Preventing Parasites in Cats | Articles and Archives |

CLINICAL RELEVANCE

The monthly administration of broad-spectrum heartworm medications can effectively prevent a variety of internal and external parasitic diseases in cats. Although not every parasite can be stopped, many of the common feline parasites are susceptible to these agents. This article discusses the epidemiology and prevention strategies for those parasites that can be controlled by the administration of ivermectin, milbemycin oxime, or selamectin, either alone or in conjunction with an external parasiticide.

Introduction

Preventive health care, including the prevention of common parasitic infections in cats, is an idea whose time has come. The importance of regular administration of broad-spectrum heartworm medications to dogs has been understood and emphasized for more than a decade. Veterinarians and owners now realize that these medications not only prevent heartworms but also eliminate and prevent a variety of internal and external parasites. This approach has been further clarified and approved by the Companion Animal Parasite Council (CAPC). CAPC recommends “year-round treatment with broad-spectrum heartworm anthelmintics that have activity against parasites with zoonotic potential.”¹ The council also states: “Pet-owner awareness of heartworms and fleas can serve as the foundation for effective prevention and control of other parasites.”¹These recommendations can be adopted for both dogs and cats. We believe that it is time to become proactive and promote year-round feline parasite control programs.

While cats may not be parasitized as commonly as dogs, there still are a large number of cats in North America that harbor parasites. The increased awareness of the deadly nature of heartworm disease in cats, the realization that the same flea species affects both dogs and cats, and the change in the status of the cat from the number two to the number one pet in the United States have led to an increase in the number of veterinary pharmaceuticals labeled for use in cats. Veterinarians now have the opportunity to provide cat owners with safe and effective methods of preventing many parasitic infections in their beloved pets. Recurring parasitic infestations may now be controlled easily and effectively.

Broad-spectrum heartworm medications prevent many of the most common parasite infestations. Because prevention of heartworm in cats using macrolides is well documented,^2,3 this article discusses the epidemiology and prevention of the other common feline parasites that can be controlled using monthly heartworm medications. Three such products currently approved in the United States are Heartgard for Cats (Merial; oral ivermectin, 24 µg/kg), Interceptor (Novartis; oral milbemycin oxime, 2.0 mg/kg), and Revolution (Pfizer; topical selamectin, 6–12 mg/kg). In addition, there is a spot-on topical formulation of imidacloprid (10 mg/kg) and moxidectin (1.0 mg/kg) (Bayer Animal Health) that has been approved in Europe, Australia, and New Zealand as Advocate and Canada as Advantage Multi. These products can be used alone or in combination with other drugs and insecticides to prevent a variety of feline parasites.

INTERNAL PARASITES

Many cat owners do not want to think their cat, a member of their family, may have preyed on a mouse or other animal. However, this natural predator–prey relationship is a common mode of transmission of many feline parasites, including Alaria marcianae, Ancylostoma tubaeforme, Isospora felis (Cystoisospora), Isospora rivolta (Cystoisospora), Physaloptera spp, Sarcocystis spp, Taenia taeniaeformis, Toxocara cati, Toxascaris leonina, and Toxoplasma gondii.⁴

Toxocara cati

Predation on infected paratenic hosts is commonly underestimated by veterinarians and likely completely unknown by most cat owners as a mode of T. Cati transmission. Larvae commonly become encysted in the tissues of chickens, cockroaches, earthworms, and mice when they accidentally ingest infective larvated eggs in feces or contaminated soil.⁴ In a similar fashion, children may also become infected with somatic tissue stages of T. Cati, which can cause visceral or ocular larva migrans.

Adult T. Cati live in the small intestine of wild and domestic cats. The worms are cream to pinkish in color and can be up to 10 cm in length. This ascarid passes large (65 to 75 µm), single-celled, thick-shelled mammilated eggs in the feces (Figure 1). In the environment, eggs develop into infective larvated eggs in 3 to 4 weeks. Cats become infected after ingesting larvated eggs or by preying on an infected paratenic host.⁵ After eggs are ingested, larvae hatch in the intestine and migrate through the liver and lungs (tracheal migration); within 5 days, some larvae can already be recovered from the intestine.⁵ Fourth-stage larvae mature and molt to the adult fifth stage in the small intestine. The adult female worms then begin to deposit eggs (prepatent period) within 56 days.⁵

