Heartworm Testing and Prevention
Heartworm Testing and Prevention
Canine Heartworm Disease
Heartworm disease is spread by mosquitoes. Cases of heartworm disease have been documented in all of the 48 contiguous states. Primary hosts and reservoirs of infection are domestic and wild canines, however, other hosts are domestic cats, non-domesticated cats, ferrets, and the California sea lion.
The disease is caused by the mosquito-borne filarial parasite Dirofilaria immitis. Endemic areas such as Texas provide adequate temperature and humidity to support a viable mosquito population year-round. Mosquitoes are outdoors and indoors; therefore .indoor. animals are affected as well.
Dirofilaria immitis (heartworms) live out part of their life cycle in the mosquito, and part in the host mammal. The non-infective stage of heartworm larvae may circulate in the blood of host for more than two years following release by pregnant female in an infected animal. This animal becomes a source of infection for other dogs and cats via transmission by a mosquito. The larva (microfilaria) must be ingested by a mosquito before development to the next life stage can continue.
The mosquito bites an infected animal and carries the infective heartworm larvae (L1)
The infective heartworm matures in the mosquito in order to infect another host
Infective larvae (L3) migrate to proboscis (nose) of mosquito; waiting to infect another host
Infected larvae (called microfilaria) are deposited on the skin and enter the host through a mosquito bite wound
Once a host is bitten, the larvae molt and mature over the next two weeks in the subcutaneous tissues (L4)
The larvae mature in the heart and lungs over the next 50-70 days post mosquito bite (L5)
Heartworms reside in the heart, lungs, and associated vessels of the host by 70-110 days post infection
Adult heartworms mate and females release their offspring by 5 to 6.5 months post infection
Adult heartworms may survive up to 8 years in the dog, and 1 to 3 years in the cat
The adult heartworm is directly responsible for nearly all of signs of disease, although some disease is related to microfilaria in lungs and kidneys. Space occupying worms in arteries lead to destruction of arteries, emboli and thrombosis (acts like blood clots), and hypertension (high blood pressure). Pulmonary hypertension leads to enlargement of right ventricle of heart, interference with heart valves, and congestive heart failure. Typically, severe inflammation will occur in the lungs.
Clinical signs of heartworm disease vary with the stage of disease and severity of disease. Most dogs do not develop signs before a year post infection (due to parasite's long life cycle). Some dogs are asymptomatic, but typical symptoms include: nonproductive cough, labored breathing, exercise intolerance, syncope (fainting), weight loss or loss of muscle mass, and ascites (fluid accumulation in abdomen).
Heartworm disease is diagnosed fairly easily in dogs. The diagnostic test of choice is an antigen test which tests for the presence of mature female heartworms. An antigen test is a simple blood test that can be run in the hospital or sent to an outside lab. This test is the most specific and sensitive test for diagnosing heartworm disease in dogs. False positive antigen tests are extremely rare and usually result from technical error. False negatives can occur with immature infections, light infections (<5 worms), or unisex infections. Other tests include microfilaria tests (tests for baby heartworms), radiology, echo cardiology. However, microfilaria tests (direct blood smears, Difil, Modified Knott's test) may be used in tandem with antigen tests, but should not be used as sole method of testing due to the fact that 20 to 75% of heartworm infected dogs do not have circulating microfilaria.
Heartworm disease in dogs must be staged to determine the severity of the disease prior to treatment. Staging includes a physical exam, a complete blood count, a serum biochemical profile, and a urinalysis. Confirmation antigen testing is typically performed (two tests) and microfilaria tests may be done in an attempt to determine worm burden.
The course of treatment of heartworm disease depends on class or stage of the disease:
Class 1: sub clinical disease- often no clinical signs, lab tests and x-rays usually normal
Class 2: moderate disease- moderate exercise intolerance or occasional cough, moderate heart and pulmonary artery involvement
Class 3: severe disease- obvious clinical signs such as exercise intolerance, respiratory distress, cough, weight loss; severe heart and pulmonary artery involvement
Caval syndrome: very severe disease (often fatal)- severe weakness and collapse; worms completely obstruct tricuspid valve of heart and obstruct major blood vessel (the vena cava)
The goal of treatment is to kill all adult worms with an adulticide and kill all microfilaria with a microfilaricide. The ideal treatment should have minimum harmful effects from drugs, and a tolerable degree of complications created by the dying heartworms. Dogs undergoing heartworm treatment must have a thorough pretreatment workup prior to initiating treatment and be hospitalized during administration of the treatment drug. Melarsomine Dihydrochloride (Immiticide) is the only drug approved for use by the FDA for elimination of adult heartworms in dogs. Immiticide is given as a series of injections depending on the class of disease. Two injections are given 24 hours apart for Classes 1 or 2. One injection is given initially, then two injections given over 24 hours one to three months later for Classes 2 or 3. The best course of treatment will be determined by the veterinarian for each individual case. Then microfilaria are eliminated by administration of macrocyclic lactones. Macrocyclic lactones include Heartgard, Revolution, Sentinel, or Interceptor. Macrocyclic lactones should be started as soon as heartworm disease is diagnosed, however, caution should be used with Sentinel or Interceptor as these products kill microfilaria very quickly which can be harmful to the dog.
