Several types of parasitic worms threaten your dog, but the only type transmitted through mosquito bites are heartworms (Dirofilaria immitis). Heartworms are deadly, blood-borne parasites that invade the circulatory system -- primarily the heart and large adjoining blood vessels. According to the American Heartworm Society, although heartworms can infect more than 30 animal species, dogs are considered their definitive host.
When female mosquitoes bite animals infected with heartworms, they ingest microfilariae -- the first larval stage of heartworms. Approximately 30 mosquito species serve as intermediate hosts for microfilariae. It takes 10 to 30 days for microfilariae to develop into infective larvae within the mosquito. When an infected mosquito bites your dog, the infective larvae are transmitted into his bloodstream. They migrate into your dog's heart and blood vessels. Within six to seven months they mature, mate and produce microfilariae.
Heartworms cause the potentially fatal disease dirofilariasis, commonly called heartworm disease. Mature heartworms clog your dog's heart and major blood vessels and interfere with proper valve function. In doing so, blood flow to bodily organs -- particularly the liver, kidneys and lungs -- is compromised, causing dysfunction and eventual organ failure.
Clinical signs of heartworm disease depend on the number, location and length of time worms have been present. Unfortunately, by the time symptoms appear severe infection has usually set in. Outwardly visible symptoms include dry cough, shortness of breath, exercise intolerance, weight loss, lethargy, nervousness, disorientation and fainting. Symptoms detectable only through a professional veterinary exam include liver enlargement, fluid accumulation in the legs and abdomen, anemia, and abnormal heart and lung sounds. Sudden death is not uncommon.
According to the American Heartworm Society, most heartworm infections are treatable. Dogs with advanced disease may not be able to withstand treatment, in which case your veterinarian may recommend palliative care. Dogs this severely advanced typically do not live more than a few months. If your dog is deemed strong enough for treatment, your vet will administer drug therapy by way of deep intramuscular injections into the lumbar muscles. Different drugs target adult heartworms and microfilariae. Injections are typically divided and administered 30 days apart. Your veterinarian will determine an injection schedule specific to your dog's needs.
Following the first injection, the adult heartworms begin to die and decompose. They are eventually reabsorbed by your dog's body, but this can take several weeks to months. Most post-injection complications are due to dead heartworm fragments clogging arteries and capillaries. This is an extremely delicate time for your dog, so it is absolutely necessary that he is confined to complete rest. There should be no physical exertion allowed until your veterinarian gives the all-clear.
Although veterinary medicine has made great strides in treating heartworm infection, it is unfortunate that any dogs have to undergo it at all, because it is completely preventable. Prevention is not only safer for your dog, but more economical as well. Monthly heartworm preventatives are readily available from your veterinarian, most of which also guard against other infective parasites. The American Heartworm Society recommends year-round heartworm prevention, even in seasonal areas. Consult your veterinarian for the best heartworm preventative for your dog.