Heartworm in Cats
Andy Pachikerl, Ph.D
Introduction of Feline Heartworm
Heartworm disease is a serious and potentially fatal disease in pets worldwide and it is caused by foot-long worms (heartworms) that dwells in the heart, lungs, and associated blood vessels of affected pets. This in turn cause severe lung disease, heart failure and damage to other organs in the body. Heartworm disease affects dogs, cats, and ferrets, but heartworms also live in other mammal species, including wolves, coyotes, foxes, sea lions and—in rare instances—humans. Because wild species such as foxes and coyotes live in proximity to many urban areas, they are considered important carriers of the disease (Ettinger, et al., 2010).
Among mammalian definitive hosts, they are best adapted to domesticated and wild dogs. If untreated, their numbers can increase, and dogs have been known to harbour several hundred worms in their bodies. Heartworm disease causes lasting damage to the heart, lungs, and arteries, and can affect the dog’s health and quality of life long after the parasites are gone (2019). For this reason, prevention is by far the best option, and treatment—when needed—should be administered as early in the course of the disease as possible.
Heartworm disease in cats is quite distinctive from heartworm disease in dogs. Since cat is an atypical host for heartworms, most worms in cats do not survive to the adult stage. Cats with adult heartworms usually have just one to three worms, and many cats affected by heartworms have no adult worms. Even if feline host of heartworm rarely occurs, cat heartworm disease often goes undiagnosed. Therefore, it is important to understand that even immature worms cause real damage in the form of a condition known as heartworm associated respiratory disease (HARD). Moreover, the medication used to treat heartworm infections in dogs cannot be used in cats, so prevention is the only means of protecting cats from the effects of heartworm disease (Society, 2007).
Figure 1. Diagram from American Heartworm Society showing how the heartworm is spread and inter-transmission can occur via different host pets.
The mosquito plays an essential role in the heartworm life cycle. Adult female heartworms living in an infected dog, fox, coyote, or wolf produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which develop and mature into “infective stage” larvae over a period of 10 to 14 days (Johnstone, 1998). Then, when the infected mosquito bites another dog, cat, or susceptible wild animal, the infective larvae are deposited onto the surface of the animal’s skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to mature into adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats. Because of the longevity of these worms, each mosquito season can lead to an increasing number of worms in an infected pet (Knight, et al., 1998).
Symptoms and signs of heartworm in cats
Signs of heartworm disease in cats can be of two extremes: very subtle or plain out dramatic. Symptoms may include coughing, asthma-like attacks, periodic vomiting, lack of appetite, or weight loss. Occasionally an affected cat may have difficulty walking, experience fainting or seizures, or suffer from fluid accumulation in the abdomen. Unfortunately, the first sign in some cases is collapse of the cat, or sudden death (Society, 2007).
How significant is my cat’s risk for heartworm infection?
Figure 2. Diagram showing the severity of heartworms in the USA as shown by the American Heartworm Society
Many factors must be considered, even if heartworms do not seem to be a problem in your local area. Your community may have a greater incidence of heartworm disease than you realize—or you may unknowingly travel with your pet to an area where heartworms are more common. Heartworm disease is also spreading to new regions of the country each year. Stray and neglected dogs and certain wildlife such as coyotes, wolves, and foxes can be carriers of heartworms. Mosquitoes blown great distances by the wind and the relocation of infected pets to previously uninfected areas also contribute to the spread of heartworm disease (this happened following Hurricane Katrina when 250,000 pets, many of them infected with heartworms, were “adopted” and shipped throughout the country).
The fact is that heartworm disease has been diagnosed in all 50 states, and risk factors are impossible to predict. Multiple variables, from climate variations to the presence of wildlife carriers, cause rates of infections to vary dramatically from year to year—even within communities. And because infected mosquitoes can come inside, both outdoor and indoor pets are at risk.
For that reason, the American Heartworm Society recommends that you “think 12:” (1) get your pet tested every 12 months for heartworm and (2) give your pet heartworm preventive 12 months a year.
Heartworm disease is a serious, progressive disease. The earlier it is detected, the better the chances the pet will recover. There are few, if any, early signs of disease when a dog or cat is infected with heartworms, so detecting their presence with a heartworm test administered by a veterinarian is important. The test requires just a small blood sample from your pet, and it works by detecting the presence of heartworm proteins. Some veterinarians process heartworm tests right in their hospitals while others send the samples to a diagnostic laboratory. In either case, results are obtained quickly. If your pet tests positive, further tests may be ordered (Yin, 2007).
