A Long Beach dog recently tested positive for heartworm disease. The dog had been on and off heartworm prevention medicine over the last couple of years. His last heartworm test was about two years ago and was negative at that time.

This means the dog was infected sometime between that negative test and six months ago. (The earliest a test will detect the disease is approximately six months after the initial infection). We suspect this dog was infected during our extremely rainy winter this year.

Heartworm disease is a serious parasite infection caused by small worms that migrate through the body, molting and growing, until they end up in the heart and lung of dogs, cats, ferrets and several other mammals. It can cause serious pulmonary and cardiac damage and is often fatal if not treated in time. Animals contract the parasite from an infected mosquito. Mosquitos reproduce and thrive in standing water.

As is often the case with recent infections, this dog did not exhibit any abnormal signs at all. The testing was part of a routine annual exam. According to the American Heartworm Society, annual testing is necessary, even when dogs are on heartworm prevention year-round, to ensure that the prevention program is working.

Heartworm medications are highly effective, but dogs can still become infected. If you miss just one dose of a monthly medication — or give it late — it can leave your dog unprotected. Even if you give the medication as recommended, your dog may spit out or vomit a heartworm pill — or rub off a topical medication. Heartworm preventives are highly effective, but not 100% effective. If you don’t get your dog tested, you won’t know your dog needs treatment.

Left untreated, a heartworm-infected dog can eventually show signs such as coughing, weight loss, swollen belly, heart failure and will often die from the disease. Cats can have asthma-like signs, periodic vomiting and anorexia, and weight loss but can appear healthy just before sudden collapse and death.

Prevention of the disease and treatment for the disease require different medicines. Monthly prevention medication will kill the immature parasite in the larval stage as it enters the dog or cat’s body from a mosquito bite. However, once the larva begins to molt into the larger adult worm, which is thought to occur as early as 50 days from infection, prevention medicine is ineffective at killing the worm. In dogs, the mature worms are killed with a series of injections.

Depending on the health of the dog and the degree of damage in the heart and lungs, the treatment protocol may involve hospitalization during the injections. A month or more of daily antibiotics also is prescribed to kill a bacteria that is symbiotic with the worm. During this treatment some dogs can experience complications from the dead worms and may suddenly die.

Unfortunately for cats, heartworm testing is very inaccurate and results are frequently false negative. This makes reporting the incidence of the disease in cats very difficult. Also, there is no effective and safe way to kill the adult worm in a cat. There is only monthly preventive medication. While there are cats that will survive the infection and eventually death of the parasitic worm in the heart and lung, it’s not a good gamble to skip the safe and effective preventive medications.

In the past, Southern California enjoyed very few mosquitos and the diseases associated with them. However, the mosquito population has boomed in the last decade, and as a result we are seeing more cases of heartworm disease in pets and West Nile in people.

Please get your dog tested every year and keep your dogs and cats on preventive medicine every month year round.

Dr. Greg Perrault owns and operates Cats & Dogs Animal Hospital in Long Beach.

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