Heartworm Prevention 101

Heartworm disease is transmitted by mosquitoes, and San Diego is one of the higher-prevalence metropolitan areas in the United States for two reasons: a mild climate that supports mosquito populations nearly year-round, and a large population of dogs who spend time outdoors. The American Heartworm Society has documented increasing case rates in Southern California over the past decade. If your dog is not on monthly prevention, the question is not whether they will be exposed — it is when.

The disease itself is caused by Dirofilaria immitis, a parasitic worm that lives in the heart and pulmonary arteries of infected dogs. Adult worms can reach 12 inches in length. A single infected dog can host dozens of worms. Early infection is completely asymptomatic — the worms are growing, and the immune system's response to them is gradually damaging the arterial lining and right side of the heart. By the time most owners notice symptoms (exercise intolerance, soft cough, fatigue), the infection is advanced and has been present for months.

Prevention is profoundly effective and inexpensive. Monthly chewables like Heartgard Plus and Interceptor Plus block larval development before worms can establish in the heart. They work by killing the larvae deposited by mosquitoes in the preceding 30 days — which is why consistent monthly dosing is essential. Missing one month does not create immediate danger, but a gap of two or more months can leave larvae unprotected long enough to develop beyond the susceptible larval stage. Annual heartworm testing at your dog's wellness exam is standard of care, because prevention is not 100% effective and because confirming a negative test before refilling the prescription is the safe, responsible approach.

Treatment — if it ever becomes necessary — is dramatically harder than prevention. The protocol involves a series of injections of melarsomine dihydrochloride (an arsenic-based compound), strict cage rest for 60 days, and careful monitoring for thromboembolic complications as dying worms break apart in the bloodstream. It is expensive ($900-1,500 at most practices), prolonged, and genuinely unpleasant for the dog. The best thing I can say about it is that it works, and most dogs treated before reaching severe disease stages recover fully. The simpler thing is to prevent the infection in the first place: one chewable, once a month, every month of the year in Southern California.