Prevention of heartworm disease is achieved by administration of a monthly preventative. It is extremely important all dogs be on a preventative in endemic areas such as Texas. Prevention includes Heartgard and Revolution dogs and cats, and Trifexis and for dogs. Ferrets can be given Heartgard or compounded ivermectin solutions. For complete protection, prevention must be given EVERY month year round in Texas.
Flea & Heartworm prevention are critically important for the health and comfort of your pets.
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CityVet Flea & Heartworm Control . Complete Summary
Feline Heartworm Disease
Heartworm disease is spread by mosquitoes. Cases of heartworm disease have been documented in all of the 48 contiguous states. Primary hosts and reservoirs of infection are domestic and wild canines, however, other hosts are domestic cats, non-domesticated cats, ferrets, and the California sea lion.
The disease is caused by the mosquito-borne filarial parasite Dirofilaria immitis. Endemic areas such as Texas provide adequate temperature and humidity to support a viable mosquito population year-round. Mosquitoes are outdoors and indoors; therefore .indoor. animals are affected as well.
Dirofilaria immitis (heartworms) live out part of their life cycle in the mosquito, and part in the host mammal. The non-infective stage of heartworm larvae may circulate in the blood of host for more than two years following release by pregnant female in an infected animal. This animal becomes a source of infection for other dogs and cats via transmission by a mosquito. The larva (microfilaria) must be ingested by a mosquito before development to the next life stage can continue.
The mosquito bites an infected animal and carries the infective heartworm larvae (L1)
The infective heartworm matures in the mosquito in order to infect another host
Infective larvae (L3) migrate to proboscis (nose) of mosquito; waiting to infect another host
Infected larvae (called microfilaria) are deposited on the skin and enter the host through a mosquito bite wound
Once a host is bitten, the larvae molt and mature over the next two weeks in the subcutaneous tissues (L4)
The larvae mature in the heart and lungs over the next 50-70 days post mosquito bite (L5)
Heartworms reside in the heart, lungs, and associated vessels of the host by 70-110 days post infection
Adult heartworms mate and females release their offspring by 5 to 6.5 months post infection
Adult heartworms may survive up to 8 years in the dog, and 1 to 3 years in the cat
There are some differences between dogs and cats with respect to heartworm disease. Cats are typically more resistant to infection than dogs; however, in any area a dog can get heartworms, cats can too. Most heartworm infections of cats involve small numbers of worms (usually less than 6 worms). Cats often have unisex heartworm infections, and circulating microfilaria are seldom found in cats. Aberrant migration of heartworms to places other than the lungs and heart often happen in cats, which cause a high mortality rate. Commonly, indoor cats can be just as easily infected with heartworm disease as outdoor cats in endemic areas.
Clinical signs of heartworm disease in cats include: Coughing, intermittent dyspnea (trouble breathing), sporadic vomiting, lethargy, weight loss, nonspecific neurologic signs, and most commonly SUDDEN DEATH.
Diagnosis is not as straight forward as in dogs because cats commonly have small worm burdens or unisex infections. The antigen tests used in dogs can be negative in cats, though the cat has heartworms. When testing for heartworm in cats, both antibody and antigen blood tests should be used to aid in diagnosis of disease. Antibody tests indicate exposure to heartworms, but do not necessarily indicate disease. When antigen tests are positive, they indicate the cat has active heartworm disease, but when negative don.t rule out disease.
Currently there is no approved treatment for heartworm disease in cats. Heartworm disease is more devastating in cats than in dogs and because of their relatively small body size, cats with only a few worms are still considered to be heavily infected in terms of parasite mass. Therefore, monthly prevention is extremely important since the disease has a higher mortality in cats and no treatment exists. Prevention of heartworm disease is achieved by administration of a monthly preventative. Types of prevention include Heartgard and Revolution . For complete protection, prevention must be given EVERY month year round in Texas.