Leptospirosis in cattle is widespread in most countries of the world and in Ukraine. The causative agents of leptospirosis are the pathogenic leptospires L. Interrogans. Despite the considerable genetic diversity among pathogenic leptospires, clinical signs of disease caused by these bacteria are similar, and range from mild course (mostly asymptomatic chronic infection) to severe form potentially lethal infection.

The vast majority of pathological changes in parenchymal organs of animals which had died or recovered from leptospirosis are the result of the leptospira exotoxin influence on the surrounding tissues. At present there are a few data on the microscopic changes in the internal organs of cows which had died or recovered from leptospirosis. The purpose of the research was to investigate the microscopic changes in the lungs, liver and brain of cows which had died or recovered from leptospirosis.

The samples of organs’ tissues were selected from 7 necessarily slaughtered and dead cows (ages from 2 to 6 years), serum of which was positive in microagglutination reaction (MAT) and had specific antibody titers of 1:100 and higher to serogroups of leptospira – Sejroe and Tarassovi.

The selected fragments of these organs were taken not later than 30 minutes for histological test after the slaughtered animals and no later than 1 hour after the death of the cattle. The material was fixed in 10% neutral formalin solution, it was dehydrated in ethanol of increased concentrations and through chloroform it was poured in paraffin. 7-9 micron thick sections were obtained at Sannomiya microtome and were colored in Karatsu haematoxylin and eosin. In a histological study of cows’ liver that died from leptospirosis, we found significant violations of the microscopic structure of this organ. The central veins were dilated; endothelium in many of them was completely or partially missing. Also most advanced internally lobed haemocapillaries and Disse’s spaces were dilated. Liver lobules were irregularly swollen; streamlined girder structure of all liver lobes in many areas was broken. All hepatocytes were in condition of the granular or hidropic dystrophy.

In the lungs of tested cows all blood vessels were distinctly dilated, filled with blood. The red blood cells in the lumen of most of the major blood vessels were glued (sludge-phenomenon). The microscopic changes in the alveoli were with two-fold character. However, regardless of the nature of these changes, the walls of the alveoli were severed.

In some areas, occupying significantly smaller portion of the organ, the blood capillaries of the alveolar walls were distinctly dilated, filled with blood. Much of alveolar epithelial cells and other cells of the alveolar walls were collapsed. There were found colonies of bacteria in the cytoplasm of many cells of the alveolar walls. In the lumen of the alveoli and their walls isolated monocytes and macrophages were registered. There were identified quite large foci of necrosis in the parenchyma of reactive lung. Such foci were surrounded by a relatively wide leukocytic shaft. The bronchial epithelial cells were in a state of granular dystrophy. Part of dystrophic cells was destroyed.

In a histological study of the cows' brain, the diffusely swollen glia was found. There was particularly pronounced swelling around the nerve cells and glial cells. The edematous fluid destruction of nerve cells was recorded in areas of their significant accumulation.

In many nerve cells there were identified microscopic features of granular dystrophy and basophilia.

As a result of the research a number of micromorphological features of pathological changes in cattle with leptospirosis were described and set of criteria which can be used to study post-mortem diagnosis for the disease was identified.

Key words: leptospira, leptospirosis, histological changes, cattle.