What is heartworm disease?
Heartworm disease is a serious and potentially fatal disease in pets in the United States and many other parts of the world. It is caused by foot-long worms (heartworms) that live in the heart, lungs and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body. Heartworm disease affects dogs, cats and ferrets, but heartworms also live in other mammal species, including wolves, coyotes, foxes, sea lions and—in rare instances—humans. Because wild species such as foxes and coyotes live in proximity to many urban areas, they are considered important carriers of the disease.
The dog is a natural host for heartworms, which means that heartworms that live inside the dog mature into adults, mate and produce offspring. If untreated, their numbers can increase, and dogs have been known to harbor several hundred worms in their bodies. Heartworm disease causes lasting damage to the heart, lungs and arteries, and can affect the dog’s health and quality of life long after the parasites are gone. For this reason, prevention is by far the best option, and treatment—when needed—should be administered as early in the course of the disease as possible.
Ferrets. Ferrets are extremely susceptible to heartworm disease and are at risk for the disease even if they are indoor pets. Just one heartworm in a ferret can cause serious disease or death. The American Heartworm Society recommends year-round prevention for ferrets as well as regular checkups with a veterinarian to ensure they stay healthy and heartworm-free.
How is heartworm disease transmitted from one pet to another?
The mosquito plays an essential role in the heartworm life cycle. Adult female heartworms living in an infected dog, fox, coyote, or wolf produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which develop and mature into “infective stage” larvae over a period of 10 to 14 days. Then, when the infected mosquito bites another dog, cat, or susceptible wild animal, the infective larvae are deposited onto the surface of the animal's skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to mature into adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats. Because of the longevity of these worms, each mosquito season can lead to an increasing number of worms in an infected pet.
What are the signs of heartworm disease in ferrets?
The signs of heartworm disease in ferrets are similar to those in dogs, but they develop more rapidly because the ferret’s heart is quite small. While dogs may not show symptoms until they have many worms infecting their hearts, lungs and blood vessels, just one worm can cause serious respiratory distress in a ferret. Symptoms of this distress include • Lethargy (i.e., fatigue, tiredness) • Open-mouth and/or rapid breathing • Pale blue or muddy gum color • Coughing
How significant is my ferret's risk for heartworm infection?
Many factors must be considered, even if heartworms do not seem to be a problem in your local area. Your community may have a greater incidence of heartworm disease than you realize—or you may unknowingly travel with your pet to an area where heartworms are more common. Heartworm disease is also spreading to new regions of the country each year. Stray and neglected dogs and certain wildlife such as coyotes, wolves, and foxes can be carriers of heartworms. Mosquitoes blown great distances by the wind and the relocation of infected pets to previously uninfected areas also contribute to the spread of heartworm disease (this happened following Hurricane Katrina when 250,000 pets, many of them infected with heartworms, were “adopted” and shipped throughout the country).
The fact is that heartworm disease has been diagnosed in all 50 states, and risk factors are impossible to predict. Multiple variables, from climate variations to the presence of wildlife carriers, cause rates of infections to vary dramatically from year to year—even within communities. And because infected mosquitoes can come inside, both outdoor and indoor pets are at risk.
For that reason, the American Heartworm Society recommends that you “think 12:” (1) get your pet tested every 12 months for heartworm and (2) give your pet heartworm preventive 12 months a year.
What do I need to know about heartworm testing?
Heartworm disease is a serious, progressive disease. The earlier it is detected, the better the chances the pet will recover. There are few, if any, early signs of disease when a dog or cat is infected with heartworms, so detecting their presence with a heartworm test administered by a veterinarian is important. The test requires just a small blood sample from your pet, and it works by detecting the presence of heartworm proteins. Some veterinarians process heartworm tests right in their hospitals while others send the samples to a diagnostic laboratory. In either case, results are obtained quickly. If your pet tests positive, further tests may be ordered.
When should my ferret be tested?
Photo courtesy of Dr. Karen Rosenthal
Diagnosis of heartworm disease in ferrets can be more problematic. Your veterinarian may recommend both antigen testing and diagnostic imaging such as echocardiography to demonstrate the presence of worm in the heart.
What if my ferret tests positive for heartworms?
