Overview of Feline Heartworm Disease
Heartworm disease is a serious and fatal disease of the heart and lungs caused by a parasite, Dirofilaria immitis, that is transmitted by mosquitoes. Historically, heartworm disease in cats has been given less consideration than in dogs because the incidence is so much lower compared to dogs and the diagnosis is more difficult.
Below is an overview of Heartworm Disease in Cats followed by in-depth detailed information about the heartworm life cycle as well as diagnosis and treatment of this serious disease.
Age is not a risk factor. Adult cats of any age can be affected, with cats as young as 1 and as old as 17 having been diagnosed. Indoor and outdoor cats can get infected, although outdoor cats have a higher prevalence. However, up to 33 percent of reported cases are in cats who are described by their owners as “strictly indoors.” Males are a bit more likely than females to be affected.
Transmission occurs when a mosquito bites an infected dog or cat and ingests heartworm larvae (baby heartworms), which live in the bloodstream. When the insect bites another dog or cat, some of the larvae are injected under the skin. The larvae grow for 3 to 4 months and eventually make their way into the heart where they develop into adults, and the process is ready to repeat itself.
What to Watch For
Symptoms don’t usually occur until damage has already occurred to the heart. Cats can have a wide range of symptoms, with some cats being completely asymptomatic (no symptoms at all). Others may show vague, generalized clinical symptoms. A small percentage, however, may show severe life threatening symptoms. Symptoms usually occur because of heart failure. These include:
Rapid breathing rate
Hemoptysis (coughing up blood)
Decreased ability to exercise
Neurologic signs such as blindness, seizures, circling, incoordination
Diagnosis of Heartworm Disease in Cats
Making the diagnosis of heartworm disease in the cat is tricky. There are several tests that your veterinarian might want to run, such as:
Complete blood count and chemistry panel
Chest X-rays (radiographs)
Serum heartworm antibody test
Serum heartworm antigen test
Microfilaria test (looking for larva in the blood)
Treatment of Heartworm Disease in Cats
Treatment of heartworm disease in cats is complicated. There are two options, basically:
Adulticide treatment (killing the worms in the heart and lungs)
Conservative treatment (i.e., intermittent corticosteroid therapy).
Home Care and Prevention
There is no specific home care other than to administer medications as prescribed.
Milbemycin (Interceptor for Cats®),Ivermectin (Heartgard for Cats®)or Selamectin (Revolution® for Cats) are the most common drugs used to prevent heartworm disease in cats.
In-depth Information on Heartworm Disease in Cats
In the past, heartworm disease has been considered a medical curiosity rather than the distinct, clinical disease that it is. Cats with heartworm infection can be found in all areas where cases of canine heartworm disease occur. The true incidence of the disease is unknown and is probably underestimated, because diagnosis is often difficult, and because some cats either spontaneously eliminate the parasite and never get diagnosed. Other cats may die acutely from the disease, again with no diagnosis ever being established.
Although there are some similarities, heartworm disease in cats is a very different disease than in dogs. The clinical importance of heartworm infection in cats has to do with the fact that even light infections are capable of producing severe disease with potentially life threatening consequences.
Cats of any age and any breed are susceptible. Since the disease is spread by the bite of a mosquito, outdoor cats are more susceptible, although cat owners should be aware that up to 33 percent of reported cases are in cats described by their owners as being “strictly indoors.” Mosquitoes can and do go indoors. Male cats tend to roam more than females and have a higher exposure; however, the incidence in males is not all that much higher than in females.
Cats are susceptible to heartworm disease, but they do not make good hosts for the worms, as do dogs. Dogs are considered “definitive hosts” because once inside the body the entire life cycle of the heartworm can be completed. Cats, however, are considered “resistant hosts.” If you were to inoculate a dog with 100 infective heartworm larvae, 70 of them would survive and grow up to become adult worms. If you give 100 infective heartworm larvae to a cat, only 10 would develop into adult heartworms. Of cats and dogs that are infected naturally, a cat usually has between one and nine worms in their heart and lungs; dogs often have 20 or more. In dogs, adult heartworms live for approximately five to seven years, but in cats, they only live two years. In dogs, you don’t usually see severe clinical signs unless you have at least 10 worms in the heart and lungs. In cats, it only takes one or two worms to cause severe disease, and possibly death.
The life cycle of the heartworm in the cat is as follows: a mosquito picks up microfilariae (baby heartworms) from an infected dog or, less likely, a cat.
