Today's Veterinary Practice

NOV-DEC 2018

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12 NOVEMBER/DECEMBER 2018 todaysveterinarypractice.com presence of mosquito species known to transmit heartworms, and (3) environmental conditions that promote the presence/breeding of mosquitoes. Repellents should be considered an adjunct to—not a replacement for—ML preventives. Putting It Into Practice The relative risk for heartworm can vary from season to season and from region to region, depending on the presence of heartworm- positive dogs and environmental and climate conditions that favor mosquito proliferation. AHS advocates the year-round administration of ML medications as safe and effective heartworm prevention, but poor and inconsistent adherence can hamper the efficacy of this approach. If veterinarians deem the relative risk for heartworm transmission to be high on a seasonal or year-round basis, they should consider the use of U.S. Environmental Protection Agency– approved repellents/ectoparasiticides to provide additional control of the mosquito vector and to interrupt the chain of transmission. HEARTWORM TESTING Heartworm screening in dogs should include antigen and microfilaria testing. Heat treatment of serum samples before routine antigen testing is not recommended. ■ Heat treatment of serum samples before antigen testing is available from reference laboratories to unmask blocked antigen; however, it should be considered only when the veterinarian suspects clinical infection in the absence of a positive test result. ■ Heat treatment of samples is contrary to label instructions for in-house tests and may interfere with the accuracy of both heartworm tests and combination tests designed to detect antibodies of other infectious agents. Putting It Into Practice The available antigen tests are highly sensitive and yield accurate results. The AHS does not recommend heat treatment for routine testing but advises veterinarians to consider it when circulating microfilaria are detected or active clinical disease is suspected in the absence of positive antigen test result. HEARTWORM TREATMENT The use of the AHS heartworm treatment protocol, which includes pretreatment with an ML and doxycycline followed by a month-long waiting period, then 3 doses of melarsomine on days 60, 90, and 91, continues to be recommended for heartworm-positive dogs. ■ Waiting until day 60 to begin melarsomine has been questioned for reasons of convenience and efficacy; however, it is hypothesized that this waiting period allows time for Wolbachia surface proteins and other metabolites to dissipate before killing of the adult worms. It also allows additional time for the worms to wither as they become unthrifty after the Wolbachia endosymbionts are eliminated. ■ The efficacy of several nonadulticide treatment protocols, including the "moxy-doxy" protocol with moxidectin and doxycycline, has been studied over the past several years. These protocols may be considered for treating dogs that are not candidates for melarsomine therapy, but they should be viewed as fallback options compared with treatment protocols of choice. Putting It Into Practice The AHS treatment protocol includes administration of an ML preventive to kill juvenile worms, followed by doxycycline to eliminate Wolbachia bacteria, with the ultimate goal of reducing the severity of complications from the adulticide therapy that follows. The waiting period before administering doxycycline HEARTWORM DISEASE INCREASING The 2016 American Heartworm Society (AHS) Incidence Survey determined that the number of infected dogs per clinic rose by 21% in the United States and its territories between 2013 and 2016, with almost 1 in 4 veterinarians reporting that heartworm disease was "on the rise" in their respective practices. Environmental and climate change, the relocation of microfilaremic dogs, and the expansion of microfilaremic wild canid territories are all considered contributing factors. shutterstock.com/Asholove

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