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 mod ified Knott test for microfilariae (mff) of Dirofilaria immitis (using 1 ml of whole blood). Direct smear examination detected all of 134 microfilarem ias greater than 50 mff ml(-1), but only 31 of 70 (44.3%) microfilaremias h aving less than 50 mff ml(-1). In a separate retrospective query of a datab ase of 963 dogs with necropsy-confirmed heartworm infections, 834 (86.6%) w ere positive by the DiroCHEK heartworm antigen test, and 504 (52.3%) were m icrofilaremic by the modified Knott test. Only 2 (0.4%) of the microfilarem ic dogs were DiroCHEK negative and another 18 (3.6%) were very weak positiv es. Although these microfilaremic dogs were not tested by direct smear, onl y one of the two DiroCHEK-negative and six of 18 weakly DiroCHEK-positive d ogs had microfilaremias so low that a direct smear may have given a false n egative result. Significant adverse reactions to either diethylcarbamazine or the macrolide endectocides have not been reported for microfilaremias le ss than 500 mff ml(-1), thus substitution of the direct smear for a concent ration 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 s light decrease (on the order of 0.1%) in the overall sensitivity of heartwo rm screening, provided a test for mff is run concurrently with an antigen t est. If a test for mff is the only screening test used, then substitution o f a direct smear for a concentration test may decrease the sensitivity of h eartworm screening by nearly 20%, depending on the prevalence of low level microfilaremias in the population of dogs tested. (C) 2001 Elsevier Science B.V. All rights reserved.