After the diagnosis of feline heartworm disease, the veterinarian and client are in a lose - lose proposition. One can let the adult worms die on their own over the next several years and run the risk of continued problems and on occassion an acute crisis. Or one can use an adulticide and eliminate the worms and run the risk of acute complications associated with the worm(s) dying all at once with severe consequences.
Mechanical removal of heartworms through surgery or special forceps and brushes has been successful. Mechanical removal should only be attempted in cats where worms have been demonstrated in the rights ventricle or pulmonary arteries by echocardiogram.
The nature of feline heartworm disease to cause chronic
vomiting, intermittent respiratory signs, or to be asymptomatic often misleads
the client into thinking the disease is not severe. Spontaneous acute complications
and death in a small percentage of cats can occur. Therefore, the client
must be warned that withholding therapy can be lethal in a minority of
cases. In the asymptomatic cat, this risk
appears to be small compared to the complications of adulticidal therapy. Because
the adult heartworm has a shortened longevity in the cat compared to the
dog, the possibility of spontaneous recovery should also be discussed.
However, the natural death of the adult worms can be associated with severe
respiratory signs. Cats which have been managed conservatively by intermittent
corticosteroid therapy have developed peracute signs and died from heartworm
disease. In the cat with recurrent dyspnea that is life-threatening or
with clinical signs that are unacceptable to the owner, adulticidal therapy
has been used safely and should be considered.
Adulticidal Therapy: Treatment of feline heartworm disease with thiacetarsamide sodium (2.2 mg/kg IV, bid, two days) is tolerated by cats without immediate complications of hepatotoxicity or renal toxicity. The use of ketamine as a sedative to aid in careful administration of thiacetarsamide is recommended in active cats. There are occasional reports of acute symptoms after thiacetarsamide injections, but slow injections have not caused acute collapse in normal healthy cats in this author's experience. Pulmonary edema as a complication during the two days of injections has been observed and oxygen therapy and corticosteroids should be considered if dyspnea and/or cyanosis occurs. This complication cannot be predicted and attempts to reproduce the acute lung injury have been unsuccessful.
In the symptomatic cat, clinical signs tend to improve after therapy. However, anorexic cats may require hyperalimentation. Although the presence of circulating microfilaria is uncommon, ivermectins have both been used successfully as microfilaricides. Imidicide at the dog dose should not be used in cats.
Post Adulticidal
Complications: Complications after therapy are usually related to
embolization.
The complication of pulmonary edema and cyanosis warrants further consideration
but has not been consistent with this author's experience. Sudden death
from embolization can occur especially within the first 10 days after adulticide
administration. Embolization can induce severe lung injury, hemoptysis
and dyspnea. Severe thrombocytopenia
and disseminated intravascular coagulation has not been noted. Based on
the assumption that
heartworm
mass is related to antigen load, a cat with a " strong positive "
antigen test would be more likely to develop post-adulticide complications
than a cat that has a low worm burden and is antigen negative or "weakly
positive." Embolization most often affects the caudal lung lobes and
thoracic radiographs may demonstrate a lung lobe with increased density.
Oxygen therapy is indicated if dyspnea occurs. High doses of corticosteroids
(1-2 mg/lb of prednisolone three times a day) with careful IV fluid therapy
will often support the cat through the crisis. The routine use of corticosteroids
is not recommended before or after thiacetarsamide in cats. Aspirin is
contraindicated in feline heartworm disease. Based on current information,
there is evidence that aspirin may inhibit
prostaglandin
formation and thus increase leukotriene production in the lung;
the result would be increased inflammatory mediators,
bronchospasm
and pulmonary hypertension.
