Page 344 - Feline Cardiology

Chapter 23: Heartworm Disease 357

Heartworm antigen
and antibody test
Negative Ag Negative Ag
Positive Ag
Positive Ab Negative Ab
Infested with adult Exposed to HW Exposure or
heart worms At risk for HARD infestation with HW
(usually 1-6) less likely
Additional diagnostics to Additional diagnostics to False negative test
stage disease: stage disease: possible. If symptomatic,
-echocardiography -echocardiography consider repeating HW AG
-thoracic radiographs -thoracic radiographs and AB in 6 months and/or
-complete blood count -complete blood count additional testing:
etc. etc. -echocardiography
-thoracic radiographs
-complete blood count
etc.

Start preventative Start preventative Start preventative
Treat clinical signs Treat clinical signs if in endemic area
as needed if evidence of HARD Treat clinical signs
-corticosteroids -corticosteroids if present
-bronchodilators -bronchodilators -corticosteroids
-etc. -etc. -bronchodilators
-etc.
Figure 23.2. An algorithm for the diagnosis and basic treatment options of heartworm disease in cats in which heartworm disease is
suspected.

on the test used (Snyder et al. 2000). Another study, Thoracic radiography and/or echocardiography are
which evaluated Ab and Ag performance in a large group the two most useful diagnostic tests after serology in
of cats, also showed significant variation in diagnostic feline patients exhibiting evidence of heartworm disease.
performance of the tests when compared to the gold Thoracic radiography is useful for diagnosis as well as
standard of necropsy (which was imperfect because it continued monitoring in cases of feline heartworm Heartworm Disease
was limited to the thorax) (Berdoulay et al. 2004). disease. The degree of caudal lobar arterial disease is best
Because migrating L 4 and L 5 larvae and/or ectopic adult assessed by the dorsoventral radiographic projection
worms would not have been detected by necropsy yet (Nelson et al. 2005). The caudal lobar arteries, especially
could still produce clinical signs and abnormal serologi- the right, are the first to enlarge and typically are the
cal results, the true number of infected cats was probably most severely diseased (Figure 23.3). The diameters of
underestimated. Moreover, cats that have recovered the caudal lobar arteries should not exceed that of the
from heartworm infection may have persistent Ag and ninth rib at the point of their superimposition.
Ab present for several months. In spite of these acknowl- Cardiomegaly with right ventricular enlargement is less
edged shortcomings, the study illustrated the limitations common in heartworm-infected cats compared to
of heartworm serology in cats and that combining Ag heartworm-infected dogs. Patchy focal or diffuse pul-
and Ab tests results in improved detection of heartworm monary parenchymal changes, in particular a bron-
infection compared to using either test alone (but it also chointerstitial pattern, are suggestive of (but not unique
resulted in more false-positive results). In this study, to) feline heartworm disease. Less commonly associated
antigen tests detected 79.3% to 86.2% of heartworm pulmonary findings include: hyperinflation of the lungs
infections and were highly specific, whereas antibody with flattening of the diaphragm, consolidated lung
tests detected 62.1% to 72.4% of heartworm infections lobes, pneumonitis (Figure 23.4), pleural effusion, and/
with 1.4% to 19.1% false-positive results (compared to or pneumothorax (Nelson et al. 2005). The radiographic
0.3% to 2% for antigen tests). Because both L 5 larvae picture often resembles feline asthma. However, radio-
and adult worms are capable of causing clinical disease graphic changes occur less frequently in feline heart-
in the cat, both antibody and antigen tests are useful worm disease than in canine heartworm disease and
tools, and the probability of making appropriate diag- the absence of radiographic abnormalities does not
nostic decisions improves when they are used together. exclude a diagnosis of feline heartworm disease. In fact,