Heartworm disease in dogs is caused by a parasite called Dirofilaria immitis transmitted through mosquito bites. Common signs include persistent coughing, reluctance to exercise, fatigue after moderate activity, decreased appetite, weight loss, and swollen belly. Diagnosis involves x-rays to check for enlarged heart, blood tests to detect the parasite, and antigen tests. Treatment consists of medications to kill the parasites along with antibiotics, rest, fluids and supportive care. Regular preventive medication is effective in preventing heartworm infection.