Heartworm has been diagnosed in dogs in all parts of the world and is actually very common. This may be due to the fact that heartworm has a virtual 100% prevalence rate in unprotected dogs living in highly endemic areas. Heartworm, also known as Dirofilaria immitis, is transmitted by mosquitoes. The mosquito injects a microscopic larvae which grows into an adult worm six to eighteen inches long inside the heart of the affected dog. The worms can cause mild symptoms, such as coughing, but with time, more severe symptoms such as congestive heart failure, weight loss, fluid build up in the abdomen, fainting spells, anemia, collapse, and death usually occur. Luckily we have several excellent medications which can prevent heartworm if given as directed. There are oral medications which need to be given monthly, and which also help protect against some intestinal parasites. There is one topical medication which is also applied monthly. An injectable medication, ProHeart, which is administered every six months, is back on the market after being withdrawn for several years. Even if a dog has been given preventatives, it is still important to have annual checkups for heartworms by doing a blood test. Many people are not totally compliant about giving the preventive medication on time, and no medication works perfectly. If a dog has heartworms and it is given a dose of preventative, there can be a reaction that is detrimental to the dog, even deadly.Heartworms were once thought to be rare in cats. Now we know the incidence is anywhere from 10% to 50% of the canine rate. Heartworm disease in cats is different than in dogs. Cats usually test negative on the routine blood test done in the hospital, the worms are smaller and usually do not produce microfilaria which are like baby heartworms that circulate in the bloodstream. Veterinarians have to do different tests, sometimes more than one, to diagnose heartworms in cats.The symptoms in cats are different also. Cats usually have asthma signs or cough, even vomit. Cats can die acutely. The treatment for adult heartworms in dogs is expensive and potentially harmful to the dog. This is why it is much better to just prevent them in the first place. There is not a treatment for adult heartworms in cats. Many veterinarians are now recommending monthly heartworms preventative for cats in addition to dogs, since heartworm can be such a serious problem.