Ferrets are extremely susceptible to heartworms. There are differences, however, in the nature of the disease and how it is diagnosed and managed. Ferrets are extremely susceptible to heartworms. Heartworms in the circulatory system also affect the ferret’s immune system and cause symptoms such as coughing, wheezing and difficulty breathing, even sudden death. Ferrets may also demonstrate fluid in the lungs, decreased appetite and weight loss, paralysis of the hind legs, or enlarged abdomen. Bilirubinuria (dark colored urine) is common in ferrets with heartworm disease.
Here’s what to expect if your ferret tests positive for heartworm:
- Diagnosis. As many as 14 heartworms have been found in a single ferret, but ferrets can be seriously affected by the presence of only one worm. Diagnosis can be complicated, requiring a physical exam, an X-ray or ultrasound exam, a complete blood count and several kinds of blood test.
- Treatment. Unfortunately, there is no approved drug therapy for heartworm infection in ferrets, and the drug used to treat infections in dogs is not safe for ferrets. Nevertheless, ferrets with heartworm disease can often be helped with good veterinary care. The goal is to stabilize your pet and determine a long-term management plan.
- Monitor your ferret. Most ferrets infected with heartworms will be showing clinical signs. If worms have been detected in the lungs, chest X-rays every 6 to 12 months may be recommended. If mild symptoms are noted, small doses of prednisolone may be administered to help reduce inflammation.
- Provide veterinary care. If the disease is severe, additional support may be necessary. Your veterinarian may recommend hospitalization in order to provide therapy, such as intravenous fluids, drugs to treat lung and heart symptoms, antibiotics, and general nursing care. In rare cases, surgical removal of heartworms may be possible.
- Maintain prevention. Ferrets are very susceptible to heartworm disease and the results of infection may be devastating. Both outdoor and indoor ferrets are at risk and your ferret should be on monthly preventive for life. Preventives keep new infections from developing if an infected mosquito bites your ferrets again.
More questions about heartworm disease
Do I need a prescription for my pet’s heartworm preventive medication? If so, why?
Yes. The U.S. Food and Drug Administration (FDA) labeling on heartworm preventives states that the medication is to be used by or on the order of a licensed veterinarian. This means heartworm preventives must be purchased from your veterinarian or with a prescription through a pet pharmacy Prior to prescribing a heartworm preventive, the veterinarian typically performs a heartworm test to make sure your pet doesn't already have adult heartworms, as giving preventives can lead to rare but possibly severe reactions that could be harmful or even fatal. It is not necessary to test very young puppies or kittens prior to starting preventives since it takes approximately 6 months for heartworms to develop to adulthood. If the heartworm testing is negative, prevention medication is prescribed.
How do monthly heartworm preventives work?
Whether the preventive you choose is given as a pill, a spot-on topical medication or as an injection, all approved heartworm medications work by eliminating the immature (larval) stages of the heartworm parasite. This includes the infective heartworm larvae deposited by the mosquito as well as the following larval stage that develops inside the animal. Unfortunately, in as little as 51 days, heartworm larvae can molt into a juvenile/immature adult stage, which cannot be effectively eliminated by preventives. Because heartworms must be eliminated before they reach this adult stage, it is extremely important that heartworm preventives be administered strictly on schedule (monthly for oral and topical products and every 6 months or 12 months for the injectable). Administering prevention late can allow immature larvae to molt into the adult stage, which is poorly prevented.
I heard that certain heartworm prevention medications can also protect against intestinal parasites. Is this true?
Yes, a number of heartworm preventives used today also are effective against certain intestinal parasites. Depending on the product, these may include hookworms, roundworms, whipworms and tapeworms. Some products are even effective in treating external parasites such as fleas, ticks, ear mites, and the mite that causes scabies. However, it is important to realize that no single product will eliminate all species of internal and external parasites, and you should consult your veterinarian to determine the best product for your pet.
At what age should young animals be started on heartworm prevention? What do I need to know about prevention in my new pet?
The risk of puppies and kittens getting heartworm disease is equal to that of adult pets. The American Heartworm Society recommends that puppies and kittens be started on a heartworm preventive as early as the product label allows, and no later than 8 weeks of age. Ferrets are started on a preventive when they weigh at least two pounds.
