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New Look at Disease Prevention

07 March 2012

Disease prevention was a theme of one of the sessions at the International Poultry Scientific Forum (IPSF) this year, writes ThePoultrySite senior editor, Jackie Linden – a session that illustrated a number of different approaches to maintaining poultry in good health.

One of the sessions of the Southern Conference on Avian Diseases (SCAD) at the IPSF in Atlanta, US in January – held in conjunction with the International Poultry Expo – included papers from several US universities with the common theme of disease prevention but taking different approaches to its improvement.

Comparison of ILT Vaccine Programmes

Researchers from the University of Georgia reported that the best protection against infectious laryngotracheitis (ILT) in commercial layers challenged at four or nine weeks of age was achieved with vaccine of tissue culture origin (TCO) at two weeks of age, with or without HVT-LT recombinant vaccine at hatch.

The main objective of this research reported by Victor Palomino and co-authors at the University of Georgia was to establish the onset of immunity and protection against ILT induced by six different vaccination programmes with recombinant and live-modified virus vaccines.

A total of 150 Hy-Line W-36 commercial layers were randomly distributed in seven and eight groups of birds, vaccinated in various ways and challenged at four or nine weeks of age. All the birds were vaccinated with the CVI988 Rispens strain of Marek’s disease virus at day-old.

For the 4th-week challenge study, the following programmes were included: non-vaccinated challenged (Group 1); Pox-LT recombinant at hatch (Group 2); HVT-LT recombinant at hatch (Group 3); TCO vaccine at two weeks of age (Group 4); Pox-LT recombinant at hatch + TCO at two weeks (Group 5); HVT-LT recombinant at hatch + TCO at two weeks (Group 6); and non-vaccinated non-challenged (Group 7).

For the 9th week challenge study, the experimental groups were similar, except that the TCO vaccination was done at four weeks of age and an additional group received a CEO vaccine at four weeks (group 7).

Tracheal swabs were collected and clinical signs were evaluated at five and seven days post-challenge (DPC).

Challenge virus concentration in the trachea was examined by qPCR. Clinical sign scores were compared statistically by Kruskal-Wallis and Dunn tests.

Five days post challenge, there was no statistical difference between groups 1 and 2. Groups 4 and 6 exhibited the highest protection against ILTV in both the four- and nine-week-old studies. In addition, Group 7 also showed the highest protection along with groups 4 and 6 in the nine-week-old challenge.

Improved Response to Bordetella Vaccination in Turkeys

A boost with oral administration of an inactivated antigen in the drinking water improved the response of turkey poults to spray vaccination against Bordetella avium (BA), according to researchers from the University of Arkansas.

The turkey disease, bordetellosis, results from an infection caused by BA, which colonises the epithelium of the trachea of turkeys causing severe respiratory disease or coryza, according to R.H. Harris of the University of Arkansas in Fayetteville.

This study’s objective, he explained, was to evaluate the oral administration of an inactivated BA vaccine in combination with either chitosan or a proprietary modification of chitosan as an adjuvant in turkey poults.

In these experiments, day-of-hatch turkey poults were vaccinated parenterally or orally with chitosan+BA adjuvated bacterin, modified chitosan (MC)+BA adjuvated bacterin, or saline control. On day 14, poults were boosted with either subcutaneous (SQ) BA+chitosan, BA+chitosan in the drinking water (DW) or BA+MC DW.

Immune response was evaluated using an ELISA to detect anti-BA IgG.

The Fayetteville group reported that in experiment 1, day 14 IgG antibody levels for groups BA chitosan SQ prime/DW boost, BA chitosan DW prime/DW boost, and BA+MC SQ prime/DW boost were significantly higher than the controls. IgG levels on day 21 followed a similar trend. However, no significant differences (P<0.05) were found.

In experiment 2, a similar trend was noted on day 21, with BA+MC SQ prime/DW boost having significantly higher IgG levels than the controls.

Currently, to prevent the disease, poults are treated with live, temperature–sensitive vaccines administered by spray at day-of-hatch and again at two weeks of age. While this technique is sometimes effective, this type of product innately has storage and administration difficulties for producers, frequently leading to compromised effectiveness and potential questions of serotype variation, according to Harris and colleagues.

They highlight that the present research was able to achieve meaningful responses following boost with oral administration of the inactivated antigen, leading to a host of opportunities for improved compliance and potential mass-application of inactivated vaccines through the drinking water.

Mycoplasma Vaccine Efficacy Investigated

The results of a study at Mississippi State University demonstrated that vaccine dosage may have direct implications on vaccination efficiency of AviPro® MG F against Mycoplasma gallisepticum (MG) for the laying flocks.

Live attenuated vaccines (LAVs) are commonly used in the table egg industry to limit economic losses associated with virulent MG outbreaks, said Roy Jacob from Mississippi State University in the introduction to his presentation.

