Respiratory affection is the most serious disease affecting poultry and causes heavy economic losses to the poultry industry worldwide¹. The main factors contributing to the severity of respiratory affections in chickens are anatomy and physiology of respiratory system, along with complex nature of respiratory pathogens. The respiratory conditions often involve more than one pathogen with management factors like ventilation, humidity, dusty environment and ammonia playing a critical role¹. The present study was carried out to study the prevalence and pathology of respiratory lesions in broilers.

During the study period of eight months stretching from July 2017- February 2018, a total of 339 post-mortem examinations were done in broilers covering twenty different poultry flocks. Broiler carcasses (n=339) were examined for the presence of gross respiratory lesions during post mortem. The cases were graded as mild, moderate and severe based on the intensity of respiratory lesions. Overall occurrence of respiratory affections was evaluated as proportion of birds necropsied. Lesions of the trachea, bronchi, lungs, and air sacs were scored for gross severity. Briefly, respiratory organs were scored altogether on scale of 0 to 3, where 0 - no lesions, l - mild or slight lesions, 2 - moderate lesions, and 3 - severe lesions².

The severity of air sac lesions were classified on the basis of air sac lesions scoring described by earlier workers³ as, no lesions, cloudiness of air sacs- mild airsacculitis, air sac membranes thickened- moderate airsacculitis, “Meaty” appearance of membranes, with large accumulation of “cheesy” exudates confined to one airsac- severe airsacculitis and lesions in two or more air sacs with severe airsacculitis. The tracheitis was classified on the basis of inflammatory exudates as catarrhal/mucous, caseous, fibrinous and haemorrhagic tracheitis⁴.

Samples of trachea and lungs were collected from broiler carcasses showing gross respiratory lesions and processed for histopathological examination. Approximately 0.5 cm in thickness of tissue samples were taken and fixed in 10 per cent formal saline and trimmed to a thickness of about 3 mm. The tissue samples were dehydrated, cleared and embedded in paraffin, and cut to obtain 5 μm thick paraffin-embedded tissue sections. The sections were mounted on glass slides and stained with haematoxylin and eosin as well as Mayer‘s mucicarmine stain for histopathological examination⁵.

A total of 339 post mortem examinations were carried out in birds from various sources, of which gross respiratory lesions were observed in 104 birds. Thus, the prevalence of respiratory lesions in broilers of different age groups was determined as 30.67%. Maximum birds with respiratory lesions were received in the month of February. The severity of gross respiratory lesions were observed and scored as mild to severe.

Trachea, lungs and air sacs differ in their susceptibility due to their response to different irritants and pathogens affecting the respiratory tract⁶. The prevalence of respiratory lesions in broilers was determined as 30.67%. Similar to our findings, previous workers also reported a higher occurrence of gross respiratory lesions (33.61%) in broilers⁷. However, the present occurrence of respiratory tract lesions was much higher than a few earlier reports⁷. The observed discrepancy may be attributed to the increased prevalence of respiratory pathogens in the study area and may also reflect the weak immune status of the birds in the study area.

The maximum cases of birds with respiratory lesions were observed in month of February where normal average temperature was 11°C, whereas the least cases of respiratory lesions were observed in month of October where the average temperature was 14°C. This finding was in accordance with previous workers⁸,⁹, who identified more prevalence of respiratory lesions in poultry during winter season. The authors opined that the increased prevalence of respiratory lesions is usually observed with the increase in moisture that favours multiplication of organism and also due to decrease in immunity of the bird due to the stress created possibly by colder climate⁹. The other reasons for occurrence of the disease seem to be poor management practices, fluctuation of temperature during winter and increased concentrations of ammonia gas due to poor ventilation⁸.

Gross lesions observed in trachea of broiler carcasses during present study varied in their nature and severity. The findings of different types of tracheitis were marginally higher than the earlier reports⁷. The present occurrence of catarrhal tracheitis of 59.03% was lower than the findings of 76.00% catarrhal tracheitis observed previously¹⁰. The differences could be attributed to stage of disease progression and nature of ailment. Tracheal congestion is the first and foremost response to aerosol irritants and infections⁶.

Air sacs are affected mostly due to the physiology of the avian respiratory system, where inspired air first reaches air sacs rather than to lungs. In accordance with findings of earlier workers¹¹ moderate type of air sacculitis with thickened air sac membranes were found in maximum (32.92%) birds.

The prevalence of gross lung lesions in our study was found to be 68.26%. This is in accordance with findings of previous workers⁷,¹² who opined that lung congestion accounted for 66.10% of all broilers affections. However, our findings are considerably higher than earlier workers, who found pulmonary oedema, classified as acute heart failure/suffocation, in 23.40% of investigated dead broilers. Hyperaemia, congestion and oedemaof lung were found to be consistent finding in broilers under heat stress⁴. A significantly higher occurrence of lesions in lungs (55.43%) in comparison to lesions in other part of respiratory tract was reported earlier⁷. The workers hypothesized that lungs having slow recovery power, when compared with other parts of respiratory tract, are apt to develop progressive lesions. In marked contrast to our findings, 1.27% occurrence of gross lung lesions were observed in a slaughterhouse study of broiler carcasses¹².

Microscopically the changes observed in the trachea were more of inflammatory nature. The common histopathological change noticed in trachea was congestion (50.00%) characterised by engorged blood vessel. Congestion is usually due to the passive accumulation of blood within the blood vessels which can be due to various pathological processes like release of vasodilator chemicals and neurogenic stimuli associated with irritation to the tissue.

Leukocyte infiltration is a fundamental immune response in which leukocytes migrate and reach the area of injury for elimination of inflammatory trigger and for tissue repair. The current study revealed leukocyte infiltration (36.66%) in trachea. Hypoxia and anoxia, toxic injury, vascular wall necrosis and vasculitis due to various aetiologies can lead to the haemorrhages in tissues. The occurrence of haemorrhages in trachea was 21.66% which was higher when compared to the findings of earlier workers⁷,¹¹. Thickening of mucosa in trachea are usually associated with epithelial hyperplasia and accumulation of leukocytes in trachea. The surface epithelial destruction, loss of cilia and necrosis of the superficial layer were also reported by earlier workers⁶,⁷.

Appreciable microscopic changes were observed in all the lung tissues examined. Congestion and haemorrhages was the most common finding noticed in lung of broilers showing respiratory tract affections. Haemorrhage was characterised by the extravasation of blood into air capillaries, secondary bronchi, parabronchial lumen and other areas of lung. The findings of oedema and necrosis in lung of affected birds were similar to those reported earlier²,⁸. Oedema occurs due to the pressure changes between interstitial space and vascular vessels altering the vascular permeability ultimately leading to extravasation of fluid. It was also explained by another author that broilers are genetically programmed for hyperphagia, and this in turn causes distension of the gastrointestinal tract and decreased ventilation and perfusion of the lungs⁶.

In conclusion, respiratory tract being the most vulnerable site for affections due to their anatomy and physiology often encountered by various pathogenic agents causing different types of pathology. Thus, the gross respiratory change along with the microscopic changes provides a better understanding on the respiratory lesions which in turn can lead to proper management of disease conditions.

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