Antigen testing is detection of a glycoprotein of fully mature adult female heartworms in circulation. Therefore it is specific
for D. immitis and will be negative in immature infections, male only infections, and dogs or cats with insufficient numbers
of mature female heartworms to produce sufficient detectable antigen. Antibody testing however is influenced by the host response
to the infection, and the antibody being evaluated varies from assay to assay as to the heartworm antigen source of the feline
antibody being evaluated. Much like checking in on a football game, some assays pick the score up earlier in the game and
others are more dependable later in the game. Despite the differences in the various antibody tests, most cats that are infected
with heartworms become positive by about two-and-half to three months after the infection. A cat that is reported to be antibody
positive has been successfully infected with heartworms, the L3 have molted to L4 and L5 and may or may not be currently present
in the heart – but they have been infected. In a project currently being conducted at Auburn University, about 25 percent
of random cats in the area that are over 1 year of age are antibody positive for feline heartworm disease. This infection
rate suggests that many of these cats have had immature adult heartworms reach the lungs and induce pathology. Most of the
research on feline heartworm disease have demonstrated that when experimentally infected with 25 to 50 infection larvae, the
majority of cats will become antibody positive. However, mounting clinical evidence has demonstrated that some cats that have
been infected and have adult heartworms demonstrated will be antibody negative, regardless of the assay method. A positive
antibody result does represent a cat that has been successfully infected, however, a negative antibody result does not rule
out feline heartworm disease infection. Antibody testing does not assure the heartworm status, either way, for the owner as
to whether adult heartworms are currently present.
Photo 2: AVD radiograph of a cat experimentally infected with L3 demonstrates that four months after the infection there can
be significant inflammatory lung disease. (Demonstrated in the right caudal lung lobe as increased opacity.)
Tentative diagnosis: Lose/lose proposition
After infection, cats have several different alternative paths for clinical disease. Some cats will have the immature heartworms
continue to develop and, with or without intermittent clinical signs, have adult heartworms present. Other cats may have all
the immature worms die over a period of several months with or without clinical signs. As noted in sedentary dogs, the majority
of dogs currently diagnosed as heartworm positive have no clinical signs. In the cat, the suggestive clinical signs are coughing,
dyspnea and vomiting unrelated to eating. However, many cats with adult heartworms will be asymptomatic for the majority of
the time the cat is infected. Acute crisis and dyspnea can occur and are most often associated with the death of heartworms
In the examination room, discussions with owners about heartworm disease can be difficult. The tentative diagnosis of feline
heartworm disease is usually in the presence of clinical and radiographic signs associated with heartworms and is usually
supported by a positive antibody test (Photo 4, p. 26). Neither a negative antigen nor a negative antibody test rule out the
possibility of heartworms. However, the tentative diagnosis places the veterinarian and owner in a lose/lose proposition.
Only a positive antigen test or confirmation of the physical presence of the adult worm by echocardiography can definitively
document the heartworm status. However, reliance on these diagnostics as being the only "confirmatory" results ignores the
three-month disease cycle and this early severe inflammatory lung disease and resultant clinical signs in cats.
Photo 3: Although enlargement of the right caudal pulmonary artery has been associated with feline heartworm infections, this
radio-graphic sign may disappear even in the presence of a continued heartworm infection. Much of the opacity is associated
with inflammation of the adjacent tissues and a periarteritis.
A tentative diagnosis of feline heartworm disease is often initiated by an owner presenting a cat with typical "asthma" symptoms.
The routine therapy of a combination of corticosteroids and bronchiodilators usually results an initial response. The addition
of an antileuko-triene such as Singular (Merck) at a dose of 5 mg/cat once a day may also reduce the dose of corticosteroid
needed to prevent clinical signs. However, the continued concern of the owner and veterinarian in this lose/lose proposition
is the fact that if this cat does have adult heartworms, there is the real risk that an acute life-threatening crisis can
develop. This can develop without any warning to the owner and the low doses of prednisolone typically used to control coughing
will not prevent this acute crisis. The owner can become constantly fearful for the cat and the veterinarian can offer little
assurance as to when and if this crisis will develop. Most cats have adult heartworms and intermittent disease and the adult
heartworm die without a life threatening event. However, like selling insurance, prediction of consequence is all but impossible.
In an attempt to blunt these fears, owners should be dispensed 50 mg prednisolone tablets to be given to their cat if they
notice abnormal breathing patterns as they are rushed to their veterinarian. In instances where emergency therapy may be initiated
by the veterinarian without current records on the cat, a vial of Solu-delta-Cortef (100 mg) is often given to the owner to
place in their refrigerator with instructions to the admitting veterinarian for IV administration of the steroid combined
with nasal oxygen therapy at the initial assessment.
Photo 4: A consequence of feline heartworm infection can be the peribronchial inflammation and coughing which is consistent
with cats presented for "asthma."
In the inability of the veterinarian to clearly define the heartworm status of a cat to the owner, the simplicity of using
a preventative has much merit. There are consequences to feline heartworm disease even if the diagnosis cannot be clearly
and inexpensively defined for the owner. Much as the religious use of a vaccination program negates the need for testing of
many viral infections, the use of heartworm preventative in cats avoids the morass associated with feline heartworm testing