The first part of this miniseries introduced dirofilariasis, also commonly known as heartworm disease, overviewing the life cycle of the parasite and discussing the impacts of climate change on the prevalence of the disease. With warmer weather becoming the new norm, there is an increasing likelihood of transmission of heartworms to dogs and cats in regions where the disease has previously not been established. As such, understanding the clinical signs and treatment options for this disease is increasingly important.
Clinical signs of heartworm
Canines
In dogs, the clinical signs of heartworm disease may include:
- Coughing
- Exercise intolerance
- Dyspnoea (difficulty breathing)
- Lethargy
- Weight loss
- Reduced appetite
In advanced cases, dogs may develop right-sided heart failure, which can cause ascites and distended jugular veins. The invasion of the worms to the right ventricle and right atrium can cause caval syndrome, a life-threatening condition that results from the obstruction of the right-heart inflow and consequent haemoglobinuria, hepatic and renal dysfunction, signs of heart failure and disseminated intravascular coagulation. Patients exhibiting these symptoms often have a high worm burden. However, only 25 percent of dogs had clinical evidence of caval syndrome in a recent study (Romano et al., 2021).
The presence of heartworms in the pulmonary artery leads to inflammatory disease (proliferative endarteritis) and pulmonary thromboembolism (PTE), which is the result of the death of the adult heartworms with the severity depending on the number of dead worms (Figure 1).
Both endarteritis and PTE can lead to pulmonary hypertension (PHT) and respiratory difficulties. Dogs can also develop immunocomplexes that can lead to glomerulonephritis; in fact, proteinuria is commonly reported (Romano et al., 2021).
Felines
In cats, the clinical signs of heartworm disease are often non-specific and may include coughing, dyspnoea, lethargy and anorexia. Cats have two manifestations of D. immitis infection: heartworm-associated respiratory disease (HARD) and adult heartworm disease (Garrity et al., 2019). The respiratory signs in HARD are also due to the presence of pulmonary intravascular macrophages that often lead to eosinophilic pneumonitis.
Dogs do not have pulmonary intravascular macrophages, therefore do not develop similar acute and severe respiratory disease.
Dirofilariasis in cats can often be mistaken for feline asthma as clinical signs are similar. In the case of the death of adult heartworms, pulmonary thromboembolism is inevitable and can be fatal
Adult heartworm disease is less common in cats as they are incidental hosts and complete larval development might not happen. Dirofilariasis in cats can often be mistaken for feline asthma as clinical signs are similar. In the case of the death of adult heartworms, PTE is inevitable and can be fatal, whereas this is better tolerated in dogs.
Humans
Dirofilariasis presents a zoonotic risk and humans, like cats, are incidental hosts; therefore, aberrant migration (ie ocular) is more common. Most commonly, D. immitis causes the formation of lung nodules secondary to PTE.
Diagnosing dirofilariasis in animals
Diagnosis of dirofilariasis can be challenging, especially in cats. Radiographic changes are often aspecific and secondary to PTE, PHT or pulmonary oedema. The presence of an interstitial pulmonary pattern with radiodense zones in the lung parenchyma and enlargement, blunting and tortuosity of one or both caudal pulmonary arteries can be suggestive of dirofilariasis in cats (Atwell and Rezakhani, 1986).
In cases of an infection of adult heartworms, echocardiography can reveal their presence in the pulmonary artery, right ventricle or right atrium (Bussadori and Borgarelli, 1993; Losonsky et al., 1988; Venco et al., 1999). However, it is often difficult to assess how many adults are present and worms can be missed if only a few are present. Signs of PHT secondary to dirofilariasis include pulmonary artery dilation and right ventricular concentric hypertrophy (Romano et al., 2021).
The presence of an interstitial pulmonary pattern with radiodense zones in the lung parenchyma and enlargement, blunting and tortuosity of one or both caudal pulmonary arteries can be suggestive of dirofilariasis in cats
Laboratory testing includes the modified Knott’s test for the visualisation of microfilariae (although not able to distinguish different species of Dirofilaria), antigen testing (SNAP test) and antibody testing. A positive antigen test is a definitive test for the presence of female adult heartworms. Possible testing scenarios are:
- Microfilariae positive/antigen positive: infected by D. immitis
- Microfilariae positive/antigen negative: possible infection with other species of Dirofilaria, patient infectedtransplacentally or patient infected via blood transfusion
- Microfilariae negative/antigen positive: patient infected with D. immitis but amicrofilaraemic (no microfilariae due to infection with only male worms and/or very old or young female worms)
Cats usually have a low burden of adults, therefore lower concentrations of circulating antigens. It is important to note that cats with negative antigen and microfilariae tests can still have HARD, therefore antibody testing can be performed to support D. immitis infection in cases showing clinical signs.
Treatment options for heartworms
The treatment of heartworm disease in dogs and cats involves several steps, including stabilisation and adulticide therapy. The treatment may vary depending on the severity of the disease and the health status of the animal. The American Heartworm Society recommends a multi-step treatment protocol for dogs, which involves the use of adulticide drugs, such as melarsomine, and a pre-treatment with a macrocyclic lactone, such as ivermectin or milbemycin oxime, to reduce the number of circulating microfilariae.
The treatment is usually administered in three injections over a period of two months (first injection on day 1, second injection on day 30 and third injection on day 31) so the potential episodes of PTE following adult heartworm death are spread over time. This is followed by a period of restricted activity to prevent thromboembolism. During this period steroids can be given to reduce inflammatory processes.
Surgical removal through venous access under fluoroscopy guidance is possible and recommended in dogs with a high adult heartworm burden or caval syndrome (Figure 2).
In cats, the treatment of heartworm disease is more challenging, as there is no approved adulticide therapy available. Adult heartworm death in cats often leads to fatal PTE and anaphylaxis, therefore adulticide therapy is generally avoided. The available treatment options involve supportive care, such as oxygen therapy, bronchodilators and corticosteroids, to manage the clinical signs and prevent further damage. In some cases, surgical removal of the worms may be considered, but it is a risky procedure that can lead to life-threatening complications. Finally, Wolbachia spp. are susceptible to tetracycline and in the absence of these symbiotic bacteria the larval development stops at L3 (Casiraghi et al., 2002; Genchi et al., 2007).
Surgical removal through venous access under fluoroscopy guidance is possible and recommended in dogs with a high adult heartworm burden or caval syndrome
Prevention of heartworms
Prevention of heartworm disease is an essential aspect of the management of the disease, as it is often easier and less expensive than treatment. The prevention of heartworm disease in dogs and cats mainly involves the use of monthly preventative medications, such as macrocyclic lactones, which kill the microfilariae and prevent the development of adult worms. In addition to preventative medications, other measures, such as mosquito control and regular testing, may be used to reduce the risk of infection.
Conclusion
Heartworm is a serious zoonotic disease that can cause significant health problems in dogs and cats as well as many other mammals. The clinical signs of heartworm disease depend on several factors and can be challenging to diagnose and treat. The treatment of heartworm disease in dogs involves the use of adulticide drugs and surgical removal of the adult heartworms. Prevention in endemic areas is a safe, effective and cheap way to avoid this life-threating disease and its therapy side-effects.
