Ostreid herpesvirus-1 microvariant surveillance in Pacific oysters (Magallana gigas, Thunberg, 1793) in Australia in 2011

OBJECTIVE

To demonstrate that OsHV-1 microvariant was limited to the known infected areas in New South Wales at the time of the survey in 2011.

DESIGN

A 2-stage survey to demonstrate probability of infection at 2% design prevalence within oyster growing regions and to detect at least one infected region (4% design prevalence) with 95% confidence.

SAMPLE POPULATION

Magallana gigas in nominated oyster growing regions in New South Wales, South Australia and Tasmania as approved by the Aquatic Consultative Committee on Emergency Animal Diseases and documented in a national surveillance plan.

PROCEDURE

Field sampling for active surveillance and laboratory selection of appropriate tissues using methods to minimize potential for cross contamination. Published methods for qPCR and conventional PCR for OsHV-1 microvariant. Stochastic analysis of survey results to demonstrate probability of detection in the areas tested.

RESULTS AND CONCLUSIONS

OsHV-1 microvariant was not detected in a total 4121 samples according to the case definition developed for the survey. However, in NSW a screening qPCR for OsHV-1 detected 13 samples that reacted. These samples were negative at 2 laboratories in the qPCR and conventional PCR assays used in the case definition for the survey. We concluded that oyster production areas of Australia outside the infected area in NSW met the criteria for self-declaration of freedom at the time of the survey in 2011.

CLINICAL RELEVANCE

This activity illustrated achievements in surveillance for an emerging emergency animal pathogen where epidemiological and test validation data were limited, but where data was required to inform the emergency disease response. It also illustrated the challenges faced by investigators in interpreting surveillance results using tests with limited validation. It was guided by and has informed improvements in surveillance and emergency disease preparedness.

Clostridial panophthalmitis - An unusual case of localised malignant oedema in Merino lambs associated with Clostridium chauvoei

The authors report an unusual case of Clostridium chauvoei causing severe panophthalmitis in Merino lambs. More than half of the lambs affected survived, which is unusual for clostridial disease; however, there have been reports in humans that the mortality risk for ocular gas gangrene is lower than when other body parts are affected. A combination of factors in this case included environmental contamination (specific to a particular paddock), genetics predisposing to entropion (lambs born of maiden 2-year-old ewes with some inbreeding), the practice of manually everting eyelids for the entropion and vaccination strategies. C. chauvoei was cultured in pure growth from the eye of a lamb affected by severe panophthalmitis. Histopathology was consistent with severe acute infection, and microscopic sections showed Gram-positive organisms associated with the inflammatory response in the eye. In the acutely affected animal examined there were no signs that the lesions were long-standing. Animals that recovered had one or both eyes destroyed. This report describes that malignant oedema in sheep due to C. chauvoei can manifest as acute and severe panophthalmitis. The case fatality rate of lambs with panophthalmitis was less than 50%, lower than normally occurs for clostridial diseases. Clostridial vaccination of the ewes may have provided a low level of protection in reducing the case fatality rate in the affected lambs as well localisation of the infection from the blood-ocular barrier.

Clinical and epidemiological features of West Nile virus equine encephalitis in New South Wales, Australia, 2011

BACKGROUND

Between February and June 2011, more than 300 horses with unexplained neurological disease were observed in New South Wales, Australia. A virulent strain of West Nile virus (WNV_NSW2011 ), of Australian origin, was shown to be the cause of many of these cases.

METHODS

We reviewed the clinical descriptions provided by veterinary practitioners and the associated laboratory results. Although there was a range of clinical signs described, ataxia was the only sign that was consistently described in laboratory-confirmed cases.

RESULTS

WNV was detected in brain samples by real-time reverse transcription PCR assay and virus isolation. For serological confirmation of clinical cases, an equine IgM ELISA specific for WNV was shown to be the most effective tool.

CONCLUSION

A state-wide serological survey undertaken after the outbreak indicated that, contrary to expectation, although infection had been widespread, the seroprevalence of antibodies to WNV was very low, suggesting that there could be a significant risk of future disease outbreaks.

Neospora caninum in beef herds in New South Wales, Australia. 1: seroprevalence study

OBJECTIVE

To determine the seroprevalence of Neospora caninum antibodies in beef breeding herds across New South Wales (NSW) and to determine if there are any differences associated with geographic location and other herd-level factors.

METHODS

Cross-sectional survey of beef breeding cows (n = 3298) from 63 properties (approximately 55 cows per herd) sampled randomly from six regions in NSW using a multistage survey design. Samples were tested by ELISA for N. caninum. Seroprevalence was determined at animal and herd levels, using an analysis approach to account for stratification, sample weighting and within-herd clustering.

RESULTS

Animal-level seroprevalence ranged from 1.8% to 11.3% across regions and the overall animal seroprevalence for NSW was 5.9%. The mean within-herd seroprevalence was 5.2%. The herd seroprevalence ranged from 50% to 92%, with an overall point estimate for NSW of 63.8% (using ≥ 1 animal positive = herd positive). The within-herd seroprevalence ranged from 1.6% to 32.7% Prevalence and associated confidence limits were adjusted for the design of the survey.

