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The changing epidemiology of human leptospirosis in New Zealand

The objective of the work published by Thornley and his co-authors (Thornley 2002) was to describe the current epidemiology and trends in New Zealand human leptospirosis using descriptive epidemiology of laboratory surveillance and disease notification data for the period 1990 -1998. During this period the annual incidence of human leptospirosis in New Zealand was 4.4 per 100,000. Incidence was highest amongst meat processing workers (163.5/100,000), livestock farm workers (91.7), and forestry-related workers (24.1).

The most commonly detected serovars in this period were L. hardjo (46.1%), L. pomona (24.4%), and L. ballum (11.9%). Although the annual incidence of leptospirosis declined from 5.7 per 100,000 in 1990 -1992 to 2.9 per 100,000 in 1996 -1998 the authors concluded that the incidence of human leptospirosis in New Zealand remains high for a temperate developed country and that targeted and evaluated disease control programmes should be renewed.

Recommendations made in the paper (Thornley 2002) include:

  1. The leptospirosis disease notification dataset collected on the case report form would benefit from ongoing improvement and a particular aspect would be the collection of accurate information on exposures to potential sources of infection.
  2. Future reviews of leptospirosis epidemiology should make use of exposure data and a study of cases matched for geographic area of residence would be necessary to determine the proportion of cases attributable to recreational exposure.
  3. The overall decline, and the decline in cases among livestock farmers, is likely to be the result of improved prevention of the disease in livestock.
  4. Several health districts have not followed the overall trend but there is insufficient data to provide explanations for the failure of these health districts to maintain the declining rates of leptospirosis.
  5. Valuable information would be gained from the inclusion of data comparing vaccination rates between health districts but this information was not currently available.

The paper published by Michael Baker and Liza Lopez (Baker 2004) updates the Thornley analysis of the period 1990 -1998, by describing the epidemiology over the most recent three-year period to 2003.

During 2001 to 2003 there were 355 notified cases of leptospirosis, an average annual total of 118 cases being a rate of 3.2 per 100,000 and for laboratory-identified cases a rate of 4.0 per 100,000, based on 2001 population census data. The average rates in this period were significantly higher than the preceding three years when notifications were 2.1 per 100,000 and laboratory identified cases were 2.8 per 100,000.

Occupation was recorded in 91.5% of the 355 cases notified in 2001 -2003. In this period there were virtually equal proportions of meat workers (37.5%) and farmers (37.4%) with a smaller group (7.0%) engaged in a wide range of occupations which potentially involve contact with animals or environments contaminated by animals. By comparison the occupational distribution of the 399 cases in 1990 -1992 included a higher proportion of livestock farm workers (57.0%) compared with meat processing workers (29.6%).

The predominant serovars among laboratory-reported leptospira in the 2001-2003 period were L. hardjo (42.7%), L. pomona (33.5%), and L. ballum (10.7%) with smaller contributions from L. tarrassovi (7.9%) and L. copenhageni (2.4%).

An analysis was made of the 90.4% of cases (321/355) in which information was reported on contact with animals or animal products prior to the illness. Of these, 47.0% reported unspecified contact with a farm. Many cases also reported contact with more than one animal species. Contact with cattle alone was reported by 23.4%, cattle in combination with other animals 9.3%, sheep on their own 5.9%, sheep in combination with other animals 4.4%, pigs on their own 2.5% and rodents on their own 1.2%. (It is interesting to note, in view of data reported elsewhere in this Report, that no cases of contact with deer were identified).

The conclusion is drawn that most infections are associated with contact with cattle, either on their own or with other animals. Such infections may be caused by any of the important serovars in New Zealand, although L. hardjo and L. ballum predominate. Sheep appear to be the second most important reservoir, on their own or in combination with other animals. Here L. hardjo and L. pomona occur in similar proportions. Cases with a history of contact with pigs are most commonly infected with L. pomona.

The authors make the point that the rise in incidence marks the end of a steady decline in incidence that had been identified over the previous two or more decades and while leptospirosis remains overwhelmingly an occupational disease of livestock farm workers and meat processing workers, as observed previously, there are marked differences in incidences occurring between different district health board areas.

District Health Boards with consistently high rates are Tairawhiti (14.4 per 100,000), Hawkes Bay (11.6), South Canterbury (7.6) Northland (7.4), Nelson Marlborough (7.3), Waikato (6.8), MidCentral (5.8), West Coast (5.5), Taranaki (4.5), Bay of Plenty (3.7), Wanganui (3.7), and Southland (2.9)

Particular mention is made of several potential sources of bias which need to be considered when interpreting the surveillance data - these include:

  1. Many individuals with leptospirosis will not seek medical attention either because the symptoms are mild and short-lived, or due to difficulties accessing medical services. This may result in a lowering of the reported rates especially of those living in rural areas.
  2. Recognition of leptospirosis by doctors was regarded by many of the people interviewed during the joint Department of Labour - ACC 'Aftermath' project as poor. The reasons for this are that the sys of Leptospirosis are not clear cut, that Leptospirosis is relatively rare (especially for urban GP's) and the need to test is not always recognised, even after admission to hospital. (Aftermath 2002.) Unrecognised cases will not receive laboratory testing and therefore are not included in the data set. Doctors may also be less likely to suspect the diagnosis in cases lacking a history of exposure to a well-known source of infection, creating a bias against detecting atypical or emerging modes of transmission.
  3. The ESR Leptospira Reference Laboratory does not receive all records from regional laboratories, and the laboratory surveillance database therefore may not represent a true count of all laboratory identified cases in New Zealand.

