Course: Tackling common poultry diseases | Last Updates: 12th October 2015
A relatively new infectious bronchitis (IB) variant has been slowly getting closer to the UK, having been found in Denmark and Germany in the past three years and there is now good evidence of its presence in some commercial poultry in other neighbouring countries in eastern Europe.
But with the right measures, UK producers and their European colleagues can help prevent it from spreading further and resulting in flocks failing to peak.
Like many IB variants it has more than one name. The Dutch isolate is named D388 while it has also acquired the common title of “Chinese strain” due to its similarity to the “QX” strain. The D388 isolate was first reported in The Netherlands in 2003 and its appearance and spread around the time of re-populating the poultry farms after the severe avian flu outbreak is unlikely to be a coincidence.
Movement of IB
IB infections are unlikely to be moved by day-old chicks or by eggs. This is the reason that, despite the large numbers of chicks and eggs moved globally, there are different sets of IB variants in different parts of the world.
Day-old chicks will only carry IB if they are vaccinated in the hatchery and carry a vaccine strain. Or if hatchery hygiene is particularly poor chicks may become infected from contaminated eggs, vehicles or equipment coming into the hatchery. It is most often spread between areas by the movement of point-of-lay pullets and birds to slaughter, and by the people and equipment involved.
Once a new strain of IB is in an area, it will most likely spread between farms, by people and equipment and other biosecurity breaches. Airborne infections are also a route for farms close together.
Course of infection
IB infections start in the trachea, then, depending on local immunity and the virulence of the virus, it can get into the blood and the resulting viraemia (circulation of the virus within the blood) enables the virus to spread. It can then localise in the kidney and oviduct. This is where the nephropathogenic strains (those that do damage to the kidneys) can do the most harm.
The extent of damage is dependent on the age of the birds and the immune status. The younger the birds the more likely there will be severe damage. In birds where the oviduct is damaged, the ovary will still become active when the bird reaches sexual maturity.
Effects of D388 on commercial poultry
D388 is a nephropathogenic strain of IB, which also targets the reproductive tract in young pullets. When the reproductive tract is infected it can be damaged. This can lead to occlusions or a partial blockage of the tract. When the birds mature, fluid filled cysts may be seen in the oviduct.
Follicles will be found on the ovary, and the pelvic bones will open, but the oviduct, due to cysts, constrictions or other damage, will not allow the proper formation or passage of eggs. Therefore, egg material, yolks and or partially formed eggs may be found in the abdominal cavity (see picture).
The extent of the damage to the oviduct and the resulting pathology appears to depend on the age at which the infection occurs and the protective antibody status of the individual. The most severe damage will be in a breeder flock which is infected in the first few weeks of life and which has no protective antibodies to D388. The condition is typically described as “blind layers” and results in a failure to peak (see graph).
The worst cases in The Netherlands had peaks of only 50%. The condition of “blind layers” in a flock may be compounded by incorrect management.
It is common for the manager to give an affected flock further feed increments to try to get them to peak. Of course, they can’t peak because of the “blind layers”. This means that the birds that are producing eggs will be over stimulated by feed and the number of double yolks will increase, as will birds with peritonitis.
In addition D388 infection in broilers can cause typical IB symptoms of respiratory disease, such as sneezing, gasping and watery ?discharge.
In many instances it is not possible to establish the presence of a D388 infection in rear. The IB infection in rear may be transient and may show few clinical signs at the time of infection. The gross pathology of the oviduct only becomes apparent at the onset of production. Diagnosis is based on clinical signs and post-mortem examination of the flock that has failed to peak. For post-mortem, try to select birds with pendulous abdomens and make a detailed examination of the oviduct.
Serum samples from a suspect house should be examined by serum neutralisation (SN) or haema-globulin inhibition (HI), which test for specific antibodies against D388 and other variant strains.
Biosecurity is vital to protection; vaccination with IB variants will only be protective in conjunction with good biosecurity. It is vital that young birds are kept isolated from potential infection until they have been fully vaccinated and, or past the age of susceptibility to D388 infection.
Birds in rear must be kept under a very high standard of biosecurity till at least eight weeks and all people entering the rearing farm must follow full biosecurity protocols – with a complete change of clothing if they have been in contact with any other chickens.
Be especially careful with beak trimming crews, people tipping new chicks, vets and other people vaccinating and production managers. Feed delivery vehicle drivers must not enter poultry houses. All people entering must change footwear or use a proper footbath.
Remember, the importance of cleaning and disinfection of the rearing farm – a common scenario is that if a flock at 16-18 weeks becomes infected with a virus like D388 it will have no clinical signs, as it is already old enough to be protected by vaccination, but the virus may multiply in the birds and leave sufficient residual infection in the house to infect the next lot of day-old chicks.
Vaccination with IB variants at a young age will enhance protection of the flocks in rear. Maternal antibodies appear insufficient to give good protection against D388. Strategic vaccination combined with good biosecurity will ensure protection against IB variants.
An option that works well is to use a combination of H120 and D274 as a coarse spray in the hatchery followed by a 793D type vaccine such as 49/1 or IB88 at about 14 days. But beware if an IB variant vaccine stain escapes into flocks in production that have not been vaccinated with that variant and are unprotected, then mild clinical signs of IB may be seen, including egg drops, vague ill health and a loss of egg quality. Live vaccines are a stress on the chicks and will make achieving good flock uniformity in rear more difficult.
Barry Thorp is a consultant for Aviagen and is also a partner at the St David’s Poultry Team at the Dick Vet, Roslin
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