Adult heartworm infection continues to be an important clinical problem in enzootic areas, despite widespread availability of numerous effective preventatives. Melarsomine dihydrochloride (Immiticide-Merial Ltd., Duluth, GA) emerged as a novel organic arsenical heartworm adulticide in the mid-90s and is the only available drug approved for this purpose. Although melarsomine dihydrochloride has advantages over its predecessor, sodium thiacetarsamide (Caparsolate-Merial Ltd., Duluth, GA ), such as better safety and more efficacy, important complications still occur.1-2 Understanding the types and causes of major complications can minimize the frequency and severity of these events and in some situations prevent them altogether.
Complications of adulticide therapy can be divided into two major categories: those associated with direct toxicity and those associated with efficacy. Direct toxicity most frequently manifests as local tissue reactions. Side effects associated with efficacy result from the demise of adult heartworms; these effects include pulmonary thromboembolism and, infrequently, congestive heart failure.
The composition of the product makes meticulous administration imperative. Although they are usually mild, local tissue reactions are among the most common adverse reactions. However, inadvertent injection of even a small amount of the product into the subcutaneous space can result in much more severe inflammation and, infrequently, local tissue necrosis (Figure 1). Hettlich and colleagues recently reported three cases in which severe neurologic signs, including marked motor and proprioceptive deficits, paraparesis, and seizures, were temporally related to administration of melarsomine.3 The article implied that imperfect injection technique may have played a role.