About Pet Heartworm Medicines
Heartworms are a serious risk to dogs. Even dogs who spend most of their time indoors are at risk, since mosquitoes are carriers and it takes just one bite to get infected. There are several heartworm medicines available, and offer inexpensive insurance against the deadly heartworm disease.
Significance
Heartworms are spread from dog to dog via mosquitoes carrying baby heartworms (called microfilaria) that are carried through the bloodstream to the heart. They lodge there and can grow into worms up to a foot long. It takes about 6 months for the microfilaria to grow into adult worms, and these weaken the heart and lungs and can eventually kill the dog. Humans do not get heartworms. Cats can, although it is very uncommon.
Identification
Heartworm infected dogs will start losing energy and vigor. They may tire quickly, be unwilling to exercise and in later stages will cough frequently. By the time these clinical signs are apparent, the dog is already in an advanced stage of heartworm disease. Whether or not the dog is on heartworm preventative medication, it should be tested every year. A simple, inexpensive blood test will determine the presence of microfilaria in the blood.
Types
There are three types of heartworm medication to prevent the disease. A daily pill called Filaribits used to be the only choice, but had to be given strictly every day to be effective. Most commonly, monthly chewable pills containing either ivermectin (Heartgard) or milbemycin oxime (Interceptor) are available through veterinarians or online. A 6-month shot called ProHeart was removed from the market because it was causing seizures and death in some dogs. Currently the safest and most convenient option is the monthly chewable, which is also available in a formula that kills several intestinal parasites.
Function
Heartworm preventative medication works retroactively by killing all microfilaria in the blood before they can mature. If a dog is already infected, immiticide is the heartworm medication of choice. It is administered in two injections, and kills the adult heartworms which break down and are absorbed into the blood. Preventing heartworms is inexpensive and safe. The treatment for an infected dog is expensive and hard on the dog.
Misconceptions
There are several misconceptions about heartworm disease and heartworm medications. All it takes is one bite by the wrong mosquito, so indoor dogs are also at risk. Long haired dogs are protected to a degree by their fur, but can still be bitten on the face or inner thighs. Some people think that feeding a natural diet or using herbal mosquito repellents will protect their dog, but no studies support this theory. Heartworm medications are extremely low risk compared to the danger of heartworm disease. Even if a dog is on the monthly pills year around, an annual blood test is advisable. All it takes is for the dog to surreptitiously throw up a heartworm dose, and they're unprotected.
Geography
Some areas, like much of the southeastern United States, are very high risk for dogs. Northern climates or very hot dry areas are less risky. Talk to your vet about the risk and follow her recommendation. The link below in Resources has extensive information on heartworm disease and its prevalence.
Expert Insight
Some dogs, collies and collie mixes, are sensitive to ivermectin. Many vets will prescribe milbemycin oxime (Interceptor) instead. There is a DNA test available to identify susceptible dogs, but it is expensive.
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