Heartworm is a serious illness that can cause heart failure, lung disease, organ damage and even death in dogs, cats and ferrets. Heartworm is most prevalent in pets living along the Atlantic Gulf coasts from New Jersey to the Gulf of Mexico, and in those living alongside the Mississippi and its main tributaries. However it has been found in pets in all of the US States.
Heartworm is caused by parasitic worm larvae that lives inside mosquitoes. When the mosquito bites an animal it transfers some worm larvae into it where the larvae then matures, mates and produces offspring inside its living host. The offspring produced by female adult heartworms is known as microfilariae and lives in the host animals’ blood stream. When a mosquito then bites an infected animal it draws microfilariae into its body where it turns it into infective larvae, beginning the cycle again.
Once an animal has been infected it takes time for the larvae to mature into adults that are capable of reproduction. In dogs this period is usually 6-7 months and in cats and ferrets around 8 months. Adult heartworms look like cooked spaghetti and can range in size from 4-6 inches in males and 10-12 inches in females. The number of worms found in a pet is known as its ‘worm burden’ and this can vary depending on the species of animal and the severity of the infection.
The lifespan of heartworms within an infected dog is between 5 and 7 years and the average worm burden is 15. However dogs have been seen with worm burdens ranging from 1 to 250.
The severity of the symptoms of heartworm in dogs is dependent on the worm burden of the animal, how long they have been infected and how well their body can cope with the disease. However it is usually broken down into four stages.
Class 1: no visible symptoms or very mild symptoms such as an intermittent cough or wheeze.
Class 2: mild to more moderate symptoms including intermittent coughing and lethargy or breathlessness after light to moderate exercise. At this time some heart and lung changes may be seen on x-rays.
Class 3: symptoms will include frequent or persistent coughing, lethargy and breathlessness after mild activity. Heart and lung changes will definitely be visible on x-rays.
Class 4: this stage is otherwise known as Caval Syndrome and is reached when an infected animal has been left untreated for an extended period of time. At this stage the animal experiences restricted blood flow to the heart caused by a blockage of worms. Heart failure is imminent and emergency surgery to remove the worms is the only course of action. However this comes with its own risks and most dogs with Caval Syndrome do not survive.