Stages of Heartworm Disease in Dogs
Heartworm disease in domestic dogs typically is classified as follows:
- Class 1: Very few or no overt signs of illness. Class 1 heartworm disease (“occult” infection, or “amicrofilaremia”) can be caused by an infection in its early stages, infection by only one gender of worms which prevents reproduction, or administration of heartworm preventatives that kill microfilaria but do not affect adult worms.
- Class 2: Moderate signs of respiratory and cardiac distress
- Class 3: Severe signs of respiratory and cardiac distress
- Class 4: Caval syndrome – extremely severe, guarded to grave prognosis
How Heartworms Affect Dogs
Many dogs infected with heartworms show no signs of illness, at least in the early stages of the disease. Often, heartworms are first detected during routine blood testing for other conditions. The most common symptoms of mild to moderate heartworm disease are coughing and general ill thrift.
Symptoms of Heartworms in Dogs
Owners of dogs infected with heartworms may notice one or more of the following signs:
- Exercise intolerance
- Tiring easily
- Respiratory distress; difficulty breathing (dyspnea; usually acute in onset)
- Rapid breathing (tachypnea)
- Weight loss (visibly prominent ribs and chest)
- Abdominal distension
- Enlarged liver and spleen (hepatomegaly; splenomegaly)
- Abnormal jugular vein pulsation
- Collapse
- Fainting
- Sudden death
Dogs at Increased Risk
Sporting breeds, and other dogs living in endemic areas that spend substantial time outdoors, have an increased risk of heartworm disease. The most severe form of the disease, Class 4 (caval syndrome), tends to occur in middle-aged, outdoor male dogs during the spring and early summer months. Most dogs are between 3 and 6 years when heartworm infestation is diagnosed. Males are about 4 times more likely to be affected by this condition, as are dogs living outdoors, especially in hot climates where mosquitoes are prevalent. Dogs living at high elevations seem less prone to infection, although the reason for this association is not well-understood.