Page 226 - Small Animal Internal Medicine, 6th Edition

198 PART I Cardiovascular System Disorders

of severe pulmonary vascular disease, especially in those severely affected dogs warrant more strict exercise restric-
with right-sided CHF or a high worm burden. Whenever tion (cage rest).
VetBooks.ir possible, other pretreatment tests should include a CBC, utes to reducing HW fertility and viability. Doxycycline
Use of doxycycline as therapy against Wolbachia contrib-
serum biochemical profile, urinalysis, and a microfilaria
test. A platelet count is important for risk assessment in
after diagnosis of HWD, before adulticide therapy. Doxycy-
animals with severe pulmonary arterial disease. Mildly to (10 mg/kg PO q12h) is recommended in dogs for 4 weeks
moderately increased liver enzyme activity may be associ- cline aids in reducing microfilaria, weakens and reduces
ated with hepatic congestion but does not preclude therapy reproductive ability of adult worms, kills developing larvae,
with melarsomine. Liver enzyme activities usually normal- and reduces pathologic immune reactions to dead and dying
ize within 1 to 2 months of HW treatment in dogs without worms. The combination of a single preventative dose of
preexisting liver disease. As previously discussed, some dogs ivermectin and doxycycline at 10 mg/kg PO q12h effectively
with HWD develop azotemia and/or severe proteinuria. Pre- eliminates microfilaria within 2 to 3 weeks, a rate that is
renal azotemia is treated with fluid therapy before adulticide appropriately rapid for reducing infectivity to other dogs but
is given. Severe glomerular disease can increase the risk for gradual enough to minimize risk of immune reaction to
thromboembolism from loss of anticoagulant proteins (see dying microfilaria. When the recommended combination of
p. 222); dogs with high UPC could benefit from ACEI treat- monthly preventative and doxycycline is used, no adjunct
ment before adulticide therapy. Echocardiography aids in treatment specifically targeting microfilaria is needed (either
confirming and estimating severity of PAH and secondary before or after adulticide therapy). Although not specifically
right-sided cardiac remodeling, as well as confirming pres- evaluated in HWD, the related drug minocycline has similar
ence of caval syndrome if suspected, but is not required in efficacy against Wolbachia and is a reasonable substitute if
most routine cases of HWD before treatment. In circum- doxycycline is unavailable; the same dosing protocol is used
stances of shelter or owner financial limitations, adulticide for minocycline.
treatment has frequently been undertaken with minimal pre- Antiplatelet therapy with aspirin or clopidogrel currently
treatment evaluation (positive Ag test only). Melarsomine is not recommended as an adjunct to routine HW treatment
injections were associated with <1% mortality rate in a high- because there is no convincing evidence for reduction in
volume shelter rescue situation after Hurricane Katrina. vascular lesion severity or risk of PTE. However, antiplatelet
The risk-benefit and cost-benefit of pretreatment diagnos- therapy should be considered in cases of documented pro-
tic testing should therefore be discussed with owners of teinuria or PTE. Corticosteroid therapy (e.g., prednisone or
HW-positive dogs. dexamethasone) is reserved for treatment of eosinophilic
Monthly HW prophylaxis with a macrocyclic lactone pneumonitis or granulomatosis (see p. 201), PTE (see p.
should begin immediately after diagnosis of HWD (if not 201), treatment or prevention of microfilaricide reactions, or
already started). The American Heartworm Society rec- to mitigate tissue reaction to melarsomine injection. Other-
ommends use of monthly HW preventative for 2 months wise corticosteroids are avoided because of their potential to
before the administration of an adulticide in dogs diag- cause fluid retention, reduced pulmonary blood flow, and
nosed with HWD. This strategy can reduce HW Ag mass hypercoagulability. Sildenafil is used in cases of severe PAH,
by decreasing or eliminating circulating microfilariae and PTE, and right-sided CHF secondary to PAH (see p. 201).
tissue-migrating larvae, stunting immature worm growth, Cardiac medications, including furosemide, pimobendan,
and damaging the adult female reproductive system. Delay- and ACEIs, are reserved for cases of right-sided CHF sec-
ing melarsomine for a few months also allows any late-stage ondary to PAH; ACEIs can also be considered in cases of
larvae to mature further, which should increase suscepti- proteinuria secondary to immune-complex glomerulone-
bility to the adulticidal effect. This 2-month window effec- phritis. Herbal or other “natural” products have not shown
tively narrows the so-called “susceptibility gap” between efficacy in treatment or prevention of HWD and are not
parasite life stages targeted by macrocyclic lactones (L 3 -L 4 ) recommended.
and adulticides (mature adult HWs). Microfilaria-positive The withholding of adulticide treatment in some asymp-
dogs should be observed in the hospital after the first dose tomatic HWD cases remains controversial and is not recom-
of macrocyclic lactone in case of adverse reaction, particu- mended. Although continuous monthly treatment with
larly if milbemycin oxime is used (the most microfilaricidal prophylactic ivermectin does eventually kill young adult
among macrocyclic lactones). Dogs with high numbers of worms, this effect occurs over a prolonged time period (over
microfilariae or those receiving milbemycin oxime might 1-2 years). Older worms are more resistant to ivermectin and
also benefit from administration of an antiinflammatory can still cause clinical disease. Furthermore, progression of
dose of glucocorticoid, with or without an antihistamine pulmonary arterial changes, pulmonary disease, and other
(e.g., diphenhydramine), an hour before the first macrolide HW-induced effects (e.g., glomerulonephritis) could increase
preventive dose. the risks associated with adulticide treatment should this be
Exercise restriction should be enforced as soon as HWD undertaken in the future. Other macrocyclic lactones have
is diagnosed and maintained until 1 to 2 months after final variable adulticidal activity, with milbemycin showing the
melarsomine treatment. Activity level is highly associated least efficacy against adult HWs at the preventative dose. It
with progression of pulmonary arterial disease and risk of also is possible that use of HW preventive medication alone
pre- or posttreatment complications, particularly PTE. More in HW-infected dogs could increase the potential for