Detecting progress in disease control

11-05-2012 | | |
Detecting progress in disease control

The annual Western Poultry Disease Conference attracts poultry scientists and veterinarians, mainly from the Americas and the Pacific Basin. Selected presentations of significance to an international readership advance knowledge concerning the detection, epidemiology and potential to control diseases of worldwide significance in chickens and turkeys.

By Dr Simon M. Shane, Durham, NC, USA
Scientists affiliated with Ceva Animal Health in Mexico, the US and Europe, conducted a series of field evaluations on alternative vaccination programmes. The trial comprised vaccination of groups of 240 broilers ranging in age from 17 to 35 days from eight commercial flocks in seven areas in Mexico, all of which are affected with endemic viscerotropic velogenic Newcastle disease (vvND).
All of the programmes included administration of Vectormune herpes virus of turkeys Newcastle disease (HVT-NDV) injected subcutaneously. In addition various combinations of spray administration of live NDV vaccine and combinations of live NDV vaccine with inactivated NDV vaccine were considered. The efficacy of immunisation was assessed by exposing representative groups of broilers to the virulent Chimalhuacan NDV strain by the ocular route in Horsfall-Bauer chambers. The challenge strain had an ICPI of 1.89 and 106 EID50 was administered. Criteria of protection included morbidity, mortality, serologic response and virus excretion.
Irrespective of the vaccination programme following administration of the Vectormune vaccine, all birds in each of the twelve treatments survived challenge compared to total mortality in the susceptible SPF controls. It was noted that Vectormune did not stimulate an antibody response which could be detected by either HI or ELISA procedures before challenge. The combination of Vectormune and live virus spray vaccinations significantly reduced viral excretion compared to conventional vaccination programmes involving combinations of inactivated and live ND vaccine. This has direct implications for control of vvND outbreaks. The studies conducted confirmed that Vectormune is an effective component of a comprehensive ND vaccination programme for commercial broilers in areas with endemic vvND.
Indonesia model for H5N1
Dr Fred Leung of the University of Hong Kong in cooperation with colleagues in the US, Belgium and the UK reported on the evolution and transmission dynamics of reassortant H5N1 influenza virus, using Indonesia as a model. Reassortment among strains occurred during the period 2003 to 2007 on the Island of Java. Sophisticated molecular analysis was applied to determine the phylogeography (evolution in relation to location) of isolates. This research is significant in that it provides an understanding of the epidemiology of HPAI which is important in relation to developing appropriate control measures.
IBD viruses from US broilers
Dr Kalen Cookson of Pfizer Animal Health-Global Poultry in cooperation with scientists at the Ohio State University applied genetic analysis to 117 isolates derived from 500 flocks aged 11 through 32 days. It was determined that AL2, AL2-like and Delaware-E types predominated in broilers under 21 days of age. In contrast AL2, CAL-6 and mixed infections were present in broilers over 21 days of age. DEL-E was able to infect young chicks in the presence of maternal antibody. Type AL2 is apparently able to compete with other strains in older broilers. Applying imaging analysis, it was estimated that all flocks were infected between 14 and 27 days of age with 82 isolates infecting before 21 days and 35 isolates after this age.
Montanide adjuvants for live vaccines
A team from SEPPIC in France evaluated alternative adjuvants for live avian vaccines with specific reference to infectious bronchitis strain H-120. Three derivatives of Montanide were evaluated. These comprised a product with an immuno- stimulating agent (Montanide IMS 1313N), Montanide ISA 201 VG and Montanide Gel 01 ST. Montanide ISA is formulated to allow manufacture of water-in-oil-in-water vaccines. Montanide Gel 01 is a dilute polymeric adjuvant containing sodium polyacrylate in water.
Vaccines incorporating the adjuvants were administered by the intranasal route. No deleterious effect was observed as a result of incorporating adjuvants to vaccine irrespective of method of delivery. Spray delivery of the adjuvant containing a polymer, stimulated higher protection compared to a similar vaccine without the adjuvant. Montanide ISA 201 VG is formulated for parenteral administration but it was effective when combined with the vaccine administered by either the eye drop or intranasal routes. It was suggested that appropriate adjuvants could improve the protection afforded by live attenuated vaccines and would stimulate higher and more uniform levels of immunity and safety.
Vectored vaccines in broilers
A cooperative study performed at North Carolina State University in conjunction with Ceva Biomune was conducted to determine the relative efficacy of HVT- and fowlpox- vectored LT vaccines in broilers. Ceva markets a herpes virus of turkeys-vectored vaccine (Vectormune-HVT LT) and a fowlpox virus vectored vaccine (Vectormune-FP LT). Chickens were vaccinated with either of the vectored vaccines and compared to chicks receiving conventional live chick embryo origin LT vaccine. Non-vaccinated susceptible broilers served as controls. All three vaccines reduced clinical scores, prevented mortality and on microscopic examination yielded lower lesion scores compared to non-vaccinated chickens.
