Page 230 - Small Animal Internal Medicine, 6th Edition

202 PART I Cardiovascular System Disorders

PAH in HWD. An additional diuretic (spironolactone or endoscopic retrieval instrument) are used to grasp and with-
hydrochlorothiazide) could be helpful in refractory cases, if draw the HWs through the jugular vein incision. The instru-
VetBooks.ir renal function is adequate. Dogs that develop right-sided ment is gently passed down the vein through the cranial vena
cava into the RA; repositioning of the animal’s head and neck
CHF before adulticide therapy should be treated for heart
failure while beginning initial American Heartworm Society
inlet. Fluoroscopic or echocardiographic guidance is helpful.
treatment guidelines (macrocyclic lactone and doxycycline). may be necessary to pass the instrument beyond the thoracic
If dogs respond well to heart failure treatment and remain The goal is to retrieve as many worms as possible without
clinically stable, they may receive the three-dose melarso- causing worm breakage; generally, five to six unsuccessful
mine protocol after the typical 2-month waiting period. attempts in sequence is the end point. Resistance to instru-
ment withdrawal from the vein may occur if too many
CAVAL SYNDROME worms are grasped at once or a cardiovascular structure is
The (vena) caval syndrome occurs when venous inflow to the grabbed by forceps. The jugular vein is ligated proximally
heart is obstructed by a mass of worms, leading to low- and distally, and then the subcutis and skin are closed rou-
output cardiovascular shock. Other terms for this condition tinely. Survival rates of 50% to 80% have been reported for
include postcaval syndrome, acute hepatic syndrome, liver dogs undergoing this procedure.
failure syndrome, dirofilarial hemoglobinuria, and vena cava Other supportive care given during and after surgical
embolism. It is an uncommon but devastating complication worm removal might include IV fluid therapy, particularly
in heavily infected animals. As the HW burden increases, in cases presenting with cardiovascular shock; however,
adult worms migrate from their preferred location in the most dogs with caval syndrome concurrently have right-
distal pulmonary arteries “backward” or “upstream” into the sided CHF, and fluid therapy will exacerbate venous conges-
RV, right atrium (RA), and caudal vena cava. Dogs with more tion. A hypotonic crystalloid solution (such as half-strength
than 40 worms or with more severe PAH have increased risk saline) is preferred, with fluid rate determined by the indi-
for developing caval syndrome. Caval syndrome occurs vidual case. Depending on the degree of anemia, blood
more often in geographic areas where HWD is enzootic. transfusion (fresh whole blood or packed red blood cells)
Most dogs that develop caval syndrome are males. Often might be required. Adjunctive therapy after HW extraction
no history of HW-related signs exists. Acute collapse is is similar to recommendations for the other complications
common, often accompanied by anorexia, weakness, tachy- of severe HWD. Treatment includes prednisone, sildenafil,
pnea or dyspnea, pallor, and hemoglobinuria. Dogs with and clopidogrel or aspirin (for pulmonary inflammation,
caval syndrome invariably have severe PAH and concurrent PAH, and PTE); pimobendan (for right-sided congestive
right-sided CHF, so additional clinical signs and findings heart failure [R-CHF]); and initiation of doxycycline and
typical of these syndromes can occur as well (see p. 201). macrocyclic lactone (as with any other HW-positive dog).
Intravascular hemolysis (from red blood cell trauma) Additional treatments for R-CHF, including furosemide and
results in Coombs-negative fragmentation hemolytic anemia, ACEIs, generally are delayed until the dog’s clinical condi-
hemoglobinemia, and hemoglobinuria. Hemoglobinuria is tion stabilizes and may not be required if venous conges-
considered a hallmark sign of caval syndrome. Other clini- tion improves dramatically after worm removal. In the acute
copathologic findings are similar to other presentations of setting, monitoring for anemia, thrombocytopenia, DIC,
severe HWD (see p. 197) and organ dysfunction is important; treatment is given as
Thoracic radiographs indicate right heart and pulmonary indicated. Severe PTE and renal or hepatic failure are associ-
artery enlargement, consistent with severe HWD. The ECG ated with poor outcome. Assuming clinical stabilization after
usually suggests RV enlargement. Ventricular or supraven- worm extraction, adulticide therapy can be started after the
tricular premature complexes are common. Echocardiogra- typical 2-month period to eliminate remaining worms.
phy reveals a mass of worms entangled at the tricuspid valve
and in the RA and/or venae cavae (see Fig. 10.3). Character- Heartworm Prevention
istic echocardiographic changes associated with severe PAH HW prophylaxis is indicated for all dogs living in endemic
(RV dilation and hypertrophy, paradoxical septal motion, areas. The American Heartworm Society recommends year-
tricuspid insufficiency, and a small LV) are also seen. round administration of an HW preventive drug to prevent
Without aggressive treatment, most dogs die within HWD, increase compliance, and control pathogenic and/or
between 24 and 72 hours as a result of cardiogenic shock zoonotic parasites. The time of year in which infection can
complicated by metabolic acidosis, DIC, and anemia. Worms occur is limited in many geographic areas because sustained
must be surgically removed from the vena cava and RA as warm and moist conditions are necessary for transmission
soon as possible. The dog is sedated (preferred) or anesthe- of the disease. Transmission usually occurs only during a
tized. A right jugular venotomy, with the dog restrained in few months in the most northern parts of the United States
left lateral recumbency, is the usual approach. Local anesthe- and Canada, and generally in fewer than 6 months per year
sia is used after the right jugular region is shaved and surgi- north of the Virginia–North Carolina border; however, local
cally prepped. The jugular vein is isolated, and loops of moist microclimates may allow larval development in mosqui-
umbilical tape or suture material are used to control bleeding toes for a much longer period. Year-round transmission is
once the vein is incised. Long alligator forceps (or another likely in the southern continental United States. Although