Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and occasionally in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms) and are filarids, one of many species of roundworms. Dogs and cats of any age or breed are susceptible to infection.
Where is Heartworm Disease?
It has been reported in all 50 states.
How Heartworm Happens: The Life Cycle
First, adult female heartworms release their young, called microfilariae, into an animal’s bloodstream. Then, mosquitoes become infected with microfilariae while taking blood meal from the infected animal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito. After that, the mosquito bites another dog, cat or other susceptible animal, and the infective larvae enter through the bite wound. It then takes a little over 6 months for the infective larvae to mature into adult worms. In dogs, the worms may live for up to 7 years. Microfilariae cannot mature into adult heartworms without first passing through a mosquito.
What Are the Signs of Heartworm Disease?
For both dogs and cats, clinical signs of heartworm disease may not be recognized in the early stages, as the number of heartworms in an animal tends to accumulate gradually over a period of months and sometimes years and after repeated mosquito bites.
Recently infected dogs may exhibit no signs of the disease, while heavily infected dogs may eventually show clinical signs, including a mild, persistent cough, reluctance to move or exercise, fatigue after only moderate exercise, reduced appetite and weight loss.
Cats may exhibit clinical signs that are very non-specific, mimicking many other feline diseases. Chronic clinical signs include vomiting, gagging, difficulty or rapid breathing, lethargy and weight loss. Signs associated with the first stage of heartworm disease, when the heartworms enter a blood vessel and are carried to the pulmonary arteries, are often mistaken for feline asthma or allergic bronchitis, when in fact they are actually due to a syndrome newly defined as Heartworm Associated Respiratory Disease (HARD).
Because heartworm disease is preventable, the American Heartworm Society (AHS) recommends that pet owners take steps now to talk to their veterinarian about how to best protect their pets from this dangerous disease. Heartworm prevention is safe, easy and inexpensive. While treatment for heartworm disease in dogs is possible, it is a complicated and expensive process, taking weeks for infected animals to recover. There is no effective treatment for heartworm disease in cats, so it is imperative that disease prevention measures be taken for cats.
There are a variety of options for preventing heartworm infection in both dogs and cats, including daily and monthly tablets and chewables, monthly topicals, and 6 month injections. All of these methods are extremely effective, and when administered properly on a timely schedule, heartworm infection can be completely prevented. These medications interrupt heartworm development before adult worms reach the lungs and cause disease.
It is your responsibility to faithfully maintain the prevention program you have selected in consultation with our veterinarian.
How Do You Detect Heartworm Disease?
Heartworm infection in apparently healthy animals is usually detected with blood tests for a heartworm substance called an “antigen” or microfilariae, although neither test is consistently positive until about seven months after infection has occurred.
Heartworm infection may also occasionally be detected through ultrasound and/or x-ray images of the heart and lungs, although these tests are usually used in animals already known to be infected.
Usually, all but the most advanced cases of heartworm disease can be successfully treated in dogs. Currently, there are no products in the United States approved for the treatment of heartworm infection in cats. Cats have proven to be more resistant hosts to heartworm than dogs, and often appear to be able to rid themselves of infection spontaneously. Unfortunately, many cats tend to react severely to the dead worms as they are being cleared by the body, and this can result in a shock reaction, a life-threatening situation. Veterinarians will often attempt to treat an infected cat with supportive therapy measures to minimize this reaction; however it is always best to prevent the disease.
Adult heartworms in dogs are killed using a drug called an adulticide that is injected into the muscle through a series of treatments. When the dog is sent home, exercise should be limited to leash walking for the duration of the recovery period, which can last from one to two months. This decreases the risk of partial or complete blockage of blood flow through the lungs by dead worms.
Re-infection during treatment is prevented by administration of a heartworm preventive. These preventives may also eliminate microfilariae if they are present. Dogs in heart failure and those with caval syndrome require special attention.
Feline Heartworm Disease
Heartworm infection takes place when a mosquito carrying infective, microscopic-size heartworm larvae, bites into a cat for a blood meal. The larvae then actively migrate into the new host and develop further as they travel through the subcutaneous tissue in the cat’s body. At about 3-4 months, they usually settle into the arteries and blood vessels of the lungs, where they continue to develop to sexual mature male and female worms (Dirofilaria immitis). The average time from when the microscopic parasites enter the host until the females develop into mature worms and produce offspring is approximately eight months and is referred to as the pre-patent period. This is about one month longer than in dogs.
As adults, the heartworms can mate and the females can release offspring called microfilariae (pronounced: micro-fil-ar-ee-a) into the blood stream. The cycle begins again when a mosquito takes a blood meal from the newly infected cat and draws the microfilariae into its system.