The microfilaria is actually a larva, at larval stage L1. Inside the mosquito’s body, the L1 babies molt into the next stage, L2, and again, to L3. The L3 larva is the infective stage, and at this point the mosquito bites an uninfected cat, and injects the L3 larvae into the cat’s skin. The L3 soon molts into an L4 larva. Sometimes the L4 larva migrate to an unusual location and cause unusual clinical signs related to the organ system. Most of the L4s molt into L5s, which are juvenile worms. L5s find their way to the pulmonary arteries in the lungs, where they cause all sorts of problems for the cat.
Approximately eight months after infection, male and female heartworms mate and produce their own baby heartworms (microfilariae). Again, these microfilariae are L1 larvae. They do not develop into adult heartworms on their own. They have to be picked up by a mosquito, undergo development into L2 and L3 stages in the mosquito and then be injected into another cat where they continue the same process.
One or two diseases have similar clinical signs as heartworm disease. The most common are asthma and lung parasites.
When clinical signs of heartworm disease are detected, a data base will need to be generated. There are a number of tests that can help make the diagnosis. Some are better than others. The following tests have been suggested.
Complete Blood Count and chemistry panel. A complete blood count can be helpful when trying to make a diagnosis. In approximately one third of cats with heartworm disease, there will be an increased number of a type of white blood cell called an eosinophil. The increased numbers of eosinophils is a transient event. It is most likely to be seen 4 to 7 months after infection. Sometimes, there is an increased number of another type of cell called a basophil. Increased basophils is much less common, however, it is more strongly suggestive of heartworm disease. Serum chemistry panels are usually normal. Occasionally, infected cats will have a higher level of proteins called globulins in the bloodstream, but this is not consistent or predictable.
Thoracic radiographs (chest X-rays). Radiographs can be an important diagnostic tool, especially in early cases, as radiographic changes suggestive of heartworm disease can be seen as soon as 120 days after infection. It takes a fair amount of expertise to interpret the X-rays correctly and many veterinarians send the films out for a second opinion by a board certified veterinary radiologist. The most common changes seen on X-rays are enlarged pulmonary arteries and changes affecting the lung tissue itself. This may result in patterns called “perivascular interstitial” or “alveolar” changes.
Serum antibody tests. When heartworms infect a cat, they release microscopic molecules called antigens. Antigens trigger the immune system to produce antibodies. This antibody test is a very sensitive test. It can detect infection in 80 percent of infected cats just two months after getting infected. If you do the test 3 months after cats are infected, it will detect it in 97 percent of them. By four months after infection, it detects nearly 100 percent. A positive test does not necessarily mean that the cat has adult heartworms in the heart and lungs, however. This is the main problem with this test. The most valuable aspect of the antibody test is that a negative test almost certainly means the cat is NOT infected with heartworms. In other words, the antibody test is a good screening test. A negative test virtually excludes heartworm disease as a cause of the cat’s clinical symptoms. A positive test means that heartworm disease is “an issue” for this cat and further testing needs to be performed, such as an antigen test.
Serum antigen tests. Antigens are molecules given off by invading organisms, such as heartworms. The serum antigen test detects these antigens. Unfortunately, the antigen it detects is a molecule produced by mature adult female worms only. The test in dogs is very reliable; a positive test means there are heartworms in the heart and lungs. A negative test means there are no heartworms. In cats, a positive test means that there is at least one mature female heartworm in the cat. A negative test, however, does not mean that the cat isn’t infected. Since the tests do not detect immature worms (worms less than seven months old), cats recently infected may go undetected. Also, since the test detects antigens given off by mature female worms, all male infections would be missed by the test. In a study of 108 naturally heartworm infected cats, 18 percent had all male infections. Finally, exactly how many female worms must be present before the test can detect them is debatable. In the past, antigen tests could not detect two or fewer worms. The newer tests claim that they can detect infections containing at least one female worm.
To reiterate, a negative antigen can mean:
The cat is not infected
The cat is infected with immature worms (less than 7 months old)
The cat has an all male infection
The cat has one or two female worms, but this particular test is not sensitive enough to detect it
A positive test means that the cat most likely has at least one mature adult female worm in his body.
