Houston PetTalk April 2011 : Page 62
LIL’ BITS HEALTH Heartworm Disease I Worming TheirWay Into Your Dog’sHeart t’s that time of year whenmosquitoes appear with vengeance. These insects not only irritate the skin but also transmit deadly parasites into our canine companions. The parasite is known as the heartworm or Dirofilaria immitis which causes heartworm disease or dirofilariasis. BIOLOGY AND LIFE CYCLE OF THEWORM The domestic dog and some feral dogs are the normal definitive host for heartworms. The life cycle is relatively long -about 7-9months. The susceptiblemosquito becomes infected when taking a blood meal from a heartworm positive dog that has baby heartworms (called microfilaria) in the blood. Once in themosquito the microfilaria rapidlymature into an infective larval stage that is transmitted into a sec-ond dog when themosquito takes its blood meal. In the new host, larvae begin a 2-3 monthmaturation and migration process ending in the right side of the heart. Here themales and females grow up to 6-12 inches long and breed to produce babies, microfilaria, that lives in the blood stream. This entire process takes about 6-9months. Large accumulations of adult worms impair circulation of blood, which ulti-mately causes serious damage to the heart, lungs, liver and kidneys. Unfortunately, a considerable amount of damage can occur before any outward signs of disease are noticed. Although a dog can lead a fairly normal, healthy life with a small worm load in its heart, in advanced stages the disease causes symptoms such as labored breath-ing, cough, lethargy, reduced stamina, weight loss and fainting episodes. If not detected and treated the disease leads to congestive heart failure and death. DIAGNOSTIC SCREENING TESTS Screening tests that detect the worm antigen have proven to be clinically use-64 www.houstonpettalk.com ful. Specificity is consistently high and differences in sensitivity are low, so false negatives rarely occur. Another older but still reliable screening test is to examine fresh blood for the presence of microfilaria or cellmovement created bymotility of the microfilaria. Themodified Knott test using whole blood is the preferredmethod for observing wormmorphology. Although screeningmay be based on antigen test-ing, antigen-positive dogs should also be checked for microfilaria, because their pres-ence validates the antigen test, identifies the dog as a reservoir of infection and alerts the veterinarian to a potential severe reaction if administering a preventive to a dog with a high microfilaria load. Other than the screening tests, additional methods are useful for confirming the diag-nosis and staging the severity of heartworm disease includingX-rays, echocardiography, laboratory tests and urinalysis. HEARTWORN PREVENTION Infection is preventable, despite the high susceptibility of dogs. Good preventive op-tions include drugs administered orally or topically. Puppies should begin prevention at about eight weeks of age and continue monthly throughout their life. Allmature dogs should be tested prior to starting a prophylactic regime. It is important to evaluate the heartworm status before start-ing a preventive drug. Themost commonly used drugs fall into a category calledmac-rocyclic lactones (ivermectin—“Heartgard”, milbemycin oxime—“Interceptor”,moxidec-tin and selamectin—“Revolution” to name a few). These are available formonthly ad-ministration. Formaximum effectiveness, prevention should be given year-around. In order to ensure that the prevention is 100% effective annual testing is imperative. Also, a yearly check ensures that treatment in the positive cases can be started quickly, thus preventing further pathology caused by both the adults and microfilaria. Kingsland Blvd.Animal Clinic By Susan Randlett,D.V.M. UNDERSTANDING TREATMENT Treatment is usually straightforward and unremarkable in dogs with undetectable or mild symptoms. However, in dogs exhibit-ingmoderate to severe signs or that have concurrent disease the treatment can be very challenging. The goals of the treat-ment are two-fold, improve the condition of the pet and eliminate all the life stages of the heartworm (microfilaria, adults and larvae) with minimal complications. To achieve this, the veterinarian must carefully evaluate each positive case to categorize the stage of disease. Depending on the stage the doctor can tell how to or whether to treat at all. Undeniably, prevention is the best option because the treatment can be expensive to the client and demanding on the patient.However, dogs still get the disease. Fortunately, the life cycle of the parasite is well understood somedication is available that is both safe and effective to kill all the life stages found in the dog. The canine heartworm prefers the dog as the primary host. In a few instances the parasite has been found in other animals such as foxes, skunks, horses and cats. These cases are considered biological oddi-ties. In the aberrant hosts the disease is very difficult to detect and treat. Fortunate-ly for the canine there are very effective drugs to prevent and treat the disease.