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Infectious hemoylytic anemias: Look for an underlying cause
Canine rickettsial infections are important causes of morbidity and mortality worldwide.
Q. Could you review current diagnostic and treatment trends for infectious hemolytic anemias in dogs and cats?
A. Dr. Adam J. Birkenheuer, at the 2006 American College of Veterinary Internal Medicine Forum in Louisville, Ky., gave a lecture on infectious hemolytic anemias. Here are relevant points from the lecture:
Most hemolytic anemias (both idiopathic as well as those associated with an underlying disease) either are documented or presumed to be immune mediated with a smaller percentage mediated through oxidative damage, red blood cell (RBC) fragility or microangiopathic destruction.
Most immune-mediated hemolytic anemia (IMHA) cases are associated with IgG antibodies attached to the RBC surface and subsequent extravascular hemolysis by the reticuloendothelial system.
Less frequently, hemolysis is mediated through the complement system resulting in intravascular hemolysis characterized by hemoglobinemia and hemoglobinuria.
Despite the frequency with which we recognize IMHA, an underlying cause is identified in a minority of cases, including those cases without restrictions on the types or amounts of diagnostics pursued.
The inability to identify an underlying cause for IMHA means that immune suppression remains the mainstay treatment. Concerns about potential negative effects of this immune suppression exist, especially if an occult underlying infectious cause of IMHA was missed during the diagnostic work-up. Recent advances in technology have improved the ability to rapidly and accurately identify several infectious diseases that can cause IMHA.
Infectious causes of feline hemolytic anemias
Idiopathic IMHA is diagnosed less frequently in cats than in dogs. It is of utmost importance to search for an underlying cause. Bacterial, viral and protozoal causes of IMHA have been described in cats.
Hemotropic Mycoplasma species (formerly Haemobartonella) are the most commonly diagnosed infectious cause of IMHA in cats. Several species can infect cats. The majority of clinical information exists regarding Mycoplasma haemofelis and candidatus Mycoplasma haemominutum. The cat flea, Ctenocephalides felis, is presumed to transmit Mycoplasma haemofelis, although recent transmission studies have failed to demonstrate this. Unlike M. haemofelis, M. haemominutum is not considered highly virulent and is not usually associated with clinical disease unless there is concurrent retroviral infection. These organisms have never been cultured in vitro, and there are no commercially available serologic tests.
The diagnostic modalities available include microscopy and molecular techniques such as polymerase chain reaction (PCR). Microscopy is generally considered to have poor sensitivity since the number of organisms present in circulation can wax and wane. It is important that PCR-based tests are able to differentiate between the M. haemofelis and M. haemominutum; a positive result with the latter species appears to be less clinically significant.
The treatments of choice are doxycycline (5 mg/kg PO BID), enrofloxacin (10 mg/kg PO Q24) or marbofloxacin (2.5 mg/kg PO Q24) for 14 days to 21 days. Since the anemia is immune-mediated, concurrent treatment with prednisone (2-4 mg/kg PO daily) may be indicated as well. To date, no treatments appear to result in complete clearance of the organisms. Therefore, infected cats are at risk of recurrence and may serve as reservoirs of infection. Flea prevention may be important for the prevention of disease transmission.
The feline leukemia virus and feline immunodeficiency virus infections have been associated with anemia in infected cats. The anemia caused by retroviruses in cats can be due to several underlying mechanisms including immune-mediated hemolysis, bone-marrow suppression, chronic inflammation and neoplasia. Of these, IMHA appears to be an uncommon cause of anemia. Usually, the diagnosis is made using commercially available in-house ELISA tests and can be confirmed in some cases by either PCR or Western blot assays. Treatment of IMHA in these cases is still directed toward the immune response and typically consists of prednisone (2-4 mg/kg PO daily).
The primary protozoal diseases associated with IMHA in cats are Cytauxzoon felis and Babesia felis. C. felis is a tick-transmitted infection that is endemic to the Southeastern, Midwestern and Mid-Atlantic regions of the United States. It is characterized typically by an acute febrile illness associated with decreases in one or more cell lines. The disease occurs most often between the months of April and September, which correlates with peak tick activity.
The diagnosis is made via microscopic identification of the parasites infecting red blood cells or macrophages. Since macrophages are the first cell line infected, tissue aspirates of affected organs such as the liver, spleen and lymph nodes may aid in the early diagnosis when the RBC forms are not present. To accurately and rapidly identify C. felis in affected cats, a PCR test was developed. Historically, the disease has been considered uniformly fatal in affected cats, but more recently there have been reports of cats surviving infection. Atovaquone and azithromycin combination therapy and imidocarb dipropionate are used for the treatment of C. felis in naturally infected cats.
