Heartworm is a parasite that lives in the heart and blood vessels. The larvae of this parasite is transmitted by mosquito bites which enter the bloodstream of the dog. Eventually the larvae develop into new worms that find their way into the heart. There are several signs of an infected dog: Coughing, lethargy, loss of appetite, poor breathing, listlessness and fainting. Annual blood tests should be done to test if your dog is positive for heartworm, especially in areas were mosquito infestation is common. There is a vaccine against heartworm but before administering the vaccine the test must be conducted, if the dog turns out to be positive for heartworm vaccinating your dog can inturn be fatal. The worms infested in the heart and bloodstream will die and clog the flow of the blood, leading to death. Vaccination should only be given if the dog is negative for heartworm. Another way to prevent heartworm in your dog is by giving monthly preventative medication that come in a pill or chewable form, it is much more safer but costly. Vaccinating your dog is done annually only but it is rare in Japan as opposed to the US and the UK to find an animal clinic that uses the vaccine due to the dangers it may present.