rusty
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Feb 16, 2005, 8:32 AM
Post #13 of 18
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Re: [scout] worms and prevention
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I felt compelled to share the lifecycle of heartworm infection again whenever I see so much of confusion as to when to start giving the first dose of heartworm preventatives, when to do testing, which type of heartworm prevention is better etc. Here goes : When we say that a dog has heartworm infection, it means that the dog has adult worms in the heart. The male and female adult worms will mate and produce babies which are called L1 (1st stage larvae) or Microfilariae. These L1 are in the blood circulation of the dog. When a mosquito bites the infected dog, it will pick up the L1. In the mosquito, the L1 will develop into L2 and eventually into L3 depending on the environmental temperature (anytime below 15 degrees Celcius, the development will halt). L1 to L3 will take a period of around 14 days. The L3 is the infective stage whereby the mosquito will look for another victim or may go back to the same infected dog and transmit the L3. Once the L3 gets into the dog, it will migrate through the tissue and become L4. L3 - L4 will take around 1-3 days. From L4, it will slowly migrate to the veins leading back to the heart and develop into L5. L4 - L5 will take around 60 days. L5 will eventually end up in the heart as adult worms and this process will take another 4-6 months. The male and female adult worms will start mating again and produce L1 and the process goes on as before. Therefore, from the lifecycle, what we can gather is that from the time a clean and healthy dog is bitten by a mosquito carrying L3, it will take at least another 6 months before they start developing adult worms in the heart. That is why a lot of people have this misconception that as long as I start giving heartworm preventatives before 6 months of age, my dog is safe. No ! No ! No ! This is not true. Just grab hold of one of the Heartgard brochures and it is mentioned in it very clearly that dogs as early as 6 weeks of age can be put on heartworm preventatives. The reason is because of how heartgard works. When a puppy is born, even at one day old it was bitten by an infected mosquito carrying L3, at 6 weeks old, the L3 would have developed into L4 and is also 6 weeks old. Heartgard given at 6 weeks old is able to kill the L4 which is also 6 weeks old from the time of infection at day old of the puppy. Beyond that, there is no guarantee of 100% efficacy. Therefore, any dogs given the 1st heartworm preventatives after 6 weeks old, will need a Heartworm test. However, the test specifically test only for adult worms and if you read back the lifecycle mentioned above, it will take at least 6 months from the time of infection by L3, before adult worms are developed and get detected by the test kit. That is why any dogs that is less than 6 months old, if we were to do a heartworm test, it will definitely be negative. Eventually, we are caught in a dilemma if we don't start giving heartgard by 6 weeks. Any heartworm preventatives given after 6 weeks of age, please go and enquire from your vet as to what you are suppose to do.
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