While female dogs commonly transmit Toxocara canis larvae to their puppies through transplacental transmission, it is well established that this mode of transmission does not occur with T. Cati in cats.^5,6 The primary mode of neonatal transmission to kittens is through the transmammary route.⁴ Infection of queens with T. Cati during pregnancy can result in overwhelming numbers of larvae being directed to the mammary glands via somatic migration and then to kittens during nursing. However, there is some doubt as to how frequently previously encysted somatic larvae are reactivated during pregnancy and then passed through transmammary transmission.⁶

Although adult cats often display no overt clinical signs of T. Cati infection, clinical disease may occur in kittens under conditions of high worm burdens or poor nutritional status. Moderate to heavy infections in kittens can produce a pot-bellied appearance, kittens smaller than normal for their age, dry skin, a malnourished appearance, vomiting, diarrhea or constipation, and (rarely) death due to acute intestinal impaction.

Two of the broad-spectrum heartworm medications currently approved for use in cats in the United States are effective in eliminating T. Cati. Milbemycin was effective in eliminating T. Cati at a dose of 0.05 to 0.1 mg/kg PO.⁷ Selamectin applied topically at 6 mg/kg produced 100% reduction in adult T. Cati.⁸ Although not currently approved in the United States, the topical spot-on imidacloprid–moxidectin formulation was 97.2% and 98.3% effective against fourth-stage and immature adult T. Cati, respectively.⁹

Ancylostoma tubaeforme

Another common intestinal parasite of cats that can be treated and prevented using broad-spectrum heartworm medications is the hookworm A. Tubaeforme. Female worms reside in the small intestine and deposit thin-shelled, four- to eight-celled eggs measuring 55 to 75 x 34 to 45 µm in the feces (Figure 2). Eggs develop quickly after being passed, and within a few days the third-stage larvae can infect cats by either the oral route or skin penetration. Ingested larvae enter the lining of the stomach or small intestine and develop over the next 10 to 12 days before moving back into the small intestine as adults.⁴ Adults mature and begin laying eggs within 18 to 28 days after initial infection.⁴ Larvae that infect the cat via skin penetration complete a tracheal migration route and mature within 19 to 25 days. As previously mentioned, rodents may serve as paratenic hosts. There is no evidence for either transmammary or transplacental transmission of A. Tubaeforme in cats.⁴ Clinical signs in cats infected with A. Tubaeforme are typical of blood-feeding hookworms and include anemia, diarrhea, weight loss, and even death.

All three of the broad-spectrum heartworm medications approved for use in cats in the United States are effective in eliminating A. Tubaeforme. Ivermectin administered orally at 24 µg/kg was 92.8% and 90.7% effective against adult Ancylostoma braziliense and A. Tubaeforme, respectively.¹⁰ Milbemycin oxime reduced the numbers of fourth-stage larvae and adults of A. Tubaeforme by 94.7% and 99.2%, respectively.¹¹ Selamectin applied topically at 6 mg/kg provided 99.4% reduction in adult A. Tubaeforme.⁸ In addition, the topical imidacloprid–moxidectin formulation was 99.64% effective in reducing Ancylostoma spp fecal eggs counts in cats naturally infected with hookworms.¹²

Based on the efficacy data, all of these formulations can be used to treat or, when applied monthly, prevent A. Tubaeforme, and all but the ivermectin formulation will also eliminate T. Cati. It should be remembered that when these formulations are administered as approved, the cat is essentially “dewormed” on a monthly basis. If a cat is frequently exposed to infective third-stage larvae of A. Tubaeforme, the worms can mature and deposit eggs between monthly applications, given that the prepatent period of A. Tubaeforme can be as short as 18 days. However, monthly application should prevent development of hookworm disease.

Overview of Internal Parasite Prevention

Broad-spectrum heartworm medications are effective in controlling T. Cati and A. Tubaeforme, but cats may become infested with a variety of internal parasites against which these formulations may not be effective. Therefore, fecal examinations should be performed two to four times during the first year of a cat’s life and one to two times annually in adult cats, depending on individual health and lifestyle factors.¹ In addition, kittens should be given biweekly anthelmintic treatments beginning at 3 weeks of age. When cats reach 8 or 9 weeks of age, they can be put on a monthly broad-spectrum heartworm medication. Nursing bitches and queens should be treated concurrently with their offspring because they often develop patent infections along with their young. As mentioned, many of these parasites are transmitted though predation, and thus cats should be fed only cooked or commercially prepared food.

EXTERNAL PARASITES

Although only a few intestinal parasites of cats can be eliminated or prevented by the regular administration of broad-spectrum heartworm medication, these formulations can control or prevent several of the most common external parasites of cats. Monthly administration of a single broad-spectrum heartworm medication with activity against both internal and external parasites, alone or in combination with an effective external parasiticide, can effectively control fleas, ticks, lice, and several species of mites.

