Heartworm in Animal Shelter Populations

Clinical signs of heartworm infection vary depending on severity of disease and the species infected.   

• Dogs: 
  • Mild: cough or no symptoms 
  • Moderate: cough and exercise intolerance 
  • Severe: In addition to the symptoms of mild and moderate cases, difficulty breathing, fluid accumulation in abdomen and episodes of loss of consciousness can occur together or individually. Death may occur at this stage of heartworm infection. 
• Cats: 
  • Clinical disease in cats is much different than dogs. Worm burdens are usually lighter and worms have shorter lifespans, but clinical signs can still be severe.  
  • Clinical signs in cats can range from none to sudden death. Chronic respiratory signs such as coughing and rapid or difficulty in breathing are most common.  

Additional information regarding physical examination findings and the results of chest radiographs in heartworm-positive dogs can be found in the American Heartworm Society’s Current Canine Guidelines for the Prevention, Diagnosis and Management of Heartworm Infection in Dogs.  

Heartworm is a vector-borne parasite transmitted by mosquitos. The lifecycle of a heartworm is complex and takes several months to complete. The spread of infection requires adequate humidity and warm temperatures to support a mosquito population.  

Mosquitos ingest an immature stage of the parasite known as microfilaria when they bite an infected dog. It takes approximately two weeks for microfilariae to develop into a later infective stage while in the mosquito’s body. At this point, the organism can be transmitted to another animal through the bite of the mosquito. Once in the body of the animal host, this immature larval stage eventually develops into adult heartworms; this process can take several months. Animals can be bitten and infected with heartworms multiple times by multiple mosquitos, leading to heavier worm burdens and increased severity of disease.  

Adult heartworms can live for a few years in cats and several years in dogs. A patent infection results in the production of circulating microfilaria in dogs that have both male and female worms within their cardiovascular system. As a result, this dog becomes a potential source of infection for mosquitos and other animals. Because a heartworm-positive dog in a household, shelter or neighborhood can serve as a reservoir of infection, treatment of infected animals is necessary to protect the health of the individual and the larger population.  

The Association of Shelter Veterinarians and the American Heartworm Society have recommended practices to minimize the risk of heartworm transmission associated with the relocation of dogs. There is a corresponding list of medications for a veterinarian to consider prescribing for a heartworm-positive dog prior to relocation of the dog.   

Annual heartworm testing (screening) for dogs older than seven months is generally recommended and is the most common way to identify infections. Testing for the presence of heartworms can be complicated and should be overseen by a veterinarian. A veterinarian can review the dog’s history, determine when testing should be performed and when it should next be repeated.  

With just a sample of blood, commercially available heartworm antigen tests will display a positive result if they detect a protein from a female heartworm. If a dog is infected with only male worms or if younger (immature) stages of heartworm are present, no antigen will be detected, and the test will display a negative result. If adult heartworms have reproduced to generate microfilaria, they may be detectable with a different diagnostic test. It is important to note that currently available tests for antigen or microfilaria can only detect infections involving adult worms.  

Furthermore, tests may fail to detect antigen and give a false negative result if they are performed early in the infection process — for example, within five to seven months of infection — which is why testing young puppies is not indicated. Although “tandem testing” for antigen and microfilaria simultaneously is recommended in some professional guidelines, shelters should carefully balance this with consideration of risk and available resources.  

Using radiographs or an echocardiogram to image the lungs and heart may help determine the extent of disease in individual dogs and plan for treatment. Imaging is not always necessary. In many cases, heartworm infection can be successfully treated using the results of screening tests and a thorough physical examination.   

Diagnosis of heartworm infection in cats is challenging and should always be performed under the direction of a veterinarian. Testing is generally indicated only for cats with consistent clinical signs and suspicion of heartworm disease. None of the screening tests currently available for dogs are reliable for ruling out infection in cats. 

