Electrocardiography may be a useful tool in the diagnosis of early mild canine heartworm disease

This study was carried out to determine the usefulness of standard electrocardiography for the diagnosis of early canine heartworm disease. Baseline electrocardiograms were recorded in 12 dogs. Thirty artificial Dirofilaria immitis worms were inserted in the pulmonary artery of each dog. New electrocardiograms were recorded on days 1-4, 7, 14, 21, 28 and 35 after insertion of worms. A significant attenuation of amplitude of Q wave in lead I was recorded on days 1-4, 7, 14, 21, 28 and 35; of R wave in lead II on days 21, 28 and 35; and of S wave in lead aVR on days 7, 14, 21, 28 and 35. Duration of the QRS complex in lead AVF was significantly prolonged on day 14. In 41.6% of the dogs, right axis deviation was recorded. These results suggest that for a dog whose normal baseline standard electrocardiogram is known, a comparison of the QRS complex of the baseline with those of subsequent standard electrocardiograms may suggest early canine heartworm disease when there is an attenuation of amplitude of Q wave in lead I, R wave in lead II and S wave in lead aVR. An additional indication can be right axis deviation. Canine heartworm, dirofilaria, dirofilariasis, ECG changes, dog Canine heartworm disease (CHD) is an important veterinary disease. It has been reported to occur in most coastal areas of the world (Otto and Jackson 1975; Knight 1983). Dirofilaria worms are the cause of CHD and the worms are spread by several species of mosquitoes. It has been suggested that the average number of adult Dirofilaria immitis worms to be found in mild cases of CHD is 25-30 (Otto and Jackson 1975). The clinical signs in early CHD are not very helpful for diagnosis since they are not specific and the pathology of the disease is usually well advanced before the disease is clinically manifested (Otto and Jackson 1975). Radiographic examination is useful only when the worms have caused some pathologic changes in the heart or pulmonary artery. Examination for microfilariae using for example the modified Knott’s method is useful if worms are mature and producing microfilariae. Angiography and ultrasonography (echocardiography) can be useful but it is expensive. Serologic methods detecting specific antigens from adult female heartworms (using for example commercial antigen detection kits; Atkins 2003) is not very useful in early cases of CHD. Molecular biology techniques using PCR have also been used for diagnosis of dirofilariasis (Rishniw et al. 2006). Because the diagnosis of early CHD is sometimes missed (Strickland 2008), there is need to test a variety of methods that can be used as early pointers to a diagnosis of early mild CHD. Electrocardiography has been used for diagnosis of CHD. However, it is usually useful when the worms have caused some pathology in the heart and pulmonary artery. There has been no report on the use of electrocardiography as an aid in the diagnosis of CHD during the early stages of the progression of the disease. The aim of this study is to use a standard electrocardiogram, by using a single channel electrocardiograph, for the diagnosis of early CHD. In this study such electrocardiograph was used because most veterinary offices, especially in developing countries and in rural areas, will most likely own one and will be able to do, at least, a basic electrocardiogram on the dogs they see. Radiographic polyvinyl chloride threads have been used as artificial Dirofilaria immitis worms (AW) to simulate