Relationship between microfilaria count and sensitivity of the direct smear for diagnosis of canine dirofilariosis.
Direct blood smear examination (using 0.05 ml of whole blood) detected 168 (80.9%) of 204 microfilaremic canine blood samples as determined by the modified Knott test for microfilariae (mff) of Dirofilaria immitis (using 1 ml of whole blood). Direct smear examination detected all of 134 microfilaremias greater than 50 mff ml(-1), but only 31 of 70 (44.3%) microfilaremias having less than 50 mff ml(-1). In a separate retrospective query of a database of 963 dogs with necropsy-confirmed heartworm infections, 834 (86.6%) were positive by the DiroCHEK heartworm antigen test, and 504 (52.3%) were microfilaremic by the modified Knott test. Only 2 (0.4%) of the microfilaremic dogs were DiroCHEK negative and another 18 (3.6%) were very weak positives. Although these microfilaremic dogs were not tested by direct smear, only one of the two DiroCHEK-negative and six of 18 weakly DiroCHEK-positive dogs had microfilaremias so low that a direct smear may have given a false negative result. Significant adverse reactions to either diethylcarbamazine or the macrolide endectocides have not been reported for microfilaremias less than 500 mff ml(-1), thus substitution of the direct smear for a concentration test for mff, such as the modified Knott test or membrane filtration, does not appear to increase the risk of an unexpected adverse reaction to heartworm prophylactic drugs. Such a substitution results in only a very slight decrease (on the order of 0.1%) in the overall sensitivity of heartworm screening, provided a test for mff is run concurrently with an antigen test. If a test for mff is the only screening test used, then substitution of a direct smear for a concentration test may decrease the sensitivity of heartworm screening by nearly 20%, depending on the prevalence of low level microfilaremias in the population of dogs tested.
Courtney
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Department of Pathobiology, College of Veterinary Medicine, Box 110125, University of Florida, Gainesville, FL 32610-0125, USA. courtneyc@mail.vetmed.ufl.edu
Zeng
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QY
D000818:Animals; D000871:Anthelmintics / Q000627:therapeutic use; D000900:Anti-Bacterial Agents / Q000627:therapeutic use; D000947:Antigens, Helminth / Q000032:analysis; D004183:Dirofilaria immitis / Q000276:immunology / Q000302:isolation & purification*; D004184:Dirofilariasis / Q000175:diagnosis* / Q000188:drug therapy; D004283:Dog Diseases / Q000175:diagnosis* / Q000469:parasitology; D004285:Dogs; D005260:Female; D007559:Ivermectin / Q000627:therapeutic use; D018942:Macrolides; D008297:Male; D012189:Retrospective Studies; D012680:Sensitivity and Specificity
D000871:Anthelmintics; D000900:Anti-Bacterial Agents; D000947:Antigens, Helminth; D018942:Macrolides; C078973:milbemycin oxime; D007559:Ivermectin