1. Cause
2. Infection route
3. Diagnostics
4. Prevalence
5. Approach to infected farm

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Cause

Infectious Bursal Disease (IBD), also known as infectious bursitis, is caused by the Gumboro virus. The very virulent Gumboro field strain (very virulent IBDV/vvIBDV) has been present in the Netherlands since 1986. Natural infection occurs through ingestion of the virus via the beak/mouth (oral route). The virus develops quickly in the bursa of Fabricius, an organ in chickens which plays an important role in production of cells for the immune system. This virus has proven capable of inhibiting chickens’ immune system and causing high mortality rates. Such high mortality has mainly been seen in layer chicks, with lower mortality rates seen in broilers of the fast-growing breeds. On average, slower-growing broilers suffer higher mortality rates than the fast-growing breeds. Immunosuppression may play a role in all types of chickens and it can be the cause of disappointing technical results.

The virus can be detected in cells of the small intestine and liver some 5 hours after infection. Strong viral development occurs in the Bursa of Fabricius from 11 hours after infection. The disease occurs worldwide. There are two IBD serotypes: serotype 1 is the most important serotype which causes disease in chickens; serotype 2 is found in chickens, turkeys and ducks, but is not pathogenic. Within serotype 1, we see the so-called ‘IBD variant viruses’. These are reported in the US, Asia and in the Middle-East. Within a serotype, a further classification can be made into genogroups. This takes place on the basis of differences within the amino acid sequence. The groups can be identified using modern DNA techniques. The virus has an incubation period of 2 to 3 days.

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Infection route

Within farms

Viral spread within a flock takes place via direct and indirect contact. Manure is the main source of infection. Infected animals excrete high volumes of virus in their faeces for up to two weeks following infection. Transfer via the air (aerogenic transmission) does not play a role. There are no indications of transfer via the hatching egg (vertical transmission). IBDV is an extremely resistant virus, resulting in the IBD problem remaining persistent for a long time once farms become infected. Following removal of an IBD infected flock, the barn will remain infectious for at least 122 days; water, feed and manure samples from an infected barn are still infectious after 52 days. The IBD virus is susceptible to disinfection using for example chloramine 2% (Halamid), formaline and for glutaraldehyde.

Between farms

Spreading to other farms takes place via persons, animals or infected materials. Due to the resistant nature of the Gumboro virus, indirect transmission occurs very easily. Wild birds, rodents and litter beetles can transfer the virus to other flocks and/or farms. The virus can survive in manure for a long time. Infected manure in the vicinity of farms is a clear risk factor.

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Diagnostics

Infectious Bursal Disease is identified via pathological examination, possibly followed by PCR testing of the bursa. Diagnostic tests poultry

The diagnosis cannot be made with certainty on the basis of the state of the flock (acute mortality, diarrhoea, lethargic, sick animals). However, pathological examination can help in the diagnosis in typical clinical cases. Pathological findings then are: muscular haematomas and haematomas in the area between the glandular stomach and the gizzard. Three days after infection, the bursa becomes enlarged and oedematous, and haematomas will sometimes colour the bursa cherry red. Five days after infection, the bursa will actually decrease in size again. Pale and swollen kidneys are often also seen. The virus can be quickly detected using the PCR, after which the virus can be further classified to determine whether it concerns a field strain or a vaccine strains.

Besides IBD causing losses and other clinical signs (so-called visible clinical form), there is also a subclinical form of IBD in which there are little or no visible clinical signs. Diagnosis of the latter form is difficult and must be made on the basis of the state of the flock (limited growth and feed conversion, thinner faeces, signs of compromised immunity), smaller bursa (histological testing is generally required for diagnostic purposes) and blood testing at the end of the fattening period.

Rapid test

A PCR test on bursa material (and genotyping if positive) is the way to find out whether and which Gumboro field virus plays a role on the farm. A fast (20 minutes) and affordable way to make a pre-selection is the Gumboro rapid test (IBDV Ag rapid test).