Cats are resistant hosts of heartworms, and microfilaremia, (the presence of heartworm offspring in the blood of the host animal), is uncommon (usually less than 20% of cases). When present, microfilaremia is inconsistent and short-lived. Some cats appear to be able to rid themselves of the infection spontaneously. It is assumed that such cats may have developed a strong immune response to the heartworms, which causes the death of the parasites. These heartworms may die as a result of an inability to thrive within a given cat’s body.
However, heartworms do not need to develop into adults to cause significant pulmonary damage in cats, and consequences can still be very serious when cats are infected by mosquitoes carrying heartworm larvae. Newly arriving worms and the subsequent death of most of these same worms can result in acute pulmonary inflammation response and lung injury. This initial phase is often misdiagnosed as asthma or allergic bronchitis but in actuality is part of a syndrome now known as Heartworm Associated Respiratory Disease (HARD).
Which Cats Are Susceptible?
Although outdoor cats are at greater risk of being infected, a relatively high percentage of cats considered by their owners to be totally indoor pets also become infected. Overall, the distribution of feline heartworm infection in the United States seems to parallel that of dogs but with lower total numbers. There is no predictable age in cats for becoming infected with heartworms. Cases have been reported in cats from nine months to 17 years of age, the average being four years at diagnosis or death.
The clinical signs of heartworm infection in cats can be very non-specific, and may mimic many other feline diseases. Diagnosis by clinical signs alone is nearly impossible, but a cat may exhibit generic signs of illness, such as vomiting intermittently (food or foam, usually unrelated to eating), lethargy, anorexia(lack of appetite), weight loss, coughing, asthma-like signs (intermittent difficulty in breathing, panting, open-mouthed breathing), gagging, difficulty breathing (dyspnea), or rapid breathing (tachypnea).
Signs associated the first stage of heartworm disease, when the heartworms enter a blood vessel and are carried to the pulmonary arteries, are often misdiagnosed as asthma or allergic bronchitis, when in fact they are actually due to a syndrome newly defined as Heartworm Associated Respiratory Disease (HARD).
Some cats exhibit acute clinical signs, with disease often related to the organs where the adult heartworms are thriving. Occasionally such infected cats die quickly without allowing sufficient time to make a diagnosis or offer appropriate treatment.
Clinical Signs Associated with Feline Heartworm Disease
Heartworm infection in cats is harder to diagnose than it is in dogs, and it is easy to overlook. Diagnostic tests have limitations, so negative test results do not necessarily rule out an infection. Antigen tests, for example, only detect adult female or dying male worms. Immature or male-only worm infections are rarely detected.
The diagnostic plan for heartworm disease in cats can include, but is not limited to, a physical examination, radiography (X-ray), echocardiography(ultrasound readings of the heart), angiocardiography (X-ray of the heart with injected contrast fluid), CBC(complete blood count), serologic testing (antigen and antibody study), microfilaria testing, and necropsy(after death).
Currently, there are no products in the United States approved for the treatment of feline heartworm infection. Most cats with heartworm infection that are not demonstrating clinical signs are allowed the time for a spontaneous cure to occur. If there is evidence of disease in the lungs and their blood vessels consistent with feline heartworm infection, such cases (possibly in the early stage) can be monitored with chest X-rays every six to twelve months as needed. Supportive therapy with small, gradually decreasing doses of prednisone (a cortisone-like drug) is recommended for cats with radiographic or clinical evidence of lung disease.
Cats with severe manifestations of feline heartworm disease may require additional supportive therapy, and may benefit from intravenous fluids, oxygen therapy, cage confinement, bronchodilators (which expand the air passages of the lungs), cardiovascular drugs, antibiotics and nursing care.
Heartworm extraction with various surgical devices has been performed in cats in which the worms can be visualized with ultrasound at the tricuspid valve or in the right atrium(of the heart), and especially in those rare instances of caval syndrome(obstruction of blood flow affecting the heart and the liver.
It is generally recommended that all cats be tested for both antigens and antibodies (serology) prior to administration of a heartworm preventive. There are four heartworm disease preventive products approved by the FDA for use in cats, Heartgard® for Cats (Ivermectin, orally) from Merial, Interceptor® (Milbemycin oxime, orally) from Novartis, Revolution® (Selemectin, topically) from Pfizer and Advantage Multi™ for Cats (Moxidectin / imidacloprid, topically) from Bayer. All of these products are considered effective in preventing the development of adult heartworms when administered properly on a monthly basis relative to the period of transmission.