IBD Symptoms and Lesions
Severity of symptoms and lesions is dependant on the virulence of the
infective virus, type of bird (layer or meat-type) and the immune status of
the infected bird. Acute and sub clinical IBD are distinguished.
Acute Clinical IBD
- Sudden onset of disease.
- Infected birds are depressed, have ruffled feathers, droopy appearance
and may be seen pecking at the vent.
- Morbidity and mortality begins 3 days post infection, peaks and recedes
in a period of 5 -7 days.
- Mortality may be negligible or as high as 90% in case of very virulent
IBDV. The more common scenario is mortality of 10 – 20%.
- In the field situation the mortality in layer type birds is generally
higher than in meat type birds.
- Birds that die are usually dehydrated (causing kidney lesions).
- Frequently petechial haemorrhages are present in the thigh and pectoral muscles.
- Haemorrhages and erosions may be present at the juncture of the proventriculus and gizzard.
- Bursal lesions are variable depending on the progress of the disease. Expected changes
that may occur are tabulated below.
Gumboro disease: Summary of expected changes in bursal size, weight and morphology
|Days post infection||Size||Morphology|
|2 - 3||Bursa increases in size and weight||Oedematous with gelatinous yellow transudate covering serosal surface.|
Colour changes from normal white to a cream colour. Petechial to extensive
haemorrhaging may be present.
|4||Bursa double the normal weight and size|
|5||Bursa returns to normal weight||Transudate and oedema disappear. Bursa turns a grey colour.|
|8||Bursa 1/3 of original weight|
Variant IBDV strains do not cause as severe an
inflammatory response. However severe bursal atrophy is characteristic and
mortality is usually less than 5%.
Very virulent IBDV strains
cause severe lesions in other lymphoid organs such as the thymus,
caecal tonsils and spleen in addition to bursal lesions.
Normal bursa and atrophied bursa post IBDV infection
Bursas from Left to Right:
- Acute stage. Enlarged oedematous bursa
- 5 days post infection bursa returns to normal size.
May be haemorrhagic as in this specific bursa
- 8 days post infection bursa atrophied and up to 1/8 of normal size
- Subclinical IBD occurs when chickens are exposed to IBDV during the
first two weeks post hatch and have sufficient maternal antibody at time
of infection to prevent clinical disease but not viral replication in
- Characterised by bursal atrophy, immunosuppression and resultant
increased susceptibility to secondary infections (such as E. coli).
- No peak mortality as evidenced with clinical IBD.
- Secondary infections in broilers, mainly E. coli, result in a
continuous above standard daily mortality and poorer feed conversions.
- Due to immunosuppression there can be a poor response to subsequent vaccinations.