By Dr. Lonnie King
The month of April has been designated as “Heartworm Awareness Month”. This designation is a good reminder for you to make sure that you understand the serious threat of heartworms and ensure that you are taking the right steps to prevent this disease in your pets.
Heartworm Disease affects thousands of dogs and cats across Georgia every year. All 50 states and many countries around the world experience heartworm disease in pets and wildlife. In the U.S. alone, there are an estimated 1.1 million active cases and Southeastern United States is a hotspot for cases. Georgia ranks #8 in the U.S. for total cases and the disease is becoming more prevalent.
A Complex Disease
Heartworm (HW) is a preventable but very serious disease that can be fatal. It can infect dogs, cats, ferrets, and a variety of wildlife. Adult HW (scientific name, Dirofilaria immitis) look like strands of cooked spaghetti and can reach lengths of 12-14 inches. These worms grow and lodge in the heart and lungs of infected animals and can cause restricted blood flow, damaged blood vessels and produce high levels of inflammation. The adult worms release microscopic offspring into the animal’s blood stream that are termed microfilaria. The microfilaria, in turn, will enter, survive and further mature in the bodies of mosquitoes when they bite an infected animal and then continue to be transmitted when the mosquito resumes biting other animals. The microfilaria take about 6 months to grow into adult worms which enter the heart and lungs. Dogs, cats, and other animals cannot transmit the disease directly, only mosquitoes are responsible for transmission.
Once a pet is infected with microfilaria, it takes about six months for them to become adults and lodge in the heart and lungs. Adult HW can live in pets for years continuing to produce microfilaria during this entire time and making them a risk and continuous source of infection. Infected dogs with advanced disease often demonstrate symptoms that include a mild cough, fatigue, reluctance to exercise, difficulty breathing, decreased appetite and loss of weight.
HW can never be eliminated because wildlife, including foxes, coyotes, raccoons, and opossums are susceptible, have the same life cycle as found in pets, and serve as sources of HW for other wildlife and domestic pets through mosquito bites. Georgia has a great environment for the maintenance and spread of HW because of our warm weather, water sources, mosquitoes, and lots of wildlife. Thus, the disease is a constant threat, and the natural cycle of infection occurs year around in our state.
HW Disease in Cats
Although dogs and cats both can contract HW, the disease in cats is very different from the disease in dogs. Cats are poor hosts for HW, and the disease often will not progress, so adult worms do not develop. Even if they do, the number of adult HW found in cats is much less that found in dogs. The immune system of cats aggressively attacks microfilaria thus preventing them from maturing into adult worms. However, through this process, a huge amount of inflammation is created that can cause serious damage to lung tissue. There are usually many fewer cases of HW in cats relative to our dogs; however, the disease in cats can be very severe and even fatal. Cats with advanced disease will often cough, wheeze and vomit. About 25 percent of cases in cats are found in indoor cats. Infected mosquitoes do get inside our homes and unknowingly put these pets at risk.
Testing and Treatment
The earlier HW disease is detected, the better the chances that your pet will recover. In dogs, diagnostic tests are accurate and relatively simple. Using a blood sample, proteins (antigens) from adult worms can be detected and the microfilaria may also be found by examining the pet’s blood sample. Your local veterinarian can usually do these tests as part of a routine office visit. HW infection in cats is much more difficult to diagnose since they commonly do not have circulating microfilaria in their blood and antigen and antibody tests are not very reliable. Sometimes, X-rays or ultrasound will be used in both cats and dogs to help diagnose HW.
Treatments are available and although they have a relatively high degree of success if used before the disease becomes too advanced, the treatments can be risky, and complications may occur. The goal of the treatment in dogs is to kill both the adult worms and the microfilaria as safely as possible. This is done through a series of injections over a period of time. It is important to consider that when treatment is taking place, HW in the heart and lungs are dying and slowly being dissolved; thus, complete rest and often hospitalization is necessary. The treatment often takes several months to complete. Glucocorticoids and antibiotics are often included as part of the total treatment. Recently, researchers have found that the adult HW are themselves infected with a bacterium (called Wolbachia) that may complicate both the course of the disease and the treatment.
Unfortunately, there are no approved treatments for cats and the treatments used for dogs, often an arsenic-containing drug, are toxic and dangerous for cats. You will need to work with your local veterinarian to plan the best treatment strategy which will often focus on reducing their very high levels of inflammation. On rare occasions, HW can be removed surgically but this is a high-risk procedure reserved for only the most severe cases.
Managing Heartworms in Animal Shelters and Humane Societies
An estimated 30 percent of pets in the U.S. are acquired from animal shelters and this number is growing. While this is wonderful trend, animal shelters do have unique challenges in dealing with HW disease. Dogs and cats are likely to be at higher risk of HW because stray and surrendered animals have often not received veterinary care nor have they been on HW preventive medications. It is costly and time consuming for shelters to diagnose and treat HW cases. However, this should not discourage you from obtaining a “forever pet” from them. Our own Oconee Regional Humane Society (ORHS) works with local veterinary practitioners to test every dog under their care and if any are found infected, they work together to treat these dogs either prior to adoption or they will help pay for the treatment after adoption. The cost of treatment can be expensive and the ORHS conducts a “Heart-to-Heart” campaign each year to obtain donations that will exclusively be used to help pay for treating HW cases. Please watch for this campaign and hopefully support the cause generously.
PREVENTION, PREVENTION, PREVENTION!
There is good news – HW disease is almost 100 percent preventable in both dogs and cats. There are numerous options available that have been approved by the Food and Drug Administration that can be used. The best choice for your pet will be based on the pet’s lifestyle and risk factors. Remember that preventives do not kill adult worms therefore your pet needs to be tested and be negative before you begin preventive treatments. Preventives may be in the form of pills, topical medications, or injections. Chewable tablets given monthly are commonly used for both dogs and cats. By preventing HW in your pet, you not only protect them, but also help break the infection cycle and protect other pets.
All the preventive medications work by eliminating the microfilaria stage of the HW parasite circulating in the blood. Once you are certain that your pet is HW negative, preventive medications can then be started. Because puppies and kittens are also susceptible to HW infection, they should be started on preventives as early as 8-10 weeks of age. However, work with your local veterinarian because not all medications are cleared for use at early ages. Our Georgia pets are constantly exposed to infected mosquitoes twelve months of the year and for the lifetime of the pet. It has been estimated that the cost of HW treatment is fifteen times greater than the annual cost of preventive medications.
There is further good news; there can be additional benefits from today’s medications. Some HW preventive treatments used today are also effective in preventing intestinal worms such as hookworms, roundworms, whipworms, and tapeworms. Some even will concurrently prevent flea, tick, and ear mite infestations. It is important to work with your local veterinarian to make certain that your pet tests negative for HW and you choose the best option to prevent the disease.
There is no vaccine for HW but the many effective preventives, if used properly, compensate for this deficiency. A final piece of good news is that HW do not have the ability to live in people and although we are bitten by infected mosquitoes like our pets, the parasite will not survive nor cause human disease.
As we acknowledge “Heartworm Awareness Month” this April, please remember that Georgia is major hot spot for HW disease and our dogs, cats and ferrets are at high risk over their entire lifetimes. However, as responsible, and caring pet owners, we have the tools to prevent this serious and often fatal infection. Our fury family members are counting on you to protect them!
Help ORHS Prevent Heartworm
April is Heartworm Prevention Month and the start of the annual ORHS Heart -to-Heart campaign. Over the last year, we have rescued 15 HW positive animals. Heartworm treatment can cost over $500 per animal. You can help us ensure that these animals go to their adoptive homes ready for long and healthy lives. Can you please help us?