Heartworms are a roundworm scientifically known as Dirofilaria immitis which is transmitted by mosquitoes and can infect more than 30 mammal species including dogs, cat and people. While dogs are considered the definitive (or preferred) host, cats, ferrets and wild mammals like coyotes can also be infected. Heartworm is found in all 50 states although the highest rate of infection is seen within 150 miles of the Atlantic and Gulf Coasts and along the Mississippi River. While some arid areas of the country such as Arizona have lower rates of infection, we do have the vector (mosquitoes) and are seeing an increase in native dogs testing positive for heartworm.

The Heartworm Life Cycle

The adult female heartworm, living in the artery or right side of the infected mammal’s heart, releases microfilariae (young heartworms) into the bloodstream. An adult mosquito bites the infected animal and ingests the microfilariae with its blood meal. Over the next 10 to 14 days the microfilariae mature into an infective larval stage. When the mosquito bites another mammal, the infected larvae leave the mosquito in the saliva and enter the new host. Over the next 6 months, the microfilariae mature into adult worms and make their way to the heart and lung arteries, where they repeat the cycle again.

Adult worms can live for up to 7 years in the host mammal, reproducing and sending more microfilariae into the blood stream.

Symptoms of Heartworm Disease

In our companion animals, the clinical signs of heartworm disease may not be seen in the early stages and can mimic a number of other diseases and conditions. In dogs, as the number of adult worms in the heart multiplies, you may notice a mild persistent, non-productive cough, exercise intolerance and fatigue, decreased appetite, weight loss, or lethargy. As these symptoms mimic other frequently seen diseases in Tucson, your veterinarian may include heartworm testing with other laboratory tests if your pet is not on consistent preventative.

In cats, clinical signs can be even more vague and non-specific. Symptoms of chronic disease can include, vomiting, gagging, difficult or rapid breathing, lethargy, coughing, panting, open-mouth breathing and weight loss. Heartworm Associated Respiratory Disease can often mimic asthma or allergic bronchitis. Cats may also exhibit acute signs including collapse, convulsions, blindness, heart rate and rhythm disruptions, syncope (fainting) or sudden death.

Heartworm testing

Routine, annual testing for the substance produced by the adult female heartworm, known as antigen testing, is the most effective method for detecting infection in dogs. Testing for the presence of microfilariae by reviewing blood smears is also used to detect early infection, the period before the adult heartworms has established residence in the arteries or heart and begun reproducing. In cats, we recommend antigen testing for the female adult heartworm and antibody testing for the male adult heartworm as cats can have male-only infections. Neither antigen, antibody or microfilariae testing is accurate until about seven months after infection due to the development period for the adult worm. False negatives by reviewing blood smears are also possible if the patient is given preventative medication inconsistently.

In addition to testing for heartworm disease, the test we commonly use here at Orange Grove Animal Hospital also tests for tick borne diseases such as Ehrlichiosis, known as Tick Fever and Lyme disease. Tick Fever is a common, life-threatening parasite carried by the tick native to our area, the Brown Dog Tick.

Treating Heartworm Disease

Treating heartworm disease is done through your veterinarian by giving a series of injections of an adulticide into a muscle. There is only 1 medication approved by the FDA for the treatment of heartworm disease in dogs. Any “holistic” or “herbal” treatments should be avoided as risk of life-threatening complications is high as these products have not been evaluated for safety or effectiveness.

Owners are then instructed to restrict activity to very short leash walks and cage rest for 1-2 months while the adult worms are processed by the dog’s body. It is very important that owners follow activity restrictions to decrease the risk of partial or complete blockage of blood flow to the lungs by the dead worms. Dogs undergoing heartworm treatment will need to be isolated from other dogs to prevent secondary infections such as upper respiratory infections (Kennel Cough) as their immune systems are under compromise from the parasite and treatment.

Untreated heartworm disease will lead to death as the heart and lungs become filled with worms that block blood flow, reduce the heart’s ability to contract and prevent the heart valves from opening and closing properly. This reduction in blood flow can also lead to blood clots in the lungs, liver or kidneys.

Prevention: safer & more cost effective

There are currently 4 medications approved by the FDA for prevention of Heartworm disease in dogs and cats which come in a number of formulations under several brand names. We recommend and carry Heartgard® Plus and Revolution®. We do not recommend any of the many “organic” or “holistic” products available as their safety and effectiveness has not been clinically evaluated and approved for use in cats and dogs nor are they recommend by the American Veterinary Medical Association or the American Heartworm Society. As Tucson has the vector (mosquitoes) that transmit heartworm disease, all dogs, even native dogs who do not leave town, are recommended to be on preventative year round. Which preventative to use will be decided by your veterinarian based on your dog’s risk factors, lifestyle and travel arrangements.

All dogs over the age of 6 months will need to be tested for heartworm before starting preventative. Products like Heartgard® Plus work by killing any microfilariae that may enter the bloodstream from a mosquito bite. Because preventatives kill microfilariae it is important to know if the dog has an adult heartworm load before beginning monthly preventatives due to the risk of side effects and the potential for false negative microfilariae tests.

Heartworm preventive medication is labeled by the FDA as veterinary only products to be used under the direction of a licensed veterinarian, therefore requires an active doctor-client-patient relationship and current prescription. Orange Grove Animal Hospital in accordance with state and federal laws and regulations requires us to have examined your pet within a year of the prescription and heartworm testing on the schedule recommended by your veterinarian.

For more information on Heartworm Disease visit the American Heartworm Society’s website by clicking the button below.