Heartworm disease (dirofilariasis) is a serious and potentially fatal disease in dogs. It is caused by a worm called Dirofilaria immitis. Because heartworm disease is carried by mosquitoes, it can be found anywhere mosquitoes are found, including in your house and back yard. Certainly, there is higher risk near standing bodies of water, where mosquitoes breed. Our doctors at the Animal Clinic at Thorndale diagnose this disease in several local Downingtown area dogs almost every year. Even those dogs whose owners insist they “Only go for a leash walk on the sidewalk outside my home” have been found to have this disease. Mosquitoes fly. Treatment has its risks, as we will discuss below. Our doctors recommend all dogs in this part of the United States stay on heartworm preventative year round. Why would you risk your dog getting this disease when it is preventable by giving a medicated chewable treat once a month?
How Heartworms Get into the Heart
Heartworms are found in the heart and large adjacent vessels of infected dogs. The female worm is 6 to 14 inches (15 to 36 cm) long and 1/8 inch (5 mm) wide; the male is about half the size of the female. One dog may have as many as 300 worms.
Adult heartworms live in the heart and pulmonary arteries of infected dogs. They have been found in other areas of the body, but this is unusual. They survive up to 5 years and, during this time, the female produces millions of young (microfilaria). These microfilaria live in the bloodstream, mainly in the small blood vessels. The immature heartworms cannot complete the entire life cycle in the dog; the mosquito is required for some stages of the heartworm life cycle. The microfilaria are therefore not infective (cannot grow to adulthood) in the dog–although they do cause problems.
As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites the infected dog and ingests the microfilariae during a blood meal. The microfilariae develop further for 10 to 30 days in the mosquito and then enter the mouthparts of the mosquito. The microfilariae are now called infective larvae because at this stage of development, they will grow to adulthood when they enter a dog. The mosquito bites the dog where the haircoat is thinnest. However, having long hair does not prevent a dog from getting heartworms.
When fully developed, the infective larvae enter the bloodstream and move to the heart and adjacent vessels, where they grow to maturity in 2 to 3 months and start reproducing, thereby completing the full life cycle.
Geography
Canine heartworm disease occurs all over the world. In the United States, it was once limited to the south and southeast regions. However, the disease is spreading and is now found in most regions of the United States and Canada, particularly where mosquitoes are prevalent. An unfortunate side effect of dog rescues bringing so many dogs north after the massive hurricane destruction in our southern states is that we have seen an influx of heartworm infected dogs. Many of these dogs were not diagnosed and treated prior to being shipped north. We certainly treat these dogs and help them on their way to a better life. However, prior to diagnosis and treatment, these dogs have acted as a reservoir of microfilaria for local mosquitoes to pick up and carry on the heartworm’s life cycle.
This movement of infected dogs northward has likely increased the distribution and prevalence of this disease. Climate change and mutation of the heartworm itself, to be better able to survive in mosquitoes found in the cooler areas of the country, have also contributed to its spread. Heartworm is very much in the Downingtown,PA, area. Dogs are a definitive host of this disease, meaning heartworms “like” to infect dogs. The doctors at the Animal Clinic at Thorndale highly recommend that all dogs be on heartworm preventative year round. Cats, too, can be infected with heartworm, but much less readily. You can read more about heartworm disease in cats by following this link.
Contagion
The disease is not spread directly from dog to dog. An intermediate host, the mosquito, is required for transmission. Spread of the disease therefore coincides with the mosquito season. The number of dogs infected and the length of the mosquito season are directly correlated with the incidence of heartworm disease in any given area.
It takes a number of years before dogs show outward signs of infection. Consequently, the disease causing illness is diagnosed mostly in 4 to 8 year old dogs. Illness from heartworm infection is seldom seen in a dog under 1 year of age because the young worms (larvae) take up to 7 months to mature following establishment of infection in a dog.
Effects on the Dog
Adult worms: Adult worms cause disease by clogging the heart and major blood vessels leading from the heart. They interfere with the valve action in the heart. By clogging the main blood vessels, the blood supply to other organs of the body is reduced, particularly the lungs, liver and kidneys, leading to malfunction of these organs.
