Pathogenicity, tissue distribution, shedding and environmental detection of two strains of IBDV following infection of chickens at 0 and 14 days of age.

Infectious bursal disease virus (IBDV) is endemic to most poultry-producing countries worldwide. Immunosuppressive classical and variant IBDV strains endemic to Australia are genetically distinct from other international strains. We report the results of infection experiments with Australian classical strain 06/95 and variant strain 02/95 in SPF chickens. We tested the effects of strain and age of infection on bursal atrophy, viral RNA (vRNA) load in bursa of Fabricius (bursa), spleen, thymus, caecal tonsils, faeces, litter and exhaust dust as determined by real-time reverse transcriptase polymerase chain reaction. The two IBDV strains did not differ in the degree of bursal atrophy induced, lymphoid organ distribution and faecal shedding but variant strain 02/95 induced a greater antibody response to the infection than classical strain 06/95 which was associated with a more rapid decline in IBDV vRNA genome copy number (VCN) in lymphoid organs and faeces. Infection at 14 days of age induced greater bursal atrophy and higher vRNA copy number in lymphoid tissues than infection on the day of hatching, indicating true age susceptibility independent of maternal antibody (Mab) status. The direction of the association between rankings for IBDV vRNA load in bursa and relative bursal weight changed from positive at 3 and 6 days post-infection to negative at 28 days post-infection. Intra-tracheal administration of dust collected from chickens infected with IBDV resulted in successful transmission of IBDV. IBDV vRNA was detected successfully at high levels in the environmental litter and dust samples.

Effects of vaccine dose, virus challenge dose and interval from vaccination to challenge on protection of broiler chickens against Marek's disease virus challenge.

OBJECTIVES: To examine the effects of varying the doses of turkey herpesvirus (HVT) vaccine and Marek's disease virus (MDV) challenge at two intervals after vaccination on the protection of chickens against challenge with MDV. DESIGN AND PROCEDURE: Experiment 1, a dose response study, consisted of 11 doses of HVT vaccine administered at hatch followed by challenge with 100 plaque forming units (pfu) of MDV 5 days post vaccination. Experiment 2, a 2 x 6 x 2 factorial design, included two HVT vaccine types, six different doses of HVT vaccine and 50 pfu and 200 pfu of MDV challenge 2 days post vaccination. All chickens were reared up to day 56 post challenge when all survivors were killed humanely. Dead and killed chickens were examined for gross MD tumours. RESULTS: Experiment 1 showed a significant positive linear relationship between dose of HVT vaccine and protective index in chickens challenged 5 days post vaccination. However the range of protective index observed was limited. In Experiment 2 neither HVT vaccine provided significant protection at any dose. There was no significant effect of vaccine type or MDV challenge dose on overall protection against challenge. Chickens challenged with 200 pfu of MDV had significantly higher mortality and MD incidence than those with 50 pfu. CONCLUSIONS: HVT vaccine dose had a significant impact on protective index, but vaccination to challenge interval appeared to have greater impact on the protective efficacy of vaccination. A fourfold increase in challenge dose increased mortality rate and incidence of MD.

In vivo characterisation of two Australian isolates of Marek's disease virus including pathology, viral load and neuropathotyping based on clinical signs.

OBJECTIVE: To evaluate the pathogenicity of Australian Marek's disease virus (MDV) isolate MPF23 (1985) against the reference strain MPF57 based on pathology, viral load and neuropathotyping on the basis of clinical signs. PROCEDURE: Two MDV challenge isolates (MPF57 or MPF23) were administered to unvaccinated specific-pathogen free (SPF) layer chicks on day 5 after hatch at three challenge doses (500, 2000 or 8000 plaque-forming units (pfu)/chick). Mortality, body weight, immune organ weights, MDV load in peripheral blood lymphocytes (PBL) and clinical signs were measured to 56 days post challenge (dpc). RESULTS: MPF23 was the more pathogenic of the two viruses, inducing higher mortality (81% vs 62%) and incidence of MD lesions (100% vs 76%). MPF23 induced earlier, more sustained and more severe neurological signs in the period 26-56 dpc. However, there were few differences during the 0-23 dpc used in the neuropathotyping classification under test. The observed pattern during this earlier period classified both viruses as neuropathotype B, consistent with a very virulent pathotype. MDV load in PBL at 7 and 44 dpc did not differ between virus isolates, but the load at 7 dpc was significantly and negatively associated with time to euthanasia or death. CONCLUSION: MPF23 appears to be as, or more, virulent than the MDV strains isolated over the subsequent two decades. The neuropathotyping system developed in the USA did not clearly differentiate between the two isolates under test; however, extension of the period of assessment of clinical signs beyond 26 dpc did reveal clear differences.

Development of a chick bioassay for determination of infectivity of viral pathogens in poultry litter.