To determine the effect of dosage of a recently released LAV, (Avipro® MG F) when applied via spray on vaccination efficiencies and in vivo MG populations, 240 mycoplasma-free Hy-Line W-36 pullets were caged individually in a commercial layer facility with four rooms, 60 birds per room, to 19 weeks of age. A randomised control study design was used.

At 11 weeks of age, birds of each room were spray vaccinated at one of four levels: 0× (negative control), 1×, 2× or 4× the manufacturer’s recommended dosage. The reconstituted LAV source titre was 2.8 × 105cfu/1×dose.

At five weeks post-vaccination, in vivo MG LAV populations were estimated via palatine fissure swabs and subsequent quantitative Taqman®-based Real Time PCR assays. At seven weeks post-vaccination, all groups were challenged with the virulent MG strain Rlow.

Vaccination efficiency was assessed pre-challenge (at six weeks post-vaccination) by measuring seroconversion rates via Serum Plate Agglutination assays (SPA) and post-challenge by measuring the degree of airsacculitis in virulent MG-challenged birds.

SPA results demonstrated a dose–dependent response as 0, 5, 37 and 42 per cent of birds showed seroconversion in the 0×, 1×, 2× and 4× dosage groups, respectively.

The incidence of detectable in vivo MG increased with higher dosages but MG population estimates did not correlate directly with dosage.

Viable in vivo MG populations were detected in all SPA–positive birds.

Following challenge, airsacculitis was observed in 36, 32, 25 and 21 per cent of birds in the 0×, 1×, 2× and 4× groups, respectively, which also showed dose dependent–protection, according to the Mississippi researchers.

Maintaining a Healthy GI Tract

Dr Stephen Collett, associate professor at the University of Georgia, presented a different approach to poultry health in a presentation entitled ‘The Avian Enteric Tract: Form, Function and Flora’, in which he described the changes in these three elements of the gastrointestinal (GI) tract that are associated with subclinical disease or dysbacteriosis.

The primary objective of any poultry production system, he explained, is to optimise the economic efficiency of converting poultry feed into human food. Highly successful breeding and selection programmes have provided the platform for annual improvements in biological efficiency as measured by feed conversion. From a biological perspective, efficiency is determined by the anatomical structure of the intestinal tract (form), and the physiological process of digestion and absorption (function). The degree to which the host genes governing intestinal form and function are expressed appears to be altered by the output of the resident microbiota (flora).


Dr Stephen Collett

The physical nature of both the intestinal lining and its content display detectable changes in the early stages of disease, said Dr Collett. Villus height to crypt depth ratios have for example been used to indicate intestinal integrity. This is possible because the length of an intestinal villus is kept constant by continuous enterocyte replacement. The delicate cells lining the intestinal tract are continuously exposed to potentially damaging luminal content and not surprisingly, they require frequent replacement.

It has been shown that the life span of a typical enterocyte is three to four days, said Dr Collett, and consequently, complete replacement of the intestinal epithelial lining occurs in this period of time by a process of cell division in the crypt area, sequential migration of the enterocyte to the tip of the villus and finally extrusion from the tip into the lumen.

The body’s first homeostatic response to accelerated enterocyte attrition is enhanced cell division in the crypt area and to achieve this, the crypt increases in size.

Dr Collett continued that it stands to reason that a slight decrease in villus height to crypt depth ratio in the absence of any change in villus height is the first indicator that the conditions within the intestinal tract have changed sufficiently to increase the rate of enterocyte attrition. This level of challenge seldom manifests as a change in nutrient assimilation or clinical disease because intestinal surface area is not affected, but does, however, indicate a shift from normal.

While it is impossible for even an experienced clinician to detect the change in the thickness of the intestinal wall induced by an increase in crypt depth, he explained that there are other changes that give insight into what is happening. Since even minor cell damage induces an inflammatory response, cell debris and inflammatory mediates, including mucus, begin to accumulate in the lumen faster than normal. Apart from causing the villi to stick together and lose optimal alignment – which is visible to the naked eye – the mucus and cellular debris accumulates to the point where orange coloured mucus forms aggregates or strings within the lumen.

As the severity of the intestinal challenge escalates, so too does the rate of enterocyte attrition, according to Dr Collett. Villus height starts to decline when the rate of enterocyte destruction exceeds the maximum capacity for replacement. At this point, the intestinal wall becomes noticeably thinner and the intestine loses muscle tone and tensile strength. The mucosal lining of the intestinal wall appears pale and dull, giving it a parboiled appearance because of the plethora of dead or dying cells on the luminal surface.

The inflammatory exudate makes the shortened villi clump together and their typical zigzag alignment is lost. At this stage, there is sufficient reduction in surface area and enough villus damage to compromise intestinal function, he said.

There is a net efflux of water into the intestinal lumen, causing what is referred to as, watery enteritis. If the irritation persists of worsens, the enteritis becomes more chronic.

