Heartworm season officially begins June 1st in Ontario
With the highly anticipated warm weather, residents of Oakville are bracing for the arrival of a pesky insect that will buzz around our ears this summer – mosquitos! As you may already know, Heartworm is spread to dogs and cats by mosquito bites. It can be prevented in this area by using a once monthly medication for 6 consecutive months between June and November. Heartworm disease is exactly what it sounds like – a worm that lives in the heart or large blood vessels. The image above, provided by the American Heartworm Society, depicts a heart containing several adult heartworms.
Here are some key points regarding heartworm disease and its prevention:
– Heartworm can be a silent disease – infected pets may show no signs of infection
– Heartworm can damage the heart, lungs, and blood vessels and lead to abnormal blood clotting
– Treatment for heartworm disease is available for dogs only and involves multiple injections of a very expensive medication, which requires hospitalization and close monitoring for adverse reactions
– Fortunately, Heartworm disease can be easily prevented by using a monthly prevention medication
– A heartworm blood test should be performed prior to using the prevention medication each year
– Heartworm preventatives are very effective, but no product is perfect and in rare cases dogs prescribed the preventative still become infected with heartworm
For more detailed information about Heartworm disease, and the importance of prevention and testing, see the frequently asked questions below. You can always call us at the clinic at 905-257-7777 if you have any questions or would like to schedule your pet’s heartworm test!
How common is Heartworm infection in Ontario?
The most recent study on this subject was completed in 2010. That study noted 431 cases in Ontario in 2010, an increase of 60% from 2002. We have successfully reduced the spread of this disease in Canada by using preventative medications and this number would be much higher if heartworm preventative medications weren’t commonly used.
How is Heartworm spread?
An animal infected with mature male and female heartworms will circulate baby heartworms (called microfilaria) in their blood stream. When a mosquito feeds on an infected animal, it takes in a live microfilaria. The microfilaria matures to an intermediate larval form within the mosquito. When the mosquito then bites another animal it deposits the larvae on the skin. The larvae enters the body through the hole left in the skin by the mosquito bite. Over a period of about 6-7 months, the larvae matures further and eventually reaches the blood stream and remains in the heart and major arteries. It is not until the larvae have fully matured to an adult form that the presence of heartworms is detectable by the heartworm test, which means it takes at least 6 months after infection for a dog to test positive for heartworm.
There is a very informative video of the heartworm lifecycle on the American Heartworm Society Website.
How is Heartworm disease different in cats?
Cats tend to be more naturally resistant to heartworm larvae. This means that the larvae only occasionally (about 20% of the time) mature to adult worms. Instead, the juvenile worms die upon entering the blood stream. This doesn’t mean that cats don’t become sick from heartworm disease. In fact, cats develop a condition called HARD or Heartworm Associated Respiratory Disease. This respiratory condition occurs because the dead juvenile heartworms in the blood stream circulate through the body and become lodged in the tiny blood vessels in the lungs. This leads to inflammation in the lungs that can cause cough, wheezing, and respiratory distress. In some cases large blood clots form, which can be fatal. There is no cure for this condition, but certain medications can alleviate the clinical signs.
Because adult heartworms are rarely found in cats that have been infected with heartworm larvae, the heartworm blood test is not as accurate in cats as it is in dogs. However there is a blood test for cats that determines if the cat has formed an immune response against heartworm larvae. This test is very helpful particularly for cats that are having respiratory problems.
The image below depicts the differences in the lifecycle for heartworm disease in dogs and cats:
Why is a heartworm test needed?
Heartworm infected dogs often show no signs of infection, meaning the blood test is the only way veterinarians can determine if a dog is harboring the parasite. However, while the worms are present in the heart and blood vessels they can cause permanent damage to the heart and can trigger the development of dangerous blood clots within the circulatory system.
Heartworm infected dogs may have large quantities of circulating microfilaria. While heartworm preventative medications generally do not kill adult heartworms, they may cause rapid death of the microfilaria. The dead microfilaria can cause severe reactions in dogs. Some heartworm preventatives are thought to be safe even in heartworm infected dogs, but we can never know for certain when this harmful reaction might occur.
Even dogs that were on heartworm preventative medication in the previous season should be tested because no product is 100% effective. There may be problems with absorption, topical medications may be rubbed off by the pet without the owner’s knowledge, and oral medications may be regurgitated. It is also not uncommon for a dose to be accidentally missed or given late. In addition, there have been rare reports of resistance in immature larvae to heartworm medications, meaning the larvae have adapted to survive despite exposure to the preventative medication.
Finally, many manufacturers of heartworm preventatives guarantee their product. This means if your pet becomes infected with heartworm while using the medication, the manufacturer will cover the costs associated with treatment of the illness. Dogs require a negative heartworm test before using the medication in order to be eligible to take advantage of this guarantee.
How do heartworm preventative medications work?
Heartworm preventatives work by killing the immature larvae before it has a change to mature to the more resistant adult. There is at least a 4 week period after an animal is infected with the immature larvae when it can be easily killed with a preventative medication. Therefore, administering a monthly medication throughout the mosquito season will kill any heartworm larvae that entered the body of an animal in the previous month, before it has a chance to mature to an adult heartworm.
If prevention is not applied in time, the larvae may no longer be susceptible to the preventative medication. This is why it is important to use the preventative medication each month.
Are heartworm preventative medications effective against any other parasites?
Yes. Depending on the product chosen, the medication may also prevent and/or treat other internal parasites like roundworms, hookworms, lungworms, and whipworms, and external parasites like fleas, lice, mites, and ticks. Your veterinarian can help you decide which heartworm preventative medication is best for your pet.
You may also want to refer to the chart below for more information on once monthly heartworm preventatives.
Heartgard Plus (dogs only)
Sentinel (dogs only)
Trifexis (dogs only)
K9 Advantix* (dogs only)
|Flea Eggs and Larvae|
|Deer Tick **(Ixodes)|
|Dog Tick (Dermacentor)||(dogs only)|
|Sarcoptic Mange (Scabies)||(dogs only)||(dogs only)|
|Demodex Mite||(dogs only)|
* – K9 Advantix is NOT a heartworm preventative. It should be used in combination with one of the other heartworm preventative medications (eg Advantage Multi). ** – The deer tick is the carrier of lyme disease