Upon reaching the heart, the young adults continue to grow. Up to now there has been no evidence of disease in the dog. It is only after adult worms mate and start to discharge tiny motile microfilariae that circulate in the blood that disease becomes apparent. Microfilariae appear in the blood about 200 days after infection. The symptoms of the disease are not noticeable in most cases, until reduced blood flow caused by adult worms damages the heart, lungs, liver, and kidneys.
Drugs are available to prevent the disease, and it is curable if diagnosed early.
Distribution of Heartworm Disease Heartworm disease occurs worldwide in most tropical and subtropical regions, with increasing frequency in temperate climates. Until the late sixties, the disease was restricted to southern and eastern coastal regions of the United States. Now, however, cases have been reported in all 50 states and in several provinces of Canada. For most of North America, the danger of infection is greatest during the summer when temperatures are favorable for mosquito breeding. In the southern U. S., especially the Gulf Coast and Florida, where mosquitoes are present year-round, the threat of heartworm disease is constant.