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Vet Connect: Heartworm Disease … Why Give Preventions?


Heartworm disease is a damaging and potentially deadly disease, but easily prevented. Learning how damaging heartworms can be may help you stay more compliant with your pet’s preventives or even seek treatment versus management.

Heartworm disease has been diagnosed in all 50 states in the U.S., with more frequent reported cases from the southeastern region. We have more months of the year where environmental conditions favor mosquito survival and reproduction. The 5 states with the highest heartworm incidence: Mississippi, Louisiana, Arkansas, Texas, and Tennessee. Despite the fact that preventive medications have been widely available for decades, the incidence of heartworm disease continues to rise across the nation.

Infection can happen in SECONDS with ONE bite from an infected mosquito. The larvae (baby worms) are deposited onto the skin from the mosquito and migrate into the “hole” left from the mosquito bite and then causes disease in 2 months (however the test may not detect infection for 6 months or more and can even have false negative results). Damage occurs early and can happen with only a few worms. Arteries within the lungs can be permanently damaged from the inflammation and even blockage caused by the worms. Masses from dead and live worms can cause embolic disease that leads to obstruction of blood flow in the arteries. The damage caused can remain even after the worms are dead. Exposure to heartworms can lead to future lung diseases such as pulmonary hypertension and heart diseases such as congestive heart failure. Unfortunately, heartworm disease is a progressive disease process.

To make matters worse, testing is not 100% foolproof either. The “color intensity” of a positive antigen test result cannot reliably be used to determine the level of heartworm burden. In addition, false negative results can occur for several reasons: the worm burden causing infection may be too low, there may be a single sex worm burden, the animal’s immune system can cause conflicts for testing, and the tests may not detect the antigen levels within the blood without special prepping of the sample prior to testing. Unfortunately, preventives have been proven to not be 100% effective, even though they are highly effective.

Cases of acute mild coughing could be due to a small single embolic incident from heartworms that resolves on its own or for example when a dog receives treatment for other conditions such as suspected kennel cough.

Once detected with heartworms, there is a difference between heartworm treatment and management. Heartworm treatment can KILL the worms, but doesn’t always cure disease due to permanent damage created. Treatment involves killing worms, while management involves preventing further infection while trying to control disease progression in the meantime. The American Heartworm Society recommends the use of doxycycline and specific types of preventives (macrocyclic lactone) prior to a 3-dose regimen of melarsomine (1 injection followed at least one month later by 2 injections of the same dose 24 hours apart) for treatment of heartworm disease in both symptomatic and asymptomatic dogs. Any method utilizing only macrocyclic lactones as a “slow-kill” adulticide is not recommended. (Ivermectin does not kill adult heartworms, it shortens the lifespan of adult heartworms, sterilizes adult heartworms, kills microfilaria (keeping the dog from being a source of contagion), and kills specific stages of larvae (preventing new infections). This means that if you opt to treat a heartworm positive dog only with an ivermectin heartworm preventive, you can expect the dog to remain heartworm positive for as long as two years and the heartworm disease will be progressing during those two years.

Veterinarians and parasitologists are now focusing on a more multimodal approach to preventing heartworms. In addition to monthly preventives, mosquito repellants should be used as an adjunct to prevent mosquitos from biting. While the bottom line is that only Food and Drug Administration (FDA)-approved preventives can effectively prevent heartworms, a number of drug-free strategies to help prevent bites from infected mosquitoes can be recommended. The key is positioning the following strategies as ways to supplement the use of preventives, not replace it: 1) Avoid mosquito exposure-keep pets indoors overnight and avoid pet walks at dusk or dawn when many mosquitoes are feeding; 2) Eliminate standing water close to the home. Mosquitoes breed in standing water, which can be anything from a birdbath to a puddle to an empty flowerpot, pail, or wheelbarrow. While mosquitoes are capable of flying a mile or more, it is common for them to stay within a few hundred feet of where they hatch. Eliminating breeding grounds can reduce the number of mosquitoes in a pet’s home environment; 3) Use natural mosquito repellents. Natural repellents such as Neem oil and CedarCide may help diminish the chances of pets being bitten by infected mosquitoes. Neem oil products should be used with caution in cats if at all.

One case of heartworms is too many – prevention is key!


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