By the time you read this, the transition from spring into summer will be well underway, and no doubt that when you're outdoors, you'll be slapping at mosquitoes. It's that time of year again when the risk of heartworm infections rises.
According to the American Heartworm Society, heartworms affect hundreds of thousands of dogs every year. They are found in every state and across the globe. Heartworm's vectors include several different species of mosquitoes found in every environmental sphere. Infected dogs capable of spreading the disease may show no clinical symptoms for several years, but the physical effects of a heartworm infection are irreversible and can lead to death.
Thankfully, heartworms can be prevented. Early diagnosis is readily available and accurate, and advancements in available pharmaceuticals and research are making treatment safer and more effective.
Dirofilaria immitis is the scientific name for the heartworm parasite. It is classified as a helminth, meaning that it is of the "worm" variety of parasites (in contrast to a protozoa such as the amoeba). The adult heartworms can be seen by the naked eye and have the appearance of "angel hair" spaghetti, while larval stages require the use of a microscope to see. The canine is the natural host of heartworms and in which they thrive; however, they have also been found in other species such as cats.
Heartworms have a rather long life cycle as compared to other parasites, sometimes taking up to nine months to complete. Starting with a pair of adults inside the cardiovascular system, baby heartworms -- microfilariae -- are birthed into the bloodstream of the host. These circulate in the bloodstream waiting to be taken up by a mosquito as it feeds. They cannot develop into adults within the host dog without first leaving and maturing inside the mosquito.
Once in the mosquito, the microfilariae develop through three or four molts before they are ready to infect a new dog. Once this happens, the next time a mosquito feeds, the larvae rupture out of the mosquito's mouthpiece and onto the victim. They then enter the puncture wound caused by the mosquito's bite and begin to migrate through the underlying muscles. After additional maturation, the larvae will penetrate a blood vessel and begin their journey toward the heart.
Initially, the first larvae – now juvenile adults – will circulate through the heart and embed in the arteries of the lung. As the number of worms increases and space begins to become limited, the worms will tend to collect in the large pulmonary artery and eventually in the right ventricle and atrium of the heart. Once very large numbers of heartworms accumulate, they might even be found within the vena cave, a large vein coming to the heart from the liver.
After heartworms of both sexes are established and mature, they will begin breeding , producing more microfilariae, and thus the cycle continues.
Heartworms cause disease due to the disruption of blood flow through the heart and lungs. Worms in the arteries of the lungs can cause slowing of blood flow, changes in arterial blood pressures, and production of micro-clots leading to microemboli, which are small clots in the bloodstream. When heartworm numbers increase within the heart itself, results include valvular insufficiency and flow disruption, which lead to reduced flow efficiency and heart enlargement. When very high numbers reside in the vena cave, this reduces blood flow to the heart and can result in liver damage. Typically the fatal aspect of heartworm infection is heart failure, but in some cases severe pulmonary emboli can cause sudden death.
Prevention of heartworms is the best policy. General efforts to reduce mosquito populations have some but little effect on the problem, and products marketed to "repel mosquitoes" as a way to prevent heartworms are even less effective. The easiest time to kill the heartworm is during its migration from mosquito bite through the dog. During this time, a low dose of parasiticide can kill larvae that have been migrating for up to 30 days. This is the reason that a program of once-a-month medication is the most common method of prevention.
As mentioned, it can be a long time after the acquisition of a heartworm infection before your dog starts to show any symptoms, since most symptoms are a result of the long-term damage from the worms. The initial symptoms will usually be a mild cough and lethargy. As the heart and lung damage increases, the symptoms will worsen into chronic cough, lack of appetite, lack of energy, and exercise intolerance. If these are not treated, the end stage symptoms include liver failure, passing out due to poor blood pressure, bloody urine, abdominal fluid, and sudden death. Testing at the first sign of any symptom gives your dog the best chance to minimize and recover from heartworms.
Historically, testing for heartworms was based on microscopic observation of microfilariae within a blood sample from the dog. This was very effective at finding dogs infected with mating heartworms, but there were gaps in how quickly the worms could be identified and the sensitivity of the test.
Heartworm tests today are quick, easy, and effective. These newer tests check for antigens produced by the female heartworm. Once the heartworm is six months of age, antigen levels are high enough for detection by common in-clinic blood tests. These tests primarily fail in the case of very juvenile worms or very low worm burdens (such as in cats) where there may not be any female worms.
Considering the preventative and diagnostic aspects, a common question that comes up revolves around the necessity of annual heartworm tests. In other words, if a dog is on preventative medications regularly – year-round – why do an annual test? Several reasons:
If the dog has only been tested once, it is possible that the test was done at a time when the heartworms were not fully mature and thus did not trigger a positive result on the test.
While preventative medications are very good, they do have occasional failures, and in some areas, there are indications that mild resistance is occurring within the heartworm population.
Unless you are utilizing an injectable heartworm prevention product, it is possible that not all the medication is getting ingested at each treatment.
Finally, though we try our best, we might sometimes miss a dose or fall behind in giving the treatments. This also presumes that the medications are being given year-round, which is not always the case, even though it is technically possible – even in the colder climates – for heartworm transmission to occur any month of the year.
Early treatments are the most successful. If you wait until you are seeing clinical signs of heart failure or pulmonary inflammation, then many times the damage has already been done. The worms might still be treated, but there is no way to reverse most damage from heartworm disease.
Another good reason to test annually lies in the public-relations aspects related to the sale of the preventatives. If you have been giving a heartworm pill and your dog comes up positive for heartworms, many times the manufacturer of the medication will assist you financially in treating your dog for the heartworms. But you have to test in order to know if you need to take advantage of their help, and the dog must have tested negative before starting on the medication.
If your dog does end up testing positive for heartworms, they may be treatable. Except for cases where the heartworms (or other heart issues) have resulted in severe cardiopulmonary dysfunction such that treatment might endanger the dog more than the infection, treatment can eradicate the heartworms. To help determine this and which therapy regimen to use, your veterinarian will probably perform diagnostics such as blood work, radiographs, and sometimes an ultrasound of the heart.
The most recent development in regard to heartworm treatment has to do with a little bacterium found on the heartworms. Researchers discovered that heartworms have a symbiotic relationship with a bacterium, which when killed weakens the worms. Therefore, the latest heartworm treatment recommendations institute a 28-day therapy of an antibiotic to kill the bacteria before starting the final treatment to actually kill the heartworms. The result is fewer post-treatment complications and greater treatment success rates.
If you do decide to have your dog's heartworms treated, the quicker the better. There are some "treatment" protocols that, while they will eventually result in eradication of the adult heartworms, they take a very long time to do so, resulting in more irreversible damage to the heart and lungs. Also, be prepared to restrict your dog's exercise during the treatment period, which can last from six to 12 weeks. Exercise restriction is instituted because if, as the worms die, the heart is pumping hard and moving around in the chest (i.e. as a result of exercise), then the worms are more likely to fragment and send emboli to the lungs -- not good. "Restriction" is whatever it takes to keep the dog minimally active with a slow heart rate and varies from dog to dog. Consulting with your veterinarian as he or she explains your dog's exact treatment protocol is the best way to decide this important aspect of heartworm treatment.
by Dr. Ben Character
Posted in: News