Heartworms

• Heartworms transmitted by mosquitoes pose a common and serious threat to dogs. The heartworm's life cycle requires both a mammal and a mosquito host, but it's the mosquito's bite that spreads the parasite. Once in a dog, the heartworm larva enter the bloodstream and, as they grow, start clogging the heart and major blood vessels leaving the heart. The lessened blood flow affects other organs, particularly the lungs. An infected dog may cough, tire out easily and look sickly. Treating heartworm disease is expensive and potentially lethal if dead or dying heartworms block vital blood vessels. Fortunately, a number of preventive medications for dogs are on the market that kill the heartworm larva, but pet owners must be vigilant in regularly dosing their dogs in order to keep them healthy.
  
  

Allergies

• Mosquito bites are a common cause of allergic reactions in dogs. Though a mosquito bite normally causes only localized skin irritation, in severe cases mosquito bites can cause diarrhea, vomiting, lethargy and other systemic health problems. Dogs that compulsively chew or scratch their skin may have allergies.
  
  

West Nile Virus

• The West Nile virus that so sickens people appears to have only mild effects on dogs, according to the Centers for Disease Control and Prevention. The CDC reports only a small number of dogs being infected with the mosquito-borne disease as of 2010. Dogs that contract West Nile from a mosquito bite may become lethargic or develop a fever but generally don't become seriously ill. A trip to the vet to treat the symptoms is in order, however.
  
  

Infections

• It's difficult to stop dogs from scratching an itchy mosquito bite. When dogs compulsively scratch, they repeatedly irritate the bite site, which can lead to an infection. These infections generally remain localized, but severe infections can spread to the blood and organs if left untreated. Give your itchy dog an oatmeal bath several times a week to ease the irritation and reduce the chance of an infection. Supplement your dog's diet with Vitamin E, which can help irritated skin. If necessary, place an Elizabethan collar around your dog's neck to prevent it from chewing at the infected bite.
  
  

Preventing Mosquito Bites

• Prevention is the best way to keep your dog safe from mosquitoes. Eliminate any standing water in your yard. Grow citronella plants in your yard to keep mosquitoes at bay. Avoid taking your dog to mosquito-infested areas such as swamps or lake shores during summer when the insects are at their peak. Spray your dog with insect repellents formulated especially for pets; never use a repellent designed for humans -- especially one containing DEET, which is toxic. Be sure to do a patch test with the repellent on your dog's skin to make sure it doesn't cause an allergic reaction.