Leptospirosis in New Zealand: "In the First Week I Thought I Would Die, and then I Wished that I Had"
Institute of Veterinary Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand
Leptospirosis is a zoonotic disease of global importance. It continues to burden New Zealand's rural communities with most cases in humans occurring in farmers and meat workers. The epidemiology of leptospirosis in New Zealand is unique with our ruminant livestock species being key maintenance hosts for serovars Hardjo-bovis and Pomona. Recently, the rodent-associated serovar Ballum has increased in frequency amongst reported human cases reinforcing the importance of a comprehensive approach to leptospirosis management.
From the 1980s to the present, there has been widespread uptake of vaccination of dairy cattle and pigs. This has been associated with a reduction in human cases from the peak of 875 in 1974 to approximately 100 cases per year over the period 1997 to 2008, a level at which case numbers remained up to 2011. Despite this large reduction in case numbers, leptospirosis continues to be a severe disease for rural New Zealanders with 50% of notified cases hospitalized. The incidence risk of 2.5 per 100,000 places New Zealand in the moderate incidence category for the Asia Pacific region and globally.
In this paper, we present an overview of our various leptospiral projects in animals and humans in New Zealand, but focus in particular on a cohort study of abattoir workers and a sero-survey of canine leptospirosis.
Cohort Study of Abattoir Workers
We conducted a cohort study in eight abattoirs slaughtering sheep (4), cattle (2) or deer (2) located in the north (6) and south (2) islands of New Zealand. Our objectives were to determine the annual risk of infection with leptospirosis in abattoir workers; the risk factors for new infection related to occupational and non-occupational activities; the incidence of confirmed clinical leptospirosis; and the proportion of 'flu-like' illness cases among all workers attributed to leptospirosis infection.
Sera and questionnaire data were collected from 592 workers over the period November 2009 to March 2010 and again over the period Nov 2010 to May 2011. Sera were tested at the Hopkirk Leptospirosis Research Laboratory (HLRL) by the Microscopic Agglutination Test (MAT) for serovars Hardjo-bovis and Pomona with a titre cut point of 1:48. Information on occupational and non-occupational risk factors and clinical history were recorded and analysed by multivariable logistic regression.
Prevalence of seropositivity and incidence of seroconversion was highest in workers in the four sheep plants. The average annual seroconversion risk across four sheep plants was 12.8%. Deer plant workers had the highest prevalence (19% and 18%) with no incidental infections. The two beef plants had 5% prevalence and a 1% annual seroconversion risk. Due to the low number of incident cases in beef and deer plants further analysis of incidence data was carried out on the four sheep plants (n = 384 subjects).
New infection was defined as either seroconversion or an anamnestic response. Forty-seven of the 384 (12.2%) sheep abattoir workers experienced new infection in the study period. In a multivariable model, worker position was identified as the main risk factor for new infection. Workers at the beginning of the slaughter line (yards, stunning and pelting) had 7.5 times (95% CI: 2.5–22.4) the odds of new infection compared to those working in the boning room, chillers or office (the reference group). Persons working at pulling kidneys, gut removal and meat inspection had 5.0 times (95% CI: 1.7–16.0) and persons in the offal room 4.0 times (95% CI: 1.0–16.4) the odds of infection, when compared with the reference group. Other significant risk factors included months worked in the meat industry and abattoir.
Three men from sheep abattoirs had confirmed clinical leptospirosis within the study period: two had seroconverted against Pomona whilst one remained seropositive against Pomona.
Of the 47 workers with new infection 22 (47%) reported experiencing 'flu-like symptoms over the study period. Of the 337 that did not have new infection, 82 (24%) reported experiencing 'flu-like symptoms over the study period. Thus new infection with Hardjo-bovis and/or Pomona increased the risk of illness with 'flu-like symptoms 1.9 times (95% CI: 1.32–2.71). The population attributable fraction was 10%. Assuming causality, this implies that 10% of 'flu-like' cases in the study population were likely attributable to a new infection with Hardjo-bovis and/or Pomona rather than other causes.
