Pets and People: Heartworm Disease is a big killer
Heartworm disease is devastating. Our practice at the Manchester Animal Hospital is focused on preventing the disease whenever possible or at lease diagnosing it in its very early stages. Fortunately, we are not a heartworm endemic area, however, the disease is still diagnosed here.
The predominance of cases I diagnose come from the south. The local patients that acquire the disease will have caught it from a mosquito harboring the disease. For that reason, we recommend an oral heartworm preventative medication and additionally we perform a blood test for heartworm disease at least annually.
Once a dog becomes infected, the migration of the young worm after it enters the dog’s body is a long one: a journey of five to seven months. In that time, the worm is growing, maturing, and preparing to mate.
Now, keep in mind that while all this is happening, nothing is happening to the dog and no one knows an invasion has taken place.
At the end of this long migration, the worm lodges in a distal pulmonary artery branch and then grows to around 12 inches (30 cm) long (females are longer than males). The dog’s immune system recognizes the foreign proteins in the worms and creates inflammation that involves the pulmonary arteries adjacent to the worms and the lung tissue surrounding the artery. The pulmonary arteries enlarge and become tortuous as a result to produce the typical pattern of heartworm disease seen on thoracic radiographs.
Contributing to the inflammation 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. The antibiotic Doxycycline is used to kill Wolbachia organisms and so to reduce inflammation, especially when administering an adulticide to kill the worms.
The inflammation calls in numerous immune cells that in turn generate even more inflammation as they attempt to destroy a parasite that is realistically too large for them to destroy. The lung itself becomes inflamed and in time becomes scarred, creating an even larger high-resistance area for the heart to pump through.
If there are many worms, the problem is multiplied. Some pulmonary arteries may become fully blocked by the crowd of worms. Blood cannot pass through these arteries as they are plugged up by a wad of worms. This in turn means the area of lung that would have been served by these arteries is rendered useless. If the lung cannot present its oxygen to a working pulmonary artery, oxygen exchange cannot take place.
The real damage comes from worms that have died in place. The body of the heartworm breaks apart and is carried through the vasculature of the lung until it lodges somewhere and obstructs blood flow. The arteries that are supposed to form delicate branches, branching tinier and tinier, are now blunted and closed off, similar to a tree branch broken off close to the tree trunk. As above, this leaves more areas of lung blocked off and unable to receive blood and participate in oxygen exchange.
All the inflammation generated by the worms as well as the inflammation generated by the areas of lung that aren't receiving proper circulation ultimately translates into scarring in the lung vasculature. Scarring and fibrosis makes it difficult for the heart to pump blood through the lung effectively. There is a point where the heart is not strong enough to pump blood through all the narrowed, stiff, damaged capillary beds. Right sided heart failure ensues.
In naturally infected dogs, the number or worms does not correlate to severity of disease, even in dogs of the same size. It's not the number of worms that matters so much as the dog’s activity level.
The factors that come into play to create severity of disease are: the dog’s activity level (the more active the dog, the fewer worms are needed to create disease), the size of the dog, and the number of worms the dog has. The infected dog that sits around at home may appear relatively normal but once some exercise or even anxiety puts more demand on his heart, symptoms erupt.
Symptoms of Heartworm Disease:
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.
A dog harboring heartworm 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 type of kidney inflammation called 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.
Caval syndrome is an especially disastrous form of heartworm disease. Here, there are so many worms (around 100) that the entire right side of the heart is filled with worms and they are backing out into the large veins that feed the right side of the heart. Usually there have been no signs of heart disease prior to the collapse, shock, and red blood cell destruction associated with this syndrome. Death usually occurs within one to two days and the only effective treatment is to open the dog's jugular vein and physically remove the worms with a clamp. If enough worms can be removed to re-establish blood flow, the dog may survive.
The most important take away is this: prevent the disease by giving your canine companion a heartworm preventative monthly and test for the disease at least once annually. Do it for yourself and for the pet you love.
Dr. Lamb is the Veterinarian at the Manchester Animal Hospital.
