Early infection

We have covered the worm, now let’s cover the dog once he or she becomes infected. As we have said, the migration of the young worm once it enters the dog’s body is a long one: a journey of 5-7 months. In that time, the worm is growing, maturing, and preparing to mate. While all this is happening, nothing is happening to the dog and no one knows an invasion has taken place.

Worms in the Pulmonary Arteries

At the end of this long migration, the worm has reached its destination: a pulmonary artery. The worm is large and takes up a lot of space in the artery. The dog’s immune system recognizes foreign proteins present and launches an attack but the immune system is designed to work against smaller invaders such as bacteria or viruses. The heartworm is too big to be killed effectively this way and the result is inflammation and thickening in the arteries. Blood flow becomes turbulent and the tendency to form abnormal clots is increased. Adding to this unpleasant mix is a bacterium called Wolbachia pipientis, which normally lives inside the heartworm but is released in large numbers every time the heartworm molts to a new developmental stage, gives birth to its young, or dies. These bacteria contribute heavily to the resultant inflammation occurring in the pulmonary arteries and lung tissue itself. Blood vessels get stiff and cannot operate normally, creating a high resistance area that the heart must pump through.

SYMPTOMS OF HEARTWORM DISEASE

• Coughing
• Shortness of Breath/Panting
• Easy tiring/intolerance of exercise
• Fluid accumulation in the abdomen or chest
• Nose Bleeds
• Sudden Death

Obviously, not all of these things necessarily occur in the same dog, nor is there necessarily a progression. An infected dog may have no symptoms at all or may develop any of the signs on the list at any time.

CHRONIC IMMUNE STIMULATION AND KIDNEY DISEASE

A dog harboring heartworms in its body basically has an infection that cannot be cleared. The dog's immune system tries and tries but the worm is too big. This means the immune system is stimulated all the time, long term, and there is damage associated with the by-products of all this stimulation. Antibodies are more than just tools of the immune system; they are inflammatory proteins and in heartworm disease they are produced in high amounts all the time. Antibodies can cause a lot of trouble when they deposit in the delicate membranes of the eye, blood vessels, joints, and kidney. Antibodies stuck in these areas, call in inflammatory cells and damage these delicate membranes thus setting up tremendous tissue damage and pain. In particular, heartworm infection is a cause of a special type of kidney inflammation called “glomerular disease.”

SO WHAT HAPPENS IN GLOMERULAR DISEASE?

The filtration system of the glomerulus keeps proteins inside the body where they belong but allows for tiny waste molecules to be filtered out and dumped in urine. When antibody : antigen complexes abound, they get stuck in the delicate filtration membranes and ultimately punch holes there. As you might guess, a leaky filtration membrane allows for important proteins to be lost. What happens after that depends on what proteins are lost (see the above link for more details) but rest assured, the results are not good. Fortunately, this inflammation and its consequences can frequently be resolved with treatment for heartworm infection.

 
This is a depiction of a kidney structure called a glomerulus.
It has a delicate filtration system for proper urine production.
Antibody : Antigen complexes will disrupt this system,
causing protein loss in urine.
(original graphic by marvistavet.com)

HEARTWORM IN THE CAT

Heartworm disease is the cat is quite a bit different from heartworm disease in the dog. Cats are so small that only one adult worm could be enough to cause heart failure plus there is much more inflammation involved with the immature worms in the cat. For details on heartworm infection, disease, treatment, and prevention in the cat, click here.