Veterinarians continue to diagnose heartworm infections regularly in dogs in many regions of the country, despite the availability
of effective and convenient preventative medications, and despite a market that has seen sales of heartworm prevention products
increase to more than $329 million dollars per year. When I speak to veterinarians, I often ask how many still diagnose canine
heartworm infections on a regular basis. I am continually surprised at the number of positive responses that I receive. It
is generally accepted that the numbers of infected dogs has decreased continually since the introduction of ivermectin in
the mid-1980s, but all continue to see infected dogs. Most veterinarians agree that they are also diagnosing fewer cases of
clinical heartworm disease compared to 10 or 15 years ago. I attribute the success that we have achieved in reducing the numbers
of infected dogs and clinical cases of heartworm disease to increased pet owner awareness of heartworm and how it is transmitted,
the advocacy of heartworm prevention by veterinarians, and the efficacy and convenience of heartworm preventative products.
Not only do newer product combinations and formulations provide veterinarians and pet owners with added convenience, some
also control additional internal and external parasites (Table 1, p 17). In the discussions that follow, I will emphasize
newer formulations for heartworm prevention and briefly review current trends in diagnosis and prevention of heartworm infections
in dogs and cats. 
After years of research, veterinarians now have a variety of options to choose from when selecting heartworm preventives.
New product combinations and formulations have paved the way for these advancements which also help control other parasites.
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New trends in heartworm preventionRecent trends in heartworm prevention have seen the introduction of some new active agents, combinations of existing agents
to increase spectrum of activity, and the development of chewable, topical and injectable formulations to provide more product
choices for veterinarians and pet owners (Table 1). We also have seen the introduction of several products for prevention
of heartworm infection in cats (Table 1). Because of the differences in heartworm infections in dogs and cats, it is sometimes
necessary to use different diagnostic and treatment strategies when dealing with feline heartworm infections (Table 2, p.
21).
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With new trends also come new technologies necessary to support their efficacy and convenience. The introduction of chewable
or flavored tablets in both single entity and combination medications has made daily and monthly prevention of heartworms,
gastrointestinal parasites, and even fleas more convenient for pet owners. Combinations of parasiticides are the basis for
broad spectrum parasite control claimed by Heartgard Plus (ivermectin/pyrantel pamoate) and Sentinel (milbemycin oxime/lufenuron).
Heartgard Plus combines the efficacy of ivermectin at 6 ������������������g/kg against L3 and L4 stages of heartworm with
the activities of pyrantel pamoate at 5 mg/kg against common roundworms (Toxocara canis, Toxascaris leonina) and hookworms
(Ancylostoma caninum, Ancylostoma braziliense, and Uncinaria stenocephala) in dogs. Heartgard Plus is the only canine monthly
broad-spectrum heartworm preventative product that also carries a label claim for U. stenocephala and A. braziliense. In addition
Heartgard Plus chewables is packaged in boxes of six or 12 chewables to provide veterinarians and pet owners with options
for prevention periods that may be more suited for their needs. Interceptor (milbemycin oxime) is an example of a single
entity product which exploits the potential broad spectrum capabilities of macrocyclic lactone (i.e. ivermectin, milbemycin
oxime, moxidectin, selamectin) compounds. At its label dosage of 0.5 mg/kg of bodyweight. Interceptor is effective in preventing
heartworm infections and also in the removal and control of roundworms (T. canis, Toxascaris leonina) and whipworms (Trichuris
vulpis), and in the control of hookworms (A. caninum). Interceptor was recently approved for use in cats at the target dose
of 2.0 mg/kg. At that dosage, Interceptor not only prevents heartworm infections but also removes roundworms (T. cati) and
hookworms (A. tubaeforme). Novartis has elected to dual label the existing product for use in either dogs or cats. For example,
the 5.75-mg tablet, now used in dogs weighing 11-25 pounds, can also be used for cats weighing 1.5-6 pounds. The 11.5-mg tablet
will treat dogs weighing 26-50 pounds, and cats weighing 6.1-12 pounds. Likewise, the 23-mg table is intended for dogs in
the 51-100 pound weight range and cats weighing 12.1-25 pounds. Dual labeling will help reduce the veterinarian's inventory