The dosage of a heartworm medication is based on body weight, not age. Puppies and kittens grow rapidly in their first months of life, and the rate of growth—especially in dogs—varies widely from one breed to another. That means a young animal can gain enough weight to bump it from one dosage range to the next within a matter of weeks. Ask your veterinarian for advice about anticipating when a dosage change will be needed. If your pet is on a monthly preventive, you may want to buy just one or two doses at a time if a dosage change is anticipated (note that there is a sustained-release injectable preventive available for dogs 6 months of age or older). Also make sure to bring your pet in for every scheduled well-puppy or well-kitten exam, so that you stay on top of all health issues, including heartworm protection. Confirm that you are giving the right heartworm preventive dosage by having your pet weighed at every visit.
Are heartworms more common in certain areas of the United States?
Heartworms have been found in all 50 states, although certain areas have a higher risk of heartworm than others. Some very high-risk areas include large regions, such as near the Atlantic and Gulf coasts, and along river tributaries. Most states have "hot spots" where the heartworm infection rate is very high compared with other areas in the same state. Factors affecting the level of risk of heartworm infection include the climate (temperature, humidity), the species of mosquitoes in the area, presence of mosquito breeding areas, and presence of animal “reservoirs” (such as infected dogs, foxes or coyotes).
I live in a northern state. How long should my pet be on heartworm prevention?
For a variety of reasons, even in regions of the country where winters are cold, the American Heartworm Society is now recommending a year-round prevention program. Dogs have been diagnosed with heartworms in almost every county in Minnesota, and there are differences in the duration of the mosquito season from the north of the state and the south of the state. Mosquito species are constantly changing and adapting to cold climates and some species successfully overwinter indoors as well. Year-round prevention is the safest, and is recommended. Remember too that many of these products are de-worming your pet for intestinal parasites that can pose serious health risks for humans.
I live in the desert where it is very dry and there are very few mosquitoes. My vet says I should use monthly prevention. What should I do?
There are different climates in Arizona, including micro-climates such as irrigated fields, backyard ponds, and man-made golf courses, which affect the severity and duration of the mosquito season. We also know that areas can have heartworm infection in wild species such as coyotes, and these infected wild animals can be a source of infection to your dog or cat as well. Despite the fact that heartworm disease may not be diagnosed as often in Arizona as in some other states, it is definitely present. In fact, heartworm disease has been found in nearly every county in the state.
The American Heartworm Society recommends year-round prevention, even in states like Arizona. And remember, if your dog or cat travels out of state with you or to another part of Arizona where mosquitoes are common, they may be at higher risk of exposure.
Is there a vaccine for heartworm disease?
No. At this time, there is not a commercially available vaccine for the prevention of heartworm disease in dogs or cats. However, research scientists are looking at this possibility. Right now, heartworm disease can only be prevented through the regular and appropriate use of preventive medications, which are prescribed by your veterinarian. These medications are available as a once-a-month chewable, a once-a-month topical, and either a once or twice-a-year injection. You should determine the best option for your pet by talking with your veterinarian. Many of the medications have the added benefit of preventing other parasites as well.
Is there an effective natural prevention for heartworm?
Only heartworm prevention products that are tested and proven effective by the U.S. Food and Drug Administration (FDA) should be used.
I have a ferret. Should I be giving it heartworm prevention?
Just like dogs and cats, ferrets can become infected with heartworms, and are at risk even if they are indoor pets. The signs of heartworm disease in ferrets are similar to those in dogs, but they develop more rapidly because the ferret’s heart is quite small. While dogs may not show symptoms until they have many worms infecting their hearts, lungs, and blood vessels, just one worm can cause serious respiratory distress in a ferret. Symptoms of this distress include coughing, fatigue, open-mouth and/or rapid breathing, and pale blue or muddy gum color. Preventing heartworm disease is much less expensive and much safer than treating it, just as it is for other pets, and your veterinarian can prescribe heartworm medication approved for use in ferrets. The American Heartworm Society recommends year-round prevention for ferrets as well as regular checkups with a veterinarian to ensure they stay healthy and heartworm-free.
The expiration date on my pet’s heartworm medication is past. Can I still use the medication?
As with all drugs or pharmaceutical products, heartworm preventives should be used before the expiration date on the package, because it is impossible to predict if it will be effective or safe. The expiration date is established by a series of tests mandated by the U.S. Food and Drug Administration to provide assurance that the product is effective and has undergone no significant deterioration.