Effect of pre-cardiac and adult stages of Dirofilaria immitis in pulmonary disease of cats: CBC, bronchial lavage cytology, serology, radiographs, CT images, bronchial reactivity, and histopathology

Abstract

A controlled, blind study was conducted to define the initial inflammatory response and lung damage associated with the death of precardiac stages of Dirofilaria immitis in cats as compared to adult heartworm infections and normal cats. Three groups of six cats each were used: UU: uninfected untreated controls; PreS I: infected with 100 D. immitis L3 by subcutaneous injection and treated topically with selamectin 32 and 2 days pre-infection and once monthly for 8 months); IU: infected with 100 D. immitis L3 and left untreated. Peripheral blood, serum, bronchial lavage, and thoracic radiographic images were collected from all cats on Days 0, 70, 110, 168, and 240. CT images were acquired on Days 0, 110, and 240. Cats were euthanized, and necropsies were conducted on Day 240 to determine the presence of heartworms. Bronchial rings were collected for in vitro reactivity. Lung, heart, brain, kidney, and liver tissues were collected for histopathology. Results were compared for changes within each group. Pearson and Spearman correlations were performed for association between histologic, radiographic, serologic, hematologic and bronchoalveolar lavage (BAL) results. Infected cats treated with selamectin did not develop radiographically evident changes throughout the study, were heartworm antibody negative, and were free of adult heartworms and worm fragments at necropsy. Histologic lung scores and CT analysis were not significantly different between PreS I cats and UU controls. Subtle alveolar myofibrosis was noted in isolated areas of several PreS I cats and an eosinophilic BAL cytology was noted on Days 75 and 120. Bronchial ring reactivity was blunted in IU cats but was normal in PreS I and UU cats. The IU cats became antibody positive, and five cats developed adult heartworms. All cats with heartworms were antigen positive at one time point; but one cat was antibody positive, antigen negative, with viable adult females at necropsy. The CT revealed early involvement of all pulmonary arteries and a random pattern of parenchymal disease with severe lesions immediately adjacent to normal areas. Analysis of CT 3D reconstruction and Hounsfield units demonstrated lung disease consistent with restrictive pulmonary fibrosis with an interstitial infiltrate, absence of air trapping, and decrease in total lung volume in Group IU as compared to Groups UU and PreS I. The clinical implications of this study are that cats pretreated with selamectin 1 month before D. immitis L3 infection did not become serologically positive and did not develop pulmonary arterial hypertrophy and myofibrosis.

Temporal Pattern of Circulating Antigens and Antibody Responses in Cats Experimentally Infected with Dirofilaria immitis

“…We propose that in this experimental model with infection of a large number of L 3 (100), the death of large numbers of immature worms may release adequate concentration of diagnostic antigen in the absence of mature female worms. Although this may lead to detectable antigenemia at time points more closely associated with HARD, similar results were not demonstrated in early detection of antigen in other experimental infections , Dillon et al 2013, Dillon et al 2014). …”

“…Cats that develop adult heartworm infection can suffer significant complications, including coughing, dyspnea, vomiting, and sudden death (McCall et al 1992, Lister et al 2008, Blagburn and Dillon 1984, Dillon et al 2014. Immature adult heartworms reaching the pulmonary arteries at approximately 2.5 -4.0 months post infection can cause significant lung pathology in infected cats, a condition referred to as heartworm-associated respiratory disease or HARD ).…”

Effects of intravenous and subcutaneous heartworm homogenate from doxycycline-treated and untreated donor dogs on bronchial reactivity and lung in cats

“…Thoracic radiographs at the completion of the 18-day IV homogenate studies were acquired on randomly selected cats and graded for pulmonary artery, bronchial pattern, interstitial pattern, and mixed bronchial-interstitialalveolar pattern using a 0-3 scoring system (0 = normal; Brawner et al, 2000;Dillon et al, 2013;Dillon et al, 2014). In addition, any abnormalities in cardiac, pleura, or diaphragm were recorded.…”

“…Isometric force of epithelium-intact third-to fourth-generation intra-parenchymal bronchioles (IPBs) (internal diameter 300-500 m) from cats was performed as described (Wooldridge et al, 2012, Dillon et al, 2013Dillon et al, 2014). Force data were recorded digitally (PowerLab, ADInstruments; Colorado Springs, CO).…”

“…The interstitial lung disease and pulmonary arterial involvement often develop in lobes and vessels where the parasite does not physically reside (Dillon et al, 2007;Dillon et al, 2014). The concept that the physical presence of the worm induces only local direct injury does not correlate with the pattern of the lung disease observed in heartworm-infected cats.…”