Because of the potential protective effects of ketamine as an
serotonin
antagonist, a single IM injection of ketamine has been recommended before
administration of the first dose of thiacetarsamide. The
peracute
nature of the post-adulticide reaction dictates that the cat be under constant
attention, especially during the first two weeks. The clinical and radiographic
signs of acute embolization can resolve over one to two days. However,
death can occur before therapy can be instituted. The client should be
aware that the risk of complications in the cat seems to be greater than
in the dog. The severity of the post-adulticidal reaction poses a dilemma
for the veterinarian and the risk of post-adulticide complications is probably
greater than the risk of spontaneous death in the asymptomatic, heartworm
infected cat. The advantage of treating a cat is being able to observe
the cat during two week period after thiacetarsamide therapy while the
worms are dying compared to not knowing when the heartworms will die on
their own in an untreated cat.
Efficacy of Treatment: Although heartworms in cats may not live as long as in dogs, clinical signs and even death may occur. The efficacy of thiacetarsamide cannot be evaluated in many client cats because of the occult nature of the disease. However, of cats that have had microfilariae, repeated attempts to eliminate microfilariae have failed and repeated adulticidal therapy has been required in some. However, current research seems to indicate that the adulticide is effective and clinical signs usually abate during the initial weeks after thiacetarsamide. As is known in the dog, immature worms are probably resistant to thiacetarsamide. If a cat was antigen positive before therapy, the antigen test should be negative 12 weeks after adulticide therapy. A positive test at this time would indicate the presence of adult heartworms after the adulticide.
Conservative
Therapy: In cats with intermittent clinical signs or if the
owner will not accept the potential risk of adulticidal therapy, the owner
should be educated as to the nature of the peracute signs of embolization.
Alternate day prednisolone therapy (5 mg/kg) has been used successfully
to prevent clinical signs of coughing and vomiting. However, progression
of radiographic lesions have been observed during corticosteroid therapy.
In addition, acute respiratory distress and death have occurred in cats
on conservative glucocorticoid
therapy. An emergency dose of oral or injectable glucocorticoid should
be dispensed to the
owner to be administered if collapse or dyspnea are noted. The onset of acute
respiratory signs in a heartworm cat is a true emergency requiring immediate
care. The radiographic signs of severe lung pathology should not be over-interpreted
as "consolidation or pneumonia." The initiation of intra-nasal
oxygen therapy, cage rest, small volumes of intravenous fluids, and injectable
prednisolone has resulted in clinical improvement and resolution of radiographic
signs within 24 hours of presentation in cats with life-threatening dyspnea
and collapse.
Preventative
Medication: In endemic areas with
vector
populations (dogs) providing the mosquito with a
reservoir,
the incidence of heartworms in cats indicates that preventive medication
are needed. Infection from D. immitis in cats can
be prevented with the newly released feline product Heartgard for Cats,
(24 mcg/kg of ivermectin; Merial Limited, Iselin, NJ, 07065).
administered per os once a month. In endemic areas, it is suggested that
preventative medication be administered as early as 6 weeks of age and continued
for the life of the cat.
Selamectin (Revolution®-Pfizer Animal Health) was approved by the U.S. Food and Drug Administration (FDA) as a heartworm preventative for cats and dogs at a dose (6 mg/kg) applied topically once a month starting within one month of the pet's exposure to mosquitoes. Although other heartworm preventatives have been combined with additional products to gain label claims for internal parasites, selamectin as a singular topical product in cats prevents heartworms, treats and controls fleas and ear mites, and is indicated in treatment of hookworms and roundworms. Selamectin can be initiated at 6 weeks of age.
Because current antigen testing is inconsistent in cats, especially those with a low worm burden, antigen testing before instituting preventative therapy in an asymptomatic adult cat would not seem to be cost effective. Heartworm positive cats may be safely placed on preventative medication. A positive antibody test reflects that the cat has been successfully infected and that the parasite has lived several months and may or may not have developed to be an adult. A positive antibody test does not preclude administration of preventative medication. Although heartworm disease may be of low incidence in many areas, the high rate of complications associated with feline heartworm disease makes preventative medication an attractive alternative. Sub-clinical signs of heartworms may precede the more obvious clinical syndromes of allergic lung disease in cats. Although heartworm disease can be self-limiting in many cats, the potential to initiate inflammatory lung disease and predispose to bronchial asthma may prove to be adequate indications for preventative medications for cats in endemic areas.
|
|