Canine Heartworms: awareness and prevention

Zika, West Nile, Dengue, Yellow fever and Malaria are all mosquito borne diseases that cause serious illness in humans. An outbreak due to any one of these diseases ignites much media attention. Current research focuses upon diagnosis, prevention and treatment of these pathogens because of their devastating affects. Generally, an affected patient willing accepts medical treatment because of the threat of appreciable morbidity and even mortality. Travelers to high risk exposure areas readily accept preventative medications that have known adverse effects because of the dire consequences of these diseases.

In dogs, Dirofilaria immitis or heartworm is the most common mosquito-borne disease in the US. Owners may not appreciate the serious consequences of heartworm infection or understand the importance of heartworm prevention for their canine. A dog infected with heartworms suffers needlessly as the disease is preventable. A bite from a single mosquito can cause serious symptoms and even kill a canine.

The dog is a natural host for heartworms. Numerous species of mosquitoes can transmit heartworms. Microfilaria or microscopic heartworms live in the dog’s bloodstream. When a mosquito bites and takes a blood meal from an infected animal, the mosquito too becomes infected with the microfilaria. Approximately ten to fourteen days later, the microfilaria develop into the “infective stage” larvae. When the infected mosquitoe bites another dog, the larvae deposit on the dog’s skin and migrate through the open wound into the dog’s bloodstream. Approximately, six months later, bloodborne larvae mature into adult heartworms. Heartworms can grow to a foot long in length with individual survival times between five to seven years. A bite from an infected mosquito can infect an unprotected canine and result in hundreds of mature adult heartworms.

In the early stages of the disease or asymptomatic stage, the canine may exhibit few to no symptoms. During this initial stage of infection, tests to diagnose heartworms may be falsely negative. This is because the microfilaria have not yet developed into mature larvae or worms.

As the disease progresses, a mild but persistent cough may occur accompanied by fatigue noted after exertion or mild exercise. Many dogs exhibit an unwillingness to participate in play. A decrease in appetite with weight loss may occur. As heartworms continue to multiply, they migrate through tissues to infect the arteries and vessels of the lungs. Adult worms flourish in the pulmonary arteries, heart and blood vessels. With continued worm growth, congestive heart, particularly right sided heart failure, may ensue. Heart failure can lead to end organ damage with a resultant enlarging abdomen or the appearance of a swollen belly. With large worm burdens a form of blood flow blockage within the heart may occur resulting in cardiovascular demise. When this occurs, death due to congestive heart failure is imminent.

Often, owners do not realize the propensity of mosquito infestation near their homes. Frequently owners are lured into a false sense of security because they believe heartworm-infested mosquitoes could not possibly survive the harsh environmental conditions of freezing or sub freezing climates where owners live. Thus, owners erroneously assume that their dog(s) do not need monthly heartworm preventative treatment. Many feel that the adverse effects from heartworm prevention are “poisonous chemicals” which are dangerous to their dogs. Still others believe that holistic remedies such as the routine feeding of garlic, other supplements or the topical use of essential oils offer equivalent protection against canine heartworm disease.

Heartworms are detected in all 50 United States. More than twenty species of mosquitoes can transmit the heartworm microfilaria. The highest rates of infected mosquitoes occur in the South, particularly along the Atlantic and Gulf coast regions where high humid and wet conditions provide optimal conditions for mosquito replication and survival. In Northern snow belt areas, mosquito infestations occur in the tepid months of spring, summer and fall particularly around large lakes or bodies of water. A marked increases in heartworm disease is documented following natural disasters. In hurricane Katrina, high winds transmitted infected mosquitoes to locations beyond Louisiana and Mississippi. Mosquito populations thrived in the massive areas of standing water dumped by heavy rains of the hurricane. “Orphaned” canines, infected with heartworms, were rescued and relocated to areas with previously low heartworm burdens. Thus, a reservoir of infected dogs and mosquitoes resulted in a marked increase in canine heartworm disease.

In the US, Canine heartworm disease is increasing. Reasons cited include lack of credible knowledge and understanding of the disease by owners, inadequate use of heartworm medications, increasing popularity of holistic approaches to prevent the disease and the marked rise of mosquito populations throughout the country.

The American Heartworm Society, founded in 1974, advocates the administration of year round medication to prevent canine heartworms. Additional measures to prevent mosquito bites include the routine use of repellants, drainage of standing water, treatment of mosquito larvae and keeping your dog indoors when mosquito feeding is at its peak. It is important to note that none of these adjuvant measures are a replacement for heartworm preventative medication.

Routine administration of heartworm prevention is an investment to protect your dog against a serious disease that causes much misery and even death. When purchasing these products from a Veterinarian, the medication has been researched, evaluated and tested by a pharmaceutical company who offers a warranty or guarantee for their product. When buying a generic equivalent on line, these warranties may be suspect. Often, adverse medication side effects may be the result of fillers or adjuvants used in the manufacturing particularly of generic drugs.

As a cost effective measure, many owners or breeders use Ivermectin for heartworm prevention. Ivermectin is a concentrated drug designed for injectable use in cattle and other large livestock. Ivermectin is more effective against larval heartworms than adult heartworms. It is difficult to dilute Ivermectin for oral use because of the technical difficulties to measure accurately small volumes, i.e. less than 2mLs, of a highly concentrated drug. Thus, the potential is high to give doses that are 10 – 100 times the recommended dose. This is particularly dangerous for small or toy dogs who can be unintentionally overdosed.

As I write, floodwaters are rising in Texas. A killer storm is in our presence. With catastrophic floods, standing water will be present for a prolonged time. Predictably, infected mosquito populations will breed and flourish and the risk of canine heartworm disease will soar.

It is a sobering thought that even if humans and canines make it out of this storm, both may suffer without appropriate medical care. Please do not let a preventable disease such as heartworms threaten your beloved canine’s quality of life. Only you can make this decision for your dog. Give them the prevention they deserve. Twelve tablets a year could be the difference between a happy healthy dog and a sickly companion or even one who succumbs to a premature death.

Barbara E. Magera MD, PharmD, MMM (Caracaleeb) is a Cavalier fancier who lives and practices medicine in Charłeston, South Carolina.