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Internal Parasites - Nematodes (Roundworms)

Terms in this set (104)

The adult hookworm lives in the small intestine of its host. It hangs on to the intestinal wall using its six sharp teeth and unlike other worms that just absorb the digested food through their skin as it passes by; the hookworm drinks its host's blood. The adult worm lives and mates within the host and ultimately, the female worm produces eggs. Hookworm eggs are released into the intestinal contents and passed into the world mixed in with the host's stool. The egg hatches in the environment and develops from a first stage larva (the hatchling) to a second stage larva and finally a third stage larva, which is ready to infect a new host. The larva can infect its new host in several ways. One way is to penetrate the host's skin directly through the feet or belly or whatever part of the skin is touching the ground. Another way for the larva to gain entry to the new host is to be present in soil that is licked and swallowed by the host as it cleans itself. Once the larvae are inside the host, they make their way to the intestine where some worms simply stay and mature into adulthood. Other individuals are bolder, tunnel out of the intestine, and migrate to the lung tissue. In the lung, the larvae develop into 4th stage larvae and when they are ready they break out of the lung, climb up the trachea, get coughed into the throat and swallowed. Once back in the intestine, these well-traveled worms will complete their maturation to adulthood. Not all the worms that begin this treacherous migration complete it. As they emerge from one tissue to move on to the next, some fall into a state of arrest where they go dormant and encyst. These larvae remain inactive but periodically some will emerge and complete their migration. The adult worms live by sucking blood from the intestine. The host passes the eggs into the environment where a new host picks them up. The developing larvae may migrate widely through the new host's body before settling down to complete their maturation.
In dogs, two treatment steps are involved. The first (1) is to kill the adult worms (adulticide therapy) using one of two very potent arcenic compounds administered intravenously or intramuscularly. If they are effective, there will be clotting of dead worms in the circulation, as well as fever, vomiting, loss of appetite, jaundice, cough, and expectoration of blood. The dog must not be allowed to exercise for two weeks following treatment because of the clotting. Must be hospitalized - especially if thromboembolic complications.
Pulmonary damage exacerbated after death of adult worms. Severe restriction of activity for 4-6 weeks after adulticide tx. Cage confinement for 3 weeks if severe. Give heart diet food. Possibly sx removal. Stabilize before adulticide. Don't give adulticide if icteric or liver failure. Recheck HWT 4 m. after tx. Old animals might not need tx if are more likely to die from something else or from tx itself. Don't treat pregnant dogs. But MUST check pups. The only product currently available for the treatment of adult heartworms is melarsomine dihydrochloride (Immiticide® by Merial).The patient receives an intramuscular injection deeply in the lower back muscles as shown above. This is a painful injection with a painful substance, and it is common for the patient to be quite sore afterwards at home. Pain medication may be needed. (2) Dogs require a second step to get rid of heartworm infection: filaricide therapy. Its purpose is to kill the microfilaria three to six weeks after the adulticide therapy. Two different drugs are available for filaricide therapy: ivermectin and milbemycin. Three weeks after filaricide treatment, a blood test is performed. If negative, the dog is ready to start a heartworm preventive program (step 3). This process is very hard on the dog and takes several months. The dog may possibly die from the first treatment, and if only given a preventative. Prognosis is good if dog survives treatment.
The cat is not a natural host for the heartworm, which means the migrating larval heartworm is not likely to complete its life cycle. The migrating worm uses molecular sign posts to tell it how to get to its host's pulmonary arteries. The worm is prepared to read CANINE directions and may get lost in the feline body, ending up who knows where. Most of the larvae that actually make it to the pulmonary artery die soon afterwards due to the massive immune attack from the feline body. Very few larval heartworms survive to adulthood in cats.
Cat owners must be aware that even a small number of Heartworms can cause sudden, serious, and even fatal disease in the cat. The main effect in cats is inflammation of the arteries and the surrounding tissues in the lungs. The amount of inflammation is much more exaggerated than what commonly occurs in dogs. The inflammation alone can result in partial to complete blocking of blood vessels. In cats, when worms die, complete blood vessel blockage often occurs. Depending upon where these blockages are located and the amount of inflammation, large portions of the lungs can be affected which results in a significant, often sudden breathing difficulty for the cat. The severity is also affected by the magnitude of the cat's immune system response. A single blocked blood vessel with a large inflammatory response can result in an emergency situation for the cat.
The cat's highly reactive immune response to heartworms may be the reason we see some of the following signs in Feline Heartworm Disease...It is rare to find circulating microfilaria in the cat. Infected cats usually do not have enough adult worms to produce microfilaria and there may be only a few adult worms and single sex infection is common. Fewer worms cause much more severe reactions. One worm can cause a severe medical emergency.
Heartworm disease is primarily a lung disease in cats, not a heart disease.