Heartworms
Many dogs will show little or no sign of infection even after the heartworms have matured. To some degree these dogs may be described as seeming to age slightly faster than normal as the worms slowly damage the lungs, kidneys and liver. These animals usually have a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections will quickly show the classic symptoms of heartworm disease.
Early symptoms include a cough, especially on exercise, and early exhaustion upon exercise. More advanced cases progress to severe weight loss, fainting, coughing up blood, and, finally, congestive heart failure.
Heartworm Transmission
Adult heartworms, about six inches long, live in the heart and large blood vessels. These adult male and female worms produce thousands of microscopic baby worms. Baby heartworms do not grow up in the dog where they were born. If they did, the dog would quickly die and so would the heartworms.
Next, a mosquito comes along and bites the infected dog, sucking up baby heartworms. It probably isn't too good for the mosquito, but this is what the worms have been waiting for. During the next month, inside the mosquito, the heartworm babies grow into heartworm teenagers, a stage partway between baby and adult.
Now, the mosquito bites another dog, infecting it with teenage heartworms, which are now ready to develop into adults. After six or seven more months, the life cycle is complete. The new adult male and female heartworms are busily producing thousands of babies.
Heartworm Treatment
Before the worms can be treated, however, the dog must be evaluated for good heart, liver, and kidney function to ensure the animal can survive the treatment. Any insufficiencies in these organs must be dealt with first, before treatment, as the eradication process can be taxing on organ function. Usually the adult worms are killed with an arsenic-based compound. The currently recommended compound, Melarsomine dihydrochloride, is marketed under the brand name Immiticide. It has a greater efficacy and fewer side effects than previous formulation (Thiacetarsamide sodium, sold as Caparsolate) which makes it a safer alternative for dogs with late-stage infestations.
After treatment, the dog must rest (restricted exercise) for several weeks so as to give its body sufficient time to absorb the dead worms without ill effect. Otherwise, when the dog is under exertion, dead worms may break loose and travel to the lungs, potentially causing respiratory failure and death.
According to the American Heartworm Society, use of aspirin in dogs infected with heartworms is no longer recommended due to a lack of evidence of clinical benefit, and may be contraindicated. It had previously been recommended for its effects on platelet adhesion and reduction of vascular damage caused by the heartworms.
The course of treatment is not completed until several weeks later when the microfilariae are dealt with in a separate course of treatment. Once heartworm tests come back negative, the treatment is considered a success.
Surgical removal of the adult heartworms is also a treatment that may be indicated, especially in advanced cases with substantial heart involvement.
Heartworm preventative is only available by prescription from a licensed veterinarian.
Heartworm Prevention
Prevention of heartworm infection can be obtained through a number of veterinary drugs. Most popular are ivermectin (sold under the brand name Heartgard), milbemycin (Interceptor) and moxidectin (ProHeart) administered as pills or chewable tablets.
These drugs are given monthly during the local mosquito season. Moxidectin is also available in a six-month sustained release injection, Proheart 6, administered by veterinarians, but the injectable form of Moxidectin was taken off the market in the United States due to safety concerns.
ProHeart 6 remains on the market in many other countries including Canada and Japan and is used with great success. Its sister product, ProHeart SR-12 is used extensively in Australia and Asia as a 12 month injectable preventative. Selamectin (Revolution), on the other hand, is a topical preventive that is likewise administered monthly. Some of these drugs also kill other parasites, including intestinal worms. In addition, Selamectin controls fleas, ticks, and mites.
Preventative drugs are highly effective, and when regularly administered will protect more than 99 percent of dogs from infection. Most failures of protection result from irregular and infrequent administration of the drug. However, the monthly preventives all have a reasonable margin for error in their administration such that if a single month's dose is accidentally missed, adequate protection is usually provided so long as the next two monthly doses are administered on schedule.
Cats may be treated with ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats). Monthly heartworm prevention should be administered beginning within a month of the onset of the local mosquito season and continued for a month after the cessation of local mosquito activity.
In warm climates, such as the warm temperate climate along the immediate Gulf coast of the United States and in tropical and subtropical regions, heartworm prevention must be administered year round. Some authorities recommend year round administration even in colder climates on the theory that mosquito activity may occur during the occasional unseasonable warm spell, but others argue that computer models indicate heartworm transmission is highly unlikely under such circumstances.
Diagnois Heartworms
Your veterinarian may perform a blood test to determine whether your pet has the disease. A blood sample is tested for the antigens (proteins) produced by adult heartworms.
The sample may also be examined under a microscope for the presence of the heartworm larvae. More laboratory tests may be required to make a diagnosis, especially in cats, as the disease can be harder to diagnose in felines.
A negative test result for the larvae does not rule out feline heartworm infection, as the larvae often are found only temporarily in an infected cat. However, if circulating larvae are found, it becomes a confirmation that heartworm disease is present.
Heartworm Resources
American Heartworm Society - mission is to be the global resource for the prevention, diagnosis and treatment of heartworm disease.
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