Pathology in Practice

Ana Margarida Alho Centro de Investigação Interdisciplinar em Sanidade Animal, Faculdade de Medicina Veterinária, Universidade de Lisboa, 1300–477 Lisboa, Portugal.

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Alessio Giannelli Dipartimento di Medicina Veterinaria, Università degli Studi di Bari, 70010 Valenzano, Bari, Italy.

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Vito Colella Dipartimento di Medicina Veterinaria, Università degli Studi di Bari, 70010 Valenzano, Bari, Italy.

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Domenico Otranto Dipartimento di Medicina Veterinaria, Università degli Studi di Bari, 70010 Valenzano, Bari, Italy.

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Luís Madeira de Carvalho Centro de Investigação Interdisciplinar em Sanidade Animal, Faculdade de Medicina Veterinária, Universidade de Lisboa, 1300–477 Lisboa, Portugal.

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Jorge Jesus Correia Centro de Investigação Interdisciplinar em Sanidade Animal, Faculdade de Medicina Veterinária, Universidade de Lisboa, 1300–477 Lisboa, Portugal.

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History

A 6-year-old 6.5-kg (14.3-lb) neutered male European Shorthair cat that had been adopted in the preceding year from a shelter in Setúbal, Portugal (an area endemic for several vector-borne diseases), was found dead by its owners in their house. The cause of death was not apparent. The cat lived with 2 other cats and had access to the outdoors. The owners mentioned that despite the fact that the cat had previously been identified as positive for FeLV antigen, it was very active and appeared healthy. The cadaver was brought to the Small Animal Teaching Hospital of the Faculty of Veterinary Medicine, University of Lisbon, in a state of rigor mortis.

Gross Findings

Postmortem examination revealed severe pulmonary edema and greater-than-normal lung tissue density. The lungs appeared shiny and red. There was aqueous fluid in the bronchial bifurcation and tracheal lumens and moderate hydropericardium in a nondilated heart. Three dead white wormlike organisms were found in the main pulmonary artery, with the longest reaching the lumen of the right heart ventricle (Figure 1). In addition, erosive gastritis, thickening of the duodenal walls, and marked splenomegaly were evident as well as bilateral mandibular and prescapular lymphadenomegaly.

Figure 1—
Figure 1—

Photographs of the lungs (A) and heart (B) of an adult European Shorthair cat that was found dead by its owners in their house. Wormlike organisms are visible in the pulmonary artery, at the base of the right lung (arrow; A) and in a cross-sectional view of the right ventricle (arrow; B).

Citation: Journal of the American Veterinary Medical Association 249, 7; 10.2460/javma.249.7.751

Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →

Histopathologic Findings

Samples of lungs, spleen, and liver were routinely processed for histologic analysis. Examination of lung tissue revealed the presence of interstitial pneumonia, with thickening of the interalveolar septa, hemorrhage, and a diffuse infiltrate of mononuclear and eosinophilic cells (Figure 2). Airway smooth muscle hyperplasia was also observed, possibly as a consequence of chronic respiratory stress. Chronic and severe obliterating endarteritis of the pulmonary artery was detected, with hyperplasia of the endothelial cells from the intima layer, which created irregular projections into the lumen. The intima layer was thickened and filled with an inflammatory infiltrate of giant cells, lymphocytes, macrophages, plasma cells, and fibrous tissue. Proliferation of the smooth muscle layer and congestion with lymphoid cell infiltrate in the adventitia were observed (Figure 3). Passive congestion of the spleen and the liver (mainly at the centrilobular level) was also evident.

Figure 2—
Figure 2—

Photomicrograph of a section of lung tissue from the cat in Figure 1. Notice the evidence of interstitial pneumonia, thickening of the interalveolar septa, hemorrhage, diffuse mononuclear cell infiltrate, and airway smooth muscle hyperplasia. H&E stain; bar = 100 µm.

Citation: Journal of the American Veterinary Medical Association 249, 7; 10.2460/javma.249.7.751

Figure 3—
Figure 3—

Photomicrograph of another section of lung tissue from the cat in Figure 1. Severe arteritis characterized by hyperplasia and thickening of the intima layer (circle), proliferation of the smooth muscle layer (square), and congestion with lymphoid cell infiltrate in the adventitia (star). H&E stain; bar = 200 µm.

Citation: Journal of the American Veterinary Medical Association 249, 7; 10.2460/javma.249.7.751

Parasitological Findings

The 3 wormlike organisms retrieved from the cat at necropsy were elongated and filiform, with thin and rounded cephalic extremities, and measured 27.2, 21.0, and 13.4 cm in length (Figure 4). Parasites were identified on the basis of their morphology.1 Briefly, the nematode had a terminal and circular oral opening, surrounded by 4 pairs of small cephalic papillae and 2 lateral amphids. Additionally, their body surfaces had delicate transverse striations. The nematodes were identified as an adult specimen of Dirofilaria immitis.

