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A negative antibody test indicates that: • It is unlikely the cat is infected with HWs • The cat is likely at less risk for HWI than a cat that tests positive. A negative test should not, however, be used to determine whether cats are free from risk for HWI. The conclusion that a cat is not at risk for HWI is based on many factors (besides a negative antibody test), including whether dogs in the community are on HW preventive, if populations of coyotes are present, and the cat's indoor/outdoor lifestyle. In conclusion, an antibody-positive cat should, indeed, be on preventive due to the: • Fact that cat has been exposed to HWs • Possibility cat is infected with HWs • Risk that cat may develop mature HWI and/or HARD Arguments Against Testing Prior to Preventive Therapy • Regardless of the test result—positive or negative—HW prevention is indicated. • The cost of the testing and associated office visits may very well deter the client from initiating preventive, particularly in the instance of a multicat household. • A negative test may be misinterpreted as an indication that no HW prophylaxis is needed. However, as mentioned earlier, a negative antibody test does not mean there is no risk for future HWI. • There is virtually no risk of an adverse reaction in cats with a mature HWI placed on a macrocyclic lactone preventive. This is because cats are rarely infected with microfilariae and, if microfilariae are present, their number is small. If, despite this, there is concern about an adverse reaction, a microfilaria test TABlE. FElINE HEARTWORM TESTING: KEY POINTS Test Arguments for Testing Arguments Against Testing Testing Prior to Initiation of Preventive Therapy • Regardlessoftestresult,HWpreventiveis indicated, if testing for hws is pursued • Costoftestsandofficevisitsmaydeter owner from initiating preventive • Anegativeresultmaybemisinterpreted as an indication that no hw prophylaxis is needed • Thereisvirtuallynoriskforadverse reaction in cats with mature hwi placed on macrocyclic lactone preventives Antibody Test Positive result indicates cat: • HasbeenexposedtoHWsandmayor may not have hwi • IsatriskforHARDormatureHWI • Requiresheartwormpreventive • Canundergofurthertestingtodiagnose HWD Negative result indicates cat: • Isunlikelyinfected • HaslowerriskforHWIthananantibodypositive cat Antigen or Microfilaria Test There are no arguments for use of antigen or microfilaria testing as screening tools in cats, unless run as a supplement to the antibody test, due to the tests' low sensitivities. These tests should be reserved for cats in which hwi is suspected. microfilaria testing lacks sensitivity for screening in both dogs and cats but is much more sensitive in dogs (> 80%) than in cats (< 15% sensitivity). Annual Testing During Preventive Therapy Antibody Test Because a positive antibody test only means that a cat has been "exposed" to hws and prevention should be continued, antibody testing is not advised for annual testing. a positive test only means that the cat continues to be at risk since infections are aborted at the same time as the test turns positive. Antigen or Microfilaria Test annual antigen testing is appropriate in dogs but not cats due to the: • Relativesensitivityandspecificityoftestingmethods—antigentestismuchmoresensitive in dogs than in cats • DifferenceinHWprevalencebetweenspecies—intheU.S.,dogsareatfargreaterriskfor hwi than cats Testing for Cats with Clinical Signs of HWI Antibody Test any cat with appropriate clinical signs should have antibody testing performed to aid in determiningtheexistenceofHWDorHARD. Antigen or Microfilaria Test a positive antibody test should be followed by (or accompanied by) an antigen test and/or microfilaria test, for which a positive result indicates mature hwi May/June 2013 Today's Veterinary Practice 51 heartworm Screening for cats american hearTworm SocieTy'S hearTworm hoTline |