Today's Veterinary Practice

MAY-JUN 2013

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| american hearTworm SocieTy'S hearTworm hoTline (direct smear, modified Knott test, or millipore filter test) can be performed. Annual Testing for Cats on Preventive Therapy An HW antibody test becomes positive early in HW development, reacting to antigens of the early larval (L3 and/or L4) stages. This is also the point at which macrocyclic lactones exert their effect in aborting developing/maturing HWI. Therefore, in exposed cats on HW preventive, antibody tests are expected to remain positive for > 6 months. • A positive antigen test means that a cat has been exposed to HWs and that prevention should be continued. • A negative antibody test neither indicates that a cat is no longer at risk for HWI nor that HW preventive should be terminated. Therefore, annual antibody testing for cats on HW preventive is not advised. HEARTWORM ANTIGEN & MICROFIlARIA TESTING There are virtually no arguments for use of antigen or microfilaria testing as screening tools (pretesting or yearly) in cats, unless run as a supplement to the antibody test. Some owners, after discussion of cost:benefit ratio, may prefer to have the tests run in tandem for peace of mind. Antigen tests react to proteins produced by mature female D immitis worms. Due the typically low worm burden in cats, the sensitivity of this test in cats is low (50%–70%).3 • A positive antigen test indicates a mature HWI and the presence of at least 1 mature female worm. • Due to the test's low sensitivity, it is not considered useful as a routine screening test in asymptomatic cats. Microfilaria testing also has low sensitivity (< 15%).4 • Microfilaria testing is very specific; a positive test indicates the presence of at least 1 mature female and 1 mature male worm. • This testing should be reserved for cats in which HWI is suspected. HEARTWORM TESTING FOR CATS WITH ClINICAl SIGNS It is important to emphasize the context of the previous discussion—screening asymptomatic cats. However, HW testing, particularly the antibody test, is underutilized in cats with coughing, wheezing and, in some instances, dyspnea. HW antibody testing is important in coughing cats 52 Today's Veterinary Practice May/June 2013 COMPARISON OF HEARTWORM TESTING IN DOGS & CATS annual antigen testing is appropriate in dogs but not cats due to the relative sensitivity and specificity of methodologies used and difference in hw prevalence between species. • The antigen test is used for screening in dogs; currently there is no canine antibody test. • The antigen test is much more sensitive in dogs than cats (but very specific in both species), providing fewer false-negative results. • IntheU.S.,riskforHWIismuchgreaterin dogs (approximately 10-fold) than cats. • The incidence of false-positive results increases as prevalence decreases:5-7 a positive antigen test in an asymptomatic cat has a substantially greater chance of being false-positive than a positive antigen test in an asymptomatic dog because there is a lower incidence of feline hw than canine hw.8 MANuFACTuRER-ADvOCATED TESTING all makers of feline hw preventives, to varying degrees, advocate testing cats prior to initiating use of their products. however, no particular type of testing is recommended and testing recommendations are softening with the passage of time. Following are each manufacturers' specific recommendations from their product handouts: • Advantage Multi for Cats: At the discretion of the veterinarian, cats older than 6 months of age may be tested to determine the presence of existing heartworm infections before beginning treatment with Advantage Multi for Cats. • Heartgard for Cats: It is recommended that cats should be tested for existing heartworm infection prior to starting treatment with Heartgard for Cats. Cats already infected with adult heartworms can be given Heartgard for Cats monthly to prevent further infection. • Revolution: At the discretion of the veterinarian, cats > 6 months of age may be tested to determine the presence of existing heartworm infections before beginning treatment with Revolution.

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