What causes heartworm disease?
Heartworm disease (dirofilariasis) is a serious and potentially fatal disease in dogs. It is caused by a blood-borne parasite called Dirofilaria immitis.
Heartworms are found in the heart and adjacent large blood vessels of infected dogs. The female worm is 6 to 14 inches long (15 to 36 cm) and 1/8 inch wide (5 mm). The male is about half the size of the female. One dog may have as many as 300 worms. (return to top)
How do heartworms get into the heart?
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 live up to five years and, during this time, the female produces millions of offspring (microfilaria). These microfilariae live mainly in the small vessels of the bloodstream. The immature heartworms cannot complete their life cycle in the dog. The mosquito is required for some stages of the heartworm life cycle. The microfilaria are 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 usually bites the dog where the hair coat 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 two to three months and start reproducing, thereby completing the full life cycle. (return to top)
Where are heartworms found?
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. (return to top)
How do dogs get infected with them?
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 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 is diagnosed mostly in four to eight year old dogs. The disease is seldom diagnosed in a dog less than one year of age because the young worms (larvae) take up five to seven months to mature after infection. (return to top)
What do heartworms do to the dog?
Adult heartworms: Adult heartworms 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 two years. Unfortunately, by the time clinical signs are seen, 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 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 heartworms): 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. (return to top)
How is heartworm infection diagnosed?
In most cases, diagnosis of heartworm disease can be made by a blood test that can be run in the veterinary hospital or by a veterinary laboratory. Further diagnostic procedures are essential 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 in about 20% of the cases. Dogs with less than five adult heartworms will not have enough antigen to give a positive result, so there may be an occasional false negative result in dogs with early infections. Because the detected antigen is only produced by the female heartworm, a population of only male heartworms will also give a false negative. Therefore, there must be at least five female worms present for the most common heartworm test to diagnose heartworm disease.
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 blood parasite that is fairly common in dogs that can be hard to distinguish from heartworm microfilariae.
Blood chemistries: Complete blood counts and blood tests for kidney and liver function may give an 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 or ECG) is a tracing of the electric currents generated by the heart. It is most useful to determine the presence of abnormal heart rhythms.
Echocardiography: An ultrasonic examination that allows us to see into the heart chambers and even visualize the heartworms. (return to top)
How are dogs treated for heartworms?
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 more frequently. A newer drug is now available that does not have the toxic side-effects. We can now successfully treat more than 95% of dogs with heartworms.
Some dogs are diagnosed 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 advanced that it will be safer to treat the organ damage rather than risk treatment to kill the heartworms. Dogs in this condition are not likely to live more than a few weeks or months.
Treatment to kill adult heartworms: An injectable drug to kill adult heartworms is given for two days. It kills the adult heartworms in the heart and adjacent vessels. In more serious infections, these injections may be divided and given thirty days apart.
Complete rest is 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 can be a dangerous period so it is absolutely essential that the dog be kept quiet and not be allowed to exercise for one month following treatment. The first week after the injections is critical because the worms are dying. A cough is noticeable for seven to eight weeks after treatment in many heavily infected dogs.
Prompt treatment is essential if the dog has a significant reaction in the weeks following the initial treatment, although such reactions are rare. If a dog shows loss of appetite, shortness of breath, severe coughing, coughing up blood, fever, and/or depression, you should notify us. Response to antibiotics, cage rest, and supportive care and intravenous fluids is usually good in these cases.
Treatment to kill microfilaria: Approximately one month following treatment to kill the adults, the dog is returned to the hospital for administration of a drug to kill the baby heartworms or microfilariae. Your dog needs to stay in the hospital for the day.
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.
Dogs with severe heart disease may need lifetime treatment for the heart failure, even after the heartworms have been killed. This includes the use of diuretics, heart drugs, 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. (return to top)
Are changes made in the treatment protocol for dogs that have severe heartworm disease?
Yes. The state of heart failure is treated as described above. However, we also treat the adult heartworms in a two-stage process. Only one treatment with the drug to kill the worms is given initially. This causes the death of approximately half of the worms. One month later, the full treatment is given to kill the remaining worms. By killing them in two stages, the severe effects on the lungs are much less likely to occur. (return to top)
How can I prevent this from happening again?
When a dog has been successfully treated for heartworms, it is essential to begin a heartworm prevention program to prevent future recurrence. With the safe and affordable heart preventives available today, no pet should ever have to endure this dreaded disease. (return to top)
Where does my pet get fleas?
The most common flea found on cats and dogs is the cat flea (Ctenocephalides felis).
The most important source of fleas is newly emerged adult fleas from pupae in your house. Adult fleas live and feed on our pets but the female flea lays eggs, which fall off into the environment. Under favorable conditions, these eggs develop first into larvae and then into pupae. The pupae contain adult fleas that lie in wait for a suitable animal host. Modern carpeted centrally-heated homes provide ideal conditions for the year round development of fleas. The highest numbers of flea eggs, larvae and pupae will be found in areas of the house where pets spend the most time such as their beds, furniture and so forth. Even though fleas may be in your house, you probably won't see them; the eggs are too small to see without magnification and the larvae, which are just visible, migrate deep down into carpets, furniture or cracks in floors away from the light. (return to top)
What effect do fleas have on my pet?
