Think heartworm disease only occurs in dogs? Think again. Although cats are not the natural host of this parasite they can still be infected and succumb to serious disease.
In fact recent studies have shown that the prevalence of feline heartworm infection exceeds the rate of feline leukaemia virus (FeLV) infection and feline immunodeficiency virus (FIV) infection in some areas of the USA.
However no recent prevalence studies in Australian cats have been conducted. The infection rate is directly related to the number of heartworm infected dogs in the area.
The good news is that your cat can be protected from this nasty disease with a simple monthly spot-on treatment.
About the Heartworm Parasite
Heartworm disease is caused by the parasite Dirofilaria immitis, which naturally infects dogs. Adult worms live in the heart and produce offspring called microfilariae. These circulate in the bloodstream of infected animals and are taken up by mosquitoes when feeding. Within the mosquito they develop into infective larvae and when the mosquito feeds these larvae are transferred to the new victim. The larvae develop into immature worms over 3 to 4 months and migrate to blood vessels in the lungs and to the heart where they mature and produce more offspring, starting the cycle over again.
Why is Heartworm Disease in Cats Different From Heartworm Disease in Dogs?
Because cats are not the parasite’s natural host, they develop a very strong immune response directed against juvenile worms. Generally it is this overactive immune response that causes the symptoms of disease in cats, rather than the direct presence of adult worms in the heart. In dogs heartworm disease is a heart disease – in cats, it is a lung disease since that is where juvenile worms are present. Hence in cats the disease is more correctly known as Heartworm-Associated Respiratory Disease (HARD).
The strong immune response generated results in an influx of inflammatory cells into the lung tissue. This inhibits lung function by preventing the transfer of carbon dioxide out of the body and oxygen into the bloodstream. This can result in lethargy and weakness, inappetence, weight loss, coughing, vomiting, breathing difficulties, fainting or even sudden death. Obviously these symptoms are not specific to heartworm infection and HARD can often be misdiagnosed as feline asthma.
If worms survive to adulthood in cats usually only a few worms are present and these die in 2 to 3 years compared to 5 years on average in dogs. When the worms die, as they disintegrate pieces can lodge in blood vessels and form clots. This can cause complications in many organs including the kidneys and the brain. An immune response is also directed against the worm pieces and this can lead to widespread life-threatening inflammatory disease. The severe consequences that can occur due to death of the adult worms is one reason why it is not recommended to treat heartworm infection in cats by killing off the worms since approximately a third of treated cats can have life-threatening reactions.
How is Heartworm Infection Diagnosed in Cats?
Diagnosis of heartworm infection in cats is more complicated than it is in dogs. The typical blood test run in dogs detects proteins produced by adult female worms. Worms often don’t survive to adulthood in cats and if they do, the number of worms is much fewer than in dogs, so this blood test can be negative even in an infected cat. Also if only male worms are present they will not be detected by this test.
Similarly tests to directly find microfilariae in the blood can be very unrewarding as this requires adult worms of both sexes to produce offspring. Additionally microfilariae are rapidly cleared by the cat’s strong immune response.
Another blood test detects antibodies produced to the worms by the immune system of an infected animal. This can be very useful in cats however a positive test result may not indicate an active infection since antibodies can persist for many years after an infection is cleared. Also the test may be negative in the early stages of infection since antibodies take time to be produced.
The current recommendations for diagnosis of HARD in cats is based on a combination of clinical signs, antigen testing and if this is negative, antibody testing and chest imaging (xrays and ultrasound of the heart).
Can Heartworm Disease be Cured in Cats?
Because of the risks associated with killing off adult worms in cats this treatment is generally not recommended. Since the clinical signs of HARD are usually due to the overactive immune response, immune suppressants such as cortisone may be prescribed to relieve the symptoms and the adult heartworms are left to die off naturally. Lung disease can be monitored by chest xrays every 6 months or so depending on the severity of symptoms.
But heartworm infection is not inevitable for your cat! Safe, effective preventative treatments are widely available to kill larvae on infection. These should be given to all cats throughout their entire lives. And although the risk may be lower in indoor cats they can certainly still be infected since all it takes is one unlucky mosquito bite. So why risk it? Make sure your cat’s heartworm prevention is up to date.
Dr Robyn Hall is a Sydney based veterinarian with a strong interest in feline medicine and well-being. She shares her life with two beautiful tabby domestic shorthairs, Elara and Callista, who she adopted from a vet clinic whilst living in Perth.