areas tend to function as “heat islands,” remaining warmer than surrounding rural areas, because man-made structures (buildings, roads, sidewalks, etc.) tend to absorb heat from the sun during the day and radiate that heat back into the air after the sun sets. These “heat islands” can allow cities that were once too cold to support sustained mosquito populations to now maintain temperatures that are warm enough to permit stable mosquito populations through cooler months. Urban heat islands can allow the spread of Dirofilaria immitis to persist outside of the area’s expected period of heartworm transmission (American Heartworm Society, 2014). Dirofilaria immitis has now been detected in all 50 of the United States, including Alaska. Heartworm disease is now considered regionally endemic (meaning that it circulates within the local Seasonality of heartworm transmission Temperature plays a key role in the transmission of heartworms. First of all, circulating air temperatures must be warm enough to allow mosquitoes to breed and survive. Mosquitoes are poikilothermic, meaning their body temperature fluctuates with environmental temperatures; therefore, the survival, breeding, physiology, and activity patterns of mosquitoes are determined by environmental temperatures (Reinhold, Lazzari, & Lahondère, 2008). Second, maturation of infective microfilaria into the L3 larva has been shown (in three separate mosquito species) to only occur at temperatures above 57°F (American Heartworm Society, 2014). Therefore, even if mosquitoes will survive and be capable of transmission at temperatures below 57°F, transmission of Dirofilaria immitis cannot occur unless environments reach or exceed this temperature. In the past, the knowledge that heartworm transmission can only occur in warmer temperatures has been regarded as a valid Incidence of feline heartworm infection Although cats are susceptible to heartworms, they are more resistant to adult heartworm infection than dogs. In experimental studies, dogs exposed to 100 L3 heartworm larvae almost always became infected with adult worms (almost 100% of dogs) and the average infected dog developed approximately 60 adult heartworms. In cats, however, a similar exposure (100 L3 heartworm larvae) resulted in an adult heartworm infection in only about 80% of cats, with an average of three adult heartworms per cat (Blagburn, Dillon, Arther, Butler, & Newton, 2011). In another contrast to canine heartworm infections, many feline heartworm infections consist of only one to two worms and these worms are often all of a single sex (American Heartworm Society, 2014). Necropsies of cats in a shelter setting, however, suggest that the prevalence of adult heartworms in cats within a given geographic area is approximately 5% to 15% of the prevalence of heartworm disease in dogs within that area (American Heartworm Society, 2014). Therefore, areas with a high prevalence of canine heartworms are also likely to have a relatively high prevalence of feline heartworms. Approximately 5% of shelter cats in the southeastern United States had visible adult worms at the time of necropsy (“A Roundtable Discussion,” 2008). While the above statistics discuss adult heartworm infection in cats, there is a key factor of feline heartworm infection that must be understood when discussing the prevalence of heartworms in cats. Cats can experience significant damage to the heart and lungs when infected with heartworm larvae, even if these larvae do not develop into mature adults. Unfortunately, diagnosing larval heartworm infections can prove quite challenging, making it difficult to determine the true incidence of feline heartworm disease. Serologic testing has demonstrated that 12-16% of cats throughout the United States have antibodies against Dirofilaria immitis , indicating previous exposure to heartworm larvae or adults. This number varies based on location, with 12% of cats
population) in all 48 of the contiguous U.S. states, as well as Hawaii, Puerto Rico, Guam, and the U.S. Virgin Islands (American Heartworm Society, 2014). Although transmission has not yet been documented in Alaska, even that state has regions with local climate conditions and mosquito populations that could sustain transmission if infected animals were introduced to the area (American Heartworm Society, 2014). Merely living in a traditionally “non-endemic” area is no longer an adequate excuse to dismiss heartworm disease, as this parasite is gradually expanding its range and has the potential to affect more patients. Therefore, all pets within the United States should be regarded as being at risk of heartworm disease and should be on regular heartworm prevention. argument for heartworm disease being of little concern outside the summer months, at least in some geographic regions. In 2004, for example, weather models demonstrated that the period of heartworm transmission for areas north of the Virginia- North Carolina state line was typically confined to six months or less, because the other six months of the year did not have temperatures conducive to heartworm transmission (Guerrero et al., 2004). As climate change has led to warmer winter temperatures across the globe, however, the transmission season for heartworm disease is increasing in length compared to what has traditionally been regarded as a typical heartworm season. When the effects of climate change are combined with the effects of other changes, such as increased irrigation and the formation of urban heat islands, it is easy to see that it is becoming increasingly challenging to continue viewing heartworm infection as a seasonal disease. in the northeastern United States having antibodies against Dirofilaria immitis and approximately 16% of cats from the southeastern United States being seropositive (“A Roundtable Discussion,” 2008). Antibody testing, as described above, only indicates exposure to heartworm larvae, not the presence of adult heartworms. While this may make it tempting to dismiss the number of seropositive cats as clinically insignificant, studies have shown that approximately half of heartworm-seropositive cats have detectable heartworm-associated damage within the lungs at necropsy (“A Roundtable Discussion,” 2008). Combining this statistic with the overall number of seropositive cats suggests that as many as 6-8% of cats within the U.S. could have heartworm-associated lung damage. Clearly, heartworm disease is a significant threat to cats. In a 2018 study performed in Texas (Norsworthy, 2018), approximately 10% of indoor cats tested positive for heartworms on either an antigen or antibody test. In that same study, nearly 25% of indoor/outdoor cats tested positive on one of the two tests. While this likely still significantly underestimates the true incidence of heartworm disease in this population, given the challenges in diagnosing heartworms, it does suggest that feline heartworm disease is more common than many veterinary clients, team members, and even veterinarians might suspect. Although many veterinarians may regard feline heartworms as an uncommon disease, these statistics demonstrate that the prevalence of heartworms in U.S. cats is even higher than the prevalence of feline leukemia, which is estimated at 3-5% in cats tested in a shelter setting (Lee, Levy, Gorman, Crawford, & Slater, 2002). Results of samples tested by IDEXX Laboratories confirm that heartworm disease is more common than feline leukemia; IDEXX has reported that 0.9% of submitted feline blood samples tested positive for feline heartworm antigen, 1.9% of submitted samples tested positive for feline leukemia virus, and 1% of samples tested positive for feline immunodeficiency
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