Commentary: An argument for year-round heartworm prevention in dogs

Find the non sequitur in the following series of statements: Heartworms cause severe lung disease in dogs and cats and can kill them. Heartworms cause zoonotic disease in people. Heartworm disease can be prevented in dogs and cats by giving them medication once a month that also controls various internal and external parasites. Heartworm infections are diagnosed in about 250,000 dogs each year.1 But there is no good reason for dogs to receive preventives all year; it is just not needed.

Dwight D. Bowman, MS, PhD

THE RISK

Now let's do some math. The risk of a dog's being infected with heartworm disease each year is 250,000 out of 50,000,000; this translates to one in 200 dogs becoming infected each year. The chance that you will be diagnosed with cancer this year is about one in 200—the same odds as a dog's acquiring heartworm disease.2 Yet heartworm disease in dogs is virtually 100% preventable. Would you take medication once a month to prevent being diagnosed with cancer this year? But there is no good reason for dogs to receive preventives all year; it is just not needed.

THE TREATMENT

Now let's look at the heartworm treatment options. Melarsomine dihydrochloride, an arsenical, is the treatment for heartworm infections in dogs; nothing is approved to treat cats. Melarsomine is given at a dose of 2.5 mg/kg. The LD50 for organic arsenic in mongrel dogs is 14 mg/kg.3 A three-fold overdose of melarsomine can be lethal.4 To put this in perspective, the epa has set the limit for arsenic in drinking water at 10 parts per billion5 —that is 10 µg/L—based on a no-effect risk of 1 to 10,000 to 1 to 1,000,000. So if you drink 10 L of water each day containing arsenic at the maximum allowable level, you would consume 100 µg of arsenic a day, or 36.5 mg of arsenic a year. A 50-kg dog receives a total dose of 250 mg of melarsomine, which contains about 37.5 mg of arsenic, in two injections. It would take one year of drinking high levels of arsenic in your water to get the same dose of arsenic given to a dog to treat heartworm disease. Thus, the dose for killing heartworms in dogs is far from a negligible amount. But there is no good reason for dogs to receive preventives all year; it is just not needed.

THE PATHOLOGY

Of course, treatment with arsenicals is far better than the long-term effects of large worms living in the pulmonary arteries of their hosts. The lungs do get better after treatment.6 The adult heartworms are long: males are 12 to 20 cm long, females are 25 to 31 cm long, and both are about 1 mm in diameter.7 If a dog has a relatively light burden of 12 worms—six males and six females—that is still a large mass of worms in the pulmonary arteries.

The disease is one of chronicity caused by the worms interacting with the surface of the pulmonary vessels.8 The presence of the worms in the bloodstream also leads to the physical rupture of red blood cells and the deposition of hemoglobin within fixed macrophages of the lungs, making the lungs of dogs with chronic heartworm infections appear brown. Villous proliferations on the vessels also lead to the formation of small thrombi that are carried deeper into the lungs and ultimately induce chronic lung disease.

But keep in mind that even with therapy, the worms are in the pulmonary arteries, not the intestinal tract. All that can happen after arsenical therapy is that the worms are driven deep into the lungs where they die in tightly coiled bundles, decay, and are ultimately cleared by the host's cellular response. However, this takes a long time and is not without effects; remember, each dead worm is about 20 cm long. But there is no good reason for dogs to receive preventives all year; it is just not needed.

THE WEATHER

Don't forget to factor in the unpredictability of nature. We know that Culex tarsalis mosquitoes can live up to two months when temperatures hold at 77 F,9 but there is a good chance they can live longer at cooler temperatures. Near George Lake in Alberta, Canada, over half of the overwintering Culex territans female mosquitoes studied survived more than 138 days at 23 F.10 These mosquitoes will continue to seek blood meals every time they are about to lay eggs, and if they are infected with heartworms in October, they could still easily transmit the infection during an unseasonably warm December, like the one we experienced this winter in Ithaca, N.Y. Such microclimate situations put dogs at risk all year long and are part of the rationale that led to the recommendation in the capc guidelines that dogs receive preventives year-round. With locally acquired heartworm transmission likely occurring in every state, it seems there is no good reason for a dog to be at risk in a nice November or in a warm March. But there is no good reason for dogs to receive preventives all year; it is just not needed.

THE PRECEDENT

Perhaps we could take a lesson from our counterparts in human medicine. Human lymphatic filariasis, a disease caused by cousins of the heartworm, is transmitted by mosquitoes that bite infected people. The disease induced—elephantiasis—is horrible. Small thread-like worms live in the lymphatics and cause severe disfiguring and immobilizing disease in their human hosts. The World Health Organization is leading the charge to eradicate these parasites through "mass drug administration strategies for disease elimination."10 These mass treatments of populations have had a significant impact on the rate of infection in people and vectors.11 We can only hope that someday we will place similar pressure on the transmission of heartworms, reducing the occurrence of the devastating disease that heartworms cause in dogs.

Dwight D. Bowman, MS, PhD, is a professor of parasitology at the Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853.

REFERENCES

1. Heartgard Plus. Heartworms are a serious threat. Available at: heartgard.us.merial.com/whyheartgard/why_problemareas.asp.

2. National Cancer Institute. Cancer Trends Progress Report—2005 Update. Available at: http://progressreport.cancer.gov/.

3. Agency for Toxic Substances & Disease Registry. Available at: www.atsdr.cdc.gov.

4. Immiticide product insert. Duluth, Ga: Merial Limited.

5. U.S. Environmental Protection Agency. Arsenic in drinking water. Available at: www.epa.gov/safewater/arsenic/index.html.

6. Rawlings CA, Losonsky JM, Schaub RG, et al. Postadulticide changes in Dirofilaria immitis-infected Beagles. Am J Vet Res 1983;44:8-15.

7. Levine ND. Nematode parasites of domestic animals and of man. Minneapolis, Minn: Burgess Publishing Company.

8. Munnell JF, Weldon JS, Lewis RE, et al. Intimal lesions of the pulmonary artery in dogs with experimental dirofilariasis. Am J Vet Res 1980;41:1108-1112.

9. Mahmood F, Reisen WK, Chiles RE, et al. Western equine encephalomyelitis virus infection affects the life table characteristics of Culex tarsalis (Diptera: Culicidae). J Med Entomol 2004;41:982-986.

10. Hudson JE. Cold hardiness of some adult mosquitoes in central Alberta. Can J Zool 1978;56:1697-1709.

11. World Health Organization. Lymphatic filariasis. Tropical disease research: progress 2003-2004. Seventeenth Programme Report of the UNICEF/UNDP/World Bank/WHO Special Programme for Research & Training in Tropical Diseases, 2005.