phagocytophilum is associated with acute febrile illness in dogs and thrombocytopenia is a common finding. Co-infection with A. phagocytophilum and Borrelia burgdorferi was associated with worse illness in on study.3
Serology is helpful in the diagnosis of ehrlichiosis/anaplasmosis. Acute and convalescent (3-4wks) titers should be performed if the clinical signs are acute and initial titers are low or negative. A four-fold change is consistent with ehrlichiosis/anaplasmosis. If the signs are chronic (> 4wks.), then a single high titer is consistent with infection. In-house tests (SNAP®) are not designed for diagnostic use, but a positive test in conjunction with appropriate clinical findings is supportive of a diagnosis of ehrlichiosis/anaplasmosis. The current in-house assays fail to detect antibodies against E. ewingii and cannot differentiate antibodies against A. phagocytophilum from those against A. platys. Occasionally a morula is identified in a white blood cell or platelet on a blood smear, but the parasitemia is often very low so the sensitivity of microscopy is poor. The low levels of circulating organisms can also hamper PCR. A positive PCR test should rule-in the presence of infection and identify which species is present. Accurate species identification is important for client education regarding zoonoses as well as expected response to treatment. A negative PCR test does not rule out the possibility of ehrlichiosis. Therefore the resolution of clinical signs in response to therapy remains an important "test."
Doxycycline (10mg/kg/day for 3-4 wk.) is considered the treatment of choice. Other drugs that are reported to be effective include; tetracycline, oxytetracycline, minocycline, and chloramphenicol. Imidocarb dipropionate does not appear to be an effective treatment.
Resolution of clinical signs after therapy is probably the most important follow-up and further specific diagnostics for ehrlichiosis/anaplasmosis are not indicated. Serology is generally a poor way to assess recovery as antibody titers may persist for months. If the animal is seroreactive, but does not respond to therapy, the PCR should be performed since some species, such as E. chafeensis, may not respond as well to therapy. If the PCR is negative, then an alternative diagnosis should be considered. Zoonosis: Dogs do not appear to transmit infections to humans but can act as a sentinel for ehrlichiosis/anaplasmosis.
Feline ehrlichiosis: The clinical signs appear similar to what is seen in dogs, and it should be considered when more common causes of disease are not apparent.
Bartonella vinsonii and B. henselae appear to be the primary causes of bartonellosis in dogs.
The full spectrum of canine disease caused by Bartonella species has yet to be elucidated. Has been shown to be a cause of endocarditis, and has been associated with granulomatous inflammation and hepatic disease in dogs. Although polyarthritis has only been confirmed in a handful of cases of canine bartonellosis, lameness and stiffness were the most common presenting signs for dogs with confirmed Bartonella endocarditis suggesting that it may be more common finding. Anemia and thrombocytopenia have been detected in nearly half of the dogs diagnosed with Bartonella vinsonii.4 The pathogenicity of Bartonella infection in cats is unclear, and it has not consistently been found to have any specific or characteristic hematological or biochemical effects. Some studies have detected an association between stomatitis and lymphadenopathy in cats co-infected with Bartonella and FIV, while other studies have not detected associations between Bartonella and clinical diseases in cats.
Serology is suggestive of exposure or infection. PCR is also available and positive test results are indicative of current infection. PCR tests should be able to identify and differentiate which species of Bartonella are present. A combination of culture (BAPGM) and PCR appears to be an excellent method for the detection of Bartonella spp. For unknown reasons, there are frequently discordant results between serological and molecular/bacteriologic assays, so a combination of these techniques is recommended.
The optimal treatment for bartonellosis is unknown. Currently patients are being treated with azithromycin (5-10mg/kg PO Q24 for 5 days then every other day for 45 days). Some cases have had additional clinical responses when rifampin was used in combination with the azithromycin.
Resolution of clinical signs and culture. Since the full spectrum of disease is unknown and a large percentage of normal animals can test positive for Bartonella cautious interpretation of test results is warranted and consideration of alternative diagnoses when patients signs fail to resolve with treatment. Zoonosis: Humans, especially immune compromised people, have been infected with Bartonella so client education is warranted.
Rocky Mountain Spotted Fever (RMSF)
RMSF caused by Rickettsia rickettsii is an acute systemic disease of dogs and humans. RMSF is generally seasonal (Apr.-Sept.) correlating with the Dermacentor sp. lifecycle.
Thrombocytopenia is the most common hematological abnormality (>85%). The degree of thrombocytopenia ranges from moderate ( 75,000plt/ul) to severe (< 5,000 plt/ul). The major mechanism is consumption secondary to vasculitis, but there is some evidence for immune mediated destruction. Leukocytosis is the second most common hematological finding. The degree of leukocytosis can be severe (> 50,000WBC/ul), and tends to increase along with the duration of the disease. RMSF is not known to commonly cause immune mediated hemolytic anemia. The anemia associated with RMSF is often mild (PCV 25-30%). The hematological effects are rarely seen without accompanying clinical signs, such as fever, lethargy, anorexia, pain, petechia, jaundice and neurologic signs. Common biochemical abnormalities identified in dogs with RMSF included hypoalbuminemia, hyponatremia and hyperbilirubminemia.5
Serology is very helpful in the diagnosis of RMSF. If the signs are acute, then paired acute and convalescent (2-4 weeks after the acute) titers must be submitted. A four-fold change is diagnostic for an active infection. If the patient is sick > 10-14 days, then a single high titer (> 1:1024) is consistent with an active infection. Positive Immunofluorescence of skin biopsies or positive nested PCR results also indicates active infection. Response to therapy (doxycycline, tetracycline, enrofloxacin, or chloramphenicol) is suggestive, but not diagnostic.
