Page 231 - Small Animal Internal Medicine, 6th Edition

CHAPTER 10 Pulmonary Hypertension and Heartworm Disease 203

monthly preventive therapy may be most important only other antiparasitic agents for broader protection against
during June through November in the majority of the United endoparasites and ectoparasites.
VetBooks.ir States, continuous chemoprophylaxis throughout the year reported, particularly in the Mississippi Delta. Reasons for
Lack of efficacy (LOE) of HW preventives has been
may be more practical, not only for animals living in loca-
tions where transmission is likely during more than half
HW prevention as directed), patient factors (failure of inges-
the year but also for those that travel to warmer regions. If LOE include owner noncompliance (failure to administer
seasonal prophylaxis is elected for financial or other reasons, tion or absorption of HW preventative), or true parasite
prevention should begin 1 month before the anticipated HW resistance. In the vast majority of cases, LOE of macrocyclic
transmission season and continue for 6 months after the lactones can be attributed to owner noncompliance, high-
season’s end. Puppies should begin HW prevention as early lighting the importance of client education and monthly
as feasible after weaning, by 8 weeks of age at the latest. reminder systems. However, in a few cases, genetic polymor-
HW Ag and microfilaria testing is recommended before the phisms have been documented that suggest true resistance
initial dose of preventive medication in dogs 6 months of age of HW microfilariae or adult worms to various macrocyclic
and older. For all age dogs, if accidental lapse in HW pre- lactones. Despite these rare and isolated instances of HW
ventive drug administration occurs, continuous prophylaxis resistance, efficacy of HW preventatives remains extremely
should be restarted as soon as possible and Ag test repeated high, and veterinarian efforts should be directed toward edu-
6 months and 12 months after the lapse. Owner education cating clients of the importance of monthly year-round pre-
about the potential seriousness of HW infection and need vention. Reports of LOE have not altered the American
for consistent administration of preventive medication as Heartworm Society’s guidelines for diagnosis, prevention, or
directed is important. treatment of HWD.
Several macrocyclic lactone (macrolide) drugs are cur- Effective HW prevention, particularly in heavily endemic
rently available for HWD prevention, including the aver- areas, also involves limiting mosquito exposure. This includes
mectins (ivermectin, selamectin) and the milbemycins removal or treatment of standing water sources, use of mos-
(milbemycin oxime, moxidectin). DEC is no longer com- quito repellants or traps, and keeping pets indoors during
mercially available; this HW preventative required daily peak mosquito hours.
dosing and could be given only to amicrofilaremic dogs.
Macrocyclic lactones induce neuromuscular paralysis and HEARTWORM DISEASE IN CATS
death in nematode (and arthropod) parasites by interacting
with membrane chloride channels. They are effective against Cats are an atypical host of HWD. HWs mature more slowly,
third- and fourth-stage larvae during the 2 months after fewer numbers of infective larvae mature to adults, and the
infection, as well as microfilariae and in some cases adult adult life span is shorter in cats. However, live worms can
worms. Ivermectin, used continuously for more than 30 persist for 2 to 4 years. HW-infected cats generally have
months, is effective against adult D. immitis; selamectin also fewer than six adult worms in the RV and pulmonary arter-
has adulticidal effect with chronic administration. Retroac- ies, and most cats with mature HW infection have only one
tive preventive efficacy (“reachback”) with these agents lasts or two worms. Nevertheless, in terms of host versus para-
at least 1 and possibly more than 2 months after a single dose. site biomass, a cat infected with two or three worms is still
These agents are quite safe in mammals when used as considered heavily infected, and even one adult worm can
directed; all commercially available macrocyclic lactones at cause death. Unisex infection is common. Most cats with
preventive doses are safe in sensitive Collies and other both male and female worms have no or only a brief period
P-glycoprotein-deficient dogs. Cases of clinical toxicity of microfilaremia, so cats rarely serve as important reservoirs
usually have been related to dosage miscalculation using a of infection. Aberrant worm migration is also more common
concentrated livestock preparation. in cats than dogs and complicates necropsy confirmation of
Macrocyclic lactones available for monthly oral adminis- infection. Aberrant sites have included the brain, subcuta-
tration include ivermectin (6-12 µg/kg; Heartgard, Iverhart, neous nodules, body cavities, and occasionally a systemic
Tri-Heart) and milbemycin oxime (0.5-1 mg/kg; Interceptor, artery.
Sentinel, Trifexis). Products available for monthly topical HW infection where the parasites never reach maturity
administration (applied to the skin between the shoulder because of the host’s inflammatory response is thought to be
blades, at dose ranges of 6-12 mg/kg) include selamectin much more common than mature HWD in cats. This HW
(Revolution) and moxidectin/imidacloprid (Advantage “exposure” with early parasite destruction has been called
Multi); efficacy is not affected if bathing or swimming is “pulmonary larval dirofilariasis” or “heartworm-associated
delayed at least 2 hours after application. A slow-release for- respiratory disease” (HARD).
mulation of moxidectin in a liposomal formulation (Pro-
Heart 6) is given by SC injection; its effect lasts 6 months. Pathophysiology
Some of these agents are effective against other parasites at The pathophysiologic changes associated with HW infec-
the doses used for HW prevention (e.g., hookworms with tion in cats can be considered to occur in two stages, with
milbemycin; fleas, ear mites, and ticks with selamectin). most infections only reaching the first. Approximately 3
These drugs are sometimes marketed in combination with to 4 months after infection, immature worms arrive in the