A dog infected by microfilariae is bitten by a mosquito If the microfilariae is not picked up by a mosquito after 2 years, they die of “old age.” Microfilariae can be transmitted across the placenta. Puppies will not develop adult heartworms because the microfilariae didn’t go through the intermediate
Life Cycle Inside the mosquito, the microfilariae develop to L2’s and finally to L3’s. The L3 is the infective stage for dogs. –Takes a few weeks –Temperature dependant Minimum of 57 ºF is required
Life Cycle The L3 is deposited in mosquito saliva next to the bite and then migrate into the body. The average mosquito can only transmit a maximum of 10 infective larvae at one time The L3 larvae then live in the dogs skin where they develop into an L4 stage. They then live in the subcutaneous tissues and muscle for 3 months before they finally molt into an adult.
Life Cycle Adults migrate to the right side of the heart and into the pulmonary arteries and lungs where they mate and produce microfilariae. –The overall maturation and migration process until mating takes approximately 5-7 months –Adult worms can live in the dog for up to 5 years
Pathogenesis of the Heart Heavy infections(over 25 worms for a 40 lb dog) –worms begin to back up into the right ventricle less blood pumped. Over 50 worms the ventricle is full and the atrium begins to contain worms. Over 100 worms the entire right side of the heart is filled –phenomenon is called "Caval Syndrome" and most dogs do not survive it.
Canine Symptoms Cough, exercise intolerance dyspnea (difficulty breathing) abnormal lung sounds hepatomegaly (enlargement of the liver) syncope (temporary loss of consciousness due to poor blood flow to the brain) ascites (fluid accumulation in the abdominal cavity) abnormal heart sounds death
Feline Symptoms Non-specific generic signs of illness –Vomiting intermittently –Lethargy –Lack of appetite –Weight loss –Coughing –Asthma-like signs –Gagging –Difficult or rapid breathing The early signs (when worms are carried to the pulmonary arteries) are often misdiagnosed as asthma or allergic bronchitis –Actually Heartworm Associated Respiratory Disease (HARD)
Diagnosis Blood Tests –Filter test to look for microfilariae Doesn’t work well for small infections –Antigen test screens for pieces of adult heartworm skin in the blood Only works for female worms Need at least 3 for it to be detected –Newer tests test for antibodies X-rays –Enlargement of lobes of lungs or right side of heart
Treatment Adulticide –Melarsomine dihydrochloride (Immiticide ® ) Intramuscular injection into lumbar muscles Complications include thrombosis (clogging) of pulmonary arteries due to dead worms –No current treatment for cats Microfilaricide –macrocyclic lactone (ML) anthelminthics, i.e.,milbemycin oxime, selamectin, moxidectin and ivermectin –Commonly used in heartworm preventative
Control/Prevention Can be given as –Monthly Tablets –Chewables –Topicals Ivermectin –Heartgard, Heartgard Plus, Tri-Heart, Iverheart Plus Milbemycin oxime –Interceptor, Sentinel Selamectin –Revolution Moxidectin –Advantage Multi