below what would be maintained if the dog and cat products were packaged separately. Interceptor is approved for use in cats
and kittens 6 weeks old and older and 1.5 pounds and greater. Sentinel is an additional example of the strategy of combining
agents to increase spectrum of activity. Sentinel combines the internal parasiticidal properties of milbemycin oxime and the
flea ovicidal and larvicidal effects of lufenuron in a single flavored tablet. Additional combination products for heartworm
prevention and internal parasite control are nearing approval and launch as this article is in preparation.The development of topically administered products such as selamectin (Revolution ) has added an additional dimension to the
convenience of regular application of preventative medications. The active agent in Revolution is selamectin, a novel semi-synthetic
avermectin compound. Revolution is an example of the use of chemical synthesis and bioengineering techniques to specifically
target both internal and external parasites in dogs and cats. Combined activity against internal and external parasites has
earned Revolution and similar products the name endecticides. Revolution contains selamectin specifically formulated in a
butylhydroxytoluene, dipropylene glycol methyl ether, isopropyl alcohol base to promote absorption when applied to the pet's
skin surface. Following absorption from the skin, Revolution enters the bloodstream, which is the basis of its activity against
heartworms. It is also excreted into the intestinal tract, resulting in activity against some gastrointestinal parasites.
Its redistribution from the bloodstream into the skin, including the sebaceous glands, provides persistent activity against
fleas, ear mites and scabies mites. Revolution was welcomed by pet owners who either could not or preferred not to administer
tablets to pets orally. Revolution is particularly convenient for those interested in internal and external parasite control
in cats but do not wish to administer oral medications. Topical heartworm preventative medications also are more likely to
be accepted by veterinarians and pet owners now that topical products have become the principal means of flea and tick control.
Heartworm prevention also includes periodic monitoring. Improvements have been made to in-clinic antigen and antibody tests
which encourage practitioners to incorporate these tests ino their monitoring strategies.
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The latest of novel formulations for heartworm prevention is an injectable suspension comprised of sterile moxidectin-containing
microspheres. Moxidectin is also related chemically to ivermectin, milbemycin oxime and selamectin. The injectable formulation
(ProHeart 6) is marketed as separate vials containing microspheres and diluent. The microspheres and diluent are combined
(constituted) just prior to injection. After mixing, the suspended microspheres are allowed to stand for five minutes, and
can then be used for four weeks if stored in the refrigerator. The ProHeart 6 formulation of moxidectin is administered subcutaneously
to dogs at the target dose of 0.17 mg of moxidectin per kg of body weight, or 1 cc of constituted product per 44 pounds of
body weight. The product insert recommends subcutaneous injection of ProHeart 6 in the subcutaneous tissues of the neck cranial
to the scapula. Subsequent injections are given at a corresponding location on the alternate side. Microsphere technology
involves the unique incorporation of the moxidectin molecule into the microsphere matrix. As the microspheres disintegrate,
the moxidectin molecule is released, resulting in an immediate (one to two week) achievement of peak measurable blood levels
of moxidectin. The amounts of detectable moxidectin decline over the subsequent six-month period. Studies supporting efficacy
of the injectable microsphere formulation of moxidectin indicate that sufficient amounts remain in dogs to prevent heartworm
infections for six months. The ProHeart 6 formulation also carries a label claim for removal of larval and adult hookworm
(Ancylostoma caninum) at the time of injection. The label does not support continued control of hookworm throughout the subsequent
six-month period as for heartworm. ProHeart 6 is approved for use in dogs that are 6 months old and older.TestingAs with all heartworm prevention products, dogs that are not currently on a heartworm prevention product should be tested
to determine their infection status. Veterinarians can then decide whether to remove adult worms with an approved adulticide.
I favor treatment of all dogs, whenever possible, to remove adult heartworms prior to placement on any preventative medication.
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