○ Negative : Suggests a lower likelihood of infection with adult heartworms, but does not rule it out. ● Heartworm antibody test: ○ Positive : Indicates exposure to heartworm larvae. ○ Negative : Suggests a lower likelihood of infection but does not rule it out. ● Microfilaria (using Knott’s or filtration test):
○ Positive : Increases the likelihood of detecting feline heartworm disease, but does not definitively confirm infection. ○ Negative : Suggests a lower likelihood of feline heartworm disease, but does not rule out infection. ● Echocardiography: ○ Positive : Confirms infection with adult heartworms. ○ Negative : Suggests a slightly lower likelihood of infection with mature adult heartworms, but does not rule out infection with larvae or immature heartworms.
○ Positive : Confirms infection with adult heartworms. ○ Negative : No significance, does not impact the likelihood of infection. ● Thoracic radiographs:
TREATMENT
In addition to the challenges that accompany the diagnosis of heartworm disease, there are also treatment challenges associated with this condition. Although canine heartworm disease has a single gold-standard treatment, which involves melarsomine injections, no such option exists for feline patients. Unfortunately, there are no controlled studies examining the effects of various heartworm treatment approaches in cats; Surgical treatment Although it is rarely performed, the only definitive treatment for adult feline heartworms is surgical removal of the worms. Basket catheters and loop snares are often used to perform this procedure; both are typically inserted through a right jugular venotomy (American Heartworm Society, 2014). Additionally, a left thoracotomy can also be used to insert alligator forceps through a right ventricular incision (American Heartworm Society, 2014). Surgical worm extraction must be performed with care, because death or crushing of the worms can lead to acute respiratory signs in the cat. This technique is considered optimal for cats Medical treatment Most cats do not receive surgical treatment for their heartworm infection. Instead, they are typically treated on a symptomatic basis. This treatment involves the administration of prednisolone, which is initiated at a dose of 2 mg/kg/day and gradually tapered (over a period of approximately two weeks) to a dose of 0.5 mg/kg every other day (Kittleson & Kienle, 2015) or even completely tapered and discontinued (ESDA, 2017). Prednisone is not recommended in cats, due to unpredictability in feline conversion of prednisone to prednisolone, the active form of the drug; prednisolone is preferred for treatment (ESDA, 2017). In cats that do not tolerate oral medication, or that seem to experience inconsistent absorption of oral prednisolone, a dose of 20 mg of methylprednisolone acetate may be administered by injection once monthly. Radiographs can be used to monitor the cat’s response to therapy and assess the need for further interventions (American Heartworm Society, 2014). If a heartworm-infected cat experiences thromboembolic disease, more aggressive therapy may be required. Options for the treatment of thromboembolic disease include oxygen therapy, intravenous fluids, injectable steroids, and bronchodilators as needed (American Heartworm Society, 2014; Kittleson & Kienle, 2015). When treating these patients, diuretics, aspirin, and nonsteroidal anti-inflammatory medications should be avoided (ESDA, 2017). Melarsomine is not typically recommended in cats, due to a high risk of toxicity. Studies in cats demonstrated no benefits with a single injection of 2.5 mg/kg melarsomine (Kittleson & Kienle, 2015). Adult worm death is enhanced with the use of two and three injection protocols, but cats treated with multiple melarsomine injections also experienced a high mortality rate
additionally, there is no evidence that any one treatment approach improves outcomes over any other treatment approach (Kittleson & Kienle, 2015; ESDA, 2017). Therefore, treatment approaches in feline heartworm infections are largely based on conjecture, with little evidence-based information available regarding why to choose one approach over another.
that are heavily infected or experiencing critical signs of adult heartworm infection (American Heartworm Society, 2014), because the potential benefits of the procedure can outweigh the risks. Unfortunately, this technique has a significant limitation. The most severe effects of feline heartworm infection are typically seen in the early stage of infection, when cats are infected with larval heartworms. Adult worms are typically not present during this stage of infection; even if adult worms are present, removal will not have any effects on reducing pulmonary inflammation. (in both controlled studies and clinical practice), making this treatment approach unacceptable (Kittleson & Kienle, 2015). Ivermectin has been shown to decrease worm burden in cats with heartworm infection. The recommended dose for ivermectin therapy is 24 µg/kg, given once monthly (American Heartworm Society, 2014). Studies have shown that after two years of treatment, cats receiving ivermectin experience up to a 65% reduction in worm burdens compared to cats in a control group (American Heartworm Society, 2014). It is unknown whether this therapy is actually beneficial for the cat, however, because many of the effects of feline heartworm disease are a result of the death of adult worms, not a result of the presence of adult worms. Doxycycline is another medical therapy that is used for the treatment of heartworm disease in dogs, for the role it plays in the treatment of Wolbachia. Wolbachia are gram- negative bacteria that reside within the body of Dirofilaria immitis parasites. In dogs, the treatment of Wolbachia with doxycycline is thought to hasten the death of adult heartworms. Additionally, the release of Wolbachia when adult heartworms die has been shown to upregulate pro-inflammatory cytokines, increase immunoglobulin levels, and increase neutrophil recruitment, indicating that Wolbachia plays a role in the inflammation associated with heartworm death. For this reason, there is some suspicion that doxycycline may also have a role to play in the treatment of feline heartworm disease. At this time, however, the role of Wolbachia in feline heartworm disease is unclear and treatment of heartworm-infected cats with doxycycline is not currently recommended (Colleran, 2017; ESDA, 2017).
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