Heartworm in Animal Shelter Populations

Heartworm is a vector-borne parasite transmitted by mosquitos. The life cycle of a heartworm is complex and takes several months to complete. The spread of infection requires adequate humidity and temperature to support a mosquito population. Mosquitoes ingest an immature stage of the parasite known as microfilaria when they bite an infected dog. These microfilariae develop into a later but still immature infective stage while in the mosquito’s body over approximately 2 weeks. After that point, they can be transmitted to another animal through the bite of that mosquito. Once in the body of the animal host, this immature larval stage eventually develops into adult heartworms over a period of several months. Animals may be bitten and infected with heartworms multiple times by multiple mosquitoes, leading to heavier worm burdens that can increase the severity of the 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 is then a potential source of infection for mosquitoes and, consequently, 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 not only the health of the individual but also 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. 

Veterinarians prescribe preventive medication that can be applied topically, given orally, or injected. Published guidelines recommend testing dogs (except for puppies under 5-7 months of age) for heartworm infection prior to starting preventive medication. In a shelter setting, this would ideally be performed upon intake, to allow shelters to identify positive animals that will require treatment and to prevent them from becoming a reservoir of infection for other animals.  While shelters are encouraged to perform testing on at-risk dogs, organizations must balance this recommendation alongside considerations such as 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 (e.g., macrocyclic lactones) used for heartworm prevention have been shown to be safe for us in infected dogs and are, in fact, a critical component of treatment protocols for dogs with heartworm disease. 

Cats and ferrets should also receive year-round heartworm preventive while in the care of the shelter. 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 mosquitoes through use of a prescribed repellent and environmental control (e.g., elimination of 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. 

Clinical Signs

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

• Dogs:
  • Mild: cough or no symptoms
  • Moderate: cough and exercise intolerance
  • Severe: signs can include difficulty breathing, fluid accumulation in abdomen, episodes of loss of consciousness in addition to signs seen in mild and moderate cases. 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 don’t live for as long, 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.

Annual heartworm testing (screening) for dogs greater than 7 months of age is generally recommended and is the most common way infections are identified. Testing for the presence of heartworms can be complicated and should be overseen by a veterinarian. Commercially available heartworm antigen tests are performed on a sample of blood and will be positive if a protein in a female heartworm is detected. No antigen will be detected if younger (immature) stages of heartworm are present or if a dog is infected with only male worms. 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 either antigen or microfilaria can only detect infections involving adult worms. Tests may fail to detect antigen and give a false negative result if the test is performed early on in the infection process (i.e., within 5-7 months of infection), which is why testing young puppies is not indicated. Although “tandem testing” for both antigen and microfilaria at the same time is recommended in some professional guidelines, shelters should carefully balance this with consideration of risk and available resources. 

Upon a review of the dog’s history, a veterinarian can best determine when repeat testing should be performed and when it should next be repeated.   Imaging the lungs and heart with radiographs or an echocardiogram may be helpful in determining the extent of disease in individual dogs and in planning for treatment. Such testing is not always necessary, and, in many cases, heartworm infection can be successfully treated based on 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 a suspicion of heartworm disease. The most commonly used type of test checks for antibodies to heartworm infection, but it cannot distinguish between current and prior infections. None of the currently available screening tests used in dogs can be relied upon to rule 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, and those recommended for management of dogs in a typical home environment may not always be feasible or optimal in a shelter setting. Alternative protocols may provide a viable option for treatment when the most commonly used drug, melarsomine, is not accessible. Organizations and shelter veterinarians must consider the overall impact of any heartworm treatment on an individual’s dog welfare (including their behavioral health). Additionally, treatment options must be considered in conjunction with potential implications it may have for the larger population in terms of resource allocation and capacity for care. 

Administration of medications included in a particular treatment protocol is generally completed over the course of 2-3 months, though conversation to a “negative” (or “no antigen detected” state) may take several months longer. The first steps of most protocols include starting the dog on an antibiotic to eliminate bacteria contained within heartworms as well as 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 both 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 such drugs. Strict exercise restriction (e.g., 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 as it will depend on the severity of the infection, the dog’s clinical condition, and the specific treatment protocol utilized. Heartworm disease causes damage to a dog’s heart and lungs that can affect a dog’s long-term health even after the parasites are killed.  

Since animals are oftentimes stressed during their stay in a shelter, it is best to have heartworm-positive dogs reside in a foster home during and following treatment for heartworms. Foster care providers should inform the shelter’s veterinarian about any abnormal symptoms and implement enrichment of the dog’s environment while restricting the dog’s exercise. 

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

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