Page 343 - Feline Cardiology

356 Section K: Heartworm Disease

disease. There are also rare reported cases of pneumo- and right heart failure occur less frequently than in the
thorax secondary to D. immitis (Smith et al. 1998). dog (Nelson et al. 2007). Caval syndrome is rare in cats
In a subset of cats, severe respiratory distress ± col- because worm burdens are usually low; however, even
lapse or sudden death may be the first clinical sign rec- single worm infections may cause tricuspid regurgita-
ognized by the owner (Nelson et al. 2005). It is a much tion and a right sided systolic murmur.
more common presentation in feline heartworm disease
than in canine heartworm disease. Acute death in DIAGNOSIS
affected cats has been attributed to pulmonary arterial
infarction following spontaneous death of adult heart- Diagnostic confirmation often requires a combination
worms; however, filariae are not always present emboliz- of tests. See Figure 23.2 for an algorithm of heartworm
ing the main pulmonary arteries and the lung lobes are disease diagnosis in the feline patient that is suspected
rarely ischemic (based on radioisotope studies). Recently, of having heartworm disease based on clinical signs.
an experimental model of acute anaphylaxis using D. Affected cats usually lack circulating microfilariae (tests
immitis–sensitized cats challenged with intravenous for microfilaremia are positive in less than 20% of
heartworm antigen demonstrated a similar shocklike infected cats), and the low worm burdens in this species
reaction; the severity of response was influenced by the result in light or absent antigen loads. Positive antigen
amount of exposure (Litster et al. 2008). Thus, it is theo- test results are the result of the presence of adult female
rized that damage to the heartworm cuticle causing worms. The antigen is associated with the worm’s
release of large amounts of heartworm antigen (Ag) reproductive tract. In cats, antigen tests have excellent
results in acute systemic anaphylaxis and death. See specificity but false-negative results are common. False-
Table 23.1 for a list of clinical signs that may be associ- negative test results are explained by low worm burdens
ated with heartworm disease. On the other hand, with male-only infections, and infection with young (<7
approximately one third of affected cats will be asymp- months old) female worms that have immature repro-
tomatic, perhaps in part due to their sedentary lifestyle ductive tracts. The sensitivity of commercial Ag tests has
and variable immune response (Nelson 2008a). improved dramatically in recent years and most are
Arterial intimal proliferation typical of canine heart- effective at detecting a singe adult female worm. In the
worm arteritis develops in feline pulmonary arteries as cat, heartworm Ag is detectable about 5.5 to 8 months
Heartworm Disease short in duration, the lesions are often localized and less a current infection with at least 1 adult female worm.
well. However, because infections are usually light and
postinfection, and a positive Ag result generally indicates
obstructive than in dogs. Therefore pulmonary hyper-
Heartworm-associated antibody (Ab) tests are useful
to rule out infections in cats when there is an index of
tension with subsequent right ventricular hypertrophy
suspicion for the disease; however, these tests also have
significant limitations. Antibodies can appear as early as
Table 23.1. Clinical signs associated with heartworm disease
2 months postinfection. Because these tests detect expo-
Acute sure to migrating heartworm larvae, they may be posi-
Sudden death tive in cats with previous heartworm exposure or
Collapse infection, so a positive antibody test indicates exposure
Shock but not necessarily active infection. Previously false neg-
ative results were thought to be rare, but recent studies
Dyspnea
suggest approximately 14–50% of infected cats are anti-
Blindness
body negative (Atkins et al. 1998; Kalkstein et al. 2000;
Seizures
Nelson et al. 2007). This wide range is probably partly
Vomiting/diarrhea
because the different antibody tests vary in their sensi-
Ataxia
tivity to the various stages of larval development, and
Chronic
evidence suggests antibody levels in the cat decrease over
Vomiting time as the worm matures. Also, heartworm-infected
Cough cats with clinical signs are more likely to be positive than
Dyspnea infected asymptomatic cats (Nelson et al. 2007). In a
Lethargy necropsy survey comparing 6 different Ab tests in an
Anorexia endemic region, Ab results varied greatly, with as many
Weight loss as 86 (26% of the cats) having positive results to as few
Weakness as 9 (3% of the cats). This correlated with a positive
predictive value ranging from 17.3 to 62.7% depending