Heartworm disease in dogs

Overview

Heartworms are transmitted through the bite of an infected mosquito.  Larvae are injected into the skin when mosquitoes are feeding.  The larvae migrate to the heart, lungs and the blood vessels near these vital organs within two to three months. The larvae complete their migration in six months, ending in the right side of the heart. Heartworms can infect dogs of all ages that live anywhere throughout the United States.  The disease is more prevalent in warmer areas where there are greater numbers of mosquitoes, like the east coast and southeastern states. Left untreated, heartworm can lead to heart and lung failure, kidney and liver damage, and eventually death. And yes, all it takes is the bite of just one infected mosquito!

Symptoms

The most common symptoms of heartworm infection are:

• Coughing
• Lethargy
• Difficulty breathing
• Weight loss
• Fluid accumulation in abdomen

Prevention

Preventing heartworm disease is easier and much cheaper than trying to treat an infection.  Preventive doses come in oral and topical versions and are only available from a veterinarian. These medications are given once a month throughout the year, especially in areas where mosquitoes are endemic.  Some heartworm preventative drugs are ineffective and potentially dangerous if heartworms are already present in your dog's body. Your veterinarian will likely require a blood test for heartworms before prescribing a prevention program. Heartworm prevention is as easy as that - but it must be done regularly to ensure continued protection. 

Treatment

If your dog has heartworms, they can usually be treated successfully with a series of injections that specifically kill adult heartworms.  These injections are given in the muscles along the spine and can cause temporary muscle soreness.  Strict exercise restriction and short leash walks for bathroom breaks area a must for 30 days after each injection.  Heartworm treatment causes the adult worms in the heart and lungs to die; aerobic exercise increases the risk of rapid dislodging of worms.  If a large worm becomes lodged in the vessels supplying blood to the lungs, it can be life threatening.  Heartworm disease is usually cureable with the current drugs.  Treatment must be done by a veterinarian.  The current recommendation by the American Heartworm Society is a three dose protocol.  In this protocol, one injection of medication is given and the patient is cage rested at home for thirty days.  At the end of the first thirty days, another injection is given and then another injection is given 24 hours later.  Another thirty days of cage resting is required after the last two doses.  This method of treatment poses the least risk for life threatening complications and is more successful than previous treatment protocols. 

After heartworm treatment is completed, your dog should be heartworm tested again  in 6 months.  This gives the body time to completely eliminate the dead heartworms.  If your dog had a large heartworm infection, it is possible that the first heartworm treatment could fail to kill all the worms.  If this is the case, a second round of treatment may be required. 

Some veterinarians still recommend starting monthly heartworm prevention in dogs with heartworm disease instead of pursuing heartworm treatment.  Owners are instructed that it will take many years to kill the heartworms with that method.  Current research shows that this route is detrimental to the dog and increases damage to the heart, which is usually irreversible.  While heartworm prevention should be started in dogs with heartworm disease  while they are undergoing heartworm treatment, it should not be relied upon as the only method of treatment. 

Some dogs are so severely affected by heartworm disease that the heartworms have to be removed from the vessels around the heart.  This is due to the fact that the worms in these dogs are blocking blood flow to the lungs and heart and causing a serious condition.  This severe form of heartworm disease is referred to as caval syndrome.  Traditional heartworm treatment is required after the dog is stable after the worm retrieval procedure. 

After complete treatment, most dogs have a long, happy, healthy life.  Remember, it is much easier (and cheaper) to prevent heartworm disease than it is to treat it!

References:

Pfizer Animal Health, Revolution (selamectin). (6 March 2008).