Most dogs infected with heartworms do not show any signs of disease for as long as 2 years. Unfortunately, by the time signs are seen outwardly, the disease is well advanced. The signs of heartworm disease depend on the number of adult worms present, the location of the worms, the length of time the worms have been present, and the degree of damage to the heart, lungs, liver, and kidneys from the adult worms and the microfilariae.
The most obvious signs are: a soft, dry, chronic cough, shortness of breath, weakness, nervousness, listlessness, and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even faint.
Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds. In advanced cases, congestive heart failure may be apparent and the abdomen and legs will swell from fluid accumulation. There may also be evidence of weight loss, poor condition, and anemia.
Severely infected dogs may die suddenly during exercise or excitement.
Microfilariae (Young worms): Microfilariae circulate throughout the body but remain primarily in the small blood vessels. Because they are as wide as the small vessels, they may block blood flow in these vessels. The body cells being supplied by these vessels are deprived of the nutrients and oxygen normally supplied by the blood. The lungs and liver are primarily affected.
Destruction of lung tissue leads to coughing. Cirrhosis of the liver causes jaundice, anemia, and general weakness because this organ is essential in maintaining a healthy animal. The kidneys may also be affected and allow poisons to accumulate in the body.
Diagnosis
In most cases, diagnosis of heartworm disease can be made by a blood test that can be run in the veterinary hospital. Further diagnostic procedures are essential, in advanced cases particularly, to determine if the dog can tolerate heartworm treatment. Depending on the case, we will recommend some or all of the following procedures before treatment is started.
Serological test for antigens to adult heartworms: This is a test performed on a blood sample. It is the most widely used test because it detects antigens (proteins) produced by adult heartworms. It will be positive even if the dog does not have any microfilaria in the blood; this occurs about 20% of the time. Dogs with less than five adult heartworms will not have enough antigen to turn the test positive, so there may be some false negative results in early infections. Because the antigen detected is produced only by the female worm, a pure population of male heartworms will give a false negative, also. Therefore, there must be at least 5 female worms present for the most common test to be positive.
Blood test for microfilariae: A blood sample is examined under the microscope for the presence of microfilariae. If microfilariae are seen, the test is positive. The number of microfilariae seen gives us a general indication of the severity of the infection. However, the microfilariae are seen in greater numbers in the summer months and in the evening, so these variations must be considered. Approximately 20% of dogs do not test positive even though they have heartworms because of an acquired immunity to this stage of the heartworm. Because of this, the antigen test is the preferred test. Also, there is another microfilarial parasite which is fairly common in dogs; on the blood smear, these can be hard to distinguish from heartworm microfilariae. in a single sex heartworm infection, microfilariae are not present.
Blood chemistries: Complete blood counts and blood tests for kidney and liver function may give an indirect indication of the presence of heartworm disease. These tests are also performed on dogs diagnosed as heartworm-infected to determine the function of the dog’s organs prior to treatment.
Radiographs (X-rays): A radiograph of a dog with heartworms will usually show heart enlargement and swelling of the large artery leading to the lungs from the heart. These signs are considered presumptive evidence of heartworm disease. Radiographs may also reveal the condition of the heart, lungs, and vessels. This information allows us to predict an increased possibility of complications related to treatment.
Electrocardiogram: An electrocardiogram (EKG) is a tracing of the electric currents generated by the heart. It is most useful to determine the presence of abnormal heart rhythms.
Echocardiography (Sonogram): An echocardiogram allows us to see into the heart chambers and even visualize the heartworms themselves. Although somewhat expensive, this procedure can diagnose heartworms when other tests fail.
Treatment
There is some risk involved in treating dogs with heartworms, although fatalities are rare. In the past, the drug used to treat heartworms contained arsenic so toxic effects and reactions occurred somewhat frequently. Now a newer drug, Immiticide, is available and does not have the toxic side effects of the old one. We are able to successfully treat more than 95% of dogs with heartworms.
Another medication, doxycycline, has been found to help in the elimination of heartworm, perhaps by killing a bacteria, call Wolbachia sp., that seems to facilitate the heartworms. The doctors at the Animal Clinic at Thorndale will generally start doxycycline before beginning the definitive treatment.
