Chapter 8: Heartworm Disease in Dogs 2 CE Hours
Expiration Date : August 17, 2027 Learning objectives After completing this course, the learner will be able to: Describe the epidemiology and pathophysiology of canine heartworm disease. Outline the available diagnostic tests for canine heartworm disease and a rationale for appropriate heartworm testing.
Compare and contrast heartworm treatments options and select an appropriate treatment for presenting canine patients. Understand the issue of macrocyclic lactone resistance and its relevance in clinical practice.
INTRODUCTION
Heartworm infection is a serious parasitic infection in dogs, with worldwide distribution. In the United States, cases are most commonly observed in specific geographic areas (for example, the Mississippi River Valley and southeastern United States). However, all dogs are at risk for this condition. Heartworm disease can have significant effects on the heart and lungs
and is also associated with severe complications such as caval syndrome. Testing for heartworms can be complicated and controversial, as can determining treatment options. In this course, you will learn about recent developments in heartworm testing, treatment, and prevention, allowing you to better serve your canine patients.
EPIDEMIOLOGY
Heartworms, of the species Dirofilaria immitis , are a filarial parasite of dogs, transmitted by mosquitoes. Over 70 unique species of mosquitoes are capable of heartworm transmission, and one or more of these mosquito species are found in most geographic areas worldwide (Companion Animal Parasite Council, 2016). This makes canine heartworm disease a significant concern with a potential for worldwide spread. Cases of canine heartworms have been detected throughout the United States, with the highest incidence observed in the southeastern United States and the Mississippi River Valley (Companion Animal Parasite Council, 2016). It is important to note, however, that heartworm disease is not strictly confined to warm climates. Increased numbers of urban mosquitoes, attracted to “heat islands” in urban centers that allow them to thrive even in cooler weather, are permitting the spread of mosquitoes even into cold-weather states. Active transmission of D. immitis has now been documented in every state except Alaska, and even Alaska has regions that contain appropriate vectors and microclimates to allow heartworm transmission (American Heartworm Society, 2014). If a microfilaremic dog were to be introduced to one of these regions, heartworms could be readily introduced to Alaska. Therefore, D. immitis is a concern for pet owners and veterinarians throughout the United States. The distribution of heartworm disease is expected to continue to spread over time, due to natural climactic change and human- induced climate and environmental changes. Mosquitoes require standing water for breeding; these pools of standing water will become more common as sea levels rise and standing water remains in place for longer periods of times after storms and Life cycle of heartworms When a mosquito containing heartworm larvae (in the L3 stage) feeds on a dog, heartworm larvae are deposited in a pool of mosquito saliva on the surface of the dog’s skin. These larvae then migrate through the bite wound, entering the skin and traveling to the subcutaneous tissues. Five to seven days after they first enter the body, heartworm larvae will molt from the L3 to L4 stage. They will remain in the L4 stage for 50 to 70 days, then molt from the L4 to L5 stage prior to entering the circulatory system. When they reach the heart, the heartworm larvae are in the young adult (L5) stage. These L5 heartworms most frequently become lodged in the right ventricle and pulmonary artery, although they can also be found in the right atrium, vena cava,
other weather events. Additionally, human activities can lead to changes in runoff and water drainage, creating new ponds and water sources that can allow for mosquito breeding. In the western United States, for example, the climate was traditionally too dry to permit large mosquito populations. In recent years, however, changes in irrigation and plant cultivation have allowed Aedes sierrensis (the western knot hole mosquito) to expand its range. This mosquito serves as the primary vector for transmission of heartworms in the western states (American Heartworm Society, 2014) and any expansion in its range can also increase the range of canine heartworm disease. In areas where heartworm disease is common (for example, the southeastern United States and Mississippi River Valley), as many as 40-60% of dogs that are not receiving regular preventive medications are infected with heartworms (Blagburn, 2013). Heartworm is most commonly seen in outdoor, male, large breed adult dogs. Heartworm is four times more common in males than females, five times more common in outdoor dogs than indoor dogs, more common in large breed dogs than small breed dogs, and most commonly seen in dogs between 3 and 15 years of age (Blagburn, 2013). The domestic dog is the primary host for heartworms. Other species can also be infected, however, including wild dogs, cats and wild cats, ferrets, marine mammals, and humans. The degree of disease caused by heartworms in these non-host species is variable (Companion Animal Parasite Council, 2016). While some of these species may develop small numbers of microfilariae, these infections are not considered a significant contributor to the spread of heartworms. or other aberrant tissues (Blagburn, 2013). Upon first reaching the heart, these young adult heartworms are approximately 2.5 cm (1 inch) in length. Over the next several months, these worms will grow and mature. The bodies of the worms will eventually extend into the main pulmonary arteries, as females reach lengths of up to 12 inches and males reach 5-7 inches in length. Heartworms reach sexual maturity approximately six months after infection and, at this time, male and female heartworms will begin to mate, reproduce, and create microfilariae. Adult females can live in the heart and pulmonary arteries, mating and releasing microfilaria, for up to 5-7 years. Worm burdens in infected dogs may range from 1-250 adult heartworms,
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