Doxycycline (5mg/kg BID or 10mg/kg Q24), chloramphenicol (15-30mg/kg TID), and enrofloxacin (5mg/kg BID) for 2 weeks are effective treatments. Resistance has not been reported, so if signs persist after treatment an alternative diagnosis should be considered.
An accurate diagnosis is important, since the dog can serve as a sentinel for human infections, so a convalescent titer is indicated even if the animal has responded to treatment. Resistant RMSF has not been reported. RMSF has not been reported in cats. Zoonosis: Casual contact should not pose a major risk, but direct exposure is a potential. Also, common vector transmission is possible (sentinel).
Cytauxzoonosis is an emerging infectious disease of cats in North America caused by the protozoal parasite Cytauxzoon felis. It is transmitted via the tick vector Dermacentor variabilis and possibly other tick species such as Amblyomma americanum. Cats typically present acutely and historically the mortality rate is very high (over 90%).6 Over 90% of the cases are diagnosed between April and September. Outdoor cats are at higher risk for infection and there appear to be hyperendemic areas of C. felis transmission. Bobcats appear to be the reservoir host and only rarely develop severe disease. Most cats have died within 5-7 days of the onset of clinical signs. The majority of clinical signs are due to obstruction of small vessels with schizont-laden macrophages which results in ischemia and thrombosis.
The most common signs are lethargy, depression and fever. Pancytopenia is the classic hematologic finding for cytauxzoonosis, but there may only be reductions of one or two cell lines in affected cats. Thrombocytopenia and leucopenia appear to be the most common hematological abnormalities. Hemolytic anemia is most prominent in days 7-14 after presentation. Hyperbilirubinemia and increased ALT/ALP concentrations (often not proportional to the degree of hyperbilirubinemia) are common biochemical findings. Physical examination typically reveals fever, and hepato-splenomegaly. Cats are often dyspneic, moribund, hypothermic and neurologic in the end stages of disease.
Cytologic diagnosis is the most common and rapid means of diagnosing cytauxzoonosis. The earliest stage of infection is the multiplication of schizonts in macrophages. These infected macrophages can be identified in tissue aspirates (particularly the liver, lung and spleen) or on the feathered edge of peripheral blood smears. These infected macrophages are frequently mistaken for platelet clumps and can measure nearly 100 microns in diameter. In endemic areas hepatic aspirates may be warranted in highly suspicious cases. The parasite may also be identified in red blood cells on Wright Giemsa stained blood smears as the classic signet ring. There are no commercially available serologic tests. PCR is now available, is sensitive, specific and can be performed rapidly to aid in the diagnosis or confirmation of cytauxzoonosis.
Supportive care with IV fluids and anti-coagulants are the standard of care for the treatment of cytauxzoonosis. Heparin is my anti-coagulant of choice (100-300 U/kg SQ TID). Anti-protozoal therapies have been administered to cats with cytauxzoonosis but the effect on outcome is not clear since no controlled studies have been performed. Imidocarb dipropionate (2mg/kg IM once every two weeks) or diminazene aceturate have been recommended. Pre-treatment with atropine (0.05mg/kg SQ once) appears to reduce the cholinergic side-effects associated with imidocarb. Atovaquone (10mg/kg PO Q24) and azithromycin (15mg/kg PO TID) combination therapy in combination for ten days with aggressive supportive care appears to be a promising treatment with survival rates approaching 60% in an uncontrolled study. Other antibiotics are frequently administered to cats with cytauxzoonosis presumably to prevent secondary infections as many cats are neutropenic, although some antibiotics (doxycycline and clindamycin) do have anti-protozoal activity.
If cats survive more than 7 days, the prognosis for long-term survival is excellent. Since there appears to be hyperendemic areas client education regarding their other cats and tick prevention are warranted. Prospective testing of cats in the same household as infected cats has identified carrier cats.
1. Suzuki K, et al. J Vet Med Sci 2007;69:563-568.
2. Birkenheuer AJ, et al. J Vet Intern Med 2004;18:494-498.
3. Beall M CR, et al. Abstract #14. ACVIM 2006. B
4. Reitschwerdt EB, et al. J Am Anim Hosp Assoc 2004;40:92-101.
5. Gasser AM, et al. J Am Anim Hosp Assoc 2001;37:41-48.
6. Birkenheuer AJ, et al. J Am Vet Med Assoc 2006;228:568-571.