Los Angeles County West Vector & Vector-Borne Disease Control District

Dog Heartworm

    Canine heartworm disease is a clinical condition in dogs caused by a roundworm, which resides within the dog's heart and lungs.  This disease is potentially fatal, and is transmitted by the Western Treehole Mosquito (Aedes sierrensis).

heartwormcycle.gif (6923 bytes)Mosquitoes become infected by ingesting very small infective stages of a microfilaria (worm).  In the mosquito, the worm molts twice and moves to the stylets of the proboscis where they remain until the mosquito feeds.  During blood feeding, the worms move out of the stylets and into the wound.  In dogs and other canines, the worms molt to the adult stage and migrate to the large blood vessels and heart where mating and reproduction occur.  Disease in dogs occur when the number of worms becomes sufficiently large enough to impede blood flow and heart function.  In humans, immature worms frequently become encapsulated in the lungs where they are detectable by chest x-ray.Heartworm disease in cats is less frequent than in dogs. Cats are susceptible but appear to be poorer hosts than dogs. The most prominent clinical signs include coughing, dyspnea, vomiting, lethargy and anorexia. Acute collapse and death can occur. Because less than 20% of infected cats have microfilariae in the blood, diagnosis is best confirmed by either X-rays or new serological methods.

    Development of heartworm in the vector starts when microfilariae are ingested by the female mosquito during blood feeding on an infected dog. Microfilariae leave the midgut of the mosquito soon after ingestion and migrate into the Malpighian tubules cells (the mosquito kidney). At a temperature of about 27 C. the parasite becomes immobile, shortens and thickens, and develop into the so-called "sausage form" larva in about 4-5 days. This larval form is followed by the first stage larva and the first molt occurs in the Malpighian tubule cells at 8 days. During the second larval stage, the internal organs of the worms are formed. The second molt occurs at 11- 12 days, resulting in third stage larvae which resemble miniature adults. During the next 2-3 days, they increase in length, break out of the Malpighian tubules, migrate through the body to the head and accumulate in the mouthparts. These third stage larvae are now called infective larvae. Thus, in 2-3 weeks, microfilariae transform into infective larva. Further development can only take place when mosquitoes feed on a dog.

    Infective larvae are concentrated in the proboscis, or mouth parts of the mosquito. As the infected mosquito feeds on a dog, the infective larvae emerge from the tip of the proboscis and on to the skin of the animal. A drop of mosquito blood protects the larvae from drying prior to their entry into the host. The infective larvae penetrate the skin through the puncture wound that remains after the mosquito withdraws her mouthparts.

    After penetrating the skin, the larvae stay close to the entry site and grow very little during the next few days. The molt from third- to fourth-stage larvae occurs 6-10 days after infection. Fourth-stage larvae migrate through subcutaneous tissue and muscle toward the upper abdomen and thoracic cavity. Fourth-stage larvae grow to about 1/10" in length during the next 40-60 days and then molt to the fifth and final larval stage, or young adults. The young adults penetrate veins to get into the blood stream and eventually, after 70-90 days in the dog, reach the heart. For unknown reasons, the percentage of infective third-stage larvae that reach maturity vary in different breeds of dogs.

    Upon reaching the heart, the young adults continue to grow. Up to now there has been no evidence of disease in the dog. It is only after adult worms mate and start to discharge tiny motile microfilariae that circulate in the blood that disease becomes apparent. Microfilariae appear in the blood about 200 days after infection.

    The symptoms of the disease are not noticeable in most cases, until reduced blood flow caused by adult worms damages the heart, lungs, liver, and kidneys.  Advanced symptoms of heartworm include: rapid tiring, shortness of breath, chronic soft dry cough, listlessness, and weight loss.  Drugs are available to prevent the disease, and it is curable if diagnosed early.

Distribution of Heartworm Disease

    Heartworm disease occurs worldwide in most tropical and subtropical regions, with increasing frequency in temperate climates. Until the late sixties, the disease was restricted to southern and eastern coastal regions of the United States. Now, however, cases have been reported in all 50 states and in several provinces of Canada. For most of North America, the danger of infection is greatest during the summer when temperatures are favorable for mosquito breeding. In the southern U. S., especially the Gulf Coast and Florida, where mosquitoes are present year-round, the threat of heartworm disease is constant.