CONCLUSIONS

Overall, about two-thirds of all herds in NSW showed evidence of infection, but the seroprevalence of N. caninum in individual beef cattle in NSW was low to moderate (1.8-11.3%). Significant differences occurred between regions. The risk for herds being positive for N. caninum was associated with geographic factors, particularly in the Mid-North Coast Region.

The first identified case of pandemic H1N1 influenza in pigs in Australia

A 300-sow farrow-to-finish herd in New South Wales was infected with influenza pandemic (H1N1) 2009 (H1N1/09) virus in July 2009 and became the first recorded case of influenza in pigs in Australia. The outbreak resulted from human-to-pig transmission. Clinical signs in affected pigs were mild compared with overseas reports of 'classical' swine influenza virus and included coughing and decreased appetite in a small proportion of non-lactating breeding stock, weaners, growers and finishers. A diagnosis of H1N1/09 influenza virus infection was confirmed using a combination of serology (haemagglutination inhibition, blocking enzyme-linked immunosorbent assay) and real-time reverse transcription polymerase chain reaction. Attempts at virus isolation were unsuccessful. Results of a longitudinal study of pigs on this farm suggested that the virus continued to circulate for 9 weeks after the onset of infection, but was not present 6 months later. This report highlights the difficulties in preventing transmission of H1N1/09 influenza virus from infected humans to pigs during a human pandemic.

Overview of the epidemiology of equine influenza in the Australian outbreak

This overview of the equine influenza (EI) epidemic as it occurred in two Australian states, New South Wales and Queensland, in 2007 describes the functions and activities of the epidemiology teams that were engaged during the outbreak and also identifies key features of the epidemiology of EI during the outbreak.

Quantitative analysis of the risk of spread of equine influenza associated with movements of vaccinated horses from infected areas during the Australian outbreak

Simulation models were developed to quantify the likelihood of equine influenza virus infection entering pre-movement isolation, persisting through pre- and post-movement isolation periods without being detected by scheduled laboratory testing, and escaping to infect susceptible horses at a destination. The mean probability of escape ranged from 1 in 1,200,000 to 1 in 600,000 depending on lot size. For 95% of iterations the probability of escape was less than 1 in 200,000, regardless of lot size. For a large group of 600 horses processed as multiple separate lots, the mean probability of escape ranged from 1 in 10,000 to 1 in 56,000 depending on lot size. As a result of this analysis, a modified protocol, with two tests during pre-movement isolation and an additional test during post-movement isolation at the Chief Veterinary Officer's discretion, was implemented.

Evaluating the effectiveness of the response to equine influenza in the Australian outbreak and the potential role of early vaccination

OBJECTIVE

To use modelling and epidemiological analyses to assess the effectiveness of control strategies employed during the equine influenza outbreak and determine if early vaccination might have had a beneficial effect.

METHODS

Transmission of infection was modelled using stochastic, spatial simulation, based on data from 16 regions in New South Wales and Queensland over the first month of the outbreak.

RESULTS

The model accurately represented the spread of infection in both space and time and showed that vaccination strategies would have reduced new infections by ∼60% and reduced the size of the infected area by 8-9%, compared to the non-vaccination baseline.

CONCLUSION

When used in conjunction with biosecurity measures and movement controls, early vaccination could play an important role in the containment and eradication of equine influenza.

Evaluating the effectiveness of early vaccination in the control and eradication of equine influenza--a modelling approach

In August 2007, Australia which had previously been free of equine influenza, experienced a large outbreak that lasted approximately 4 months before it was eradicated. The outbreak required a significant national response by government and the horse industries. The main components of the response were movement controls, biosecurity measures, risk-based zoning and, subsequently, vaccination to contain the outbreak. Although not initially used, vaccination became a key element in the eradication program, with approximately 140000 horses vaccinated. Vaccination is recognised as a valuable tool for managing EI in endemically infected countries but there is little experience using it in situations where the objective is disease eradication. Vaccination was undoubtedly an important factor in 2007 as it enabled movements of some horses and associated industry activities to recommence. However, its contribution to containment and eradication is less clear. A premises-level equine influenza model, based on an epidemiological analysis of the 2007 outbreak, was developed to evaluate effectiveness of the mitigation strategies used and to investigate whether vaccination, if applied earlier, would have had an effect on the course of the outbreak. The results indicate that early use of strategic vaccination could have significantly reduced the size of the outbreak. The four vaccination strategies evaluated had, by 1 month into the control program, reduced the number of new infections on average by 60% and the size of the infected area by 8-9%. If resources are limited, a 1 km suppressive ring vaccination around infected premises gave the best results, but with greater vaccination capacity, a 3 km ring vaccination was the most effective strategy. The findings suggest that as well as reducing clinical and economic impacts, vaccination when used with biosecurity measures and movement controls could play an important role in the containment and eradication of equine influenza.