Monograph

Thu, 01/27/2011 - 14:21 — Anonyme
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definition: 

Highly contagious viral disease characterized by inflammation of the bursa of Fabricius causing immunosupression accompanied by diarrhoea and dehydration.

Situation in America: 

Declaration of the disease to the OIE in 2004 by the following countries: Argentina, Barbados, Bolivia, Brazil, Canada, Chili, Columbia, Cuba, Dominican Republic, Haiti, Honduras, Jamaica, Martinique, Mexico, Paraguay, Peru, USA, Uruguay and Venezuela.

Susceptible species: 

Only chickens fully express the disease.

Turkeys, ducks, geese, pheasants, etc., present subclinical infection.

Wild birds and rodents may act as asymptomatic vectors of the disease. 

 

Receptivity and susceptibility factors

• Species: the chicken is highly susceptible

• Age: between 3 and 6 weeks

• Descendants: great susceptibility of the White Leghorn

The younger the animal the more significant the immunosupression. Susceptibility reaches a maximum level at 30 days when the animal is no longer protected by maternal antibodies and is not yet protected by its own. The death can reach up to 100%.

Etiological agent: 

The IBDV "Infectious Bursal Disease Virus", belongs to the family Birnaviridae, Avibirnavirus genus.

Only serotype 1 is pathogenic and mainly affects the B lymphocytes of the bursa of Fabricius.

It is a bisegmented RNA, highly resistant virus. It is inactivated at pH = 12 and after 30 minutes at 70°C.

Methods of transmission
Source: 

Virulent matter

Droppings, water, food, litter (30 days)

Method of contamination: 

Primarily via oral route but probable via upper respiratory route and via conjunctival route.

Symptoms: 

Incubation period: 2-3 days

 

The symptoms vary depending on the species, age and virus strain.

 

Acute form

This is caused by a hypervirulent strain with the following symptoms:

- Prostration

- Inappetence

- Elevated water consumption (tripled)

- Watery diarrhoea, dehydration

 

50% of affected animals die in 2 to 3 days. Sudden disappearance of symptoms after 8 days. Each batch is affected even after disinfection and vaccines are ineffective.

 

Subacute form

This form appears after depletion of maternal antibodies and is caused by a less virulent strain. As the lymphoid system is affected other diseases also appear causing severe economic consequences.  

 

Immunosuppresive form

This is due to infection occurring early on - before 15 days in chicks not having acquired maternal antibodies.

Lesions
Macroscopic lesions: 

Acute form

- Benign lesions, temporary lesions: spleen and thymus

- Haemorrhagic lesions (muscles, proventricule)

- Nephritis

- Affected bursa of Fabricius:

• Hypertrophied (x3)

• Bruised and congested aspect

• Yellowish jelly-like transudate

• Petechia and haemorrhage

• Followed by atrophy as of the 5th day following infection 

 

Chronic form

Retarded growth, bursa of Fabricius of normal or reduced weight, respiratory disorders.

Microscopic lesions: 

- Bursa of Fabricius:

• Degeneration

• Lymphocyte necrosis (follicles)

• Followed by absence of lymphocytes in the medulla

• Hypertrophy of inter-follicular conjunctive tissue

- Plasmocyte infiltration of Harder's gland

- Non-specific renal lesions

Diagnostics
Clinical diagnosis: 

Acute form

 Diarrhoea and lesions of the bursa of Fabricius. 

 

Subacute form 

 More difficult to diagnose.

Differential diagnosis: 

Infectious Bursal Disease should not be confused with (depending on the form):  

 

Acute

nephritis-nephrosis syndrome (Coronavirus)

acute coccidiosis

Newcastle disease 

 

Subacute

chronic respiratory diseases

malabsorption syndrome

Laboratory diagnosis: 

Histological diagnosis

Carried out on the bursa of Fabricius (follicular lesions), 2 to 5 days following appearance of symptoms.

Virological diagnosis: 

Virus detection in crushed organs (spleen or bursa of Fabricius). In vivo, in ovo or cell culture detection.

Gel agar immunodiffusion test

RT-PCR test

Serological diagnosis: 

ELISA test

Gel agar immunodiffusion test

Seroneutralization

Treatment: 

No specific treatment exists.

Prophylaxis: 

Sanitary prophylaxis

Cleaning - disinfection (fumigation) and depopulation to reduce viral load. However the virus is highly resistant. Non susceptible species such as Guinea Fowl may be used alternately.

Vaccines: 

Two types of vaccine are available: attenuated live virus vaccines (for chicks), inactivated virus vaccines (breeders).

- Three types of attenuated live virus vaccines are available according to the virus strain used:

• mild

• intermediate

• hot 

 

NB: Maternal antibodies play a protective role during 15 days and may inhibit the vaccine. Chicks should be tested serologically to determine the level of maternal antibodies present as well as to determine the date of vaccination and the type of vaccine to be used.

 

- Inactivated vaccines increase the level of maternal antibodies (animals already vaccinated or having been infected) to protect chicks.  

 

Vaccination strategies 

Clinical form

- disease-free farms in disease-free areas: vaccination at 18 days using an intermediate attenuated strain.

- contaminated farms in contaminated areas: vaccination using a hot strain.

 

Subclinical form

Vaccination using an attenuated strain after serotesting to define the optimal date for vaccination.

 

Immunosuppressive form

 Vaccination of breeding chickens. Extended vaccination programme for chicks then a booster injection at 16-18 weeks using an inactivated vaccine. Possible serotesting for chicks less than 5 days' old.


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