No less an authority than the U.S. Food and Drug Administration recommends giving dogs heartworm preventive medication year-round. That's because it's hard to tell when the first mosquito of the season appears or the last one disappears, and mosquito bites are how heartworm transmission occurs in canines. If you live in a warm climate, your veterinarian will almost certainly advise year-round medication. In cold climates, any medication gap would last just a few months.
Canine Heartworm Disease
Heartworm disease in dogs is often fatal. While it's possible to treat a dog diagnosed with adult heartworms, it's a difficult, time-consuming process that can prove fatal in itself. An adult heartworm can reach a length of 10 to 12 inches. An infected dog can have dozens -- or more -- of these parasites in his body. Dogs with heartworm disease usually die of heart failure or pulmonary thrombosis, or blood clots to the lungs.
Mosquitoes are the vectors for canine heartworm transmission. If your dog is bitten by an infected mosquito, microscopic heartworm larvae enter his bloodstream. Although the larvae mature more quickly, it takes between six months and a year for the adults to invade the heart. Even if your dog never or rarely leaves the house, he's vulnerable, because mosquitoes often enter buildings. Even when it's too cold outside for mosquitoes, there's always the chance that some are still thriving in your warm house.
Heartworm preventives are given monthly, either orally or in topical form. In the latter, you squeeze the medicated solution on the skin between your dog's shoulder blades. Both types of medication work by eradicating heartworm larvae from your dog's bloodstream, before they can grow to adulthood. It takes only two months for larvae to mature into adults. That means that, in a worst-case scenario, your dog is bitten by an infected mosquito about a month after your stopped giving him heartworm preventive in the fall and develops a heartworm infestation by the time you plan to start him back on preventive in early spring.
Even if you keep your dog on heartworm preventive year-round, your vet will require an annual blood test to detect the presence of adult heartworms before writing you a new prescription. Giving preventives to infected dogs can prove fatal. The millions of larvae swarming throughout the bloodstream can suddenly die en masse, putting the affected animal into shock. Although it's unlikely that a dog on year-round preventive would have this reaction, it's a possibility. For example, your pet could throw up his medication without your knowledge, leaving him without protection.