Many pets live with fleas but show minimal signs. However, the following problems can occur:
• Some pets develop an allergy to flea bites. If these pets are bitten by fleas they may groom or scratch excessively causing skin rashes and infection.
• Adult fleas live on animals and feed on blood. In kittens, puppies and debilitated animals this may cause anemia.
• The flea acts as the intermediate host for the tapeworm (Dipylidium caninum). Tapeworm eggs, which are shed within tapeworm segments in cat and dog feces, are eaten by flea larvae that develop into infected fleas. Pets become infested by eating infected fleas during grooming. Any pet with fleas is likely also to have a tapeworm infestation. (return to top)
How can I get rid of fleas on my pet?
This can be a demanding task and requires a two pronged approach. Fleas need to be eliminated from all of your pets and from your home. (return to top)
What products are available to treat my pet?
Many over the counter flea treatments are either too toxic or ineffective. Certain topical treatments have a considerable toxicity to pets and should definitely be avoided! Reactions to these insecticides could range from neurological twitching to seizure and death. Other flea treatments, like shampoos and collars, are simply ineffective. Newer prescription flea treatments are not only more effective, they are also safe to use and veterinary recommended. Please ask us about these products and how to use them.
ALWAYS READ THE LABEL CAREFULLY - apply the product as instructed and repeat at the intervals stated. (return to top)
How can I treat my home environment?
A number of different products are available which will kill the stages of the flea life cycle present in your home such as:
• Insecticide sprays for use in the house
• Sprays containing insect growth regulators for use in the house
• Insecticides applied by professional pest control operatives in your house.
Sprays for use in the house should be used in places where the flea eggs, larvae and pupae are likely to be. It is recommended that you treat the entire household first and then concentrate on the hot spots - your pet's favorite dozing spots - such as soft furniture, beds and carpets. Flea eggs are like particles of sand and fall into the nooks and crannies of the environment. Once they hatch from the egg, flea larvae move away from the light and burrow deep into carpets and into other places where it is difficult to treat. Be sure to move cushions, furniture and beds to spray underneath. Other places larvae are likely to live include baseboards and the cracks in wooden floors.
Your pet's bedding should be regularly washed in hot water or replaced. Regular and thorough vacuuming of your carpets, floors and soft furnishings can remove a large number of flea eggs, larvae and pupae that are present in your home. You will need to throw away the vacuum bag to prevent eggs and larvae from developing inside the vacuum cleaner. Vacuuming prior to the application of a spray to the house is recommended because the vibrations will encourage newly developed fleas to emerge from pupae, which will be killed by the insecticide. (return to top)
How do I choose which products to use?
A flea control program needs to be individually tailored based on the lifestyle of your pets. Your veterinarian is the best person to advise you about safe and effective flea control products. (return to top)
Are insecticides safe for my pet and my family?
Insecticides for flea control should be safe both for pets and humans provided the manufacturer's instructions are carefully followed. One should be particularly careful to avoid combining insecticides with similar modes of action. Always seek your veterinarian's advice if you are unsure about this and always tell your veterinarian about any flea control products you may be using other than those which he has prescribed.
Certain types of pets (e.g. fish, amphibians, reptiles and invertebrates) may be particularly susceptible to some products. Do not use any flea control products in the room in which these pets are kept without first consulting your veterinarian for advice. (return to top)
I have not seen any fleas on my pet. Why has my veterinarian advised flea control?
Fleas are easy to find if an animal is heavily infested. If fleas are present in smaller numbers, it can be harder to see them and fleas move fast! Try looking on the pet’s stomach, around the tail base and around the neck. Sometimes adult fleas cannot be found but "flea dirt" can be seen. This is fecal matter from the flea that contains partially digested blood and is a good indicator of the presence of fleas. Flea dirt is seen as small black specks or coiled structures; when placed on damp white tissue, they dissolve leaving a reddish brown blood color. Flea dirt may be found in bedding even when they cannot be found on the pet.
In pets that develop an allergy to fleas one of the symptoms is excessive grooming and scratching. Cats especially are very efficient at removing debris from their coat's using their tongues and may succeed in removing all evidence of flea infestation i.e. adult fleas and flea dirt. One of the commonest causes of pet allergic skin disease is flea allergy. To investigate this possibility your veterinarian may advise rigorous flea control even though no fleas can be found. If your pet’s skin problem improves with flea control then it suggests that flea allergy is involved. (return to top)
I noticed my pet had fleas after his return from boarding. Did he get fleas there?
Not necessarily. Newly hatched adult fleas can survive for up to 140 days within the pupa. When you and your pets are absent from home for extended periods of time these adult fleas remain in the pupae because no host is available. As soon as you or your pet returns home, these fleas will emerge in large numbers and jump onto cats, dogs and even people in the search for a blood meal. (return to top)
Despite treating my pet for fleas he still has them. Is there a “super flea”?
There is no evidence of fleas developing resistant to insecticides. Apparent failure of treatment almost always results from inadequate treatment of the home or exposure to other infested pets or environments. Consider treating sheds, cars and any outdoor sleeping spots. Most of these problems can be overcome by using an effective product on the pet to kill adult fleas in addition to treating your home. (return to top)
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