Infectious bursal disease is an acute, highly contagious immunosuppressive viral disease of young chicken damaging the bursa of Fabricius and impaired growth rate causing great economic loss to poultry industry¹. IBD is caused by non-enveloped dsRNA virus of the genus Avibirnavirus of Birnaviridae family². Chickens and turkey are considered to be the primary host species for IBD virus³. Chickens in the age group of 3-6 weeks are most prone to clinical infection⁴. Mortality due to IBD ranges from 1 to 40%^5,6,7. IBD occurs throughout the year, but more in the winter season followed by rainy, summer and spring seasons^7,8,9.

Incubation period of 3-4 days lead to a clinical disease which is manifested by high morbidity, mortality and marked immunosuppression of the affected ones¹⁰. The disease is characterized by anorexia, depression, dullness, reluctant to move, ruffed feathers with watery diarrhoea, trembling, severe prostration followed by death^7,11. Gross lesions include enlarged, edematous bursa with petechial hemorrhages on its luminal surface. Microscopic changes include complete lymphoid depletion, extensive hemorrhages in interfolicular tissue, follicular atrophy and severe heterophilic and lymphocytic infiltration in the interfolicular connective tissue^7,12,13. For confirmatory diagnosis, nucleic acid tests like RT-PCR and nucleic acid hybridization are used for the detection and differentiation of various IBD viruses^14,15. Serological test like AGPT and ELISA can also be used for detecting IBDV antigen and antibody^7,16. The present study reports the incidences of infectious bursal disease in and around Aizawl district of Mizoram.

Both commercially organized and unorganized household backyard poultry farms of Aizawl district and different localities of Mizoram were regularly visited from April, 2020 to March, 2021 to study the incidence, pathology and diagnosis of IBD. During the present study, the suspected flocks were clinically examined and the clinical signs and symptoms were recorded. All the relevant epidemiological information like name and location of farm, strength of affected flock, age, sex and breed of affected flock, morbidity and mortality rate, history of vaccination if any previous outbreaks of diseases, clinical sings and necropsy findings were recorded in details from the farm attendants/persons. In case of mortality/outbreak in the poultry population, the clinical signs which were exhibited by individual birds during the illness were recorded in detail in a proforma. In addition, sometimes some sick/moribund birds were kept under careful observation with feed and water ad libitum till death to record the detailed clinical signs along with other abnormalities.

Detailed post-mortem examination of all dead birds of the suspected cases of IBD was performed and all the gross pathological lesions in different organs were systematically recorded. Representative tissue samples (bursa of Fabricius, spleen, kidney, proventriculus, lung, liver, heart, caecal tonsils, thymus) showing lesions were collected and preserved in 10% neutral buffered formalin for histopathological examination. Tissues were processed as per the standard procedure, sectioned and stained with Mayer’s hematoxylin and eosin¹⁷. Tissue samples were collected aseptically in sterile polypropylene zipper bags, stored first at -20°C and then transferred to -80°C for further analysis for molecular diagnosis of IBD. The diagnosis was made mainly on the basis of clinical signs, characteristic gross and microscopic changes. However, molecular techniques like RT-PCR were employed to detect the viral antigens (i.e. VP2) of IBDV which was performed as per the procedure described by OIE manual, 2018 with modification¹⁸.

In the present study, the maximum cases of IBD (30 nos.) were recorded in 3-6 weeks old birds, which is in support of earlier record¹ as well as the reports of the previous authors^19,20 who found maximum cases (52.80%) in 21-30 days old birds followed by (33.13%) in 31-40 days old birds in Haryana. The younger chicks of 1-3 weeks (5 nos.) as well as 6-9 weeks old (2 nos.) were also found affected during the investigation (Table 1), which is in conformity with the earlier reports^21,22. The disease was found to occur all around the year with highest incidence during winter season followed by summer and rainy seasons. Similar observations were also recorded by some researchers^7,20,23,24. The percent morbidity varied from 39.17-47.25%, while percent mortality varied between 16.11-23.73% during the period under study (Table 1) which was similar to previous reports²⁵. The high morbidity and mortality rates recorded during the present study might be due to the improper vaccination of chicks and poor managemental practices. The most typical signs exhibited by the affected flocks were acute onset of depression, dullness, dehydration and anorexia, slight tremors on the onset of disease, declined to move, closed eyes, droopy appearance, ruffed feather along with whitish loose or mucoid diarrhoea (Fig. 1), which were in agreement with earlier findings^{7,20,26,27,28}. Most of the affected birds were having tendency for vent pecking, soiled vent feathers and inflammation of cloaca as similarly described by other workers^7,20,29,30.

