There are 3 serologic methods which have been used for feline heartworm disease. Rapid advances are constantly changing these assays.
IFA Testing: The confusion over the interpretation of the different tests and the variability of the methodology of individual laboratories has caused the practicing veterinarian serious problems in making a definitive diagnosis. With the high incidence of occult disease in the cat, the use of serology is a valuable asset. The IFA test (detecting antibodies to microfilarial cuticular antigen) is diagnostic in about 33% of positive cases, but the presence of immature or sterile worms, worms of only one sex, or the absence of host response to antigen does not produce a diagnostic titer. Use of the somatic IFA (detecting antibodies to microfilarial somatic antigen) is non-specific. These assays have been helpful in researching the biology of the parasite-host interaction, but have limited application to clinical diagnostics. New adaptation of this technology awaits independent validation.
Antibody
Testing: The ELISA test (detecting feline antibodies to adult heartworm
antigen) shows promise and initial concerns related to false positives
from cross-reactivity have not been detected. The use of the ELISA
(as adapted from the canine ELISA) in the cat to confirm a clinical diagnosis
has been very helpful and false positives from cross-reactivity have not
been observed. The canine methods for measuring dog antibodies to heartworms
cannot be used on cat sera. Initial studies of cats that have eliminated
the adult parasite naturally or after adulticidal reveals that a negative
ELISA titer develops when the host antibody
gradually decreases to negative concentrations (4-6 months). The ELISA test denotes
a method of analysis, therefore antigen preparation, antibody sources,
and techniques can vary between diagnostic laboratories and titers may
differ accordingly. (AAVP abstract on clinical study.)
Because the antibody being detected in produced by the cat in response to the early migration of the L3 or L4 larvae, positive titers are detected about 2-3 months after a successful infection. Detection of antibodies by the antibody test is of clinical significance in symptomatic cats which have negative antigen tests due to the presence of immature worms. However, with the use of macrolides as a preventative medication, the larvae in a cat can initiate a positive antibody response and then be killed by the macrolide; producing a antibody positive but heartworm negative cat. Additionally, the death of adult heartworms may produce a strong antibody response after release of large amounts of antigen. Some of the highest titers are associated with severe clinical signs in cats where the worms have died and the disease may be resolving.
In rare cases, the antibody test can be negative even in the presence of adult worms. There are several different antibody tests currenly available, and there are differences in the specific antibody each appears to quantitate.
Antigen Testing: Heartworm antigen detection tests utilizing blood or serum have been successful in dogs and have been positive in cats within days of transplantation of mature adult worms from dogs into cats. Since the antigen being detected seems to be derived primarily from the adult female reproductive tract, immature infections, a low worm burden, a male infection, or sexually immature worms may not produce enough antigen to be detected. The elimination of the adult parasite will also cause a negative antigen test. Cats may develop positive antigen tests 6 months after the experimental introduction of large numbers of infective larvae.
However, clinical cats and experimentally infected cats with active heartworm disease and high antibody titers can be negative on antigen testing. The low number and slow maturation of adult worms in clinical infections and the clinical signs associated with immature worms make it prudent to consider a positive antigen test diagnostic but not to rule out heartworms based on a negative antigen test. Most cats with heartworm disease are antigen negative.
There are now several different assays for antigen in the blood, and each should be reviewed for strengths and weaknesses. However, currently the antigen tests appear to be detecting the same basic glycoprotein.
Electrocardiogram: Although subtle signs of right ventricular enlargement are occasionally noted (with unipolar chest leads) a right axis vector (>120 degrees) on a standard 6 lead EKG is rare. Ectopic ventricular beats and other arrhythmias have been infrequently seen after adulticide in asymptomatic cats.
Radiography:
Radiology is a screening tests for feline heartworms. The pulmonary
parenchymal
changes are non-specific and can change rapidly in infected cats. The
lung changes include diffuse or coalescing infiltrates, perivascular densities,
and lung atelectasis. The most distinctive radiographic sign is enlarged
pulmonary arteries with ill-defined margins.This is most prominent in the
caudal lung lobes on the VD view.
Blunting and tortuosity of the pulmonary arteries are occasionally seen,
but not as common in cats as in dogs. An enlarged main pulmonary arterial
segment extending beyond the cardiac border on the VD or DV
view is not a classic feature of feline heartworms.
Arteriograms
as a diagnostic tool may demonstrate the enlarged pulmonary arteries and
embolus. 
A non-selective angiocardiogram is a simple and safe method of confirming
a tentative diagnosis of heartworms. A radiographic exposure 5-6 seconds
after injection of a contrast material into the jugular vein will provide
good visualization of the pulmonary vasculature and on occasion the presence
of worms. There does not seem to be a correlation between the severity
of lesions based on angiocardiogram and the severity of clinical signs
or post-adulticide reaction. Some cats with heartworm disease have normal
radiographs. Because of the changing nature of the disease over time, repeated
radiographs are often necessary.
Echocardiogram: Parallel
hyperechoic
lines, representing an image from the heartworm cuticle, may be observed
in the pulmonary arteries (PA), right ventricle (RV), or 
rarely
the right atria (RA). These lines are generally not over 0.5-1 cm in length
because of the angle of the probe and curved nature of the worms in the
heart. Heartworms in the most distal pulmonary arteries often cannot be
visualized (distal to LPA & RPA). Echocardiography (images: Dr. C.
Atkins, NCS) is useful to confirm a tentative diagnosis of heartworms.
Actual videos of the echocardiogram pictured above are available in
several different formats below. These are large files and may take a
considerable length of time to download over a modem connection.
The file size is listed next to each available version.
| large .avi (13 M) | small .avi (4 M) | large .mpg (5 M) | small .mpg (1 M) |
The right parasteral view gives the best view of the pulmonary outflow tract.
The most common location of the adult worm is the pulmonary outflow tract.
Tracheal Cytology:
The finding of eosinophils on
a tracheal wash is common
in heartworm disease, asthma and
parasitic lung diseases. In feline heartworms, the presence of eosinophils
on the wash seems to occur 4-7 months after L3 infection and often may
not be present later in the infection even when adult worms are present.
Tracheal cytology typical of chronic inflammation may be present after the eosinophilic
reaction resolves. Careful fecal examination should be performed before
the tracheal wash. Fecal flotation and direct smears may reveal the large
operculated egg of Paragonimus kellicotti or the larvae of
Aelurostrongylus abstrusus.
In the cat with respiratory signs, heartworm disease must be differentiated from Aelurostrongylus abstrusus or Paragonimus kellicotti infection, asthma, cardiomyopathy, and other diseases associated with dyspnea (pyothorax, pleural effusions, pneumothorax, anemia, etc). Although each in various stages can mimic the clinical and radiographic pulmonary parenchymal changes, the pulmonary arterial changes of heartworm disease are unique, if present, and can be enhanced by contrast procedures.
The changing clinical and radiographic pattern of disease make the diagnosis difficult and over the time course of the disease, there will be differences in diagnostic results. The peripheral eosinophilia, eosinophilic tracheal cytology, and chronic cough of feline heartworms is consistent with a diagnosis of "bronchial asthma." However, an apparent higher incidence of asthma has not been reported in heartworm endemic areas. The enlarged pulmonary arteries and muscular hypertrophy of Aelurostrongylus abstrusus and Toxocara cati infection is clinically uncommon.
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Table 1: Clinical Signs of Feline Heartworm Disease
| Chronic Signs
|
Acute Signs
|
Table 2: Diagnostic Testing for
Suspected Feline Dirofilariasis
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