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Preventing feline heartworm disease

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Heartworm in cats is not limited to outdoor cats -- indoor cats are also at risk of the disease. When snow was still on the ground in early March, Elena O’Curry received a postcard from the Cat Hospital of Chicago, urging her to protect her cats from heartworm, a disease transmitted by mosquitoes. 

“I didn't realize indoor cats could be at risk – the card very clearly spelled out the risk factors and the options available,” said O’Curry, 27, an anthropologist who works for a marketing firm in Chicago. “My grandfather recently lost a cat to a heart defect, which made me more aware of potential problems.”

For the first time, she’s giving once-a-month chewables to prevent heartworm in her cats, Saucer and Pekoe, brown striped tabbies. 

Colleen Currigan, DVM, who for more than 20 years has devoted her practice to cats, said heartworm has received new attention in cats because research has shown heartworms – typically considered a problem in dogs – can be a threat not only to cats’ hearts, but also to their lungs.

What is feline heartworm disease?

Feline heartworm disease is caused when a mosquito carrying microscopic heartworm larvae bites a cat, Currigan said.

“The larvae are injected into the cat’s bloodstream. When the heartworms become sexually mature, they settle into the cat’s heart and lungs and then release their offspring into the bloodstream,” she said.

The parasitic worms, which are classified as nematodes, or roundworms, cause inflammation in the heart, lungs and arteries, especially when they die. Adult worms can cause heart failure and sudden death.

“What’s new as far as cats are concerned is that now we’re realizing that in all those cats who never develop adult worms, the immature worms can do a lot of lung damage during their development prior to reaching the heart,” Currigan said.

Heartworm tests for cats, unlike those for dogs, are unreliable. Currigan said that as a result it is difficult to know how common the problem is in cats. However, she noted that one study has shown, for example, that 13 percent of cats in Chicago and 25 percent of cats in Miami may be infected, adding that the problem has been observed in all 50 states.

“Research suggests that the condition is certainly higher than what we used to think and the current thinking is that the prevalence of heartworm is probably higher than it is of feline leukemia virus,” she said. 

Difficult to diagnose 

Currigan said often adult worms or the immature larvae lead to a newly defined syndrome called heartworm associated respiratory disease (HARD). She urged cat owners to look for the following signs:

  • Difficult breathing 
  • Heavy or fast breathing
  • Coughing or gagging 
  • Lethargy 
  • Weight loss 
  • Collapsing 
  • Seizures 
  • Loss of coordination 
  • Rapid heartbeat 
  • Diarrhea or vomiting 
  • Blindness

“HARD can be difficult to diagnose because many cats do not show symptoms,” Currigan said. “And because heartworm disease imitates ailments such as asthma, it can be difficult to distinguish.”

She said HARD is almost impossible to diagnose without conducting one or more of the following tests, often multiple times:

  • A physical examination
  • X-ray 
  • Angiocardiography (X-ray of the heart with injected contrast fluid) 
  • Echocardiography (ultrasound of the heart) 
  • Complete blood count (CBC) 
  • Serologic testing (antigen and antibody study)
  • Microfilaria testing

Educating the public

Until a year and a half ago, Currigan said she rarely discussed heartworm prevention for cats – with the exception of cats that roamed outside in the warmer months.

“In those cases, I was telling owners that the risk of fleas and intestinal parasites was quite high, the risk of disease from ticks and heartworms was much lower (and very low for heartworm),” she said. “But we know the risk for heartworm is much higher than we thought…at least for lung damage.”

The American Heartworm Society has partnered with the American Association of Feline Practitioners on a campaign to educate cat owners about the dangers of heartworm.

Preventive medicines  

There currently are no approved treatments for feline heartworm infections. Some infected cats recover on their own. Heartworm disease in cats may be difficult to diagnose and difficult to treat, but Currigan said it is easy to prevent with medicine given each month during mosquito season, or year-round in warmer climates.

She suggests that owners give the medication April through November. But, “a lot of veterinarians now suggest owners give it year-round, because owners tend to remember to give it better that way,” she said. “It really does not have to be given year-round, except in areas where mosquitoes are present year-round."

Currigan recommends topical medicine applied to the neck for outdoor cats. She noted that topical medicine not only prevents heartworm by killing the larvae, but also treats and prevents fleas, ear mites and intestinal parasites.

This treatment received some attention a year ago when the Food and Drug Administration approved Advantage Multi For Cats, a monthly topical medicine. Currigan recommends chewable medicines for indoor cats, which are also vulnerable to mosquito bites.  

Other parasites

Currigan said medications are prescribed based on the risk to exposure to parasites, owner preferences and the medications owners can most easily administer. 

Options for prevention include:

  • Heartgard for Cats (ivermectin): monthly chewable, prevents heartworm only
  • Advantage Multi for Cats (imidacloprid + moxidectin): monthly topical, prevents heartworm, fleas, ear mites, intestinal parasites
  • Revolution (selamectin): monthly topical, prevents heartworm, fleas, ear mites, intestinal parasites
  • Interceptor Flavor Tabs for Cats (milbemycin oxime): monthly chewable, prevents heartworm and intestinal parasites

Currigan said these products do not protect against ticks. For cats at risk of exposure to ticks, she advises use of Frontline (fipronil), a monthly topical, combined with Heartgard or Interceptor for heartworm protection.

Heading into the mosquito season, O’Curry said she feels her cats are safer now that they are taking the heartworm preventive. “My cats love sitting by the window and I would hate myself if I didn't do anything and something did happen.”

Credit: Reviewed by Amy I. Attas, V.M.D.
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