Should Dogs and Cats Take Heartworm Preventatives?

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Yes, your dog or cat should take heartworm preventative medication, because it is very effective. For dogs, prevention is far superior to heartworm treatment—and for cats, there is no treatment for heartworm. Preventing your pet from becoming infected is better for his health, less expensive, and less time-consuming than treating heartworm itself.

Heartworm infection has the potential to occur wherever there are mosquitoes. It has been found in all 50 states and even in climates that people assume are safe. (Even desert areas may have irrigation drainage and other opportunities for mosquitoes to propagate.) Some pet owners think that they can stop giving their dogs and cats heartworm preventatives in the winter, but because of the parasite’s long maturation process, this puts pets’ health at risk.

What is heartworm disease?

[Heartworm disease] is a serious and potentially fatal infection in pets. It is caused by foot-long worms that live in heart tissue, lung tissue, and blood vessels, damaging the internal organs.

Heartworm is transmitted by mosquitoes that have drawn blood from infected animals that carry microfilariae, or baby heartworms. Over 10 to 14 days, while inside the mosquito, microfilariae mature into infective stage larvae. When an infected mosquito bites a susceptible animal, the larvae are deposited on the skin and use the bite wound to enter the new host.

Once inside a host, larvae develop into mature adult heartworms in about 6 to 7 months. Heartworms can live for 5 to 7 years in dogs and 2 to 3 years in cats.

Dogs are the natural host species, which means that heartworms that live inside a dog mature into adults, mate, and produce offspring. Left untreated, heartworm numbers can increase, and infected dogs can potentially harbor hundreds of worms. Heartworm treatment takes an average of about 6 months to complete, during which time your dog has to be on virtual bed rest. The treatment itself is also not without risk, because the process of killing heartworms can cause a severe, life-threatening anaphylactic (allergic) reaction. Finally, heartworm treatment is expensive and the injections are painful. For these reasons, heartworm prevention for dogs is by far the best option, and treatment—when needed—should be administered as early as possible.

Cats are an atypical host for heartworms, and most worms in cats do not mature to the adult stage. Cats are usually infected with only a small number of immature heartworms, which means that heartworm can often go undiagnosed. Unfortunately, even a small number of these parasites can cause a medical condition called [heartworm-associated respiratory disease]. There is no treatment for heartworm disease in cats, only supportive care, which makes heartworm prevention even more important.

Preventing your pet from becoming infected is better for his health, less expensive, and less time-consuming than treating heartworm itself.

What should I know about heartworm preventatives?

It wasn’t until the 1980s that heartworm preventatives became readily available to pet owners. Lucky for us, and our pets, preventatives are highly effective, and, along with testing, allow us to protect our dogs and cats from a devastating disease.

Heartworm preventatives are available as a pill, a topical medication, or an injection. They work by eliminating the immature larval stages of the heartworm parasite, including larvae deposited by a mosquito on the skin and the following stages that develop inside the host animal.

Unfortunately, heartworm larvae can molt fairly quickly (sometimes in as few as 50 days) into the juvenile or immature adult stage, at which point they cannot be eliminated by preventatives. It’s because heartworm preventatives work only on immature larvae that the medication must be administered on a strict schedule.

Oral and topical heartworm preventatives must be administered monthly, all year round, while injections are given every 6 months.

Can pets still get heartworms while on a preventative?

Yes, your pet can still get heartworm even if she is on a heartworm preventative. Most vets recommend that your pet take a heartworm preventative and also be tested for heartworm every 12 months.

Heartworm preventatives are highly effective, but not 100%. Not following a strict schedule of administering preventatives raises the risk of infection. Sometimes pills are not swallowed, or they may be thrown up. Topical creams can be washed—or licked—off. (Keep in mind that if your dog is a swimmer, she should be on an oral instead of a topical preventative).

Heartworm is a serious, progressive disease, with few early signs of infection, so testing is necessary to catch it as early as possible. Heartworm testing requires a small blood sample, which is tested for proteins that indicate the presence of heartworms.

What is the safest heartworm preventative for dogs?

The two most common, and generally safe, heartworm preventative ingredients used today are [ivermectin, selamectin, and milbemycin oxime]. These drugs have the potential to cause side effects in some pets, but when balanced against the dangers of heartworm disease, using preventatives is much safer than not.

There are a few specific considerations about what preventative will be safest for your particular dog. For instance, some breeds of dogs, such as collies and other herding dogs, can carry a genetic mutation that may result in serious side effects if they receive ivermectin or similar medications. If you own one of these breeds, or if you suspect your mutt has herding dog lineage, talk to your vet about which preventative your dog can take safely. Your dog’s vet can also answer any other questions or concerns you may have about heartworm preventatives.

An older, daily heartworm preventative, called diethylcarbamazine or DEC, is available as a generic. It has few side effects, but because missing a single day of medication might allow an infection to occur, it’s rarely used. It can also be life-threatening if given to a heartworm-infected dog with circulating microfilariae, because a rapid die-off of microfilariae can result in an anaphylactic reaction.

It’s best to avoid herbal and homeopathic heartworm preventatives and treatments, because there are no scientific studies to prove their effectiveness. Some, including wormwood and black walnut, are even toxic in large doses.

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