The article presents the results of macroscopic, histological and histochemical studies of dogs liver for isoniazid poisoning. The pathoanatomical section of 18 corpses of dogs of all ages and breeds has been carried out, fragments of different parts of the liver have been selected. Liver samples were fixed in a 10% aqueous solution of neutral formalin, Carnoua and Buena liquids. The dewatering in ethanol of increasing concentration was conducted, sealed in paraffin. The sections were made on a microtome MC-2 in the thickness of 7 microns, stained with hematoxylin and eosin, according to the technique of McManus, Pearles. The produced histoplasts were examined under a light microscope Leica DM 2500 (Switzerland), photocycled by the Leica DFC450C camera using the Leica Application Suite Version 4.4 software. Macroscopically, the liver in all studied corpses of dogs is enlarged, the edges are rounded, inhomogeneously colored (from dark cherry to light brown), loose consistency, a light bloody fluid is secreted on the cut, the structure of the organ is smoothed out. Optically light in the liver of most dogs for isoniazid poisoning noted profound violations of all structures of angioarhitectonics, stroma and parenchyma of the organ. Prevalent non-fibrotic changes in the endothelium, dilatation and deformation of central veins and sinusoidal mesh with the formation of cells filled with blood plasma. The expressed dystrophy and necrobiosis of hepatocytes and Kupferov cells were combined with erythrocyte hemolysis, which was reflected by the deposition of hemosiderin in the cytoplasm of macrophages and bilirubin in hepatocytes. In rare cases, in the liver of poisoned dogs, acute congestive hyperemia, diapedeous hemorrhages in peripartal regions and non-fibrotic changes in hepatocytes were observed. On the basis of conducted morphological studies, it can be argued that derivatives of isoniazid compounds in the dog's organism have a pronounced hepatotoxic effect characterized by a severe violation of blood and lymph microcirculation and all metabolic processes in liver cells, manifested by dystrophic-non-fibrotic processes in the body.

The article presents the results of macroscopic, histological and histochemical studies of the heart and lungs of dogs for isoniazid poisoning. A pathoanatomical section of 19 carcasses of dogs of different ages and breeds was performed; heart and lung fragments were selected for morphological examination. Heart and lung samples were fixed in 10 % aqueous formalin neutral solution, Carnua and Buen fluids. Dehydration was carried out in ethanol of increasing concentration and embedded in paraffin. The sections were made on a MC-2 microtome 7 μm thick, stained with hematoxylin and eosin, using the McManus and Brasche technique. Made histopreparations were examined under a Leica DM 2500 light microscope (Switzerland), photo-fixation was performed with a Leica DFC450C using Leica Application Suite Version 4.4 software. The results of morphohistochemical studies have shown that in acute poisoning of dogs with isoniazid  in the heart prevailed acute disorders of the vascular circulation, manifested by a sharp expansion and destruction of the walls of the venous vessels, plasmorrhagia, microtromb of the capillaries and spasm of the arterioles, which led to the development of swelling, caused acute decrease of protein synthesation function of muscle fibers and dystrophic-necrobiotic changes of cardiomyocytes. It was noted, in the lungs of dogs on the background of acute congestive hyperemia, diapedesis of erythrocytes and accumulation of transudate in the lumen of the alveoli, foci of emphysema and bronchiectasis with rupture of the epithelial layer of the mucous membrane, elastic skeleton and muscular tubules of bronchial tubules that indicated the violation of gas exchange and the development of anoxemia. It is known that spontaneous isoniazid poisoning in dogs leads to an acute course and often fatal consequences. In the comprehensive study of the effects of chemicals on the body, and in particular isoniazid on the body of dogs, toxicological and pathomorphological studies are of great importance. The use of morphological and histochemical methods will reveal some of the mechanisms of intoxication.

Most publications on isoniazid poisoning in dogs are devoted to clinical diagnostics, treatment, and prevention of the disease. Histological and histochemical changes are not fully described, though they are important in assessing the toxic effects of isoniazid. Isoniazid is used to treat tuberculosis in humans. Dogs are hypersensitive to this drug. The article highlights the results of macroscopic, histological, and histochemical studies of the dogs’ lymph nodes and spleen in cases of isoniazid poisoning. A pathological examination of 19 corpses of dogs of different ages was performed, during which isoniazid poisoning was posthumously diagnosed, based on anamnesis, clinical signs, pathological autopsy, histological, and histochemical examination. Samples of lymph nodes and spleen were fixed in a 10% aqueous neutral formalin solution, Carnoy’s solution, and Bouin’s fixative. Histoсuts were prepared using a sled microtome and stained with hematoxylin and eosin. Staining was also performed according to the techniques suggested by McManus, Brachet, and Perls. The pathomorphological changes in lymph nodes and spleen were characterized by disorganization of vascular walls and connective tissue fibers of the stroma, dilatation of veins, their overflow with hemolyzed blood, and, in cases of the long clinical course, thrombosis of small vessels. Intravascular hemolysis of erythrocytes resulted in an excessive formation of hemosiderin. Histochemically, the spleen and lymph nodes showed a significant increase in the number of hemosiderophages in the spleen’s red and white pulp and the lymph nodes’ central sinuses and pulp cords. In the spleen, mucoid swelling and necrobiotic changes in the wall structures of the arterioles and arteries progressed with a narrowing of their lumen in dogs suffering from the long clinical course. Increased permeability of the microcirculatory tract vessels of the spleen and lymph nodes, transudate formation, and the destructive changes in the reticular skeleton accompanied hemodynamic violations. A sharp change in blood rheology caused the violation of trophism and metabolism in the immune system. Lymphoid elements of the lymph nodes and white pulp of the spleen were in a state of karyorrhexis and karyolysis. The morphological study of the immune system’s peripheral organs suggests that dogs poisoned by isoniazid demonstrate hemodynamic disorders, changes in the physicochemical properties of blood (hemolysis of erythrocytes and thrombosis). This is the basis of trophic disorders, metabolic malfunctions, and the development of dystrophic processes in all structural elements of the spleen and lymph nodes.