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Heartworms (HW) are parasites that live in the arteries of the lungs (pulmonary arteries) of dogs and wild canines. They cause an increased pressure in these vessels (pulmonary hypertension) which increases the work-load of the right side of the heart, resulting in damage and eventual failure.
HWs (Dirofilaria immitis) are endemic to Hong Kong and are spread from an infected animal to others by mosquitoes. Adult female HW produce larvae (microfilariae) that migrate through the body and are picked up by mosquitoes when they feed on the dog.
Once a dog is bitten by an infected mosquito, the HW larvae moult and travel to the lungs, where they develop into adults, a process which takes about 6 months.
HWs produce a substance that irritates the arteries causing them to harden, and as worms die they can also lodge in and block smaller lung vessels.
If a dog is bitten by many infected mosquitoes at once, many HWs can become adult simultaneously and if there is no room for them in the pulmonary arteries, they will live in the actual heart chambers and the large vein (posterior vena cava) leading to it. This is called caval syndrome and alters blood flow and damages the red blood cells. This is rapidly fatal!
Most infected dogs have no signs yet, and the disease is detected by routine HW blood tests
The first signs of problems are coughing and dullness. As the disease worsens, breathing rate (more than 50 breaths per minute) and effort increase and some dogs may faint when stressed.
Dogs with advanced disease will show the effects of right-sided heart failure, with fluid build-up in the abdomen and weight loss.
A simple blood test can be performed within 5 minutes at Acorn to check for the presence of antigens given off by the adult female HWs and is the most common test used.
If a dog is positive for heartworm, chest radiographs are advised to look for changes to the heart and lungs and an echocardiogram (heart ultrasound) is indicated to look for presence of worms in the heart and vena cava, and for evidence of right-sided heart failure.
Prior to treatment routine lab tests are advised to check other body organs are functioning normally, as the drug used to kill adult worms can adversely affect the kidneys and liver
If right-sided heart failure, liver disease or kidney disease are present they must be treated before treatment to kill the adult HWs.
No single drug kills all the stages of the parasite.
Adult worms are killed with melarsomine (“Immiticide”) and 2 protocols are used
- The most common protocol composes 2 injections intramuscularly 24 hours apart killing worms slowly over 2-3 weeks
- In more severely affected animals, one dose is given, followed by a rest of 4 weeks before two more shots 24 hours apart.
Killing the worms slowly reduces the risk of parts of parasite breaking up in the lungs causing sudden death.
Circulating microfilariae are usually killed 1 month after adulticide therapy with an injection of Ivermectin or milbemycin.
Once all stages have been treated, oral HW-preventive drugs are started monthly. These include ivermectin (“Heartguard”), selamectin (“Revolution”) and milbemycin .
Alternatively a yearly slow-release injection of moxidectin (“ProHeart 12") can be given and is more convenient for many owners.
Follow-up and Prognosis
When adult HWs die they break up and lodge in the smaller lung vessels (embolism) but this should cause no significant problems in dogs with no or few lung changes.
However it is VITAL all dogs treated are kept quiet for 6 weeks after treatment to reduce the risks of embolism and often anti-inflammatory drugs are prescribed to reduce risks of complications.
Sadly even with the most careful management a dog may die when the adult worms die.
4-6 months after adult HWs are treated a repeat blood test for adults is performed and occasionally an animal may require repeat adulticide treatment for this
Dogs with no clinical signs of HW infection have a very good prognosis although unfortunately there is the occasional fatality after treatment.
Dogs with right heart failure have a more guarded prognosis as their significant lung changes may not reverse, and will require life-long therapy