Prevention and treatment of Heartworms in our canine companions has become quite complicated in the last several years. As you can see by looking at the following Heartworm Incidence Survey Map published by the American Heartworm Society (AHS), the rate of Heartworm infected dogs has greatly increased since 2001. These surveys are requested every 3 years by the AHS from over 40,000 veterinary clinics around the nation. Another survey will be released in 2010.http://www.heartwormsociety.org/veterinary-resources/slide.html

In November of 2008 the AHS issued a warning attributed to a rise in mosquito populations for pet owners to continue year-round heartworm protection. “The risk from the increased mosquito population makes the need for prevention even greater this year,” says Dr. Sheldon Rubin, American Heartworm Society president and Chicago area practitioner.

http://www.heartwormsociety.org/inthenews/6-05-08.html

What is not being released by the AHS, which is sponsored by large animal pharmaceutical companies such as Merial, Pfizer, and Bayer, is that many of these heartworm positive dogs were already on heartworm preventatives.

While Ivermectin resistance has already been reported in many nematodes, previous studies show no reports of resistance in filarial nematodes such as Dirofilaria immitis (parasitic roundworm called heartworm). The Merck Veterinary Manual already states, “…there are reports of ivermectin resistance in Cooperia oncophora , C punctata , and Haemonchus placei . Anthelmintic resistance among nematode parasites of cattle is possibly more widespread than realized and needs more investigation.” Is the recent outbreak of heartworms in dogs currently on preventatives a sign of filarial nematode resistance to Ivermectin in particular areas of the country?

Many studies must be done in order to determine if the current epidemic is due to Avermectin resistance in D. Immitis, but we can be sure that there is something going on in the Mississippi delta with large numbers of dogs on preventatives becoming heartworm infected.

If this is the case, then what are a pet owner’s choices for prevention of heartworms? As stated in the Merck Veterinary Manual “When resistance to the recommended dose rate of ivermectin appears, moxidectin, at its recommended dose rate, is usually still effective. However, there is cross-resistance between the avermectins and the milbemycins, and the use of either subgroup will select for macrocyclic lactone resistance.”

http://merckveterinarymanual.com/mvm/index.jsp?cfile=htm/bc/191506.htm

Commercially Available Preventatives

MOXIDECTIN based: (can be given to heartworm-positive pets with no adverse side affects)

• Advantage-Multi by Bayer (2.5 mg/kg Moxidectin) Topical application controls Heartworms, Fleas, Hookworms, Roundworms, Whipworms
• Pro-Heart 6 injectable (previously removed from the market by the FDA) made by Fort Dodge a division of Wyeth which has merged with Pfizer. Controls Heartworms and Hookworms.

IVERMECTIN based: (no adverse reaction if this medication is given to a dog that is positive for heartworms)

• Heartgard and Heartgard Plus by Merial (.006 mg/kg Ivermectin) Oral heartworm preventative. Heartgard Plus also controls roundworms and hookworms
• Iverhart Plus by Virbac (.006 mg/kg Ivermectin) Oral – heartworm, roundworm and hookworms. Iverhart Max also controls tapeworms.
• Tri-Heart Plus by Intervet/Schering Plough (n/s) Oral – heartworm, roundworm and hookworm.

MILBEMYCIN based: (can cause problems if given to a dog that is positive for heartworm infection. Because the larvae die much faster with Milbemycin, it can create a shock to the dog’s system)

• Interceptor by Novartis (.5 mg/kg) Oral – heartworms, roundworms, hookworms and whipworms
• Sentinel by Novartis (.5 mg/kg) Oral – heartworm, flea, roundworm, hookworms, and whipworms

SELAMECTIN based: (safe to use in heartworm-infected animals)

• Revolution by Pfizer ( 6 mg/kg) Topical – heartworms, fleas, ticks, ear mites, sarcoptic mange

Treatment

Recent research has led to the discovery of a parasite called Wolbachia that lives symbiotically inside heartworms. Studies indicate that this parasite contributes to the adverse effects of both heartworm infection and heartworm treatment, including inflammation, embolism and allergic reaction. Treatment with doxycycline to kill the Wolbachia parasite weakens the heartworms and makes them unable to reproduce which lessens their adverse effects on the body and greatly reduces the chance of adverse reaction during heartworm treatment.

New studies published in late 2008 clearly indicate that treatment with a combination of weekly ivermectin and daily doxycycline given intermittently will sterilize the heartworms, prevent the dog from being infective to other dogs, speed up the death of the worms prior to (or in place of) Immiticide treatment, limit inflammation and damage caused by the worms presence, and reduce the chance of serious adverse reaction from Immiticide treatment. All of these effects are greater when the two drugs are used together than when either is given alone.

Three studies used a protocol of ivermectin (Heartgard) given weekly at the normal monthly heartworm preventative dose (6 mcg/kg), combined with doxycycline at the rate of 10 mg/kg/day for weeks 1-6, 10-11, 16-17, 22-25, and 28-33 (the end of the studies). The findings show these results:

Reduction of adult worms was as follows:
• 8.7% for Doxycycline only
• 20.3% for Ivermectin only
• 78.3% for Ivermectin + Doxcycline
• 92.8% for Ivermectin + Doxycycline + Immiticide
• 100% for Immiticide alone

Mosquitoes that fed on blood from DOXY-treated dogs had L3 normal in appearance but were not infective for dogs. Preliminary observations suggest that administration of Doxycycline and Ivermectine for several months prior to (or without) MEL will eliminate adult HW with less potential for severe thromboembolism than MEL alone.

http://www.ncbi.nlm.nih.gov/pubmed/18930598

What’s the Future?

With the current decline in the economy and rising pet health care costs, many families simply cannot afford to maintain their pets on the expensive heartworm medications currently available much less provide treatment for heartworm positive pets when the average veterinary clinic charges $600 per pet. Clinics state that less than half of the dogs diagnosed with heartworms will undergo the expensive treatment.

This in addition to the rising mosquito population creates a dilemma for our pet population not to mention the many homeless pets which eventually will wind up in shelters. The results are rising populations of heartworm positive dogs which has been documented already by the American Heartworm Society.

Hopefully the veterinary community will recognize a need for ‘affordable’ and ‘effective’ heartworm prevention and treatment methods…..SOONER THAN LATER!