Serosurvey of Canine Leptospirosis
We conducted a survey of canine sera from the North and South Islands of New Zealand and tested by MAT for Copenhageni, Hardjo-bovis, Ballum and Pomona with a titre cut point of 1:96. The study population was a convenience sample of 655 sera submitted consecutively to New Zealand Veterinary Pathology laboratories and the Massey University Veterinary Clinic, Palmerston North between January 2005 and May 2005. The majority of samples (606/655) were from the lower half of the North Island and the remainder were from the South Island. We explored the association between the prevalence of seropositive samples to leptospirosis and breed group, age group and sex.
The count and prevalence of positive titres to any one of the four serovars nationwide was 98/655, (15%, 95% CI: 12.3%–18.0%). Copenhageni was the predominant serovar with 67 of 648 dogs positive, 10.3%, 95% CI: 8.1%–12.9%. Twenty-three of 649 were positive to Hardjo-bovis (3.5%, 95% CI: 2.3%–5.3%), 7/651 to Pomona (1.1%, 95% CI: 0.4%–2.2%) and 5/654 to Ballum (0.8%, 95% CI: 0.2%–1.8%). There was a significant association between age quartile (<6, 6–9, 10–11, >11 years) and the prevalence of positive MAT tests to any one of the four serovars tested. Dogs aged 12 years and older were less likely to have positive leptospiral titres when compared with the reference group (< 6 years) (OR 0.41, 95% CI: 0.21–0.79).There was a significant association between the prevalence of positive titres to serovar Hardjo-bovis and breed group. Breeds of dog typically used as farm working dogs were at significantly higher risk for positive Hardjo-bovis titres than other breed groups (OR 10.14, 95% CI:2.34–52.09). There was no statistically significant difference in prevalence of positive leptospiral titres between the North and South Islands. This finding refutes the traditionally held belief that exposure of dogs to leptospires is largely confined to areas north of Taupo.
The study has confirmed positive titres to serovar Copenhageni do occur, though it remains unclear whether the positive titres are related to natural exposure or, in some cases, from vaccination. The study also confirms that dogs in the South Island are exposed to Leptospira organisms, and that dogs of breeds typically used on farm are at higher risk for exposure to serovar Hardjo-bovis suggesting that transmission from livestock occurs. Further studies in New Zealand are planned for 2013 to confirm the prevalence of leptospiral exposure in South Island and working dogs, and to determine the species and serovar of leptospires shed in canine urine in order to further assess the zoonotic risk, aid our understanding of the epidemiology of leptospirosis and inform canine vaccination strategy.
Update on Other Leptospirosis Projects
Production Effects of Leptospirosis in Sheep and Beef Cattle
Vaccination of livestock against leptospirosis should reduce human exposure. Furthermore, vaccination of livestock could reduce the adverse effects of leptospiral infection within the stock, due to acute disease or chronic infection. Recent work has shown that leptospirosis control through vaccination increases profitability in deer farming but the cost benefits of leptospirosis control on productivity of sheep and beef cattle need to be considered for farmers to make informed risk-based decisions.
Sampling commenced in August 2011 on eight volunteer commercial/stud sheep and beef farms to quantify the effects of natural leptospiral infection on growth and reproduction. Specifically, we will measure the growth of lambs and calves and reproductive performances of ewe lambs and heifers. A second aim of this work is to quantify the effect of leptospiral commercial vaccination. There are six North and two South Island farms. In all farms the presence of leptospiral infection (Hardjo-bovis and/or Pomona) has been confirmed. In each of the farms, 3 cohorts (female lambs, calves and heifers) have been enrolled and one third of the animals have been vaccinated. Growth rates and reproductive performance are recorded and compared between seronegative and seropositive animals (in the control group) and between vaccinated and control animals. These data will be used for modelling of the economic efficiency of vaccination as a control measure at the farm level. Interim results in the ewe lamb cohort show no seroconversion to date in six farms. In one farm 12/124 control lambs seroconverted between docking (day 0) and weaning (day 53); 13/98 seroconverted between weaning and 3rd sampling (day 188). The vaccinated ewe lambs grew significantly faster (13 g/day) between docking and weaning. The heifer cohort comprises 851 enrolled (1 year old heifers), with 691 of these mated. Twenty seven percent of heifers had Hardjo-bovis titres and 12.7% had Pomona titres at enrolment. The complete results are expected for January 2014.
