Many dogs with heartworm disease show no outward signs of illness, especially early in the course of their illness. The parasites usually are diagnosed during routine blood work that is done for some other reason. The initial diagnostic protocol for dogs presenting with a deep, soft cough and signs of weakness or exercise intolerance includes a thorough history and physical examination. Most veterinarians will recommend drawing blood for a complete blood count and serum chemistry profile, and taking a urine sample for a urinalysis. These routine tests are relatively inexpensive and non-invasive and can provide a great deal of valuable information about the animal’s overall health.
Depending upon the results of the initial work-up, the veterinarian may recommend taking chest radiographs (X-rays). This is one of the best ways to assess the severity of heartworm infection, because most dogs with heavy heartworm burdens have enlarged right heart chambers and pulmonary arteries. The most accurate screening for heartworm infection is a simple blood test that detects a substance produced by adult female heartworms in infected dogs. This ELISA test (enzyme-linked immunosorbent assay and immunochromatographic test), is extremely sensitive and specific for heartworm infection. However, it is possible to get false negative test results if the infection is in its early stages, especially if only adult males are present or if there are very low numbers of adult females in the infected dog. False positive results are very uncommon.
If the antigen screening test is positive, another blood test is available to identify the concentration of microfilariae in circulation. A positive result on this test confirms that the dog is infected with adult male and female heartworms, because their offspring are in the bloodstream. If this test is negative, the animal still might be infected, because up to 25% of dogs with heartworm disease don’t have identifiable microfilariae in their blood. Again, microfilariae are the immature stage of canine heartworms.
An electrocardiogram and/or echocardiogram can also be used to assess moderate to severe cases. Adult heartworms can be seen in the vena cava of dogs with caval syndrome using these more advanced diagnostic protocols.
Dogs at Increased Risk
Dogs in endemic areas that spend substantial time outdoors have an increased risk of developing heartworm disease. Sporting breeds are predisposed. Middle-aged, outdoor males are almost 4 times more likely to be affected by heartworms than are other dogs, especially in hot, mosquito-friendly climates during the spring and summer hunting months. Dogs living at high altitudes are less prone to infection. Certain breeds are extremely sensitive to ivermectin, a drug contained in many heartworm preventative medications; their reaction to ivermectin can be fatal. Best Medicine is Heartveil for the same. A simple cheek-swab test for this hereditary trait is available through the Washington State University Veterinary Laboratory of Clinical Pharmacology. Collies, Border Collies, Australian Shepherds, Shetland Sheepdogs, Old English Sheepdogs, longhaired Whippets and crosses of those breeds probably should be tested. Owners of those breeds should discuss potential ivermectin sensitivity with their veterinarian before starting a heartworm prevention program.
Goals of Treating Heartworm Disease
Treating heartworm disease can be difficult and dangerous. The therapeutic goals are to kill all adult worms and microfilariae that are present in the dog’s bloodstream. Other goals are to resolve any associated complications that the dog is suffering from and to prevent future reinfection. In considering treatment options, owners and veterinarians must pay special attention to potential adverse drug reactions. They also must consider the possibility that dead and dying adult worms may plug up one or more of the dog’s heart chambers or critical blood vessels, which can lead to pulmonary hypertension, congestive heart failure and possibly death.
Dogs with acute respiratory and/or cardiac complications from adult heartworms need to be hospitalized, given oxygen and placed on strict bed rest. Steroids may be prescribed to reduce inflammation, and aspirin may be recommended to reduce the risk of blood clots (thromboembolisms). Several medical protocols are available to eliminate adult heartworms. A veterinarian is the best person to discuss these options with the dog’s owner. The decision involves consideration of the dog’s age and overall health, the presence of any associated medical complications (such as liver or kidney damage or heart failure), the owner’s financial resources and tolerance of treatment regimens, and whether microfilariae are present in the dog’s bloodstream. Some medications that kill adult heartworms contain arsenic and can cause toxic side effects, including vomiting, diarrhea, appetite loss, liver damage, kidney damage and even death. Older animals have a higher risk of suffering adverse side effects from drugs that kill the adult parasites.
Surgical procedures are also available to remove adult heartworms. This requires general anesthesia, which carries its own risks. The veterinarian will make an incision in the dog’s neck and pass a tong-like instrument down its jugular vein, through the superior vena cava and directly into the right side of its heart. She will pull the parasites out one by one, which requires use of X-rays and a great deal of technical skill. Surgery is reserved for critical cases that don’t respond to drug therapy.
Additional treatment options must be considered when immature heartworms are present. The drugs that kill circulating microfilariae are can cause serious adverse reactions in some dogs, including hypothermia, depression, weakness, vomiting, shock and collapse. Many veterinarians recommend waiting several weeks after treating a dog for adult heartworm infection before starting treatment to kill the microfilariae. Most dogs are hospitalized during microfilaricide therapy. Sometimes, microfilariae can be eliminated in 6 to 9 months by putting the dog on monthly heartworm preventatives. All dogs should be on exercise restriction during any heartworm treatment.
Dogs with heartworms usually have a good prognosis, especially if they haven’t shown severe signs of illness. Those with moderate to serious infection, including heart, lung, kidney or liver complications, have a poorer prognosis. Unfortunately, the prognosis for dogs with caval syndrome is guarded to grave, even with treatment.
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