CAT UpdateA NESTLE PURINA PUBLICATION DEDICATED TO CAT ENTHUSIASTS VOLUME 14 | WINTER 2016

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Historically, heartworm disease has beenoverlooked in cats, yet it has proved to beserious and potentially fatal. Becausethere is no approved treatment for heart-worm infection in cats as there is in dogs,prevention of infection is the best protec-tion. Other differences between felineand canine heartworm disease make itimportant for cat breeders and ownersto be aware of the risk of the disease andhow to prevent it.

Alice Lee, DVM, the Novartis clinicalresident in veterinary parasitology atCornell University, attributes the priorlack of awareness about feline heartwormdisease to the fact that mature heartworminfection can be difficult to diagnose incats. “Until recently, it was not knownthat heartworms cause disease in catseven before they’re fully grown,” shesays. “This lack of awareness translatedinto low rates of heartworm preventiveuse in cats.”

Dr. Lee cites other factors that maycontribute to a lack of the use of heart-worm preventives by cat owners. “Somepeople think that if their cats don’t gooutside, they won’t be bitten by mosqui-toes and they won’t get infected. In reality,infected mosquitoes can enter homes, soboth outdoor and indoor cats are at risk,”she says. “Cats also are not brought tothe veterinary clinic as often as dogs,thus there are fewer opportunities forthe veterinarian to educate cat ownersabout the risk of heartworm.”

In recent years, feline heartworm dis-ease has been diagnosed more frequently,

says Patricia A. Payne, DVM, PhD, a vet-erinary parasitologist and member ofthe board of directors of the AmericanHeartworm Society. “With increasedtesting for heartworms in cats by veteri-narians, new ‘hot spots’ for the diseaseare being found in previously nonendemicplaces such as southern Michigan andnorthern California,” she says. “Height-ened awareness of the disease in catsand improved diagnostic methods playa role as well.”

Every year, heartworm disease spreadsto new regions of the country, with thedisease now having been diagnosed inall 50 states. Environmental changes cantrigger spread. This occurred followingHurricane Katrina when 250,000 pets, manyinfected with heartworms, were adoptedand shipped throughout the country.

Urban sprawl creates “heat islands”as buildings and parking lots retain heatduring the day and as water drainage isaltered, thus fostering a microenviron-ment that supports the development ofheartworm larvae and thereby lengthen-ing the transmission season. Mosquitoesblown great distances by the wind alsocontribute to the spread of disease topreviously uninfected areas.

Heartworm disease can occur in catsanywhere dogs are infected, though theprevalence of infection in cats is from 5to 20 percent of the level in unprotecteddogs in the same geographical location.The prevalence rate in cats is believed tobe underrepresented due to diagnosticlimitations related to cats exhibiting

CAT Update

“Until recently, it was notknown that heartworms causedisease in cats even beforethey’re fully grown. This lackof awareness translated intolow rates of heartworm preventive use in cats.”Alice Lee, DVM, Novartis clinical resident inveterinary parasitology, Cornell University

Feline HeartwormDisease

CAN BE POTENTIALLY FATAL

Winter 2016

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CAT Update CAT Update

short-term clinical signs andoften dying without confirma-tion of infection.

Heartworms, known by thescientific name Dirofilaria im-mitis, are parasites. The bite ofa mosquito carrying a larvalform of the heartworm sets inmotion the process that causeslung disease and heart failurethat can lead to death. (See “Feline Heartworm Life Cycle,”left) Unlike dogs, cats are nota typical host for heartworms,and thus most worms in catsdo not survive to the adult stage.When worms do survive, catstypically have one to eight wormscompared to dogs that can have30 or more.

Recognizing Heartworms in CatsSigns of heartworm disease

in cats can be subtle or dramatic.Coughing, asthma-like attacks, periodicvomiting, lack of appetite, and weightloss are signs. Occasionally an infectedcat may have difficulty walking, experi-ence fainting or seizures, or suffer fromabdominal fluid accumulation.

Two phases of an infection are par-ticularly dangerous. They occur when: • Immature heartworms reach the pul-

monary system three to four months

post-infection, causing inflammationin the lungs, a condition known asheartworm-associated respiratory dis-ease (HARD).

• Adult heartworm(s) die, plugging theblood vessels in the lungs, resulting inrespiratory distress and death. In cats,adult heartworms are more likely to befound in atypical locations such as thebrain, causing neurological problems. A definitive diagnosis of feline heart-

worms is based on blood antigen andantibody tests and in some cases cardiacultrasonography. The antibody test detectswhether an infection has occurred asearly as two months post-infection, andthe antigen test picks up the presenceof adult worms. Positive tests should be followed up with cardiac diagnosticimaging tests to help visualize the worms.