Seroprevalence of Leptospirosis in Veterinarians, Veterinary Students and Farmers
A study of leptospirosis in 298 New Zealand veterinarians who attended the New Zealand Veterinary Association conference in Hamilton was conducted in June 2012. The aim of this study was to determine the seroprevalence of leptospirosis in New Zealand veterinarians, and identify and quantify the risk factors that may lead to the exposure to leptospirosis infection. Veterinarians were blood sampled and sera tested by MAT with three serovars (Hardjo-bovis, Pomona and Ballum) and a titre cut point of 1:48. An on-line risk factor questionnaire was completed covering both occupational and leisure-related risk factors. There were 114 females, 177 males and seven who did not record their gender. Veterinarians came from 15 regions of New Zealand. There were 13 positive samples (4.4%): six to Pomona, five to Hardjo-bovis, one to Ballum and one with dual infection to Hardjo-bovis and Pomona. More detailed analysis of results is pending.
A study was conducted to determine the seroprevalence of leptospirosis in veterinary students from Massey University, and identify and quantify the risk factors that may lead to the exposure to leptospirosis infection. The leptospirosis component is one part of a larger vet student health study. Other components include respiratory health and allergies and exposure to toxoplasmosis. It is hoped that this study will springboard a cohort of veterinary students who in the future will come forward for further studies into "veterinarian survival" and assist in investigating questions around why veterinarians leave the profession and in particular rural practice. From a population of 500 vet students (years 1 to 5 inclusive) at Massey, 327 students were enrolled in the study from September 2010 to October 2011. Within this group, blood samples were collected from 311 vet students, and tested by MAT with three serovars (Hardjo-bovis, Pomona and Ballum) with a titre cut point of 1:48. All samples tested negative allowing us to use this group as a baseline with which to compare with our other groups (meat workers, veterinarians and farmers) at risk.
Latest data on notified cases show that of the 70 cases notified in 2011, 62 had occupation recorded. Forty-six of these were in occupations considered at high risk for leptospirosis, 36 of these were farmers or farm workers. Since 2009, farming has been the occupational group most represented amongst notified leptospirosis cases. We will undertake a study of seroprevalence and risk factors for leptospirosis exposure in farmers in 2013. This has the potential to inform risk mitigation strategies and to provide data for the development of an ecological model about on-farm infection dynamics and means of prevention.
Update on Study of Leptospirosis Diagnostics in Humans
The aim of this study is to identify the best diagnostic test or combination of tests for acute cases of leptospirosis that will satisfy the need for early diagnosis, treatment, and also meet ACC requirements. In addition to the traditional diagnostic test for leptospirosis (the MAT and culture), collected blood samples will also be tested with DNA based tests from ESR, Canterbury Health Laboratory and our own leptospirosis research lab (HLRL). Our collaborators in human health are an integral part of this study. To meet this aim we started recruiting people with suspected leptospirosis presenting at general practitioners (GPs) in rural Waikato. We have augmented the GP recruitment with recruitment through phlebotomists in the Waikato and also by receiving serum samples directly from ESR's Leptospiral Reference Laboratory.
To date we have recruited 14 patients through GPs or phlebotomists and received 23 serum samples from ESR representing 14 patients. The majority of GP/phlebotomy recruited patients have had negative MAT, culture and PCR test results from all three labs showing excellent agreement between negative tests in these patients. This study is ongoing and should be completed late 2013.
Strain-typing of Leptospires
We are performing a genetic comparison of leptospirosis strains to see if identical types are found between vertebrate hosts. Genetic typing (molecular techniques) are reported to have more discriminatory power than the traditional serotyping of leptospires, especially when the latter failed to distinguish within a particular serogroup. We have used multilocus sequence typing of 23 isolates from sheep and cattle from an abattoir in the Waikato region. Interim results from 21 isolates show no genetic variation within serovars from this sample. This has potential implications for diagnostic testing and vaccination manufacture in New Zealand. However, we are seeking further isolates from a broader host species (including humans) and serovar range to better interpret these findings.
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