Heartworm Info

Many of the dogs we rescue and would like to rescue turn out to be heartworm positive. It’s not surprising, but because the treatment for heartworms is expensive and time-consuming, this usually means a death sentence for the shelter dog. We have been able to rescue and treat many heartworm positive dogs in the last 5 years, but we would rescue and treat many more if we had the financial capacity to do so. We invite you to read the story of Mark, a beautiful little Sheltie whose condition was too far progressed to be successfully treated. In his memory, we have set up Mark’s Gift to aid other shelter dogs who would not have a chance otherwise. We also would like to see people better educated about heartworms and how to prevent them, so we have put together some information below.

Heartworm Lifecycle

What are heartworms?

Adult heartworms live in the right side of the heart which can literally fill up with hundreds of worms. These adult heartworms, which resemble spaghetti, can grow to be 14 inches long and live for 5 years. Dogs infected with heartworms may have decreased appetite, weight loss, listlessness and lack of endurance. Often the first sign of heartworm infection is a cough. If the flow of blood to the heart is blocked or slowed by the presence of heartworms, the heart and the liver can be damaged. If the worms die, they may travel through the bloodstream to smaller blood vessels in the lungs and completely block the flow of blood causing an embolism. In some instances, heartworms can cause heart failure and death.

How does an animal get heartworms?

To contract heartworms, an animal must be bit by a mosquito which has previously bitten a heartworm-infected dog. Adult heartworms living in the animal’s heart lay tiny larvae called microfilariae. These microfilariae live in the bloodstream of the animal and when a mosquito bites and sucks blood from the animal, it ingests the microfilariae that is in the blood. Once inside the mosquito (and only then), the microfilariae begin to grow and in about 2-3 weeks, they migrate to the mosquito’s mouth. The next time the mosquito bites an animal, the microfilariae are deposited into the animal’s skin, where they stay for 3 months and grow to a length of 3 inches. The larvae then migrate to the heart where they grow into adults and begin to lay their own microfilariae. The entire process from mosquito bite to adult heartworms takes 6 months.


How can I prevent heartworms?

There are several types of heartworm preventatives available from your vet. The preventatives work by killing the microfilariae deposited in the blood by infected mosquitos which prevents them from ever reaching the heart and becoming adult heartworms. They do not kill adult heartworms already residing in the heart. The monthly preventatives kill any microfiliariae that were deposited during the preceding month. There are also daily preventatives which kill any microfiliariae deposited that same day. And now there is a new semi-annual preventative that kills microfiliariae for six months. It is important that your pet be tested for heartworms prior to beginning a heartworm preventative.

What if my dog already has heartworms?

The treatment for killing adult heartworms already present in the heart is expensive and time-consuming. Overnight stays at the vet’s office are usually required, as well as blood work and x-rays. The medications used to kill heartworms are called adulticides and they contain arsenic. The first drug developed to kill heartworms was called Caparasolate®. Dogs treated with this drug usually became very ill, lost their appetite, lost their fur, and had to be hospitalized for several days. It was given by IV into the vein and if any got on the animal’s skin, it killed the tissue. Some animals simply could not withstand the therapy and it had to be stopped or death occurred. Fortunately, a new drug has been developed called Immiticide® which, while it also contains arsenic, is safer, more effective, and has much fewer side effects. Two injections are given deep in the back muscles 24 hours apart. The animal must be kept quiet and crated for 2-4 weeks after treatment to prevent a large mass of dead worms breaking free from the heart at once and blocking a blood vessel and causing an embolism. If the animal has a severe case of heartworms, Immiticide® is usually given in three doses by administering one injection, waiting 30 days, and then giving two more injections 24 hours apart. This means your pet must be kept confined and quiet for a period of up to two months.

As you can see, it is a much simpler and safer process to keep your dog on heartworm preventative. In Texas, we give the preventatives year-round. In other parts of the U.S., it may be given seasonally since mosquitos will not be present during the colder months. Even inside-only dogs are suspectible to heartworm because mosquitos can and do get inside the house, or they could bite the dog during outside potty breaks. Puppies can be started on heartworm preventative between the ages of 6 – 8 weeks depending on the type of preventative given and the weight of the puppy. Some types of monthly preventatives are Sentinel, Heartguard, Interceptor and Revolution. Some preventatives also contain medications that control other types of worms such as hookworms or roundworms, and even fleas. The tablets are tasty and most dogs think they are treats.

You can get a heartworm test and heartworm preventatives for a low cost at Animal Trustees of Austin. Call (512) 450-0111 or visit the website for more information.

Note that most veterinarians (including those at Animal Trustees of Austin) will not prescribe heartworm preventatives unless the animal has been tested for heartworms within the last year and is heartworm negative.