The Heartworm is a blood parasitic worm, not associated with the common gut worm parasites (round, hook and tapeworms). Heartworm normally live free floating in a section of the heart (right ventricle) and nearby vessels.
Even if your dog never comes in contact with other dogs, Heartworm can still be passed to your dog, as Heartworm is transmitted by mosquitoes. Prevalence in unprotected dog populations living along rivers is often much higher than in populations living away from rivers.
The life cycle of the Heartworm is complex. Adult Heartworm once mature (at six months) produce “babies” called microfilaria, which swim around the bloodstream.
As a mosquito bites your dog, it sucks out blood containing these microfilaria and this is what will be injected into the next dog the mosquito bites. Again these larval stages take 6 months to mature into adults.
Tests are unable to detect this larval stage, so in the first 6 months of infection, Heartworm cannot be tested for.
Therefore Heartworm preventatives are initiated first and tests to detect Heartworm in your dog are carried out 6 to 12 months after prevention has been commenced.
Dogs not on preventatives, will eventually succumb to heart failure and die. The earliest pathological changes associated with Heartworm are usually due to inflammatory processes that occur in and around the arteries of the lower portion of the lungs. Later, heart enlargement will result and the heart will become very weak due to an increased workload and congestive heart failure may occur, which over time will lead to death.
This is easily achieved with a yearly injection, which can be commenced in your dog as early as 3 months old without any testing procedures.
Older dogs who have not been on Heartworm preventatives before, will need a Heartworm test done prior to commencement of the yearly Heartworm injection and a repeat test 12 months later. If yearly Heartworm injections are kept up thereafter, no further testing will be required.