On necropsy, most of the carcasses showed dark discolouration of thigh and pectoral musculature (Fig. 2), with paint brush hemorrhages on the breast and thigh muscles (Fig. 3), which supports the findings of several workers^{1,7,20,26,31,32}. In most of the cases, bursa were swollen, congested and edematous with accumulation of thick mucoid exudates (Fig. 4). There were presence of thin layer of gelatinous exudates covering the serosal surface of bursa, while in few cases, presence of caseative mass inside bursa could be seen (Fig. 5 & 6). These findings were in agreement with the previous reports^7,12,20,33. Congestion, enlargement of liver, hydropericardium and catarrhal enteritis were also noticed in a few cases; however, these findings were not consistent. Spleen in most of the cases were enlarged, mottled and very often small grey foci uniformly dispersed on the surface. Most of the birds showed congestion and hemorrhages on mucosa of proventriculus, while some cases revealed congestion and hemorrhages at the junction of proventriculus and gizzard (Fig. 7). The gross lesions in liver, spleen and proventriculus recorded during the present study were found almost similar to those described by previous researchers^7,12,20,26. In most of the cases, kidneys were congested, enlarged and swollen which might be due to deposition of urates caused by the enlarged bursa. Similar observations have been reported by some workers^7,12,20,26. Thymus in most cases was found to be enlarged, congested and hemorrhagic (Fig. 8), which might be due to involvement of virulent form of IBDV and secondary infections.

Microscopically, the bursa of Fabricius showed congestion, complete lymphoid depletion in follicles leading to formation of cysts filled with necrotic debris, heterophils (Fig 9), hemorrhages in the interfolicular tissue and follicular atrophy. In few cases, exudates comprised of necrotic debris with severe heterophillic and lymphocytic infiltration in bursal lumen. These findings were in the line of earlier observations of several workers^{1,7,12,13,20,33}. In most of the cases, spleen showed depletion of lymphocytes, congestion and focal or diffused areas of hemorrhages (Fig. 11), which were in support of the previous reports^7,12,20. The kidney lesions of tubular epithelium degeneration and congestion in interstitium were supported by the findings of earlier workers^7,12,19. There was congestion, degeneration of hepatocytes and lymphoid aggregations in portal areas in the liver sections and these were supported by the previous findings²⁰. Lymphoid depletion in caecal tonsils recorded during the present study period supports the findings of earlier researchers^19,34 who observed significant reduction of lymphocytes in caecal tonsils, proventriculus, duodenum, jejunum, ileum and caecum. Severe congestion in parabronchial area of lungs and microscopic changes in liver such as congestion, degeneration of hepatocytes and lymphoid aggregations in portal areas might be due to involvement of virulent form of IBDV and secondary infections.

The disease was clinically diagnosed on the basis of clinical history, clinical signs, gross and microscopic lesions of affected chickens. RT-PCR, a nucleic acid-based detection test, was used as confirmatory diagnosis for the detection of IBD viral genome. Tissue samples comprising of bursa, spleen, thymus and liver from a total of 62 clinically IBD suspected cases were tested for detection of the VP2 gene. Out of 62 IBD suspected cases, 37 (59.67%) cases were found positive for IBD virus (Table 1). Similar diagnostic techniques have also been performed by several workers^20,26,35,36. The present RT-PCR positive results 37 (59.67%) is lower than that of earlier workers³⁷ who could detect 81 (95.29%) samples positive out of 85 suspected bursal samples, which might be due to improper clinical diagnosis of IBD suspected cases (62) in our study.

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