Heartworm infection in cats is harder to detect than in dogs, because cats are much less likely than dogs to have adult heartworms. The preferred method for screening cats includes the use of both an antigen and an antibody test (the “antibody” test detects exposure to heartworm larvae). Your veterinarian may also use x-rays or ultrasound to look for heartworm infection. Cats should be tested before being put on prevention and re-tested as the veterinarian deems appropriate to document continued exposure and risk. Because there is no approved treatment for heartworm infection in cats, prevention is critical (Atkins, 1999).
Treatments or other things to do for a cat tested positive for heartworms
Like dogs, cats can be infected with heartworms. There are differences, however, in the nature of the disease and how it is diagnosed and managed. Because a cat is not an ideal host for heartworms, some infections resolve on their own, although these infections can leave cats with respiratory system damage. Heartworms in the circulatory system also affect the cat’s immune system and cause symptoms such as coughing, wheezing and difficulty breathing. Heartworms in cats may even migrate to other parts of the body, such as the brain, eye and spinal cord. Severe complications such as blood clots in the lungs and lung inflammation can result when the adult worms die in the cat’s body (Atkins, et al., 2005).
Here is what to expect if your cat tests positive for heartworm:
- While infected dogs may have 30 or more worms in their heart and lungs, cats usually have 6 or fewer—and may have just one or two. But while the severity of heartworm disease in dogs is related to the number of worms, in cats, just one or two worms can make a cat extremely ill. Diagnosis can be complicated, requiring a physical exam, an X-ray, a complete blood count and several kinds of blood test. An ultrasound may also be performed.
- Unfortunately, there is no approved drug therapy for heartworm infection in cats, and the drug used to treat infections in dogs is not safe for cats. Nevertheless, cats with heartworm disease can often be helped with good veterinary care. The goal is to stabilize your cat and determine a long-term management plan.
- Monitor your cat. Heartworm-positive cats may experience spontaneous clearing of heartworms, but the damage they cause may be permanent. If your cat is not showing signs of respiratory distress, but worms have been detected in the lungs, chest X-rays every 6 to 12 months may be recommended. If mild symptoms are noted, small doses of prednisolone may be administered to help reduce inflammation.
- Provide veterinary care. If the disease is severe, additional support may be necessary. Your veterinarian my recommend hospitalization to provide therapy, such as intravenous fluids, drugs to treat lung and heart symptoms, antibiotics, and general nursing care. In some cases, surgical removal of heartworms may be possible.
- Maintain prevention. A cat that has developed heartworm disease has demonstrated that it is susceptible to heartworm infection, and both outdoor and indoor cats are at risk. It is important to give your cat monthly heartworm preventives, which are available in both spot-on and pill form. Preventives keep new infections from developing if an infected mosquito bites your cat again.
- Atkins C The diagnosis of feline heartworm infection [Journal]. – [s.l.] : Journal of the American Animal Hospital Association, 1999. – 3 : Vol. 35.
- Atkins Clarke E. and Litster Annette L. Heartworm Disease [Book]. – [s.l.] : Elsevier Saunders, 2005. – Vol. 5.
- Ettinger Stephen J. and Feldman Edward C. Textbook of Veterinary Internal Medicine [Book]. – [s.l.] : W.B. Saunders Company, 2010. – 7.
- Johnstone Colin Hearworm [Report]. – Pennsylvania : Parasites and parasitic diseases of domestic animals, 1998.
- Knight H. D. and Lok B. J. Seasonality of heartworm infection and implications for chemoprophylaxis [Book]. – [s.l.] : Clinical Techniques in Small Animal Practice, 1998. – Vol. 13.
- Prevention, CDC-Centers for Disease Control [Online]. – 2019. – 07 05 2020. – www.cdc.gov.
- Society American Heartworm 2007 Guidelines For the Diagnosis, Treatment of and Prevention of Heartworm (Dirofilaria immitis) Infection in Cats [Online]. – American Heartworm Society, 2007. – 21 07 2020. – https://web.archive.org/web/20070614130430/http://www.heartwormsociety.org/article.asp?id=47.
- Yin Sophia Update on heartworm infection [Report]. – [s.l.] : Veterinary Forum, 2007.