More and more dog owners are becoming aware of heartworms and the severe damage they can cause to their pets.
Although this world-wide parasite was first reported over a century ago, until recently it was only well known in warmer climates in areas such as Florida and other warm southern locations.
Due to increased travel of pets and better diagnostic techniques, heartworms are now being diagnosed in almost every corner of the nation.
How The Enemy Works
Since the parasite resides within the circulatory system, the serious consequences should be obvious. The adult heartworm, Dirofilaria immitis (which literally means “evil thread”) belongs to a group of parasitic worms called filariae. A well-known relative, Wuchereria bancroft, causes elephantiasis in humans.
D. immitis commonly lives in the right side of the heart and the blood vessel leading to the lungs (pulmonary artery). Dogs seem to be the preferred host, however, this worm has been found in many other carnivores, including the cat.
Some human infections have been reported, mainly from the southern coast of the United States. Mammals, such as the muskrat and otter, have also been found to harbor D. immitis.
The adult worms are approximately seven to ten inches in length and quite thin. The female produces live young known as microfilariae. These “baby” heartworms circulate in the blood until they are picked up and swallowed by a mosquito during its bite.
Once inside the mosquito, the microfilariae start maturing and migrate up to the mouth of the insect. This part of the life cycle takes about two weeks. From here they are transferred to a new host when it is bitten by the mosquito. The microfilariae remain at the site of injection for three to four months as they undergo further development.
Finally, the immature worms migrate to the heart by entering small blood vessels. Another two to three months are spent in the heart to reach full maturity as reproductive adult heartworms.
Three important points to be derived from this brief sketch of the life cycle are:
1) Microfilariae which are circulating in the blood will eventually die if not picked up by a mosquito. They will not become adult heartworms without the intermediate insect host. This in turn means areas with severe mosquito problems are most likely to have a high incidence of heartworms.
2) In most cases, microfilariae will not be detectable in the blood until six months after a dog has been infected by a carrier mosquito. Thus in colder areas which only have a summer mosquito season, an infected dog might not be discovered until the next spring.
3) A heartworm-positive dog can act as a carrier or source of infection for many other dogs if left outside during the mosquito season.
The Disease Up Close
Any clinical signs of heartworm disease will depend on the number of worms present, their location, the length of time in the host, and the individual susceptibility of the host. Early infections or those only involving a few worms will probably remain asymptomatic. With longer infections or large numbers of worms, abnormalities are usually detected.
The changes which occur appear to be due in large part to some type of allergic reaction between parasite and host. Since most worms are found in the right ventricle of the heart and the pulmonary artery, the clinical signs are most often those of heart and lung failure.
Initially, dogs will exhibit coughing, loss of stamina, drop in weight, and poor hair coat. As the disease progresses, the coughing becomes more severe, occasionally with blood, and breathing becomes more difficult and fatigue is almost constant.
The abdomen may swell up due to fluid retention (ascites) while the muscles shrink making the skeleton appear prominent. As the body runs down from the impaired circulation, other organs, especially the liver and kidneys, will start to deteriorate. If untreated, this disease of often fatal.
There are many variations of this typical clinical picture. Sometimes the heartworms collect in the posterior vena cava, the large vein leading into the heart from the abdomen. This usually results in sudden collapse and death. A similar situation if a mass of worms lodges in the pulmonary artery which carries blood from the right ventricle of the heart to the lungs for oxygen. Again, the onset is rapid and death within a few hours is usual.
Diagnosis
As with any disease, the history and physical examination are the foundation for a diagnosis. For example, a hunting dog from a mosquito-infested area which has developed a persistent cough would certainly be a prime suspect for heartworm disease. The best method of detecting heartworms in a dog is by finding the microfilariae (baby heartworms) in the blood.
The most rapid test is simply to scan a thick fresh blood smear under the microscope. Other techniques involve the concentration and preparation of blood samples to make detection easier, especially in cases where the number of microfilariae in the blood is very low.
There is another filaria parasite of dogs in the United States which lives primarily under the skin but causes virtually no problems. The worm is of the Dipetalonema species and, unfortunately, it produces microfilariae in the blood which is very similar to those of the heartworm. There are subtle differences, however, (such as length) which distinguish the two and make positive identification under the microscope possible.