Microfilaria test. A definitive diagnosis of heartworm infection can be made by detecting microfilariae in the circulation. Adult heartworms mate and produce microfilariae approximately 8 months after infection. Circulating microfilariae are detectable for only about a month, however, before the cat’s immune system eliminates them from the bloodstream. (This doesn’t happen in dogs; microfilariae can circulate for years in dogs). As a result, about 80 percent of cats with heartworm infection do not have circulating microfilariae, so there are many false negatives with this test. If you do see them, however, the cat is definitely infected.
Echocardiography (cardiac ultrasound). This test is more sensitive in cats than in dogs for detecting heartworm infection, especially if you’re specifically looking for worms, and if the test is performed by an experienced cardiologist.
Angiography. This test requires injecting a special dye in the jugular vein and then take X-rays a few seconds later. This test can demonstrate enlarged pulmonary arteries, other characteristic vessel changes and, occasionally, the heartworms themselves. Performance of the test and interpretation of the angiograms usually requires referral to a specialist.
Transtracheal wash. This test involves putting a small amount of sterile fluid down the trachea into the lungs and then retrieving some of the fluid and analyzing it. The retrieved fluid usually contains cells that can help make a diagnosis. Similar to the results of a complete blood count, the findings of many eosinophils on a transtracheal wash is supportive of feline heartworm disease. It is also supportive of feline asthma and/or lung parasites, so other tests should be performed to rule in heartworm disease and rule out other diseases.
The decision whether or not to treat feline heartworm disease is complicated, mostly because of the unpredictable nature of the disease and the risks of treatment side effects. There are no approved therapies.
Adulticide Therapy. Using drug therapy to kill the adult worms is a common way to treat heartworm disease in dogs, however it is risky and not routinely used in cats. The drug thiacetarsemide (Caparsolate®) given intravenously works effectively in dogs to kill adult heartworms. Caparsolate® has the potential to cause severe reactions in cats. Liver problems, kidney problems, pulmonary edema (fluid in the lungs) and a terrible skin reaction can occur if the drug leaks out of the vein during injection. The most worrisome side effect, however, is development of an embolus caused by a piece of dead worm traveling through the lungs and getting lodged. This causes sudden signs of respiratory distress that may be severe enough to cause death. Another drug, melarsomine (Immiticide®) is also used in dogs. Preliminary studies of Immiticide® on heartworm infected cats have yielded poor results and is not commonly recommended in cats. Melarsomine may also be toxic to cats in low doses. Adulticide therapy should probably be reserved for cats who have recurrent life threatening bouts of dyspnea as a result of their heartworm infection and are not responding to corticosteroid therapy. According to the American Heartworm Society, to date, there are no studies that indicate any form of medical adulticidal therapy increases the survival rate of cats harboring adult heartworms. There is no standardized agreed upon treatment for adulticidal heartworm therapy for cats.
Please note that thiacetarsemide (Caparsolate®) is currently unavailable/off the market.
Corticosteroid Therapy. Intermittent therapy with corticosteroids (for example, prednisone) has been recommended in cats with evidence of lung disease resulting from the heartworms. Cats with no symptoms, or mild symptoms may be managed with intermittent corticosteroids as needed. The life span of a heartworm in a cat is approximately two years. If the cat can get through several recurrent bouts of coughing and respiratory difficulties on corticosteroid therapy for two years after the initial diagnosis, there is a good chance that all the worms have died and the cat may be fine thereafter.
Supportive Care. Cats with severe manifestations of heartworm disease may be treated in the hospital with oxygen, intravenous fluids, cage confinement, drugs to expand the airways (bronchodialators), drugs to improve heart function, and/or antibiotics.
Manual worm removal. Special surgical devices are sometimes used to manually snare and remove the worms. This is most commonly done when there is obstruction of blood flow affect the liver and heart.
No therapy. Cats that are stable may be monitored only. Monitoring may include chest radiographs every 6 to 12 months and given time for a natural cure.
Follow-up Care for Cats with Heartworm Disease
Optimal treatment for your pet requires a combination of home and professional veterinary care. Follow-up can be critical, especially if your pet does not rapidly improve.
Administer all prescribed medication(s) as directed. Alert your veterinarian if you are experiencing problems treating your pet.
Consider administering milbemycin (Interceptor for Cats) or ivermectin (Heartgard for Cats®)or selamectin (Revolution® for Cats) to all cats in the household. Milbemycin, ivermectin and selamectin are the only heartworm preventatives approved for use in cats and should be considered for cats living in highly endemic areas, where the incidence of heartworm infection is higher than most people realize.