Creature Feature: Prevent heartworm infection

10 Aug

BY RHONDA OWEN
I’ve been told that I have to give my dog medicine all year long to keep him from getting heartworms. Is that true?

Pet owners often question the necessity of giving their pet heartworm preventive medication year-round, but it’s the only way to protect an animal from getting heartworms.

All it takes is one bite from one mosquito to infect a dog or cat with heartworms, says veterinarian Wallace Graham, president of the American Heartworm Society.   

Mosquitoes breed in standing water so given this year’s heavy rain and flooding, Arkansas could have more mosquitoes than normal. Plus, the state lies in the area of the country that the society describes as “heartworm endemic,” meaning there’s a high rate of heartworm cases reported annually.

“Anything that will enhance the mosquito population will enhance transmission of heartworms,” Graham says. “We know that weather conditions do impact, to some degree, heartworm transmission.”

How heartworms develop: Mosquitos pick up heartworm microfilariae (tiny larvae) when they bite an infected animal, then those mature into an “infective larval stage” in 10-14 days. At that point, if a mosquito bites a susceptible animal, the larvae enter though the bite wound.

Heartworm larvae then migrate through the body until they reach the heart and lungs, where they mature. Adult heartworms — thin, stringlike  and 10 to 14 inches long — clump together in arteries serving the lungs. As their numbers grow, they spread into the right chamber of the heart, which could weaken the heart and cause congestive heart failure in dogs. (Photos of this are on the heartworm society’s website; be warned, they’re gruesome.)

Cats are just as susceptible as dogs to contracting heartworms, Graham says, but heartworm disease in cats is a “whole different ballgame.”

“In dogs, the primary effects are in the lungs and heart while in cats, it’s almost completely a lung disease. The pathology in the lungs of cats is completely different. Cats have severe respiratory signs generally at two times during infection,” he says.

The first “crisis period” for cats is when a heartworm first emerges in its lungs. At that point, the infection can appear to be asthma or allergic bronchitis but is actually a syndrome known as “heartworm associated respiratory disease.”

“If a cat survives that, then the heartworm will die of old age,” Graham says. “But the death of the heartworm causes tremendous problems.” When a cat’s body tries to get rid of the dead heartworms, the cat may go into shock and die.

Dogs with heartworms can be treated successfully in all but the most advanced cases, but there’s no treatment for cats, Graham says. ‘That’s why it’s important for cats to be on a heartworm preventive even in they live indoors.”

A dog in the early stages of heartworm infection won’t show any symptoms but as the number of heartworms grow (sometimes to more than 30), signs appear. They include coughing, tiring easily, lethargy, loss of appetite and weight loss. If they aren’t treated, it becomes harder and harder for them to breathe — death by heartworms is slow torture for an animal.

A cat’s symptoms mimic those of other diseases and can include rapid breathing, gagging, vomiting and weight loss.

Blood tests will reveal if a dog or cat is infected, Graham says. Owners sometimes resist having their pets tested because they are certain that they never missed giving their pet the preventive. Often, they misremember so it’s better to test and be certain that the dog or cat hasn’t been infected.

For infected dogs, treatment is expensive and takes at least two months during which the dog must be kept inactive — no running, jumping or playing although walking on a leash is allowed.

During treatment, the heartworms die and decompose and “are eaten up by the white blood cells,” Graham explains. “That process takes some time and we have to do what we can to manage the side effects.” 

The main side effect is that the decaying heartworms may block the animal’s arteries and lower blood flow to the lungs. “We have drugs and protocols that minimize effects on the dog, but it’s critical that the dog not get any exercise throughout the entire course of the treatment.”

Most dogs can be treated successfully, but some experience complications and don’t survive. Vigilance by an owner can prevent this, Graham says.

Preventives work simply by killing the infected larvae when they enter an animal through a mosquito bite. Dead larvae mean no heartworms.

Heartworm preventive is available in pill form, as a monthly topical treatment applied to the skin, and as a shot given every six months. Your vet can explain how each works and what may be best for your dog or cat.

While heartworm preventives have always been considered effective, the American Heartworm Society is currently conducting a study to determine if there are any strains of heartworms that have become resistance to a specific chemical compound now being used.

The study doesn’t cover a specific type or brand of preventive, but there has been controversy about the effectiveness of HeartGuard Plus. In 2005, the federal Food and Drug Administration ordered the maker, Merial LLC, to stop claiming 100 percent effectiveness on its labels.

Graham says he can’t address the controversy because there is no scientific evidence to “definitively answer” all the questions about the effectiveness of any specific product. Scientific studies in this area are only in the early stages.

“People ought to be having conversations with their veterinarians about what they ought to do in terms of preventives,” he says. “This needs to be between the pet owner and veterinarian. He may have reason to believe one product is better in their area.”

He notes that veterinarians tend to recommend the preventive they’ve had success with.

Creature Feature appears weekly in the Arkansas Democrat-Gazette. This column was published June 15, 2011.   

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