There is an influx of inflammatory cells, causing the gut associated lymphoid tissue to appear congested and the luminal content becomes dominated by mucus giving rise to a typical mucoid enteritis.


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"Dysbacteriosis, as it is commonly referred to in the poultry industry, became common place after the moratorium on in-feed antibiotics was introduced in the European Union"
Dr Collett

At this stage, continued Dr Collett, enzymatic digestion and nutrient absorption is sufficiently compromised for bacterial fermentation of undigested nutrient to result in gas accumulation. Initially, the intestinal content becomes foamy but as the ecology of the intestinal lumen deteriorates, the destabilisation of the microbiota manifests as the accumulation of free gas.

Changes in the composition of the intestinal microbiota have been associated with deterioration in intestinal function as measured by feed conversion efficiency, explained Dr Collett. Dysbacteriosis, as it is commonly referred to in the poultry industry, became common place after the moratorium on in-feed antibiotics was introduced in the European Union.

These undefined shifts in the intestinal microbiota are difficult to diagnose, even with advanced molecular techniques and yet they appear to be associated with visible intestinal changes. There are changes in the thickness, appearance, muscle tone and tensile strength of the intestinal wall. Signs of inflammation are evidenced by a parboiled appearance of the mucosal surface, the accumulation of inflammatory cell aggregates, congestion and the development of a watery to mucoid exudate in the intestinal lumen. Gas by-products of bacterial fermentation provide confirmation of ecological disturbance.

Interestingly, the phenotypic expression, or community output, of the intestinal microbiota contributes to bird performance by influencing host gene expression and feed efficiency, according to Dr Collett. This makes it is easy to see why a detrimental change in the intestinal microbiota structure and composition, regardless of cause, can result in a deterioration in gut health and bird performance.

The intestinal tract and, more specifically, the caeca serve as a stable bioreactor that sustains a complex web of nutrient substrate conversion facilitated by secreted enzymes and resident organisms, he explained. The stability of the intestinal microbiota is consequently governed by the amount and type of substrate. As with any hindgut–fermenter, the chicken caeca are designed to support organisms that aid in digestion of the non-digestible components of the diet but unfortunately, such conditions are very suitable for many of the common enteric inhabitants that are potential pathogens.

An oversupply of nutrient to the hindgut rapidly changes the composition of the microbiota since the resident organisms are able to shift from steady-state to exponential growth phase. Potential pathogens such as Clostridium perfringens gain competitive advantage under such circumstances and rapidly dominate the microbial community, thus compromising intestinal health.

Dr Collett concluded by saying that astute observation on the part of the clinician can provide enough information to detect and diagnose subclinical disease in apparently healthy birds if necropsy is performed on a small sample of individuals on a regular basis at very little cost.

Correct Vaccination Techniques Advised

At the Hatchery & Breeder Clinic, also held in conjunction with the International Poultry Expo in Atlanta, Terry Bruce of Tip Top Poultry warned his audience about a new issue related to vaccination, which is leading to downgrading of breeder carcasses in the processing plant.

Granulomas were first seen on or in the breast muscle of breeder chickens in the 1990s, he said. These were thought to be related to cholera vaccines but more recently, the defects are being seen more commonly again, now associated with Salmonella vaccine incorrectly administered in the breast muscle.

The lesions, described by the USDA as ‘occult vaccination lesions’, are seen at the processing plant as cheesy areas, which vary in colour.

“They are a quality defect that we don’t want to see,”, said Mr Bruce, adding that consumers are likely to react strongly against the lesions.

In fact, they are a natural response to vaccination, he said, but he warned that a 10-bird sample at the plant could lead to the disposition of the whole flock.

Mr Bruce said that the correct vaccination site is under the skin at the back of the neck.

If the industry does not act quickly to prevent these defects, he warned that these lesions will devalue breeder hens, increase egg costs and regulations may be introduced to penalise offending flock owners.

References

The following papers were presented at the 2012 International Poultry Scientific Forum in Atlanta, US, in January 2012:

Collett S.R. 2012. The avian enteric tract: form, function and flora.
Harris R.H., N.R. Pumford, M.J. Morgan, A.D. Wolfenden, S. Shivaramaiah, O.B. Faulkner, L.R. Bielke and B.M. Hargis. 2012. Evaluation of immune response to chitosan-based adjuvated Bordetella avium vaccines.
Jacob R., D. Peebles, S. Leigh, S. Branton and J. Evans. 2012. Effects of spray dosage of a live attenuated Mycoplasma gallisepticum vaccine on the vaccination efficiency and associated in vivo M. gallisepticum populations in layers.
Palomino V., G. Zavala and S. Cheng. 2012. Vaccines against ILT recombinant and modified live virus vaccines in commercial layers.

Further Reading

- Find out more information on the diseases mentioned in this article by clicking here.


March 2012



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