Morphologic Diagnosis and Case Summary

Morphologic diagnosis and case summary: acute pulmonary thromboembolism, circulatory collapse, and sudden death as a result of D immitis infection in a cat.

Comments

The clinical and pathological abnormalities along with the morphological characteristics of the parasite found in the cat of the present report were all supportive of D immitis infection. Following the extensive postmortem examination, the causes of death were considered to be circulatory collapse and respiratory failure as a result of acute pulmonary embolism caused by D immitis nematodes.

Cardiopulmonary dirofilariosis, also referred to as heartworm disease, is severe and life threatening. The prevalence and geographic distribution of this disease have increased during the past few years.2 Although cats are susceptible hosts for this nematode, they are considered more resistant to infection than dogs.3 Infections with D immitis in cats occur in the same regions in which canine dirofilariosis is endemic, albeit with a lower prevalence (between 5% and 20% of that among dogs).4,5 In cats, infection with D immitis may induce a severe condition, the clinical signs of which are part of a syndrome known as heartworm-associated respiratory disease.6,7 This syndrome is an acute vascular and parenchymal inflammatory response usually caused by the presence of immature adult worms and subsequent parasite death in the pulmonary arteries.2 On the other hand, feline dirofilariosis may lead to death in the absence of clinical signs.

Figure 4—
Figure 4—

Photograph of 3 adult nematodes, identified as Dirofilaria immitis, collected from the main pulmonary artery of the cat in Figure 1. The nematodes were 27.2, 21.0, and 13.4 cm in length.

Citation: Journal of the American Veterinary Medical Association 249, 7; 10.2460/javma.249.7.751

Antemortem diagnosis of cardiopulmonary dirofilariosis in cats is difficult especially when clinical signs may not be indicative of the presence of the nematode, or when those related to the heartworm-associated respiratory disease syndrome are absent.3,4 Respiratory and gastrointestinal tract signs (eg, chronic coughing, labored breathing, or vomiting) may often mimic other more common diseases, leading practitioners to often misdiagnose this infection as asthma or allergic bronchitis.2 In addition, most cats are amicrofilaremic or have a small microfilarial burden, which frequently leads to false-negative results of parasitological and serologic testing.3 Electrocardiography may detect the presence of the parasite, although alterations are minimal (eg, usually only a double-lined echo density in the main pulmonary artery or its branches). Also, radiographic alterations (ie, the so-called enlarged caudal pulmonary arteries and pulmonary parenchymal changes) may be inconsistent or transient.8 For these reasons, the American Heartworm Society suggests that the diagnosis of feline heartworm disease should be made on the basis of a combination of medical history along with findings of 4 test methods (circulating antibody and antigen testing, thoracic radiography, and echocardiography).3 In the case described in the present report, the cat was adopted from a shelter in a highly endemic area of canine dirofilariosis,9 which stresses the importance of being acquainted with epidemiological data to effectively diagnosis heartworm disease in cats. It is important to note that clinical signs of the disease are often more severe in cats than in dogs. This can be explained by the presence of intravascular pulmonary macrophages, which causes an exacerbated pulmonary reaction including induction of acute respiratory distress or respiratory failure in cats.10 Other factors are the anatomic organization of the pulmonary arterial tree in cats (which is smaller than the homologue in dogs) and the lack of collateral circulation. Accordingly, embolization can easily result in infarction and death, even when the worm burden is low,8 as in the cat of the present report.

In cats, Dirofilaria infection usually becomes clinically apparent in its chronic form, with anorexia, weight loss, lethargy, exercise intolerance, cough, dyspnea, and vomiting as the major findings. However, an acute form may also become noticeable, either as a result of worm death and embolization or aberrant migration. Clinical signs of this acute form are salivation, tachycardia, shock, dyspnea, hemoptysis, vomiting, diarrhea, syncope, ataxia, seizures, and death.8

Considering the observed blockage of the pulmonary artery in the case described in the present report, the causes of death were most likely circulatory collapse and respiratory failure resulting from acute embolism of the pulmonary artery secondary to Dirofilaria infection.

Treatment options for cats infected with D immitis are limited and are usually based on supportive care. Currently, there is no adulticidal medication approved for the treatment of feline dirofilariosis. Nonetheless, highly effective prevention can be achieved through monthly chemoprophylaxis.4 These measures are important, given the increasingly favorable climatic conditions for D immitis vectors coupled with frequent animal movements worldwide. Taking into account the subclinical and lethal course of the infection (as illustrated by the case described in this report) and the lack of approved treatment in cats, prevention of dirofilariosis is crucial. There is a need for an increase in the general awareness of owners and practitioners about the threat of feline dirofilariosis and the necessity of routine diagnostic testing and targeted preventive treatment against heartworm infection in cats.

Acknowledgments

Research was funded by Fundação para a Ciência e a Tecnologia (FCT): PhD Research Grant reference SFRH/BD/85427/2012 and by CIISA, Faculdade de Medicina Veterinária, Universidade de Lisboa, Project reference UID/CVT/00276/2013.

References

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