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  • Dog Heartworm


    Florida is a year round paradise for the filarial nematode, Dirofilaria immitis, commonly called the “dog heartworm.” Florida has 16 species of mosquitoes that can act as hosts for heartworms. Unfortunately, Lee County has at least 8 of them. These mosquitoes are found in fresh and salt water habitats throughout Lee County and unlike many northern areas, they are found all year long. If you have provided heartworm protection for your pet only during the summer time, you may be putting that pet at significant risk. D. immitis is not just a problem for dogs. Please ask your vet about dog heartworm for your dog or cat. For more information click on http://www.heartwormsociety.org/.

    Although very rare, pulmonary infections with D. immitis in humans have been reported. According to J. K. Nayar and C. Roxanne Rutledge of the University of Florida, “The parasite is usually found in the lung (pulmonary dirofilariasis) and less often in the heart. Although the worms form “coin lesions” in the lung, which may be confused with other diseases on x-rays, such as carcinoma, its clinical significance in man has not been fully determined. During the last 40 years about 100 cases of human pulmonary dirofilariasis have been reported in Florida.” (http://edis.ifas.ufl.edu/mg100). The disease is detected and diagnosed in males twice as often as females and the usual age range is 40 to 60 years old. Clinical diagnosis is essentiallly impossible, but diagnosis can be definitively made by histologic examination for the presence of the worm. This information is provided through the U.S. National Library of Medicine, National Institutes of Health. (Watson J, Wetzel WJ, Burkhalter J. 1991. Human disease caused by dog heartworm. Journal of Mississippi State Medical Association. Nov; 32(11):399-401. For further information click on http://www.ncbi.nlm.nih.gov/pubmed/1784039.

    The following information is from the American Heartworm Society.

    Life cycle of heartworms (Dirofilaria immitis):

    • Microfilariae (mff) are consumed by a female mosquito when she takes a blood meal from an infected dog. The mosquito requires blood before it can lay eggs.
    • These larvae molt 2 times in the mosquito and becomes the infective stage (L3).
    • When this mosquito bites another victim, it injects a small amount of saliva. The L3 larvae are passed in this saliva.
    • The larvae molt again (L4) in the host tissues before the larvae migrate to the heart (L5).
    • These adult heartworms begin producing larvae (microfilariae) by 6.5 months after the original infection.
    • Disease caused by heartworms:
    • Adults can live 8 years in the canine host, and symptoms rarely develop before at least 1 year after infection.
    • The first evidence of infection may be a “poor doer” (dull coat, weight loss, etc).
    • Cardiac and respiratory signs are usually evidence of advanced disease.
    • A deep chest cough may develop, along with exertional fatigue.
    • Significant disease occurs in the heart, lungs, liver and kidneys.
    • Pulmonary hypertension varies with severity of infection and number of pulmonary vessels affected and level of physical activity.
    • Kidneys affected by mff and immune complexes.
    • Advanced symptoms may include cough, dyspnea, ascites, jugular pulses, collapse and the vena caval syndrome.
    • Acute death may be the first sign the owner reports.
    • Testing procedures for detecting heartworm infection:
    • Eosinophilia, increased ALT, Creatinine. Bilirubinuria.
    • Blood smear – screening test for the presence of larvae – < 50% accuracy
    • Knott’s test or filter tests – larvae concentrating tests – also <60% accurate
    • Occult test – usually detect adult female heartworm antigen – >90% accurate. Antigen may persist for up to 6 months after treatment for the adults. 20 – 40% of positive dogs have no mff. If < 5 adult worms = false negative test. The strength of the test result is proportional to the number of adult worms, but not the severity of the disease.
    • Well testing – low cost testing procedure for multiple pets at one time – human error is a concern.
    • Individual tests (Idexx Snap, etc) – virtually eliminates human error. Expensive testing procedure.
    • Some dogs with mff do not have enough antigen to show positive on serologic tests.
    • Occult antibody test – tests for the patient’s response (antibody) to the disease – antibody may persist well after the infection has been eliminated.
    • Treatment for heartworm disease:
    • The patient must be thoroughly evaluated prior to treatment. A complete PE, and Profile/CBC are minimal requirements. In addition, radiographs are strongly recommended to evaluate the significance of the changes in the heart and lungs. An ill patient must be supported and brought to an optimal level before treatment is considered.
    • Caparsolate – arsenic containing chemotherapy that kills the adult worm. Statistics indicate that treatment will eliminate >80% of the adults. The patient may be pre-medicated with Heartgard for 2-3 months prior to treatment to eliminate some of the tissue stages of the heartworms. Aspirin (5gr/60#) is also given throughout the process to avoid blood clots. Treatment of the adult worms consists of a series of 4 IV injections given over 2 days. The patient is closely evaluated prior to each injection to ensure no adverse reactions are occurring.
    • Immiticide – melarsamine chemotherapy that treats for all stages of the heartworm. Statistically eliminates >98% of the adults. Since all stages are affected, pre-med with Heartgard is not required but should be considered for 1-6 months prior to Immiticide. . Two epaxial muscle injections are given 24 hours apart. Discomfort and swelling can occur post injection, and will be more severe if the patient moves during the injection. Modified treatment should be considered in advanced disease or concern over patient’s general health (1 injection, followed by 2 injections one month later).
    • Treatment with either drug must be discontinued if icterus, vomiting, depression or fever occur.
    • Can usually retreat 4-6 weeks later with no complications.
    • Regardless of the selected treatment, the protocol may be modified if the patient is at a greater risk (disease stage, concurrent illness, age, etc).
    • Prevention of heartworm disease: DEC (diethylcarbamazine) – daily preventative. Dangerous if mff present!
    • Filaribits. – contains DEC
    • Filaribits Plus – Prevents adult hook- roundworms from developing
    • Heartgard (ivermectin) – monthly preventative
    • Heartgard Plus – (ivermectin + Strongid) – also treats for adult hook- and round worms
    • Interceptor – (milbemycin) – monthly heartworm preventive, plus hook-, round- and whipworms. Can cause reactions in dogs with high numbers of mff.
    • Sentinel – Interceptor + Program (for flea control)
    • Ivomec – may produce dose-dependent side effects.
    • Minor signs within 10-12 hrs – salivation, vomiting, ataxia, stupor and disorientation
    • Severe signs within 4-6 hrs – tremors, seizures, marked ataxia, coma and death
    • All heartworm preventatives are prescription items and cannot be dispensed as over the counter medications. The label REQUIRES a current test within 12 months!
    • For more information on dog heartworm click on http://www.heartwormsociety.org.
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