OBJECTIVE: To develop a chicken bioassay to detect infective viral pathogens in poultry litter and to determine the effects of type of chicken and age of exposure, as well as the effect of simulated litter transportation, on the level of viral infectivity detected. DESIGN: A 5 × 2 × 2 factorial design, plus negative controls. Five chicken litters, including two with deliberate contamination (one transported and one not), two chicken types (specific-pathogen-free (SPF) Leghorns and Cobb broilers) and two ages at initial exposure (days 1 and 8). Two replicates of each treatment combination. METHODS: The 10 chickens in each of 22 isolators were either exposed (20 isolators) or not (2 isolators) to 8 L of previously used or deliberately contaminated poultry litter in two deep scratch trays. At day 35 post-exposure, sera were assayed for antibodies against chicken anaemia virus (CAV), infectious bronchitis virus (IBV), infectious bursal disease virus (IBDV), Newcastle disease virus (NDV) and fowl adenovirus (FAV). Spleen samples were tested for Marek's disease virus (MDV) using real-time polymerase chain reaction. RESULTS: The bioassay detected CAV, IBDV and FAV, but not NDV, IBV or MDV, in chickens exposed to infected litters. Infection in SPF chickens was detected with greater sensitivity than in the broiler chickens. Sensitivity increased with age at exposure in broiler but not SPF chickens. Simulated transportation for 24 h had little effect on pathogen detection. CONCLUSION: A bioassay based on the exposure of day-old SPF chickens to poultry litter and measurement of seroconversion at day 35 post-exposure is a useful semi-quantitative assay for viral infectivity in poultry litter, with overnight transportation of litter having little effect on the level of viral infectivity detected. This bioassay has applications in research on litter treatment protocols.

Pathotyping of Australian isolates of Marek's disease virus in commercial broiler chickens vaccinated with herpesvirus of turkeys (HVT) or bivalent (HVT/SB1) vaccine and association with viral load in the spleen and feather dander.

OBJECTIVE: To pathotype Australian isolates of Marek's disease virus (MDV) in commercial broiler chickens using standard methods and to evaluate early markers of pathotype. METHODS: A complete 3 × 4 factorial experiment with two replicates was conducted using 648 Cobb broiler chickens in 24 isolators. The experimental factors were vaccination (unvaccinated, herpesvirus of turkeys (HVT), bivalent (HVT + SB1 strain of serotype 2 MDV)) and MDV challenge (unchallenged or 500 plaque-forming units of isolates MFP57, 02LAR or FT158). Mortality, body weight, immune-organ weights and viral load were measured to 56 days post challenge (dpc). Vaccinal protective index (PI) and virulence rank (VR) were calculated based on gross Marek's disease (MD) pathology. RESULTS: The PIs provided by the HVT and bivalent vaccines against challenge with MPF57, 02LAR, and FT158 were 84.6% 56%, 61.4% and 82.2%, 60.8%, 57.7%, respectively, leading to putative pathotypes of virulent MDV for MPF57 and very virulent MDV for 02LAR and FT158. Significantly more of the unvaccinated chickens (85.7%) had MD lesions than chickens vaccinated with either the HVT (26.8%) or bivalent vaccine (27.6%). Strong linear relationships were observed between the incidence of MD at 56 dpc and MDV load in the spleen at 7 dpc (R(2) = 0.71) and MDV load in the isolator exhaust dust at 14 dpc (R(2) = 0.57) and 21 dpc (R(2) = 0.51). Immune organ weights had a weaker association with subsequent MD incidence. CONCLUSION: Pathotyping results in broiler chickens with maternal antibody broadly agreed with those in specific-pathogen-free chickens in other studies, with some important differences. MDV load in the spleen at 7 dpc and in isolator dust at both 14 and 21 dpc was a powerful early predictor of subsequent MD incidence.

Immunosuppressive effects of Marek's disease virus (MDV) and herpesvirus of turkeys (HVT) in broiler chickens and the protective effect of HVT vaccination against MDV challenge.

Much of the impact of Marek's disease in broiler chickens is considered to be due to immunosuppression induced by Marek's disease virus (MDV). The present study evaluates the effects of an Australian isolate of pathogenic MDV (strain MPF 57) and a non-pathogenic vaccinal strain of herpesvirus of turkeys (HVT) (strain FC 126) on the immune system of commercial broiler chickens for 35 days following challenge at days 0 or 3 of age. It also investigates the extent of protection provided by HVT vaccine against MDV-induced immunosuppression. Immune system variables, including relative lymphoid organ weight, blood lymphocyte phenotype (CD45+/CD3+, putatively T, and CD45+/LC+, putatively B) and antibody production following vaccination against infectious bronchitis (IB) at hatch, were used to assess the immune status of chickens. Immunosuppression was also assessed by susceptibility to secondary challenge with pathogenic Escherichia coli on day 29 post-MDV challenge. MDV infection reduced the weight of the thymus and bursa of Fabricius, the numbers of circulating T lymphocytes and B lymphocytes, and IB antibody titre. The timing of these effects varied. MDV infection greatly increased susceptibility to E. coli infection. HVT alone caused mild depletion of T and B lymphocytes but no effect on immune organ weight or IB titre. Vaccination with HVT provided good protection against most of the immunosuppressive effects of MDV but not against MDV-induced growth impairment and reduced responsiveness to IB vaccination, suggesting that recent Australian strains of MDV may be evolving in virulence to overcome the protective effects of HVT.