If you are considering putting your dog on heartworm prevention for the first time or if your veterinarian has prescribed your dog a year worth of heartworm preventative pills and you are now down to the last pill, very likely your vet will request a blood test to ensure your dog is free of heartworms. A blood test is important because, if your dog has heartworms and he is administered heartworm pills, he may be exposed to a very dangerous situation that is better understood by taking a closer look into how heartworms affect dogs.
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Identification
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Heartworm disease is caused by mosquitos. Heartworm disease takes place when heartworm infected mosquitoes bite the dog, delivering heartworm (Dirofilaria immitis) larvae into the dog's bloodstream. With time, juvenile heartworms will reach the lungs, where the the blood pressure will force them to settle into the small pulmonary arteries, according to American Heartworm Society. Then, as these heartworms grow and increase in size, they will occupy larger arteries and, in severe cases, even the heart's right ventricle, right atrium and the caudal vena cava.
Stages
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Once heartworms are adults, they will be sexually mature and start to reproduce. This means that these adult heartworms will give birth to millions of microscopic heartworms, which are referred to as microfilaria. These large quantites of microfilariae are released into the dog's bloodstream and are ready to be picked up by more mosquitoes looking for a blood meal. Ten to 14 days later, the microfilariae mature into larvae, which are injected into another dog by the same mosquito. The life cycle therefore starts over.
Function
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Once-a-month heartworm medication works by killing the microscopic heartworm larvae deposited by the mosquito. It is important to point out that heartworm medications work backwards--in other words, they eliminate all the heartworm larvae that the dog has acquired in the previous 30 days. This helps eradicate the heartworm larvae population by preventing them from growing into adults and causing all the common symptoms typical of heartworm disease. As effective as heartworm preventatives are in killing larvae, they are ultimately not capable of killing adult heartworms.
Effects
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Giving a heartworm-positive dog a heartworm preventative may cause rare, yet severe reactions that can prove quite harmful or even fatal to the dog, according HeartwormSociety.org. Indeed, if a dog is infested with adult heartworms, very likely millions of microfilariae will be circulating in the bloodstream. Giving a heartworm preventative at this stage will cause the sudden death of all these microfilariae, which may cause the dog to have a serious shock-like reaction.
Considerations
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A simple blood test should be carried out before starting heartworm preventatives. If the dog does not develop this shock-like reaction, it is important to consider that the dog still remains infected with adult heartworms. The heartworm preventative will not kill the adults but will only shorten their life span. Heartworm disease will still progress, ultimately leading to significant and often life-threatening damages to the heart and lungs. It is therefore of primary importance to determine if the dog is infected with heartworms prior to starting or resuming the administration of heartworm pills. This is accomplished by having the dog undergo annual heartworm antigen testing.
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