This document discusses the use of broad-spectrum heartworm medications to prevent common internal and external parasites in cats. It describes how these medications can effectively eliminate the intestinal parasites Toxocara cati and Ancylostoma tubaeforme in cats through monthly administration. It also explains that while these medications may not prevent all internal parasites, they can control many common external parasites like fleas and ticks when used alone or combined with other treatments. The document emphasizes that establishing consistent preventive treatment programs is important for parasite control in cats.

1. Veterinary Therapeutics • Vol. 6, No. 3, Fall 2005
260
become proactive and promote year-round fe-
line parasite control programs.
While cats may not be parasitized as com-
monly 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 op-
portunity to provide cat owners with safe and
effective methods of preventing many parasitic
infections in their beloved pets. Recurring par-
asitic infestations may now be controlled easily
and effectively.
Broad-spectrum heartworm medications
prevent many of the most common parasite in-
festations. Because prevention of heartworm in
cats using macrolides is well documented,2,3
CLINICAL RELEVANCE
The monthly administration of broad-spectrum heartworm medications can ef-
fectively prevent a variety of internal and external parasitic diseases in cats. Al-
though 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 adminis-
tration of ivermectin, milbemycin oxime, or selamectin, either alone or in con-
junction with an external parasiticide.
Preventing Parasites in Cats
Michael W. Dryden, DVM, PhD
Patricia A. Payne, DVM, PhD
Department of Diagnostic Medicine and Pathobiology
College of Veterinary Medicine
Kansas State University
Manhattan, KS 66506-5606
I INTRODUCTION
Preventive health care, including the preven-
tion 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 un-
derstood and emphasized for more than a
decade. Veterinarians and owners now realize
that these medications not only prevent heart-
worms but also eliminate and prevent a variety
of internal and external parasites. This ap-
proach has been further clarified and approved
by the Companion Animal Parasite Council
(CAPC). CAPC recommends “year-round
treatment with broad-spectrum heartworm an-
thelmintics 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.”1
These recommendations can be adopted for
both dogs and cats. We believe that it is time to

2. M. W. Dryden and P. A. Payne
261
this article discusses the epidemiology and pre-
vention of the other common feline parasites
that can be controlled using monthly heart-
worm medications. Three such products cur-
rently approved in the United States are Heart-
gard 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 imi-
dacloprid (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.
I 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 nat-
ural predator–prey relationship is a common
mode of transmission of many feline parasites,
including Alaria marcianae, Ancylostoma tubae-
forme, Isospora felis (Cystoisospora), Isospora riv-
olta (Cystoisospora), Physaloptera spp, Sarcocystis
spp, Taenia taeniaeformis, Toxocara cati, Toxas-
caris leonina, and Toxoplasma gondii.4
Toxocara cati
Predation on infected paratenic hosts is com-
monly underestimated by veterinarians and
likely completely unknown by most cat owners
as a mode of T. cati transmission. Larvae com-
monly become encysted in the tissues of chick-
ens, cockroaches, earthworms, and mice when
they accidentally ingest infective larvated eggs
in feces or contaminated soil.4
In a similar fash-
ion, children may also become infected with so-
matic 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 lar-
vated 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 intes-
tine. The adult female worms then begin to de-
posit eggs (prepatent period) within 56 days.5
Figure 1. Toxocara cati eggs (65 × 75 µm). Figure 2. Ancylostoma tubaeforme egg (61 × 40 µm).

3. Veterinary Therapeutics • Vol. 6, No. 3, Fall 2005
262
While female dogs commonly transmit Tox-
ocara canis larvae to their puppies through
transplacental transmission, it is well estab-
lished 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 direct-
ed to the mammary glands via somatic migra-
tion and then to kittens during nursing. How-
ever, there is some doubt as to how frequently
previously encysted somatic larvae are reacti-
vated during pregnancy and then passed
through transmammary transmission.6
Although adult cats often display no overt
clinical signs of T. cati infection, clinical dis-
ease 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 med-
ications 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
Se-
lamectin applied topically at 6 mg/kg pro-
duced 100% reduction in adult T. cati.8
Al-
though not currently approved in the United
States, the topical spot-on imidacloprid–mox-
idectin formulation was 97.2% and 98.3% ef-
fective 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 hook-
worm A. tubaeforme. Female worms reside in
the small intestine and deposit thin-shelled,
four- to eight-celled eggs measuring 55 to 75 ×
34 to 45 µm in the feces (Figure 2). Eggs de-
velop quickly after being passed, and within a
few days the third-stage larvae can infect cats
by either the oral route or skin penetration. In-
gested 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 in-
testine as adults.4
Adults mature and begin lay-
ing eggs within 18 to 28 days after initial in-
fection.4
Larvae that infect the cat via skin
penetration complete a tracheal migration
route and mature within 19 to 25 days. As pre-
viously 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 ane-
mia, 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 provid-
ed 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 natu-
rally infected with hookworms.12
Based on the efficacy data, all of these for-
mulations 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 ba-

