Page 227 - Small Animal Internal Medicine, 6th Edition

CHAPTER 10 Pulmonary Hypertension and Heartworm Disease 199

resistant infections. The American Heartworm Society does
not recommend the use of macrocyclic lactones alone (the BOX 10.1
VetBooks.ir so-called “slow kill” method) as an adulticide strategy. Heartworm Disease Management Protocol for Dogs*
However, in selected cases where adulticide therapy cannot
be given for one reason or another (such as during melarso-
• Positive antigen (Ag) test verified with microfilaria (MF)
mine drug shortages), dogs should be treated continuously Day 0: Dog diagnosed and verified as heartworm-positive
(for at least 2 years) with ivermectin or possibly selamectin. test
Recent availability of a generic melarsomine product • If no microfilariae are detected, confirm with second
(Diroban) should reduce concerns about shortages of the Ag test from a different manufacturer
brand-name product (Immiticide). If the “slow kill” method • Begin exercise restriction
• The more pronounced the signs, the stricter the
is used, doxycycline (10 mg/kg PO q12h) still should be used exercise restriction
for a 4-week period after diagnosis, and exercise restriction If the dog is symptomatic:
should be maintained throughout the entire duration of the • Stabilize with appropriate therapy and nursing care
treatment process. HW Ag testing is recommended every 6 • Prednisone prescribed at 0.5 mg/kg PO q12h first
months. The HW infection is considered cleared when two week, 0.5 mg/kg q24h second week, 0.5 mg/kg
consecutive negative test results are obtained. Lifelong HW q48h third and fourth weeks
prophylaxis is then used to prevent reinfection and disease Day 1: Administer heartworm preventive
transmission to other animals. • If microfilariae are present, pretreat with antihistamine
and glucocorticoid (if not already on prednisone) to
ADULTICIDE THERAPY IN DOGS reduce risk of anaphylaxis
Melarsomine dihydrochloride (Immiticide, Diroban) is the • Observe for at least 8 hours for signs of reaction
adulticide of choice. It is effective against both immature Days 1-28: Administer doxycycline 10 mg/kg PO q12h for
and mature HWs; male worms are more susceptible than 4 weeks
females. The American Heartworm Society recommends • Reduces pathology associated with dead heartworms
the three-dose “split” protocol for all HW-infected dogs in • Disrupts heartworm transmission
Classes 1 to 3 (see Box 10.1). Compared with the previ-
ously employed two-dose protocol, the three-dose protocol Day 30: Administer heartworm preventive
promotes a more gradual worm kill. It involves one initial Day 60: Administer heartworm preventive and first melar-
dose, followed a month (or longer) later with two doses somine injection, 2.5 mg/kg intramuscularly (IM) †
24 hours apart. The first dose leads to death of approxi- • Prescribe prednisone 0.5 mg/kg q12h first week,
mately half of the worms, resulting in less chance for severe 0.5 mg/kg q24h second week, 0.5 mg/kg q48h
PTE or immune reactions associated with massive worm third and fourth weeks
die-off. After the subsequent two doses, the worm kill is Decrease activity level even further
about 98%. This protocol does entail greater expense, higher • Cage restriction/on leash when using yard
arsenic exposure, and a requirement for more prolonged Day 90: Administer heartworm preventive, and second
exercise restriction (from initial dose to at least a month after melarsomine injection, 2.5 mg/kg IM
the final dose) compared with the two-dose protocol. The
two-dose protocol remains an alternative option, especially Day 91: Third melarsomine injection, 2.5 mg/kg IM
• Prescribe prednisone 0.5 mg/kg q12h first week,
for shelter situations, minimally affected dogs when owner 0.5 mg/kg q24h second week, 0.5 mg/kg q48h
financial constraints are an issue, or when marked renal third and fourth weeks
or liver disease is present (because of the greater potential • Continue exercise restriction for 6 to 8 weeks after
for arsenic toxicity). The two-dose protocol reportedly kills last melarsomine injections
about 90% of adult worms. Dogs with caval syndrome (Class
4) should not be given adulticide treatment until after worms Day 120: Test for presence of microfilariae
• If positive, treat for microfilariae with additional
are surgically removed (see p. 202). 30-day regimen of doxycycline and retest in 4 weeks
Melarsomine is absorbed rapidly from the intramuscular • Establish year-round heartworm prevention
(IM) injection site. Unchanged drug and a major metabolite
are rapidly eliminated in the feces; a minor metabolite is Day 271: Antigen test 6 months after completion; screen
excreted in urine. The drug should be given by deep IM for microfilariae
injection into the epaxial lumbar muscles (L 3 to L 5 region), *2014 American Heartworm Society Recommendations (www.
exactly as recommended by the manufacturer. The lumbar heartwormsociety.org).
muscle site provides good vascularity and lymphatic drain- † Follow manufacturer’s instructions closely for all melarsomine injections.
age with minimal fascial planes. Furthermore, gravity may
help prevent the drug from leaking into subcutaneous each melarsomine injection can help reduce local soreness,
tissues, where it can cause more irritation. The drug does inflammatory reaction to dying worms, and clinical effects of
cause a local reaction at the injection site; this is clinically pneumonitis (if present). Melarsomine is available as a sterile
noticeable in about a third of treated dogs. A tapering course lyophilized powder in 50-mg vials. The rehydrated product
of antiinflammatory prednisone beginning at the time of is fully stable for 24 hours if kept refrigerated in the dark.