Learning more about respiratory disease
Leading experts in the field of respiratory diseases joined specialist poultry vets in explaining to producers and trade representatives more about one of the industry’s major health challenges.
This gathering of specialist knowledge was organised by vaccine manufacturer Intervet UK in the shape of their ’Respiratory Roadshow’ which was hosted at four venues across England and Scotland.
The major viruses affecting poultry, as outlined by Dr Richard Jones of Liverpool University, are Infectious bronchitis (IB), Newcastle disease (ND), Avian influenza (AI), Infectious laryngtracheitis (ILT) and Avian pneumovirus which is commonly referred to as ART or TRT.
The effects on production of these viruses range from a failure to peak, which can involve a gradual or sudden onset, and a failure to return to expected production. Egg quality can be affected with thin, abnormal shells, a loss of pigment and, very commonly, watery whites. In some cases meat and blood spots will be more prevalent.
According to Dr Jones these symptoms were either the result of a direct action by the virus on the reproductive tract or the debilitating effect of the disease—or a combination of both.
Research by Dr Jones's team at Liverpool had established that whereas both mechanisms were involved with IB, when it came to ILT, any impact on production was a result of debilitation and the virus had not been shown to directly affect the birds’ reproductive tissue.
“But there is still much to learn about how respiratory viruses affect layers and the mechanisms are not fully understood,” said Dr Jones.
Also admitting to not always having the answer was Ranger vet Stephen Lister who described a lot of egg-drop problems he sees as “multi-factorial”.
“This maybe the current buzz-word for us vets but it’s not a kop-out. Environmental factors will influence how disease manifests itself in a flock,” said Mr Lister. “For instance,” he went on, “the bird’s best line of defence against virus infection is the hair-like cilia that line the respiratory tract but high ammonia levels can badly damage these and render the bird vulnerable to infection.”
As well as ventilation, the vet reeled off a long list of factors that could all influence a flock’s vulnerability to disease, including stocking levels; nutrition; water; vermin; parasites; and general housing conditions.
“The picture can be further complicated by the proximity of the site to other laying farms or other types of poultry,” he said. “Rearing pullets on the laying site will have an effect on the disease situation and a multi-age site can create additional challenges when it comes to maintaining flock health.” Fellow vet Alistair Johnston, who chaired the roadshow held in Cheltenham, also confessed to a feeling of frustration when it came to investigating egg-drop problems, particularly on free range farms which make up most of his client base.
“Diagnosis is often difficult but there is no doubt that IB variants are at the top of the list when it comes to viral problems experienced this year,” said Mr Johnston, “and a lot of producers are now administering booster vaccines during the laying period.”
On a brighter note he reported that ART and ILT had been causing few problems in recent months.
Imparting her specialist knowledge on the complex subject of variant strains of IB was consultant microbiologist Jane Cook.
And while there will always be new variants being discovered, Ms Cook said that this did not necessarily mean a new vaccine was required every time.
“ In most circumstances the cross protection offered by existing vaccines, particularly those containing the IB 4/91 and Massachusetts strains, should give adequate protection against variant strains of IB,” said Ms Cook, adding that it was too early to say if the recently identified Italian 02 strain would have any significant impact in the UK.
On the question of whether to administer live vaccines during the laying period, Intervet’s Goossen van den Bosch said that while there was no scientific evidence to back it up, results from the field suggested it could be beneficial.
“Poultry farmers are very practical people,” said Mr Bosch, “and they don’t spend money if they don’t have to.
“I know of many producers who started using live vaccines in lay when they had a problem and they have carried on ever since. That tells me it must be of some benefit.”
But Mr Bosch was not recommending widespread use of this strategy and in his view, in the absence of a proven IB problem, a simple programme incorporating a live vaccine at day-old and again at five to six weeks, followed by an inactivated vaccine on transfer to the laying house should give adequate protection.
“By taking blood samples six weeks after the dead vaccine it will indicate whether you have achieved good vaccine immunity,” explained Mr Bosch. “Check the bloods again at the end of lay and if the titres haven’t risen you know you haven’t had a problem.
“Even if the titres are higher but you have not seen any production problems, the birds are likely to have had a field challenge which has done no harm and can be ignored.
“But if titres are higher and you’ve seen obvious production problems then you may need to consider using vaccines in lay to top-up protection.”
In this case it was recommended that vaccines are applied by spray every six to eight weeks. Applying them at night would avoid disturbing the birds and it was important to vaccinate all the birds on the site, said Mr Bosch. Used in this way the live virus should not invade beyond the birds’ upper respiratory tract provided that good vaccine cover had been achieved in rear.
Aside from viral infections causing respiratory disease, Mycoplasmas are of increasing concern to the poultry industry and on hand to discuss these was Janet Bradbury, also from the University of Liverpool.
With twenty three varieties of the disease known to affect birds, egg producers could at least take comfort from the fact that only two are found in layers—Mycoplasma synoviae (MS) and Mycoplasma gallisepticum (MG).
MG is the most economically important of the two and studies have shown that infection with MG alone can result in the loss of up to sixteen eggs per laying cycle. But, said Ms Bradbury, the real significance of MG is how it can interact with other diseases, such as IB and E-coli, making their effect far more severe. Control measures outlined by Ms Bradbury included only using ‘clean’ parent stock; implementing strict biosecurity measures; and vaccination depending on the likelihood of challenge and the virulence of the strain.