Occasionally, for several different reasons, a dog can have heartworms living in the heart and lungs but not have microfilariae in the blood. If the clinician suspects heartworms, he may take several blood samples over a time period to try to demonstrate microfilariae. An X-ray of the chest will usually be taken. This disease causes certain radiographic changes which may be sufficient to make a diagnosis in spite of negative blood samples.
It should be mentioned that, although the presence of heartworms can be established, there is to date no practical method for determining the actual number of worms in the body. The severity of the infection must be determined by clinical signs, laboratory findings, and X-ray evaluation.
Treatment: What To Do If Your Dog Has Heartworms
Having established the fact that a dog has heartworms, the next step is what to do about them. Treatment can be either medical or surgical, bur prior to this a careful assessment of the dog's physical condition must be made. No therapy should be started before conditions such as heart failure, congestion, liver disease, kidney disease, and concurrent infection are brought under control.
It should be stated that either treatment is risky. Younger dogs who are not showing clinical signs are much better candidates for treatment than older, symptomatic ones. But it cannot be forgotten that they are all suffering from a type of heart disease.
Cost is another factor which cannot be ignored. Lab tests, X-rays, hospitalization, drugs, or even surgery often run up a considerable bill. It is very important for a dog owner to fully discuss and understand the risk and expenses of treating heartworms prior to admitting his pet for treatment.
Medical treatment consists of the administration of an arsenic compound into the blood stream (intravenous). This is usually done twice a day for two days. This is the only effective drug available. The liver and kidneys must get rid of this arsenical and if either is not in good working order then a toxic build-up can occur.
Laboratory work is done is done before treatment to evaluate these organs and minimize risk. This treatment has been proven highly effective in killing adult heartworms.
Since the worms are trapped inside the circulatory system, once they are destroyed they cannot be eliminated from the body. As the heartworms dies from the arsenical treatment, they disintegrate and are carried into the lungs. The lungs then absorbs these fragments and it is during this process that coughing, fever, and loss of appetite are often noticed.
This occurs about seven to fourteen days after the last arsenical injection. It is very important that owners keep their pets as quiet as possible for three to four weeks following treatment to minimize extra stress.
Surgical removal of the worms may be the only option for certain dogs. The primary indications for surgery are dogs with severe heart and lung changes or those in which arsenical therapy would be too dangerous.
The procedure involves the manual removal of heartworms with a small pair of forceps through and opening in the right ventricle or pulmonary artery. One advantage to surgical removal is that the body does not have to undergo absorption of dead worms as with the arsenical treatment. If large numbers of worms are present, this can be very important.
Although either method of treatment is certainly not without danger, they are both proven and should be given careful consideration in each case.
Regardless of the means used to remove the adult worms, a drug must be given after recovery to destroy the microfilariae in the blood. These young worms will eventually die, but this can take many months and during this time the dog will act as a source of infection for other dogs.
Detection of re-infection is also impossible if the microfilariae are not eliminated. Several drugs are effective in removing microfilariae and each veterinarian has his or her own preference.
The Best Cure: Prevention
Obviously such a serious disease would be better to prevent than to treat. Drugs are now available which, if given continuously during the mosquito season, seem effective in preventing heartworms from developing in the dog.
Mosquito control is one of the best methods of protection. This can be accomplished by screening outdoor pens or keeping dogs indoors as much as possible. Some veterinarians in high-incidence areas like to treat dogs once or twice per year with the intravenous arsenical.
An annual or bi-annual blood check should be routine in endemic areas. This will often detect infections prior to the onset of clinical signs. These cases usually have the best response to treatment since damage to organs is minimal.
Since a dog with heartworms is a source of infection for the surrounding canine population, diagnosis and treatment are an essential part of control.
Many institutions and individuals are currently engaged in the study of this disease. Part of this is due to the similarity of heartworm disease to the human filarial diseases, such as elephantiasis.
Continual upgrading of diagnosis, treatment, and control is the reward of this work. Without owner awareness and concern, however, the disease will continue to flourish.
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