Causes & Prevention of Heartworm Disease in Dogs

Dog Breeds

Introduction

Causes & Prevention of Heartworm Disease in Dogs: Veterinary reviewed information that discusses the main causes of Heartworm Disease in dogs, including how it may be prevented.

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Causes of Heartworm Disease in Dogs

Heartworms are transferred from infected dogs to non-infected dogs by adult female mosquitoes. To understand this illness, it is important to understand the parasites’ life cycle. Infection begins when a female mosquito feeds on a wild or domestic dog whose blood contains microfilaria. Microfilaria are the immature offspring of adult male and female heartworms that are living and reproducing within a dog’s bloodstream (in its cardiovascular system). If microfilaria are not ingested by a female mosquito, they will never develop into infectious larvae. In other words, the female mosquito is an essential intermediate host in the heartworm life cycle.

Heartworm microfilaria go through several stages of development inside of the female mosquito, molting through larval stages L1, L2 and L3. It is only during the brief L3 phase, which occurs between 1 and 2½ weeks after they are ingested by the mosquito, that the larvae are infective to dogs. When a female mosquito containing L3-stage microfilaria bites a dog, the immature heartworm parasites are transferred from the mouthparts of the mosquito into the site of their bite. The larvae burrow into the dog’s skin and eventually enter circulation in its bloodstream. They then lodge in the pulmonary arteries and right heart chambers, where they mature into L4 and finally L5 stage parasites, which are adult heartworms. The pulmonary arteries are the blood vessels between the heart and the lungs. They are essential for the transfer of oxygen and other nutrients to all bodily tissues, and for the removal of carbon dioxide and other waste products out of circulation.

Heartworms reach adulthood by about 6 or 7 months after a dog is inoculated by a female mosquito. As adults, they reproduce and release their own microfilaria. This is when heartworm infection usually becomes obvious, or patent (called “microfilaremia”). A single mature female heartworm can produce up to 5,000 microfilaria in a single day, each of which can survive in the dog’s bloodstream for many years. Adult heartworms can grow to 15 inches in length, and they can live inside the infected animal for up to 8 years, continuing to produce microfilaria. The cycle starts over each time an adult female mosquito feeds on an infected dog that has microfilaria in its bloodstream.

The causes of symptoms of heartworm infection in dogs are related to the location, number and size of the living and dead adult worms in the dog’s cardiovascular system. Heartworms irritate and injure the sensitive lining of blood vessels by direct physical contact. They cause pulmonary hypertension by obstructing blood flow through the pulmonary arteries, and they can become thromboembolisms, essentially acting as blood clots. Heartworms can cause allergic or autoimmune hypersensitivity reactions, arterial inflammation and edema (abnormal fluid accumulation in tissues). In very severe chronic cases, heartworm infestation can cause irreversible fibrosis of (damage to) the lungs. Up to one-half of dogs with chronic heartworm infection develop right-sided congestive heart failure. Kidney damage and liver disease are also commonly associated with heartworm infection.

Heartworms that block the superior or inferior vena cava and/or liver blood vessels cause an extremely serious and often fatal condition called “caval syndrome.” Caval syndrome involves liver failure, anemia, jaundice, bleeding, blood-clotting disorders and fluid retention. Death can occur in as few as 2 or 3 days.

Dead heartworms can be as problematic as living ones. If a dog is treated with medications that are designed to kill heartworms (called “adulticides”), the dead or dying adult worms can become physical barriers to block flow through affected arteries and veins. Dead and dying heartworms also can become wrapped around heart valves and vessels and interfere mechanically with the beating and overall function of the heart.

Prevention of Heartworm Disease

Heartworm disease has occurred in all 50 states and is especially common along the southeastern Atlantic and Gulf coasts – particularly within 150 miles of the coast from Texas to New Jersey, and along the Mississippi river and its tributaries. It is common in many other countries, as well. Fortunately, heartworm infection is almost always preventable. It is far better to prevent this serious condition rather than trying to treat it once it happens. A dog’s local veterinarian is in the best position to recommend an appropriate prevention protocol.

In endemic areas, dogs should be on a heartworm prevention program for life, typically starting at about 6 to 8 weeks of age. Many owners choose to keep their dogs on heartworm preventatives year-round. Dogs can be tested for heartworms starting at about 7 months of age. If they test positive for the heartworm antigen, a second test typically is performed to detect the levels of immature microfilaria in circulation, if any are present. Heartworm preventatives should not be given to a dog that is infected with the parasites unless done under strict veterinary supervision. Most veterinarians recommend retesting dogs annually, even if they are on heartworm preventative medication.

Heartworm preventatives have become quite convenient for owners to administer. Most are given once a month, either orally or topically between the shoulder blades. They each work differently, and some also control fleas, roundworms, hookworms and/or whipworms.

Other preventative techniques include bringing dogs indoors in the late afternoon and early evening, when mosquitoes feed and are most prevalent. Since mosquitoes require standing water to reproduce and can travel up to ¼ mile per day, ponds, puddles or other sources of brackish water should be avoided. Areas around the house and yard can be sprayed for mosquitoes. Obviously, these approaches will not guarantee that a dog will be free from bites by infected mosquitoes, but they certainly should help.

Special Notes

Heartworm disease has become a serious threat to pets in areas where mosquitoes are prevalent. Heartworm prevention is highly advised in these areas. Not all animals are susceptible to heartworm infection, but cats and dogs are. Human infection is possible, but thankfully is extremely rare.

Certain breeds have an inherited predisposition to developing severe and potentially fatal reactions to some drugs, including some heartworm preventatives that include ivermectin, such as Ivomec and Heartguard. A simple, cheek-swab test for this genetic defect is available through the Washington State University Veterinary Laboratory of Clinical Pharmacology. Collies, Border Collies, Australian Shepherds, Shetland Sheepdogs, Old English Sheepdogs, longhaired Whippets and crosses of those breeds probably should be tested. Owners of these breeds will want to discuss potential ivermectin sensitivity with their veterinarian before starting a heartworm prevention program.

Symptoms & Signs