Heartworm disease in cats 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. Recently, in a survey done by the American Heartworm Society, 24% of veterinarians reported that they saw more heartworm cases in 2016 than 2013.
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.
Heartworm Disease in Cats
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.
The disease can be spread from animal to animal by mosquitoes. The mosquito bites a cat with heartworm infection, collects some of the microscopic heartworm offspring, and then after a couple of weeks, passes these on to another cat. Inside the cat, the microscopic heartworm can grow into a parasite exceeding a foot in length. The life cycle is somewhat complicated. The important thing is to prevent worm development using safe and effective preventative drugs.
Heartworms are present (endemic) in most parts of the United States and in many parts of North America. Mosquitoes are the key — without them the disease cannot spread. The highest rate of infections are found in subtropical climates like those of the southeastern United States, the Gulf States, and Hawaii. However, heartworms are also found throughout the central and eastern United States, particularly near oceans, lakes, and rivers. When compared to dogs, cats are naturally resistant to heartworm disease (estimated at about one-fifth as likely to become seriously infected as dogs in the same region); however, heartworm disease in cats is often more severe than in dogs. The presence of even a single adult heartworm in a cat can result in very serious consequences.
Heartworm disease injures the lungs, the arteries within the lungs, and the heart. Symptoms include tiring, coughing, vomiting, weight loss, difficulty breathing, and even sudden death. Heartworm infection in cats can be difficult to diagnose. Blood tests are available, but the results may sometimes be misleading.
Preventing Heartworm Disease in Cats
Prevention of heartworm disease is simple. “Preventatives” kill microscopic larvae that are left behind by mosquitoes when they bite a cat. In most cases, a once-monthly pill or topical ointment is effective in preventing heartworms from taking hold.
Owners of all cats living in areas endemic for heartworms should discuss the pros and cons of preventative care with their veterinarian. If dogs in the area receive heartworm prevention, it is likely that cats also may benefit from this protection. Do NOT use your canine heartworm medicine in your cat. The drug dosing is very different between species. Speak to your veterinarian about the need for preventative therapy, administration guidelines and when to start and stop prevention treatments.
Testing for Heartworms
There are two different tests used to check for heartworms in dogs, but only one of these has been found effective in cats.
Microfilaria Test. Microfilaria are “heartworm babies.” Screening for them is rarely done in cats because it is often negative even when heartworms are present. The reasons for microfilaria being absent from the bloodstream when adult worms are present include:
- Only a single heartworm being present (thus no mating of worms is possible).
- The infection is with either all male or all female worms.
- The cat’s immune system is actively destroying the microfilaria as they are being produced.
Antigen Testing. Heartworm antigen tests detect antigen derived primarily from mature female worms. Immature infections, low worm burden, male only infection, or sexually immature worms may not produce enough antigen for detection. Also, cats may take a while after infection to develop positive antigens.
Heartworm testing involves drawing a blood sample. The blood is placed immediately in a glass container with a substance that prevents clotting of the sample. The sample is then submitted to a laboratory. The canine heartworm test cannot be used to detect feline heartworm disease. It takes 1 to 2 days to obtain the results of the test.