INTRO
Happy July everyone, I'm going to briefly discuss heartworm disease in the dog and cat. In short, heartworm disease is a parasitic disease that is spread by the mosquito into an animal's blood stream where it grows to be an adult worm and often resides within the heart of a dog or cat. It is often times fatal, but can be prevented.
DISEASE LIFE CYCLE
An infected mosquito will land on a dog or cat and in the process of obtaining a blood meal transmits heartworm larvae into the animal via saliva. The larvae continues to develop for approximately 2 months within the animal's tissue. The larva then develops into an adult in the animal's blood stream which takes around 4-5 months. This is a period when sometimes a blood smear can be done to see "microfilaria" or tiny worms in the blood stream. This is also the same time when worms will start to reside within the pulmonary artery of the heart. After about 6 months the adult worms become mature adults and can reproduce and produce more larvae. Mosquitoes can then "retrieve" these larvae and go on to infect other dogs or cats. Heartworm in cats follows the same life cycle however larvae in cats seldom develop into adult worms because they are atypical hosts of heartworms (like humans). Sometimes this means cats may have heartworm and show no clinical signs. However, even the immature worms can cause death in cats.
WHY IS IT SO BAD?
The heart, as we all know, is critical in life. The pulmonary artery is a vessel that exits the heart in order to transport blood that needs to retrieve oxygen from the lungs. When it's occluded in any way, that means that blood that needs oxygen, has a harder time getting it. The heart will want to pump harder or faster which causes the heart muscle to enlarge resulting in heart disease. It also means that if any worms get swept "downstream" they can lodge themselves into the lungs which is a fatal condition known as pulmonary thrombosis (lung clot). Take home message is that these worms, when in a high enough concentration, will cause heart disease.
WHERE IS IT?
Heartworm has been found in all 50 states. It definitely is more prevalent in certain regions (Gulf Coast, Atlantic Coast, etc) but the prevalence is increasing. To add, each state has its own microclimates throughout like irrigation ponds, reservoirs, and backyard ponds making it difficult to say "heartworm doesn't exist in my county". There have been 80 cases in Oregon in 2015, most noted in southern Oregon, but a few (7) even in Marion county. I should add this is only a number from a study done by Idexx Laboratories, which means this does not include a lot of the in-house heartworm testing done in veterinary clinics. The number is likely higher.
You may wonder why is heartworm more prevalent in certain areas of the U.S. The answer is climate, temperature and humidity. In order for larva in a mosquito to become infective it must go through several temperature changes. 8-10 days at 82-86 degrees, followed by 11-12 days at 75 degrees then 16-20 days at 71 degrees. This is not exact and just a set of ranges. If you think about it, most of Oregon could fit this picture, especially due to this year's warm weather. Luckily development stops at around 57 degrees, though if the temperature warms up again the cycle can restart itself.
PREVENTION
Prevention is crucial because treatment of heartworm disease can be costly and carry a significant amount of risk. We recommend year round prevention for dogs. There are several forms of prevention that are offered and most often is as easy as a monthly pill. Currently, we carry Interceptor which prevents heartworm disease as well as treats hookworm, roundworm and whipworm infections.
This was just a brief summary about the important aspects to know on heartworm disease. If you have any questions, please don't hesitate to give us a call! Thanks for reading.
-Dr. C
References:
Key factors influencing canine heartworm, Dirofilaria immitis in the United States.
H. Brown, L Harrington, et al
http://www.capcvet.org/parasite-prevalence-maps/
http://www.morrisvetcenter.com/morrisvet/companion%20animal/Wellness-Care/Heartworm.asp
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