Successful heartworm treatment eliminates all life stages of heartworms, prevents further internal damage resulting from the presence of worms and can improve the dog’s clinical condition especially when symptoms are present. A variety of protocols exist. Those recommended for dogs in a typical home environment may not always be feasible or optimal in a shelter setting.  

Alternative protocols may be a viable treatment option when the most commonly used drug, melarsomine, is not accessible. Organizations and shelter veterinarians must consider the overall impact of heartworm treatment on an individual dog’s welfare, including their behavioral health. Shelters must also consider resource allocation and capacity to care for the larger population as they assess the potential implications of heartworm treatment. 

It generally takes two to three months for administration of medications to treat a dog with heartworms, though conversion to a “negative” or “no antigen detected” state may take several months or more. Most protocols start with placing the dog on an antibiotic to eliminate the bacteria contained within heartworms. Additionally, the dog will be given a monthly preventive to kill any circulating microfilaria and protect the dog from additional infections that could otherwise occur with subsequent mosquito bites. Then, drugs to kill adult heartworms and immature stages of the worms are administered. Most protocols utilize the drug melarsomine.  

Infected dogs who are ill may need to be stabilized prior to administration of these drugs. Strict exercise restrictions (leash walks, cage rest, no strenuous activity) during treatment and for the recovery period that follows is necessary to decrease the risk of treatment complications. A veterinarian should determine the duration and nature of exercise restriction because it depends on the severity of the infection, the dog’s clinical condition and the specific treatment protocol. The damage heartworm disease inflicts on a dog’s heart and lungs can affect the dog’s long-term health even after the parasites are killed.   

In collaboration with the ASPCA, Emancipet has published a study that documents a heartworm treatment protocol for owned dogs that is based on the American Heartworm Society guidelines. The concepts discussed in this technician-leveraged protocol can help shelters safely and effectively treat heartworm in sheltered dogs at a lower cost.  

If a heartworm-positive dog is on track to be relocated, a shelter veterinarian can determine the best time for this to occur, considering the dog’s clinical condition and specific state regulations regarding transportation of dogs with heartworm.  

Currently, there are no drugs available to kill adult stages of heartworm in cats or ferrets, and treatment is generally targeted at managing the specific clinical signs present in an individual animal.  

Veterinarians prescribe preventive medication that can be applied topically, given orally or injected. Published guidelines recommend testing dogs (excluding puppies under seven months old) for heartworm infection prior to starting preventive medication. In a shelter setting, this would ideally be performed during intake to identify positive animals who will require treatment and to prevent them from becoming a reservoir of infection for other animals. While shelters are encouraged to test at-risk dogs, organizations must balance this recommendation with considerations like resource allocation and their capacity for care. Even when testing is not performed, administration of year-round heartworm preventive is still recommended for all shelter dogs. Current medications used for heartworm prevention, like macrocyclic lactones, have been shown to be safe for use in infected dogs and are a critical component of treatment protocols for dogs with heartworm disease.   

Cats and ferrets should also receive year-round heartworm preventive while in the shelter’s care. Testing prior to starting preventive medications is not recommended or necessary because of the different nature of infections and the limitations of available tests in these species.   

Reducing exposure to mosquitos through use of a prescribed repellent and environmental control, like eliminating standing water, is also helpful in preventing infection with heartworms.   

In animal shelters, development of protocols regarding testing, prevention and treatment of heartworms should be overseen by a veterinarian familiar with the population and knowledgeable in shelter medicine.   

Since animals are often stressed in a shelter, it is best to have heartworm-positive dogs reside in foster homes during and following treatment. To enhance the dog’s well-being while their exercise is restricted, foster providers should seek out ways to enrich the dog’s environment. Any abnormal clinical signs should be reported to the shelter’s veterinarian. 

The Association of Shelter Veterinarians and the American Heartworm Society collaborated to produce educational brochures  on various topics related to heartworm disease and its treatment in newly adopted animals. Sharing such information will best prepare adopters regarding the health needs of dogs recently treated for heartworm and animals who are heartworm positive at the time of adoption.