Wednesday, July 20, 2011
Heartworm: The Most Important Blood Parasite in the U.S.
Heartworm is a parasite called Dirofilaria immitus. It's host is primarily dogs, wild canids, sea lions, seals, ferrets, horses, beavers, bears, raccoons, pandas, and occasionally cats. Humans are accidental hosts, and can contract a younger 'form' of this worm, though it never impacts the human host negatively.
This parasite is found world wide, and in the US is very bad in the south and south east. In Colorado, the south and western parts of the state. 60% of dogs in Japan have heartworm, and Great Britain has no cases.
Adult males are up to 16cm and females are up to 30 cm. Adults can live up to 8 years. Microfiliaria, the "babies", can live up to 2 years in the blood. The microfilaria are ingested by mosquitos after a blood meal. After, they move up to the mouth of the bug and enter through a bite wound. At this point, they are considered "migrating microfilaria" in your animal's blood. Then, they live in the connective tissue for several months before maturing into adults. Afterwards, they migrate into the pulmonary arteries through the tissues. They reach the right heart ventricle as adults, and then start producing more "babies" 6-9 months post-infection. Their babies, the microfiliaria, can live up to 2 years in the blood.
While often the parasites enter their host through the skin via the mosquito, our animals can be infected in utero if their mother has both adults and babies in her blood. Puppies will only get "circulating" (the babies) which usually will not become adults, but they can still pass the parasite through the mosquito.
There are 2 kinds of infestation, "full blown" and "occult". Full blown means that the animal has both adults and microfiliaria. Occult means they have the adult only. One of the reasons why they may only have adults is because the worms may all be male or female, and cannot reproduce. Sometimes, the microfilaria can be destroyed by the immune system. Your pet can be reinfected multiple times, causing different stages of the worm to be present in the body.
The worms infest the right ventricle (de-oxygenated blood enters the heart into the right atria, then moves to the right ventricle before going to the lungs for oxygen) and pulmonary arteries (they take the blood to the lungs for oxygen replenishment from the right ventricle), potentially resulting in failure of the right heart, hepatic cirrhosis (hardening of the liver), ascites, pulmonary embolism, acute respiratory distress with coughed up blood, and worms. Basic signs are exercise intolerance, leathargy, and dyspnea (difficulty breathing), coughing, blue/purple skin discoloration, fainting, and nose bleeds. With adult worms, they obstruct the vessels, heart chambers, and heart valves. Smaller dogs tolerate heart worm worse than larger dogs, merely due to the size of the vessels and of the heart.
Your vet can test for heartworm through blood analysis. Heartworm tests are not expensive. If your dog has not been on heartworm preventative, always test them before putting them on preventative. If their preventative regime has missed some weeks or months, test them again. Heartworm is too serious to leave anything to chance, and I've seen too many owners pay 100 bucks for blood work, and yet skip out on the 30 bucks to test for heartworm. Please just pay for the test!
Heartworm is treatable, but the fact it is treatable should never cause an owner to not take it seriously. Heartworm can be fatal, and treatment has a lot of risks. There are a couple of forms of treatment regarding dosing. Lately I have been reading several different approaches regarding frequency of injection of the arsenic drug which kills them, and how long the treatment is. It's best to just ask your doc. Because the method is using arsenic, there are some real risks including death. They are hospitalized to watch for symptoms such as vomit, diarrhea, and jaundice: remember that all medications take their toll on the kidneys and liver.
Following treatment to kill the worms, your vet will advise that your animal should be restricted from exercise or excess exertion for some months. How long that activity is restricted will be based upon level of infestation as well as the age and overall health of your dog, as well as any other conditions. The worms, now killed, are still being "broken down" by their body and are still circulating (recall what I wrote above, about how BIG they can be!) You want to give your dog's body time to expel these worms, and too much excitement (thusly, high blood pressure) may push those dead worms through your dog's blood steam at a rate that is too much for their bodies to bear. Always ask your vet every question you have in your mind, and never trust any informational outlet over the advice of your vet. Following the treatment to kill the worms and/or microfilaria, your vet will have your dog on drugs for several months. I have seen varying approaches to what meds docs have sent owners home with, anything from anti-inflammatories to a doc who was sending dogs home with anti-biotics.
If your dog was treated for heartworm and sent home with you with all their meds, you restrict their activity but you also look for other symptoms of complications. The idea of restricting activity is met by many owners with a 'sigh'. Some have high energy dogs, and they 'feel bad' confining them. Several weeks of changed routine for them is far better than complications or death resulting from treatment not followed by the owner following the advice of the doctor. Complications may look a lot like the symptoms your dog had *before* you treated them. Our doctors know that heartworm is very scary, so call your clinic any time you feel concerned. Never fear they will think you are 'over reacting'. You know your pet best, and it's our job to reassure you and educate you.
There is a lot of information available regarding prevention and treatment. Felines can also contract heartworm, though less frequent. I think it's less about understanding the life cycle of this parasite, and more about understanding how incredibly awful it is, and how easy it is to prevent. Ask your vet about a once-a-month preventative. Don't assume that since the area you live in has few mosquitoes that your animal is safe. It only takes one infected mosquito, and the risks to your animal can be fatal. Take another look at the picture of what a heartworm infested heart looks like, and ask yourself if it's something you should take lightly.
Some doctors I've spoken with say they saw cases of heartworm increase considerably in the last year. While we may not think that mosquitoes are an issue in dryer climate, consider all the dogs rescued from out of state who have the disease. Just one monthly pill may save your dog's life, and most pills also prevent other parasites at the same time. Let's get all our dogs on preventative and tell this parasite we don't want it in our dog's hearts any longer.