If there are clinical or subclinical problems due to Gumboro, the rapid test result is positive. After a positive rapid test, it is important to send samples for a PCR test (and genotyping if positive). This makes the diagnosis and determines the type of virus. Bursa samples can be sent to GD using tissue, swabs or FTA cards.

You can also use the rapid test to check the Gumboro vaccination. The rapid test should be positive three to five days after a vaccination with a live vaccine via drinking water. Ten days after a successful vaccination, the rapid test is negative. If the vaccination is successful, the rapid test remains negative even after a field infection at a later time.

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Prevalence 

Situation in Northwestern Europe

We have seen genetic mutations in the vvIBDV strains in the Netherlands. The strains that have been found since 2017 differ from the older strains. The recent strains seem to be dominant, with 98.1% DV86 homology concerning the sequenced VP2 part versus the older strains (100% DV86). The monitoring data shows that the mutated vvIBDV strains have been detected at dozens of farms (in broilers and layer pullets) since 2017. The farms affected were located all over the country. The flocks in which the mutated vvIBDV strains were detected, displayed varying clinical signs. The most commonly detected issues were latent increased losses, wet litter, reduced growth and disappointing technical results. The pathogenicity (capacity to cause disease) of this virus for chicks was researched and reported in 2019 (AVINED field study). The virus did not result in mortality among layer chicks and broilers, though it did cause long-term (non-recovering) and serious damage to the bursa.

In 2020, in another study, the capacity was determined of the older field strain (100% DV86) and the recent field strain (98.1% DV86) to break through the maternal immunity induced by vaccination. This experiment showed that both the classical Gumboro strain (vvIBDV) and the more recent Gumboro strain (98.1% DV86) were capable of causing an infection in all groups. This was apparent in the serious damage to the bursa and a PCR test which detected the presence of the Gumboro virus. The breakthrough titre is higher than an ELISA titre of 1,000 (titre group 2 in the GD results). The field virus is therefore capable of resulting in infection before a live attenuated vaccine can protect the chicken.

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Approach to infected farms

The approach to Infectious Bursal Disease consists of measures taken for prevention and for control.

Prevention

• Without vaccination: The choice not to vaccinate is only possible if there is no risk of infection in the region, in combination with an Early Warning System (EWS) and excellent farm hygiene. The EWS will quickly warn of any possible outbreaks, enabling farms in the vicinity to take suitable measures, for example the immediate vaccination of a flock or the administration of a stronger vaccine instead of an intermediate vaccine.
• Vaccination: The available vaccines are an effective method of preventing infections. Blood tests can provide additional information in order to estimate the optimal vaccination age.

Control

Control of the disease consists of supplementary measures taken in the event of an outbreak. Following an outbreak, it is important to:

• pay extra attention to cleaning and disinfection;
• pay extra attention to biosecurity;
• vaccinate accompanied by blood testing (to estimate the optimal vaccination age);
• preferably administer a stronger (e.g. intermediate plus) vaccine;
• have the administering of the vaccination checked.

Treatment

There is no treatment for this disease. Antibacterial agents to combat secondary infections are generally not necessary.

Vaccination

The required timing of one or more IBD vaccinations depends on the volume and variation of the maternal antibodies and the vaccine to be used, in all poultry categories. It is recommended to administer an intermediate vaccine using a full dose per animal, based on a blood test of the maternal immunity, as long as there is no increased risk of infection at the farm and in the region. As soon as there is an increased risk of infection, the temporary use of an intermediate plus vaccine is recommended.

The most suitable timing for the vaccination can be calculated on the basis of a blood test. Blood is tested from at least 18 animals at a young age, using the IBD antibodies ELISA. The calculation differs depending on the animal type: regular broilers, slow-growing broilers, typical layers and pullets for meat reproduction. It may be necessary to vaccinate more than once. When submitting samples, please take into account the time required for the test and the theoretically expected vaccination timing. Currently, there are also hatchery vaccines available which do not require a blood test. These are immune-complex vaccines (which contain a live attenuated IBDV-strain) and vector vaccines (which are based on a HVT-vaccine with a part of the IBD-virus built-in). The effectiveness of the immunisation can be checked by means of blood testing at the end of the round.