Strategically Timed Heartworm Prevention is Risky

Do you recommend heartworm prevention products all year?


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In some northern states, heartworm prevention for pets is a relatively new necessity in “mosquito season,” much less during winter months, which probably prompted a letter published in January’s Journal of the American Veterinary Association.

“Heartworm development unit data suggest a 3.5- to 4-month heartworm transmission season in Wisconsin and indicate that although mosquitoes may be seen during the winter, they almost certainly will not contain infective larvae,” states the letter to the editor by James C. Frank, DVM, of Milwaukee in the Jan. 1 issue. “What, then, is the evidence to support current recommendations for year-round administration of heartworm preventatives in the north half of the country?”

The thoroughly-researched and well-written letter by Dr. Frank cited referenced research studies in Georgia, Louisiana and northern Florida that demonstrated a zero percent heartworm transfer rate between Dec. 15 and April 15.

“This winter, absence of heartworm transmission was also found during a second and a third winter, despite having microfilaremic dogs housed in kennels adjacent to uninfected dogs,” Frank wrote. The study found an 86 percent heartworm transfer rate from April 15 to Aug. 15, and a 73 percent transfer rate from Aug. 15 through Dec. 15, the letter stated.

Moreover, Frank cited another study of mosquito life cycles, in two Florida locations and one in Louisiana, demonstrating a lack of the L3 larval stage—the mosquito life stage that transmits heartworms—in mosquitoes captured between December and March, which could be interpreted as a lack of winter heartworm transmission.

“Drug companies themselves acknowledge that most owners on 12-month preventative programs do not give all of the recommended pills,” Frank wrote. “Is there evidence that 12-month preventative usage improves compliance over seasonal use?”

This subject is a sticky wicket for many practitioners who want scientific validation for recommending that clients purchase heartworm prevention products all year. Veterinary Practice News turned to veterinary parasitologist Susan E. Little, DVM, Ph.D., president-elect of the Companion Animal Parasite Council, for input.

“Local climatic factors can vary dramatically from year to year, and even day to day or place to place, due to regional heat islands,” she said. “And pets travel with their owners, usually in the winter months, to somewhere much warmer."

Dr. Little, regents professor of parasitology at Oklahoma State University’s Center for Veterinary Health Sciences, noted that both the American Heartworm Society and the CAPC recommend year-round administration of heartworm preventives.

“Consistent monthly doses have also been shown to be more effective against more challenging strains of heartworm,” she said.

“In addition, the monthly preventives also protect against much more than heartworm. Intestinal parasites like ascarids and whipworms are transmitted all year long and pets deserve to be protected from these all year, as well,” Little said.

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