Feline heartworm disease can bechallenging to diagnose. “It may beconfused with other diseases that havesimilar signs. For example, HARD may beconfused with feline asthma or allergicbronchitis,” says Dr. Lee. “Many catsshow no signs at all, and the infectionremains undetected until a worm diestriggering respiratory failure and suddendeath in a cat previously presumed tobe healthy.”

Cats diagnosed with heartworms aretreated using medications, such as pred -nisone, a steroid, to manage the clinical

signs. In rare cases, surgery can be per-formed to remove worms though thisprocedure is expensive and can be risky.Guided by ultrasound, a surgeon makesan incision in the neck to pull a worm(s)out of the pulmonary artery and/or heart.

Heartworm prevention is the best wayto prevent an infection. “Cats should betested before starting on a heartwormpreventive,” advises Dr. Payne. “Becausethere is no approved treatment for heart-worm infection in cats, prevention iscritical. The American Heartworm Societyrecommends testing cats every 12 monthsfor heartworms and giving heartwormpreventive 12 months a year.”

Topical and oral heartworm preventiveshave been available commercially in theU.S. since the 1990s, though they have notbeen used widely by cat owners. Gradu-ally, their use is increasing, thanks togreater awareness about heartworm dis-ease. Monthly preventives, such as the

oral preventives Heartgard® and Inter-ceptor® and topical preventives Revolu-tion® and Advantage Multi®, require aveterinary prescription. Some preventivesalso are effective against parasites, suchas roundworms, hookworms, fleas, andear mites.

Due to the difficulty diagnosing heart-worms in cats, Dr. Payne advises breedersto have a necropsy performed when thereis an unexplained death in the cattery.“A necropsy can alert you to a problemsuch as heartworms, which potentiallycould devastate your breeding stock,”she says. “Still, the most important thingis to practice using preventives and testingannually. It is well worth the effort.” �

Feline Heartworm Life Cycle Mosquitoes become infected whenthey bite a dog with immature wormscalled microfilariae. In the mosquito,the microfilariae develop into larvae.When the infected mosquito bites acat, the larvae enter through the biteand develop in tissues under the skin.The immature worms go through severaldevelopmental stages, eventually find-ing their way to a blood vessel wherethey are carried to the arteries in thelungs. Many immature worms die at thispoint, causing intense inflammation.

Worms that survive to adulthoodprimarily live in the pulmonary artery,though sometimes they can be foundin the right side of the heart as well.From the time the mosquito bites andinfects the cat to the development ofadult worms takes about eight months.Cats rarely produce microfilariae because they have few adult wormsand because their immune systeminhibits the development of microfilar-iae. Thus, cats rarely transmit heart-worms to other species via mosquitoes.

Comparing Feline & Canine Heartworm Disease

Mature heartworm infection in cats usually consistsof one to eight adult worms, which is considered aheavy, potentially life-threatening infestation due tocats’ small size. On the other hand, dogs may have30 or more worms.

Although heartworms do not grow as long in cats asin dogs, they migrate into other body tissues, such asthe nervous system, blood vessels and body cavities,at a much higher proportion than in dogs.

Heartworm disease is harder to detect in cats than indogs, usually requiring a combination of blood antigenand antibody tests and cardiac ultrasonography.

Clinical signs of heartworm infection are not alwaysobserved in cats but usually are recognizable in dogs.In cats, heartworm-associated respiratory disease isoften misdiagnosed as asthma or allergic bronchitis.

Cats are more resistant to heartworms (Dirofilariaimmiitis) than dogs. It is estimated that cats are infected at a rate of 5 to 20 percent of the rate in dogs.

Cats with mature heartworm infection rarely showmicrofilariae (baby worms) circulating in their

bloodstream compared to 70 to 80 percent ofdogs that do.

Heartworms can live two to four years in cats, compared to five to seven years in dogs.

Purina appreciates the support of theWinn Feline Foundation, and particularlyGlenn A. Olah, DVM, PhD, DABVP (feline),president, in helping to identify this topicfor the Cat Update newsletter.

© 2014American Heartworm Society

Wilmington, DL

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Want to Reach the Editor?Have comments about the Cat Update?Send them to: Barbara Fawver, Editor,Nestlé Purina PetCare, 2T CheckerboardSquare, St. Louis, MO 63164 or viaemail at editor@purina.nestle.com.

Looking to Reprint?Purina Pro Club Update articles may be reprinted provided the article is used in itsentirety and in a positive manner. To request permission to reprint an article, pleasecontact the editor at: editor@purina.nestle.com. Reprints should include the follow-ing attribution: Used with permission from the Purina Pro Club Update newsletter,Nestlé Purina PetCare Company.

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