Fleas

The most common external parasite of cats is Ctenocephalides felis, the cat flea. The cat flea lays 0.5-mm pearly white oval eggs in the haircoat of its host. The eggs fall from the pelage to be deposited into the surrounding indoor and outdoor environments. Larvae hatch from the eggs within 2 to 7 days and develop over 1 to 3 weeks in protected microhabitats that combine moderate temperatures, high relative humidity, and a source of adult flea fecal blood. Larvae spin silken cocoons in which they transform into pupae and then adults. Adult C. Felis can begin emerging 8 days after initiation of pupal development but, if undisturbed, may remain quiescent in the cocoon for up to 30 weeks.¹³ Development from egg to adult typically ranges from 3 to 8 weeks, depending on temperature and humidity level.

Once adult C. Felis acquire a host, they begin feeding almost immediately. The total volume of blood ingested daily by males is unknown, but female C. Felis can consume 13.6 µl of blood daily.¹³ After a rapid transit through the flea, excreted blood dries into reddish-black fecal pellets or long tubular coils (“flea dirt”). Mating occurs after fleas have fed, and egg production begins within 24 to 36 hours after females take their first blood meal; females can produce up to 40 to 50 eggs/day.

Cat fleas cannot tolerate freezing; in cold climates, adults most likely survive the winter on untreated dogs and cats or small wild mammals (e.g., opossums, raccoons) in the urban environment.¹³ As these animals pass through yards in the spring or set up nesting sites in crawl spaces, under porches, or in attics, the eggs laid by surviving female fleas drop off and subsequently develop into adults. Cat fleas may also survive the winter as preemerged adults in microenvironments protected from the cold.

Cat fleas can cause severe irritation and are responsible for the production of flea allergy dermatitis, often observed as miliary dermatitis in cats. Heavy infestations can result in life-threatening anemia. Cat fleas can also serve as the vector of murine typhus, caused by the organisms Rickettsia typhi and Rickettsia felis, and are considered the vector of Bartonella henselae among cats. Cat fleas are also the intermediate host for Dipetalonema reconditum and Dipylidium caninum, the double-pore tapeworm. D. Caninum is the most common tapeworm of dogs and cats in North America. Thus, prevention of flea infestations can prevent a variety of disease conditions in cats.

Monthly applications of selamectin can be used to effectively eliminate and prevent flea infestation in cats.^14,15 The other two broad-spectrum heartworm medications currently approved in the United States do not have activity against fleas but can be administered in conjunction with an effective flea-control product to achieve comprehensive control and prevention of fleas and heartworms. Monthly application of the topical imidacloprid–moxidectin formulation is also effective in eliminating and controlling flea infestations on cats.³

An important point of flea control is that on-again, off-again treatment programs are rarely effective regardless of the product used. By the time a pet owner notices fleas on their cat, the fleas have already begun laying eggs that infest the premises. Since cat fleas can initiate egg production within 24 hours after infesting a host, reactive treatments by pet owners typically occur too late to stop infestations of the premises. That is why establishing a preventive flea-control program is the best way to safeguard cat health.

Ticks

While cats are not as commonly infested with ticks as dogs are, cats can be parasitized by ticks, including such species as Amblyomma americanum, Dermacentor variabilis, and Ixodes scapularis.¹⁶ Because these blood-sucking parasites can transmit pathogenic organisms to cats, tick prevention may be vital in certain areas of North America. None of the broad-spectrum heartworm medications are approved for tick control in cats, and an effective acaricide such as fipronil must be added to the prevention program if tick control is needed.

Lice

Cats, especially those that were recently purchased from pet stores or catteries, may be infested with lice. Since most flea adulticides readily kill lice, lice are not commonly encountered in areas where fleas occur. The only species of lice that infests cats is Felicola subrostratus. It should be remembered that lice are very host specific: The cat louse will not infect dogs or humans. Lice are usually transmitted between cats by direct contact. All life stages reside on the host; females attach their eggs (nits) to the hair. Development from egg to adult takes about 1 to 2 months. Clinical signs of infestation in cats include pruritus and associated alopecia and seborrhea sicca (dandruff). Since the louse that parasitizes cats is a chewing louse (Mallophaga), anemia is unlikely to be a presenting clinical sign.