4. M. W. Dryden and P. A. Payne
263
sis. 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. How-
ever, monthly application should prevent de-
velopment 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 formula-
tions may not be effective. Therefore, fecal ex-
aminations 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 ad-
dition, kittens should be given biweekly an-
thelmintic 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 heart-
worm medication. Nursing bitches and queens
should be treated concurrently with their off-
spring because they often develop patent infec-
tions 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.
I EXTERNAL PARASITES
Although only a few intestinal parasites of
cats can be eliminated or prevented by the reg-
ular administration of broad-spectrum heart-
worm medication, these formulations can con-
trol or prevent several of the most common
external parasites of cats. Monthly administra-
tion 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 ef-
fectively 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 hair-
coat 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 com-
bine moderate temperatures, high relative hu-
midity, and a source of adult flea fecal blood.
Larvae spin silken cocoons in which they trans-
form into pupae and then adults. Adult C. fe-
lis 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 typi-
cally 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 fe-
cal pellets or long tubular coils (“flea dirt”).
Mating occurs after fleas have fed, and egg pro-
duction begins within 24 to 36 hours after fe-
males take their first blood meal; females can
produce up to 40 to 50 eggs/day.
Cat fleas cannot tolerate freezing; in cold cli-
mates, adults most likely survive the winter on
The primary mode of neonatal transmission of
T. cati to kittens is through the transmammary route.

5. Veterinary Therapeutics • Vol. 6, No. 3, Fall 2005
264
untreated dogs and cats or small wild mammals
(e.g., opossums, raccoons) in the urban envi-
ronment.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 subse-
quently develop into adults. Cat fleas may also
survive the winter as preemerged adults in mi-
croenvironments 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 or-
ganisms 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 Dipylid-
ium caninum, the double-pore tapeworm. D.
caninum is the most common tapeworm of
dogs and cats in North America. Thus, preven-
tion 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 in-
festation in cats.14,15
The other two broad-spec-
trum heartworm medications currently ap-
proved in the United States do not have activity
against fleas but can be administered in con-
junction with an effective flea-control product
to achieve comprehensive control and preven-
tion of fleas and heartworms. Monthly applica-
tion 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 ini-
tiate egg production within 24 hours after in-
festing a host, reactive treatments by pet own-
ers typically occur too late to stop infestations
of the premises. That is why establishing a pre-
ventive 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 para-
sites can transmit pathogenic organisms to
cats, tick prevention may be vital in certain ar-
eas of North America. None of the broad-spec-
trum heartworm medications are approved for
tick control in cats, and an effective acaricide
such as fipronil must be added to the preven-
tion program if tick control is needed.
Lice
Cats, especially those that were recently pur-
chased from pet stores or catteries, may be in-
fested with lice. Since most flea adulticides
readily kill lice, lice are not commonly en-
countered in areas where fleas occur. The only
species of lice that infests cats is Felicola subro-
stratus. 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 asso-
ciated alopecia and seborrhea sicca (dandruff).
Since the louse that parasitizes cats is a chew-
ing 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 heart-
worm medications currently approved in the

6. M. W. Dryden and P. A. Payne
265
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 dis-
ease 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 × 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, pro-
ducing 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 extremi-
ties. Disease can progress to self-mutilation
and even death. Selamectin has been shown to
be effective in eliminating this parasitic dis-
ease,20
and monthly administration should pre-
vent occurrence of these disease in cats.
Ear mites (Otodectes cynotis) are very com-
mon in cats. The mites spend their entire life
in the ears and are primarily transmitted be-
tween 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. Clas-
sic otitis externa is characterized by a dark
brown to black coffee-ground exudate occur-
ring 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. Se-
lamectin 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 protrud-
ing hook-like mouthparts or palpal claws (Fig-
ure 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, al-
though adults may live up to 10 days off the
host. Infestations are most common in catter-
ies 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 cli-
mates of certain flea products that also have ac-
tivity against Cheyletiella spp. Afflicted cats can
have variable pruritus, crusty papules along the
Figure 3. Notoedres cati recovered from a scraping
from a cat.

7. Veterinary Therapeutics • Vol. 6, No. 3, Fall 2005
266
head, neck, and back, and a dry, scaly sebor-
rhea. Owners of affected cats are also com-
monly 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 effi-
cacious. 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 formula-
tions may be combined with other broad-spec-
trum heartworm medications to control
Cheyletiella spp.26
I CONCLUSION
Historically, many veterinarians and pet
owners attempted to administer broad-spec-
trum heartworm medications seasonally. Al-
though seasonal prevention of heartworm and
other parasites may seem appropriate in many
regions of North America, it is actually diffi-
cult to accomplish. Veterinarians must first at-
tempt to estimate heartworm larvae develop-
ment rates in mosquitoes so they can
seasonally time the administration of heart-
worm preventives. Then they are faced with
the difficulty—if not impossibility—of at-
tempting to estimate the rate of flea develop-
ment, tick questing patterns, mite and louse
transmission, and development of infective
roundworm eggs and hookworm larvae. Be-
cause of differences in the biologic require-
ments 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 seasonal-
ly timed prevention programs, but it is our ex-
perience that determining start and stop dates
for seasonally timed applications of broad-
spectrum parasite prevention programs is diffi-
cult. 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
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