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.

Tissue distribution, shedding and environmental detection of infectious bursal disease virus genome following infection of meat chickens at two ages.

OBJECTIVE: To compare the effects of infectious bursal disease virus (IBDV) infection of commercial meat chickens at 0 and 16 days old (d.o.) and determine if IBDV vRNA is quantifiable in litter and dust samples. METHODS: Ross meat chickens (n = 60) were orally infected or not with IBDV at 0 or 16 d.o. Blood and faecal samples were collected longitudinally to 28 days post infection (dpi) from six chickens and tissues collected weekly from three euthanased chickens. Relative bursal weight was recorded postmortem. IBDV antibody titres in sera were measured using ELISA and VCN was determined in tissues, faeces, litter and dust using qRT-PCR. RESULTS: Chickens infected at 16 d.o. had earlier and more severe bursal atrophy, earlier and higher IBDV vRNA load in lymphoid organs and an earlier and greater antibody response to infection than those infected at 0 d.o. Faecal shedding of IBDV between 2 and 6 dpi was observed in both groups followed by cessation with the 0 d.o. group and re-initiation of shedding at 28 dpi. IBDV was readily detected and quantified in litter and dust samples. CONCLUSIONS: The presence of significant maternal antibody (MAb) titres in 0 d.o. chickens provided protection against IBDV replication and bursal atrophy at 7 and 14 days post infection. The reduced titres of MAb present at 16 d.o. did not prevent rapid IBDV replication and early marked bursal atrophy. The observed resistance of 0 d.o. chickens is likely to be a combination of MAb inhibition of IBDV and true age resistance of neonatal chicks. Measurement of IBDV in litter and dust may have research or diagnostic application.

Field studies of the detection, persistence and spread of the Rispens CVI988 vaccine virus and the extent of co-infection with Marek's disease virus.

OBJECTIVE: To use specific real-time qPCR to determine (1) the vaccination success of Rispens CVI988 vaccine in feathers and dust; (2) persistence of Rispens infection in vaccinated layer chickens; (3) extent of co-infection with wild-type Marek's disease virus (MDV) in vaccinated layers; and (4) presence of Rispens virus in unvaccinated broiler flocks. METHODS: Feather, dust and serum samples were collected from birds aged 3 days to 91 weeks from three layer farms. qPCR was used to detect MDV and Rispens in DNA extracted from dust and feathers. Previously tested MDV-positive dust samples from 100 broiler flocks were tested for the presence of Rispens using qPCR, while serum samples were used to detect anti-MDV antibody using ELISA. RESULTS: Overall, 66% and 93% of feather and dust samples, respectively, from Rispens-vaccinated layers were Rispens-positive. Viral load in these samples varied between farms during early life, reaching readily detectable levels at 2-3 weeks of age. Vaccinated chickens maintained a high Rispens load in feathers and dust and high MDV antibody levels until 91 weeks of age. MDV infection was detected in 6.7% of feather samples from vaccinated chickens. Rispens virus was detected in 7% of samples from unvaccinated broiler flocks. CONCLUSION: Vaccine take can be measured effectively by Rispens-specific qPCR of feathers or dust from approximately 3 weeks post vaccination. Infection with Rispens is persistent, with lifelong shedding and serological response. The detectable infection rate of vaccinated chickens with MDV is low and there is preliminary evidence of escape of Rispens virus to unvaccinated flocks.

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.

Differentiation between pathogenic serotype 1 isolates of Marek's disease virus and the Rispens CVI988 vaccine in Australia using real-time PCR and high resolution melt curve analysis.

Two real-time PCR assays were developed which enable quantitation and differentiation between pathogenic Australian isolates of Marek's disease virus (MDV) serotype 1 and the serotype 1 vaccine strain Rispens CVI988. The assays are based on a DNA sequence variation in the meq gene between pathogenic and vaccinal MDV1 which has been confirmed by sequencing of 20 Australian field strains of MDV. Complete specificity has been demonstrated in samples containing pathogenic MDV (n=20), Rispens (3 commercial vaccine strains), or both. The limit of detection of both the Rispens-specific and the pathogenic MDV1-specific assays was 10 viral copies/reaction. The tests successfully differentiated and quantified MDV in mixtures of pathogenic and vaccinal Rispens virus. A high resolution melt curve analysis targeting the same SNP used for the real-time PCR assays was also developed which successfully detected sequence variation between Md5, six Australian MDV1 isolates and the three Rispens vaccines. However it was ineffective at differentiating mixtures of pathogenic and vaccinal MDV1. The real-time PCR assays have both diagnostic and epidemiological applications as they enable differentiation and quantitation of Rispens CVI988 and pathogenic MDV1 in co-infected chickens in Australia.