Viral excretion was determined by RT-PCR assay to determine the extent and duration of shedding. Neither of the vectored vaccines reduced recovery of virus as compared to unvaccinated controls. In contrast the CEO vaccine was more effective in this respect. Administration of the HVT vectored vaccine in ovo reduced clinical manifestation of ILT as noted under field conditions. Real-time PCR detected viral DNA in tracheal exudates. This does not necessarily mean that entire LT virus was present as denoted by failure to isolate and propagate LT virus from infected birds. The authors stressed that quantification of infectious virus rather than viral DNA should serve as a standard to assess immunity stimulated by recombinant LT vaccines.
Current trends in turkeys
Dr Helen Wojcinski of Hybrid Turkeys provided a review of the current status of turkey health, welfare issues and vaccination. This was guided by the results of an annual survey conducted among US turkey veterinarians ranking major issues. Consistently over past surveys, the lack of approved and efficacious drugs was regarded as the most important detractor from efficient production. Conditions of significance include clostridal dermatitis and cellulitis, colibacillosis, heat stress, poult enteritis and Salmonella infections. Lower ranked problems included histomoniasis, low pathogenicity H3N2 influenza, pasteurellosis and skeletal deformities late in the growing period.
Industry veterinarians consider that the withdrawal of dimetridazole in 1987, enrofloxacin in 2005 and the recently announced ban on cephalosposins have impeded the ability to treat and prevent infections. Clostridal dermatitis which emerged in 1992 now affects all geographic areas in the US and is responsible for losses in breeders and commercial-level flocks. Research on the causation and contributing factors is in progress. Administration of penicillin is ameliorative. Improvements in biosecurity and the administration of autogenous vaccines are implemented on farms which demonstrate repeat infections. Colibacillosis continues as a significant cause of losses affecting between 10% and 20% of flocks from 5 through 12 weeks of age.
Extensive losses in toms
Late mortality in excess of 1.5% per week in toms older than 17 weeks is responsible for extensive losses. Genetic selection for rapid growth may well have contributed to aggression, cannibalism, hypertension and skeletal deformities. There has been a decline in the prevalence of late mortality and “leg problems” since 2005 partly due to enhanced selection emphasising livability and delayed maturity. The prevalence of “leg problems” including reoviral arthritis and tendonitis, osteomyelitis, tibial dyschondroplasia and spondylolisthesis has decreased in parallel with “late mortality”.
The turkey industry in the US makes extensive use of autogenous vaccines including products to prevent H3N2 influenza predominantly acquired from swine reservoirs.
Due to the pressure imposed by the USDA-Food Safety and Inspection Service, pre-harvest control of Salmonella now extends to administration of both commercially available generic and autogenous vaccines.
Increased emphasis on welfare has resulted in the evaluation of housing and management practices. There is now a zero tolerance for abuse in handling and transport. Issues of concern include beak trimming, molting of turkey hens and scoring skeletal abnormalities. There is a lack of consistency and scientific rigor with respect to recommendations made by advocates of “welfare”. Suggestions that turkeys should be reared on range will undoubtedly result in increased mortality and deterioration in food safety, both of which would be detrimental to the industry and consumers.
The latest issue of concern to turkey producers is the emergence of a reovirus responsible for tenosynovitis and rupture of the digital flexor tendon.
Characterisation of reoviruses
In 2009 a locomotory syndrome emerged in tom turkeys raised in the Midwest states. Affected flocks demonstrated lameness with swollen hocks and shanks. On post-mortem examination tenosynovitis was evident with some birds showing rupture of the digital flexor tendon. The condition was exceptionally severe during the winter of 2010 but the incidence rate declined by mid-2011. Tenosynovitis was also associated with aortic rupture which contributed to an elevation in mortality of 3% to 10% above standard.
Responding to the widespread and serious condition, turkey veterinarians representing the major producers, biologics companies and private laboratories established a task force to investigate causation. Reovirus was consistently isolated from the lesions of affected turkey flocks. The criterion for infectivity was the ability of a viral isolate to induce arthritis and tenosynovitis following injection of suspensions into the foot pad. Reovirus isolated from the intestinal tract was innocuous. The virus derived from joint and tendon lesions could reproduce the condition both in turkeys and in chickens. Early studies showed that the turkey reoviruses were distinct from the chicken viral arthritis virus S1133. Cross protection studies indicated that the viruses were antigenically similar although differences existed in pathogenicity.
A specific isolate was selected and was used to prepare an autogenous inactivated vaccine which is currently administered to breeder flocks in an attempt to transmit maternal antibody to progeny. A chicken-origin reovirus isolated from broilers has been shown to be antigenically similar to the turkey isolate but is not pathogenic to turkeys. This strain is being evaluated as a candidate for a live vaccine.
Vaccine interaction and protection
Studies conducted by scientists affiliated to the Universities of Bologna and Padova confirmed that administration of a live AMPV vaccine to day-old poults protected against subsequent infection. The efficacy of AMPV subtype B live vaccine (strains VCO3) was not affected by concurrent hatchery administration of either Newcastle disease B1 or VG/GA strains. The Newcastle disease vaccines have either respiratory (B1 strain) or enteric (VG/GA strains) tropism. There was no difference in ND antibody titer as a result of concurrent vaccination of poults with the AMPV strain.

M- Shane