Heartworm: How One Mosquito Bite Can Kill Your Dog


Heartworms are parasitic roundworms that are spread from dog to dog through the bites of mosquitoes. Heartworms in dogs can cause serious health complications and even death, if not treated properly.

The Life Cycle of Heartworm

Heartworm is a parasitic round worm (Dirofilaria immitis) which lives in the heart and lungs of dogs and occasionally in cats, ferrets and other mammals. Mosquitos transmit the parasite from dog to dog.

The parasite is a major problem in many regions of the North America and in other countries, especially in areas where mosquitoes are present year-round and areas with warm temperatures.

It takes 6 to 7 months after an infected mosquito bites a dog before the larvae develop into adult worms and are present in the heart and lungs where they cause severe disturbance to function.

The adult worms lodge in the heart, lungs, and surrounding blood vessels and begin reproducing. Adult worms can grow up to 12 inches in length, can live 5 to 7 years. A dog can have as many as 250 worms in his/her system!

Dogs may go into heart failure, and marked coughing and loss of health are common. Dogs with very light infestations may be free of symptoms.

The adult worms produce tiny “microfilariae” which enter the general blood circulation from where they can be acquired by another mosquito when it sucks blood. The life cycle requires an incubation period in the mosquito and is completed when the infected mosquito goes on to bite another dog (the next host).

Symptoms of Heartworm Infestations

Initially, there are no symptoms as it takes several months for the larvae to develop into adult worms. But as more and more worms grow and enter the heart and lungs, most dogs will develop a cough. The dogs will also tire easily and won’t be able to exercise as much as before.

General listlessness, labored breathing and weight loss will often evince as the disease progresses. With severe heartworm disease, veterinarians can hear abnormal lung sounds as the dog starts to retain fluids and dogs can pass out from the loss of blood to the brain.

Eventually, most dogs will die if the worms are not treated. Without regular testing, heartworm disease is usually only detected after the disease has progressed, in which case irreversible damage may have already been done to the internal organs.


Treatment of an established case of heartworm is involved, expensive and not always effective, but preventative medication is effective, safe, easy to administer and can be used in puppies from 6 weeks of age. Not treating heartworm in timely fashion will lead to heart and lung damage and eventually death.

A veterinarian treating heartworm will usually carry out an extensive pre-treatment diagnosis, which includes X-rays, blood work, and tests in order to determine how serious the infection is. Then the dog is given the injections. The drug used is called Immiticide, an arsenic-based product. The dog is given two or three injections that will kill the adult heartworms in the blood vessels of the heart.

In the past, plain arsenic was used, which had many side effects. The product used now is safer with fewer side effects. It is a safe product if used correctly, but please consult with your veterinarian about the latest possible treatment options and which is best for any dog about to be treated.

A dog must be kept quiet and rested for the several months during his/her treatment. This is because after an injection treatment the worms will start dying. As they die, they break up into pieces, which can cause a blockage of the pulmonary vessels and cause death. That is why dogs have to be kept quiet during the treatment and then for several months afterward. Studies have shown that most of the dogs who die after heartworm treatment do so because the owners let them exercise!


Prevention is by far the best course of action and is recommended by veterinarians and by the American Heartworm Society. Heartworm preventative is not costly (about $35 to $80 per year, depending on the dog’;s size).

Although all states in the US and many provinces in Canada have had cases of heartworm, some areas are more severely affected than others.

Before prescribing any preventative, consult with your veterinarian to determine if heartworm preventative is recommended for where you live. Also if you plan to travel with your dog to an area with heartworm, then having your dog on preventative would be a good idea.

Macrocyclic lactones are highly effective parasiticides used in preventing heartworm infections. Heartworm preventatives include monthly pills, monthly topicals that you put on the skin, and there is also a six-month injectable product.

Selemectin (brands Revolution by Pfizer or Stronghold) and Moxidectin (brand Advantage Multi by Bayer) heartworm medications are applied to the skin once a month. Ivermectin (brands Stromectol, Ivermec, Heartgard and Heartgard Plus by Merial, Iverhart Plus and Iverhart MAX by Virbac and Tri-Heart Plus by Schering-Plough) and Milbemycin oxime (brands Interceptor and Sentinel by Novartis) are given by mouth (orally) once a month. Moxidectin is also available as a six-month injectable product for dogs (ProHeart Sustained Release Injectable for Dogs, by Fort Dodge Animal Health).

These medications are effective at preventing development of the damaging adult worms after infection, so long as the medications are given within one month of exposure (some of these products are also effective at controlling fleas and other parasitic worms for one month).

In dogs that have been exposed to potentially infected mosquitos, a simple blood test is recommended prior to use of preventative medication in order to avoid potential side effects. Testing is repeated before the start of the next mosquito season (usually April or May) or at the recommendation of a veterinarian.

Heartworm Medication and Adverse Drug Reactions:

We recommend that you consult with your veterinarian prior to administering a heartworm preventative due to the potential for adverse drug reactions. For example, Ivermectin is a common heartworm preventative, however some dog breeds (such as the Rough Collie, the Smooth Collie, the Shetland Sheepdog and the Australian Shepherd) that have a high incidence of a certain mutation within the MDR1 gene (defects in the P-glycoprotein gene), are particularly sensitive to ivermectin. If this mutation is present, ivermectin can severely poison the dog. It should be noted that ivermectin use in heartworm preventative is usually in low enough dosage as to not affect sensitive dogs.


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