Page 221 - Small Animal Internal Medicine, 6th Edition

CHAPTER 10 Pulmonary Hypertension and Heartworm Disease 193

phosphodiesterase-5 inhibitors (sildenafil and tadalafil). Canada; the disease is prevalent in other regions of the world
These drugs decrease the inactivation of cyclic guanosine as well. Infection with Dirofilaria immitis causes a spectrum
VetBooks.ir monophosphate (GMP), a second messenger of the nitric of disease ranging from mild, subclinical changes to severe
pulmonary disease and secondary right-sided CHF. Dogs
oxide pathway, leading to vasodilation. Phosphodiesterase-5
inhibitors are relatively specific for the pulmonary vascula-
cats also are affected by HWD, they are more resistant to
ture and thus act as selective pulmonary vasodilators. Treat- and other canids are the preferred host species. Although
ment with sildenafil can improve clinical signs and quality infection than dogs. The overall prevalence of mature HW
of life in dogs with severe PAH, although effects on echo- infection in cats in the United States is 0.4%, and regionally
cardiographically estimated pulmonary arterial pressures is thought to be 5% to 15% of that in dogs in the same geo-
are variable. Dose adjustments typically are made based graphic area. However, exposure to and subsequent clear-
on clinical status. Adverse effects are uncommon but can ance of larvae by host reactions is estimated to be much more
include cutaneous flushing, hypotension, and nasal conges- common.
tion. Other therapies used in people with PAH (endothe-
lin receptor antagonists, prostacyclin analogs, and nitric HEARTWORM LIFE CYCLE
oxide substrates) usually are cost-prohibitive for veterinary The HW (D. immitis) is transmitted by various species of
patients, and many require delivery via inhalation or con- mosquitoes, which act as its obligate intermediate host. A
tinuous intravenous (IV) infusion. mosquito initially ingests the microfilariae, or first-stage
Management of patients with PAH also involves exercise larvae (L 1 ), which circulate in the blood of an infected host
restriction and treatment of the underlying disease (if identi- animal. The L 1 develops into an L 2 and then enters the infec-
fied). Treatment of HWD is discussed later in this chapter; tive L 3 stage within the mosquito over a period of approxi-
treatment of pulmonary thromboembolic disease is dis- mately 2 to 2.5 weeks. Symbiotic bacteria of the genus
cussed in Chapter 12; and treatment of chronic bronchopul- Wolbachia are important for larval development within the
monary disease is discussed in Chapter 21. mosquito. Infective larvae enter the new host when the mos-
For dogs with postcapillary PAH secondary to left-sided quito takes another blood meal. L 3 larvae migrate subcutane-
heart disease, treatment focuses on decreasing left atrial ously within the new host, molting into an L 4 stage within 9
(and thus pulmonary venous) pressures. This generally to 12 days, and then entering the L 5 (final) stage by 2 to 3
involves balanced systemic vasodilation with pimobendan months postinfection. The juvenile L 5 worms enter the vas-
(a phosphodiesterase-III inhibitor) and an angiotensin- culature within about 100 days of infection, where they
converting enzyme inhibitor (ACEI), as well as preload migrate preferentially to the peripheral pulmonary arteries
reduction with diuretics (furosemide). Further systemic of the caudal lung lobes. It takes at least 5 to 6 and usually
arterial vasodilation (afterload reduction) with amlodipine between 7 and 9 months before these worms develop into
also can be considered; amlodipine has some vasodilatory mature adults; after mating, gravid females release microfi-
activity in pulmonary arterioles as well. If clinically rele- lariae (L 1 ) and the infection becomes patent. Mature male
vant PAH persists despite therapy for pulmonary venous worms grow to approximately 15 to 18 cm, whereas adult
hypertension and CHF, sildenafil can be used as adjunctive females can reach 25 to 30 cm in length. Adult worms can
therapy. Dogs with right-sided CHF secondary to PAH (cor survive for 5 to 7 years in dogs. HW transmission is limited
pulmonale) are managed similarly to those with other causes by climate. An average daily temperature of greater than 64°
of CHF (furosemide, pimobendan, ACEI, dietary sodium F for about a month is necessary for the L 1 larvae to mature
restriction) with the addition of sildenafil. within a mosquito to the infective stage. HW transmission
Prognosis for dogs with PAH varies with underly- peaks during July and August in temperate regions of the
ing disease. Other than HWD, most causes of PAH are Northern Hemisphere.
advanced and incurable, and pulmonary vascular remodel- Microfilariae passed to another animal by blood trans-
ing is irreversible. The prognosis for dogs with severe PAH fusion or across the placenta do not develop into adult
generally is poor, with median survival times between 3 worms because the mosquito host is required to complete
and 6 months; treatment with sildenafil improves survival, the parasite’s life cycle. Therefore puppies younger than
with one study reporting nearly 75% survival at 1 year 6 months of age that have circulating microfilariae most
postdiagnosis. likely received them transplacentally and do not have patent
HWD. Survival of microfilaria for up to 30 months has
been reported.
HEARTWORM DISEASE HW development proceeds more slowly in the cat, which
is not the natural host, and infection does not become
HWD is an important cause of PAH in regions where the patent (mature) until at least 7 to 8 months postinfection.
disease is endemic. HW infection is widespread throughout Adult worms can live for 3 to 4 years in cats. Microfilar-
the United States, especially along the Eastern and Gulf iae are evident only in a minority of cats. Nevertheless,
coasts and in the Mississippi River Valley. The infection rate infection with L 3 through immature L 5 can cause sub-
in unprotected dogs can be up to 45% or higher in some stantial pulmonary disease as the host attempts to reject
areas. Sporadic cases occur in other areas of the country and the parasites.