Selamectin was 100% effective against F. Subrostratus when applied once topically at 6 mg/kg.¹⁷ If the other broad-spectrum heartworm medications currently approved in the United States are used, an insecticide effective against lice, such as fipronil or imidacloprid, must also be administered.^18,19

Mites

Notoedres cati is an uncommon yet highly pathogenic parasitic mite (Figure 3). The disease produced by this mite is commonly called face mange and occurs more commonly along the Gulf Coast and southeastern United States. The adult female mite measures approximately 240 x 200 µm and has a “thumbprint” pattern of folds on its cuticle. The mites are easily transmitted through direct cat-to-cat contact. The mite burrows through the epidermis, producing a severe inflammatory reaction likely associated with hypersensitivity to mite feces, salivary proteins, and shed cuticles. Clinical signs include pruritus, alopecia, and scaly crusts on the face, ear tips, and distal extremities. Disease can progress to self-mutilation and even death. Selamectin has been shown to be effective in eliminating this parasitic disease,²⁰ and monthly administration should prevent occurrence of these disease in cats.

Ear mites (Otodectes cynotis) are very common in cats. The mites spend their entire life in the ears and are primarily transmitted between cats by direct contact. The mites feed on epidermal debris; it is thought that salivary proteins and feces induce hypersensitivity. Cats exhibit pruritus and may have raw sores behind the ears as a result of excessive scratching. Classic otitis externa is characterized by a dark brown to black coffee-ground exudate occurring in the external ear canal.

Ear mites can be treated and prevented by the monthly application of a broad-spectrum heartworm medication such as selamectin or the imidacloprid–moxidectin formulation. Selamectin has been evaluated in several studies in both North America and Europe and has been found to be 94% to 100% effective in eliminating natural O. Cynotis infections in cats when applied at a minimum single topical dose of 6 mg/kg.^21–23 The topical imidacloprid–moxidectin formulation was 80% effective against ear mites after a single application and 100% effective after two applications 4 weeks apart.²⁴ Topical otic formulations of ivermectin (0.01%) and milbemycin oxime (0.1%) have also been approved for treatment of ear mites in cats.

Cheyletiellosis, or “walking dandruff,” in cats in North America is primarily caused by Cheyletiella blakei. This is a relatively large (300 to 500 µm long) mite with large protruding hook-like mouthparts or palpal claws (Figure 4). The entire life cycle is spent on the host. These mites are considered nonburrowing as they live on the surface of the epidermis. Mites are transmitted primarily by direct contact, although adults may live up to 10 days off the host. Infestations are most common in catteries and pet stores. The mites appear to be more common in temperate climates, where fleas are not as prevalent. This may be related to the widespread use in subtropical and tropical climates of certain flea products that also have activity against Cheyletiella spp. Afflicted cats can have variable pruritus, crusty papules along the head, neck, and back, and a dry, scaly seborrhea. Owners of affected cats are also commonly afflicted with a papular pruritic rash.

Although the broad-spectrum heartworm medication selamectin is not FDA approved for use against Cheyletiella spp in cats, it is efficacious. In one study,²⁵ cats parasitized by Cheyletiella spp received topical selamectin (45 mg total dose) on days 0, 30, and 60. Within 30 days, no mites were observed on hair or skin debris samples; clinical signs had subsided by day 60 in all cats.²⁵ Fipronil-based formulations may be combined with other broad-spectrum heartworm medications to control Cheyletiella spp.²⁶

Conclusion

Historically, many veterinarians and pet owners attempted to administer broad-spectrum heartworm medications seasonally. Although seasonal prevention of heartworm and other parasites may seem appropriate in many regions of North America, it is actually difficult to accomplish. Veterinarians must first attempt to estimate heartworm larvae development rates in mosquitoes so they can seasonally time the administration of heartworm preventives. Then they are faced with the difficulty—if not impossibility—of attempting to estimate the rate of flea development, tick questing patterns, mite and louse transmission, and development of infective roundworm eggs and hookworm larvae. Because of differences in the biologic requirements of each parasite, specific transmission “seasons” will vary among parasites. Simply put, when does the transmission season of one parasite start and another end? In addition, changing climatic conditions from one year to the next can have marked effects on flea and tick “seasonality.”

Studies have not been conducted to compare the effectiveness of year-round versus seasonally timed prevention programs, but it is our experience that determining start and stop dates for seasonally timed applications of broad-spectrum parasite prevention programs is difficult. Combine these epidemiologic issues with historically poor pet owner compliance and a need to protect pet and human health, and it is understandable why the CAPC recommends year-round treatment with broad-spectrum heartworm medications.¹

References

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