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12 AHS HEARTWORM HOTLINE HEARTWORM HOTLINE It is hypothesized that doxycycline had eliminated Wolbachia from microfilariae. Therefore, Wolbachia expansion after the L3 to L4 molt could not occur and heartworms could not fully develop. In essence, the use of doxycycline in heartworm-infected dogs prevented heartworm infection in subsequent animals. The fact that doxycycline can essentially prevent heartworm transmission makes its use mandatory when macrocyclic lactone resistance is suspected. HOW SHOULD PRACTITIONERS TIME DOXYCYCLINE ADMINISTRATION? Under the current AHS guidelines ( Table 1 ), a 4-week dose regimen of oral doxycycline should be initiated immediately after diagnosis and concurrent with the first dose of a macrocyclic lactone heartworm preventive. A second dose of heartworm preventive is given at the end of the doxycycline regimen. 11 A month later, the first dose of melarsomine is administered. However, if there are extenuating circumstances, melarsomine may be administered immediately after the last dose of doxycycline instead of waiting an extra month because at least some of the Wolbachia population will be eliminated by this time. It is unknown whether the extra month between the end of the last doxycycline dose and the beginning of melarsomine is necessary for the observed reduction in lung pathology associated with Wolbachia elimination. In Kramer et al, 2 a lack of arterial lesions was demonstrated after waiting for 2 months after the completion of a 30-day doxycycline regimen. On the basis of this TABLE 1 American Heartworm Society Treatment Protocol for Dogs DAY TREATMENT Day 0 Dog diagnosed and verified as heartworm positive: • Positive antigen (Ag) test verified with microfilaria (MF) test • If no microfilariae are detected, confirm with 2nd Ag test from a different manufacturer Begin exercise restriction. • The more pronounced the signs, the stricter the exercise restriction If the dog is symptomatic: • Stabilize with appropriate therapy and nursing care • Prednisone prescribed at 0.5 mg/kg BID 1st week, 0.5 mg/kg SID 2nd week, 0.5 mg/kg EOD 3rd and 4th weeks Day 1 Administer heartworm preventive. • If microfilariae are detected, pretreat with antihistamine and glucocorticosteroid, if not already on prednisone, to reduce risk of anaphylaxis • Observe for at least 8 hours for signs of reaction Days 1–28 Administer doxycycline 10 mg/kg BID for 4 weeks. • Reduces pathology associated with dead heartworms • Disrupts heartworm transmission Day 30 Administer heartworm preventive. Day 60 Administer heartworm preventive. First melarsomine injection 2.5 mg/kg intramuscularly (IM) Prescribe prednisone 0.5 mg/kg BID 1st week, 0.5 mg/kg SID 2nd week, 0.5 mg/kg EOD 3rd and 4th weeks. Decrease activity level even further. • Cage restriction/on leash when using yard Day 90 Administer heartworm preventive. Second melarsomine injection 2.5 mg/kg IM Day 91 Third melarsomine injection 2.5 mg/kg IM Prescribe prednisone 0.5 mg/kg BID 1st week, 0.5 mg/kg SID 2nd week, 0.5 mg/kg EOD 3rd and 4th weeks. Continue exercise restriction for 6 to 8 weeks following last melarsomine injections. Day 120 Test for presence of microfilariae. • If positive treat with a microfilaricide and retest in 4 weeks Establish year-round heartworm prevention. Day 271 Antigen test 6 months after completion; screen for microfilariae. BID, twice daily; EOD, every other day; SID, once daily. Reprinted by permission from the American Heartworm Society.