    The primary vector of dog heartworm in Southern California, Aedes sierrensis, is found locally throughout Los Angeles County.  Dogs travelling with their owners to these endemic areas may acquire heartworm and bring it back to Los Angeles County where the vector mosquito occurs.  Under ideal conditions, small outbreaks of dog heartworm could be observed.

Click on picture below for distribution map.

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    A veterinarian can usually detect heartworm infection by finding microfilariae in a blood sample drawn from the dog by either 1) looking for microfilariae using a microscope or 2) using several new serological diagnostic tests which can detect of heartworm antibodies even when microfilariae can not be seen under a microscope. Early detection is important to enable prompt treatment to destroy the adult heartworms. Another reliable method is to take X-rays. When heartworm disease is confirmed, a treatment program is set up to remove both adult worms and microfilariae.

Treatment, Prevention And Control

    Heartworm disease in dogs and cats is a serious and potentially fatal disease. Always consult your veterinarian for treatment and prevention of the heartworm disease in dogs and cats.

    The adulticide treatment is expensive and involves some risk to the animals. Much of the damage caused by heartworms occurs before there are any outward signs of the disease. Heartworm disease in dogs and cats cannot be eliminated but it can be controlled or prevented. Therefore, prevention is the key element in protecting a dog. A dog not on preventive medication can be infected with numerous worms.

Kill the adult worms: The first step in ridding a dog of the parasites is to administer a chemical to kill the adult worms. Capasolate (Arsenamide, Thiacetarsamide), and Immiticide R (Melarsomine, dihyrdochloride) are arsenical compounds used to kill adult heartworms in both dogs and cats. These compounds are given as an intravenous injection and one or two doses are given each day for two days followed by restriction of physical activity for one to two months. As the worms die they are carried by the bloodstream to the lungs. One dog in twenty may be expected to die as a result of complications from this therapy. There are fewer complications with cats. Adult female worms and immature forms are somewhat resistant to Capasolate and, consequently, this drug may be less than 100 percent effective.
Kill the microfilariae: After the adult worms have been eliminated, the next step is to kill the microfilariae. Dithiazanine iodide is the recommended microfilaricide. The compound is given orally; the dosage varies with the weight of the dog. After 7 days on this program, the dog's blood should be re-examined. If microfilariae are still present, and there has been no drug reaction, the dosage may be continued until the blood is microfilariae free.
Prevent larval development: Only after the adult heartworms and the microfilariae have been eliminated may an infected dog be put on a program to prevent reinfection. Filaribits (Diethylcarbamazine--DEC, Caricide, Hetrazan) and Filaribits Plus kill the infective larval stage, but is less effective against adult heartworms. It should be administered daily throughout the mosquito season when dogs are exposed to infective larvae. It is a safe and efficacious drug in noninfected dogs, but may cause a fatal shock reaction if given to dogs with microfilariae. Sudden destruction of large numbers of microfilariae occasionally causes severe shock-like symptoms that may kill the dog. Cypip can be mixed with dog meal for continuous administration of DEC in feed.

    Heartgard-30 (ivermectin) and Interceptor (Milbemycin oxime) Flavor Tabs are indicated for usein the prevention of heartworm disease, controls adult hookworms, and removes adult roundworms and whipworms only if administered on a monthly dosing schedule. They eliminate infective larvae before they reach the heart. They also kill microfilariae. In certain dogs, particularly of the Collie breed, Heartgard has been reported to cause serious adverse reaction. The major advantage is that it need be given only once every 30 days. The disadvantages are that the dog owner may forget to give the medication and that there are risks of a mild reaction and transient diarrhea if given to a heartworm infected dog.

Control mosquitoes: In addition to drug therapy for dogs and cats, mosquito control in residential areas where dogs and cats live can break the transmission cycle of heartworm disease. Dog owners should keep their animals out of mosquito infested areas. Dog's living quarters should be mosquito-free. Indoor dogs usually show much lower incidence of infection. Insect repellents can be used on dogs and are safe and effective when applied properly.




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