Heartworm is a parasitic worm that may make a home and grow in your dog’s heart. Left untreated, heartworm is fatal.

It can take a year or more after infection for your dog to have any outward symptoms of heartworm. These symptoms will include coughing, weight loss and lethargy.

Heartworm is transmitted by mosquitoes. When an infected mosquito bites your dog, the larval form of the worm enters your dog. The larvae will live under the skin for about 3 months before entering your dog’s blood vessels. Once in the blood vessels, the larvae will travel to the right side of your dog’s heart. The larvae will remain in the right side of your dog’s heart for about five months until they reach maturity. When growth is complete, females will begin reproducing. The newly born worms are called microfilariae. The microfilariae do not grow, but live and move throughout your dog’s bloodstream for about three years. The microfilariae, however, cannot become infective to your dog without first developing in a mosquito. Thus, if a mosquito bites your heartworm-infected dog, it will consume some of the microfilariae, allowing the microfilariae to live and grow within the mosquito, so that the next time the mosquito bites any dog, it will transmit heartworm.

Heartworm can be detected with a blood test. Your vet may also be able to see the actual worms in your dog by taking X-rays or an ultrasound.

If caught early enough, heartworm may be treated with medications such as Immiticide. In some cases, your vet may surgically remove the worms.

There are many oral and topical medications that prevent heartworm from developing in your dog. Prior to being put on one of these medications, your dog will have to be tested to make sure he does not already have heartworm. If your dog does have heartworm and you give him heartworm preventative medication you can cause your dog to have an anaphylactic reaction triggered by the massive destruction of worms that the preventative will initiate. 

Many heartworm preventatives are designed to be administered monthly to your dog. And, unfortunately, dog owners sometimes forget and skip a month. What to do?  Don't fret. You can safely restart your heartworm preventative medication without heartworm testing if you have not skipped administering the preventative for longer than 4 months. After 4 months, enough worms may have built up so that the medication's destruction of the worms can potentially trigger the anaphylactic reaction.

Some dog owners in northern climates only give their dogs heartworm preventative medication from April to October figuring that these are the only months in which mosquitoes are present. However, as many parts of the world are experiencing milder winters, before you decide to give your dog heartworm preventative for only a portion of the year, you must really ensure that the weather is such by you that no mosquitoes can possibly be in your area. Also, since many heartworm preventatives also protect your dog against some gastrointestinal worms, keeping your dog on a heartworm preventative all year can help stave off these worms. And, of course, if your dog is on heartworm year-round your dog will not have to have a heartworm test each year. 

No, you cannot get heartworm from your dog.