The doctors at the Animal Clinic at Thorndale will also, in most cases, dispense prednisone, for you to have on hand when your dog goes home after a heartworm treatment. Because these large worms are dying and breaking down inside the blood vessels around the heart and lungs, some dogs will have a reaction to the dying worms. Prednisone is an anti-inflammatory drug that can quiet this reaction and help your dog if it begins coughing or having any increased respiratory effort.
We see some dogs with advanced heartworm disease. This means that the heartworms have been present long enough to cause substantial damage to the heart, lungs, blood vessels, kidneys, and liver. A few of these cases will be so far advanced that it will be safer to just treat the organ damage rather than risk treatment to kill the worms. Dogs in this condition are not likely to live more than a few weeks or months. Fortunately this is a rare occurrence and we would recommend consultation with an area cardiology specialist before deciding not to treat a dog.
Treatment to kill microfilaria: Approximately 1 month prior to treatment to kill the adults, we will request that your dog be brought to the hospital for administration of a drug to kill microfilariae, when they have been diagnosed as present. Your dog needs to stay in the hospital for the day. Prior to treating your dog for the adult worms, we will begin an anti-microfilarial drug to reduce the risk that your dog will remain indirectly infective to other dogs. Your dog should take this heartworm preventative once a month from this point forward. We may also prescribe and antibiotic, doxycycline, to help treat a secondary bacterial infection commonly found called Wolbachia, which contributes to the symptoms of heartworm disease.
Treatment to kill adult worms: An injectable drug, Immiticide, is given to kill adult heartworms. It kills the adult heartworms in the heart and adjacent vessels. Generally we stage the treatment so there is not a massive breakdown of adult worms flooding the bloodstream, at once. The first treatment consists of one injection and monitoring in the hospital for the day. One month later, 2 injections, given on sequential days in the hospital, destroy the remaining adult worms. After each of these treatments, it is imperative that your dog be kept quiet, leash walked outside only to urinate and defecate, and then back in the house to rest again. Consider that these very large worms are dying inside the great blood vessels around the heart. We do not want to send floods of worm fragments through the lungs to plug large blood vessels (pulmonary embolism), so rest is imperative.
Complete rest essential after treatment: The adult worms die in a few days and start to decompose. As they break up, they are carried to the lungs, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This is a dangerous period, and it is absolutely essential that the dog be kept quiet and not be allowed to exercise for 1 month following treatment. The first week after the injections is very critical because the worms are dying. A cough is noticeable for 7 to 8 weeks after treatment in many heavily infected dogs. We may send home a steroid medication, prednisone, to be given if you dog develops problems after treatment.
Prompt treatment is essential if the dog has a significant reaction in the weeks following the initial treatment, although such reactions are not common. If a dog shows loss of appetite, shortness of breath, severe coughing, coughing up blood, fever, and/or depression, you should notify us immediately. Response to antibiotics, cage rest, and supportive care, such as intravenous fluids, is usually good in these cases.
Other treatments: In dogs with severe heartworm disease, it may be necessary to treat them with antibiotics, special diets, diuretics to remove fluid accumulations, and drugs to improve heart function prior to treatment for the heartworms. An evaluation by a cardiologist is also recommended to assess the extent of damage to the heart by the presence of heartworms. Dogs with severe heart disease may need lifetime treatment for the failing heart, even after the heartworms have been killed and or removed. This includes the use of diuretics, heart drugs, aspirin, and special low salt, low protein diets.
Response to treatment: Dog owners are usually pleasantly surprised at the change in their dog following treatment for heartworms, especially if the dog had been showing signs of heartworm disease. The dog has a renewed vigor and vitality, improved appetite, and weight gain.
Prevention
When a dog has been successfully treated for heartworms, you cannot sit back and relax because dogs can be reinfected. Therefore, it is essential to begin and continue a heartworm prevention program. We recommend HeartGard Plus or Sentinnel. Both are chewable tablets that are given only once monthly on the same day (i.e. the first of the month, or the 15th of the month, or our mortgage payment date – whatever you remember). Revolution is a topical product that is applied once monthly. Because it is trickier to correctly apply this product than to give a chewable treat preventative, better prevention may be obtained with the oral medications. Their costs are similar.
For more information follow this link: American Heartworm Society