Summer 2007 Newsletter
Canaan Club of Ontario
Health Report

The Heart of the Matter �
Dirofilaria immitis infection in dogs


by CCO Columnist
Dr. Kathy Chorneyko


After a long winter, spring and summer bring warmth and sunshine.  However, they also bring mosquitos and for our dogs this means heartworm season.

Heartworm, also known as Dirofilaria immitis, is a parasite carried by mosquitos and transferred to animals during mosquito blood meals.

The Heartworm Cycle (click here to view diagram)

Adult worms can be up to 30 cm in length.  Symptoms in an infected dog will vary with the number of adult worms present.  A dog may be asymptomatic if there are only a few worms, but with 50 or more worms there will be circulatory problems and there may even be circulatory blockage.  Symptoms include respiratory difficulty, coughing, vomiting, exercise intolerance, fatigue and in the most extreme case � death.

Heartworm in Canada

Although heartworm is not common in Canada with a documented overall prevalence of 0.16% in 1998, there are areas where is it more frequently seen � southern Ontario, southern Manitoba, southern Quebec and around the Okanagan Valley in B.C.  Heartworm data from Vita-Tech shows that Toronto had the highest number of positive heartworm tests in 2006.

The length of the heartworm transmission season depends on development of sufficient heat for incubation of larvae to the infective stage in the mosquito. In the Northern Hemisphere the peak transmission months are July and August.

Heartworm Testing

The earliest that the heartworm antigen or the microfilaria can be detected in the blood is about 5 to 7 months post infection.  Therefore testing in the late spring will likely detect infections from the previous year.  Antigen testing is generally considered the most sensitive diagnostic test method and should be the one used by your veterinarian.

Heartworm Treatment


If a dog tests positive for heartworm, treatment will depend on the number of worms present, their location and the severity of clinical symptoms.  At least one medication will be used that will kill the adult worms, however even after the worms are dead they can cause circulatory blockage and failure.  Therefore monitoring and supportive treatment is also very important.  On occasion direct removal of the worms might be necessary.  After a thorough medical and radiological examination treatment options will be recommended and discussed by your veterinarian.

Heartworm Prevention

Although dogs are highly susceptible to heartworm, extremely effective drugs exist which can prevent infection.  Before starting the prevention regime however, it is extremely important that your dog have a heartworm test first.  If the test is positive then they must be treated for heartworm infection and the prevention regime is not appropriate. If the test is negative then the prevention regime can be started.

The most commonly used heartworm prevention drugs belong to a group of agents called macrocyclic lactones � ivermectin, milbemycin oxime, moxidectin and selamectin.   These agents have formulations that can be given orally or topically.  They are highly effective for one month and efficacy remains high for at least an additional month.  This extended efficacy is a good safeguard in the event of inadvertent delay or omission of regularly scheduled doses.  Heartworm prevention decreases the reservoir of heartworm in the population and therefore can decrease infection even in unprotected dogs. 

Heartworm in Humans


Can humans get heartworm?  Unfortunately yes, however humans are relatively resistant and infection is quite rare.  Humans acquire this disease from infected mosquitos, not from infected dogs.  

So don�t forget to give your dog their monthly heartworm prevention from June to November. Although it can be a devastating infection, it is entirely and easily preventable.


References:

1.  2005 Guidelines for the diagnosis, prevention and management of heartworm (Dirofilaria immitis) infection in dogs.  Veterinary Parasitology 133 (2005): 255-266.
2.  Canine Heartworm testing in Canada � are we being effective.  Can Vet Journal 41 (2000): 929-937.
3.  American Heartworm Society Website.
www.heartwormsociety.org
4.  Bayer Healthcare Website.
www.advocate.spot-on.com
5.  Vita � Tech website.
www.vita-tech.com
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