Feline babesiosis caused by Babesia felis has not been identified in the United States but is a common cause of anemia in cats in South Africa. The diagnosis of feline babesiosis is dependent upon microscopy, but PCR can be performed on feline samples using broad-range primers designed to amplify most Babesia species. The treatment of choice is primaquine, but the safety margin for the use of this drug in cats is narrow. At least one other species of Babesia has been identified in cats and novel Babesia species have been found in cougars.
Recently cats with Ehrlichia infections have been associated with anemia. While IMHA has not been clearly defined in cats with ehrlichiosis, veterinarians should consider testing cats with IMHA for ehrlichiosis since the full spectrum of disease associated with ehrlichiosis in cats has not been defined.
Infectious causes for canines
Idiopathic IMHA is still the most-common diagnosis for dogs presenting with hemolytic anemia. However, there are a number of infectious diseases that can be associated with IMHA in dogs.
Canine babesiosis is an important cause of canine IMHA worldwide. At least eight genetically distinct piroplasms have been amplified by PCR from the blood of dogs.
Babesia gibsoni and the three subspecies of Babesia canis remain the most important of these pathogens. Canine babesiosis is characterized by hemolytic anemia, thrombocytopenia and hyperglobulinemia. It is important to note that anemia is not present in every case.
Up to 85 percent of dogs with babesiosis will have a positive Coombs' test attesting to the immune-mediated nature of the anemia. In most areas, babesiosis is transmitted by ticks, and dogs exposed to ectoparasites are at risk. Several studies have implicated dog bites as risk factors for B. gibsoni infections. Other studies have shown that perinatal transmission of Babesia can occur as well. Additionally, there have been several documented cases of babesiosis in dogs that have been recipients of blood transfusions from infected dogs.
There are breed associations with canine babesiosis [Greyhounds (B. canis) and American pit bull terriers (B. gibsoni)] that should prompt veterinarians to have an increased incidence of suspicion.
The three diagnostic tests available for babesiosis include microscopy, serology and PCR. None of these tests has 100 percent sensitivity or specificity, and in many cases more than one modality is needed. Of the three tests available, PCR is the only method to differentiate accurately which species is present in a given dog. Since there can be substantial genetic variation between species, it is important to know which species will be detected by individual laboratory. Accurate differentiation is important because the treatment will vary among species.
Clearing the parasite
Imidocarb dipropionate (6.6 mg/kg IM once and repeat in 2-3 weeks) is the treatment of choice for B. canis. This treatment appears to be effective in clearing the parasite. For B. gibsoni, a combination of atovaquone (13.5 mg/kg PO TID administered with a fatty meal) and azithromycin (10 mg/kg PO Q24) administered simultaneously for 10 days appears to be effective. Treatment failures appear most common in dogs that have been splenectomized or that have undergone prolonged (weeks to months) immune suppression prior to the diagnosis of babesiosis. Reserve immune suppression therapy for the cases that are not rapidly responding (three to five days) to anti-protozoal therapy.
Canine rickettsial infections are important causes of morbidity and mortality worldwide. They can be caused by Rickettsia rickettsii, Ehrlichia species and Anaplasma species. Of these, E. canis appears to be the organism most frequently associated with IMHA.
Although anemia is one of the most common clinical signs that accompany ehrlichiosis, nonregenerative anemias secondary to bone marrow suppression appear to be more common than IMHA. Occasionally, dogs will have acute hemolytic crisis that is immune-mediated and an associated regenerative anemia. Dogs with ehrlichiosis will frequently have concurrent thrombocytopenia and hyperglobulinemia. Diagnoses are primarily based on serology and PCR.
Microscopic identification of morulae can be helpful when present, but microscopy is not sensitive for the diagnosis of infection. Some species of Ehrlichia such as E. ewingii have never been cultured in vitro; consequently, no specific serologic tests exist. PCR and microscopy are the only tests available for the diagnosis of E. ewingii infection. Treatment with doxycycline (10 mg/kg/PO daily) for three weeks is the treatment of choice for canine ehrlichiosis. IMHA appears to be a rare complication of the other rickettsial infections in dogs.
Closer look at bacterial diseases
A few bacterial diseases are associated with anemia in dogs. Definitive associations between these diseases and IMHA have not been made. Candidatus Mycoplasma haemocanis (Haemobartonella canis) and at least one other species of haemoplasma have been identified in anemic dogs. It does not appear, however, that these species are virulent pathogens in the absence of concurrent diseases.
Bartonella species are emerging as an important cause of disease in dogs. While endocarditis remains the primary clinical disease recognized, there is a growing body of evidence that bartonellosis is associated with IMHA in dogs. Leptospirosis has been associated with IMHA in dogs; however, renal and hepatic failure continue to be the primary clinical problems that need to be addressed in dogs with clinical leptospirosis.
Dr. Hoskins is owner of DocuTech Services. He is a diplomate of the American College of Veterinary Internal Medicine with specialities in small animal pediatrics. He can be reached at (225) 955-3252, fax: (214) 242-2200 or e-mail: firstname.lastname@example.org.