The conclusion is drawn that the analysis of trends over this 2001 - 2003 period provide some evidence that the epidemiology of leptospirosis in New Zealand is changing. The proportion of cases among meat workers now equals that among farmers. There is evidence that sheep are emerging as a more important reservoir than previously.

Ongoing disease surveillance would provide more sensitive and complete information if laboratory and notification data were fully integrated. More intensive epidemiological study would enable better definition of the risk factors of the disease on the farm and in the meat processing environments which in turn would help identify effective interventions.

Leptospirosis in sheep and deer

When the Guidelines were published in June 2001 it was acknowledged that knowledge of the role of all the individual livestock species as sources of human exposure to Leptospira pathogens was incomplete. In a paper to the New Zealand Veterinary Association meeting in May 2005 Dorjee et al (Dorjee 2005) reported preliminary results of work undertaken in response to the incidence of leptospirosis in meat workers in sheep only plants.

The study was conducted between May 2004 and June 2005 with a total of 1966 sera from 68 lines of sheep comprising at least 65 different properties from various areas around New Zealand being collected by systematic random sampling of 30 carcasses from each of 3 - 5 randomly selected lines (i.e. farms) per week. The results were that 33 of 68 (48.5%) lines had one or more carcasses with titres of either L. hardjo or L. pomona or both. The prevalence of positive lines to both serovars was significantly higher for hogget lines (85.7% and 28.6%) than for lamb lines (23.4% and 10.6%) for L. hardjo and L. pomona respectively.

A strong positive association was observed with the kidneys lesions (i.e. distinct white spots) and seroprevalence for both serovars. Carcasses with at least one distinct white spot on one or both kidneys were 4.6 times and 15.1 times more likely to test positive to L. hardjo and L. pomona respectively, than those without white spots. However the prediction of the sero-status of a carcass based on kidney lesions is low with positive predictive values of 16.7% and 9.2% for L. hardjo and L. pomona respectively.

In a second study, sheep from five farms with reported clinical outbreaks of leptospirosis attributable to L. pomona were investigated and sampled for serology. The outbreaks presented as a weakness, lethargy, and death of either 1 - 4 week old lambs or 6 - 9 month old lambs with up to 5 - 15% of the lambs in the affected groups dying. Severe jaundice was a feature of all cases. Three cases were associated with periods of high rainfall that caused surface flooding or poor drainage. In two cases a worker became ill and leptospirosis was suspected. On another farm where an outbreak of leptospirosis was suspected 14 successful isolations of L. pomona were made from 16 lambs (13 seropositive and 3 seronegative) sent for slaughter.

The third study followed cohorts of lambs, hoggets, and mixed age ewes by serology and culture on a commercial farm on which a clinical outbreak of leptospirosis had occurred in the previous season. Results indicate that the seroprevalences in lambs up to eight months of age was low, while it was high in hoggets and ewes exposed to the outbreak in the previous year.

The authors conclude that this study indicates L. pomona can cause significant economic losses on affected properties and at the same time that lambs affected with this serovar can be clinically normal. The on-farm case study data shows that although the clinical outbreaks were related to L. pomona, the prevalence of L. hardjo was equally or widely prevalent in both affected and unaffected mobs on the same farm. This is probably because the risk factors for transmission between species, and between sheep, are the same for both serovars. It also indicates that subclinical infection with both of these serovars was very common, again highlighting the higher levels of leptospirosis circulating on these properties.

While the evidence available to date indicates that meat workers have a higher risk of exposure to live leptospires during the processing of sheep carcasses coming from farms where there were recent clinical or unrecognised outbreaks of leptospirosis, the risk of exposure from lamb carcasses appears to be lower than that for older sheep. The authors comment that more work is needed to understand the risk to meat processing workers and other persons working with sheep and that the additional information gained from this further work will be important to contribute to the development of measures to address the risks faced by these workers.

In his paper "Leptospirosis in sheep - a call to action?" Dr Peter Davies (Davies 2003) stresses the necessity to understand which serovars predominate among human cases of leptospirosis that have been attributed to exposure to sheep and more information is required of the prevalence of the relevant serovars in different classes of stock and regionally. The importance of interspecies transmission of leptospirosis to sheep (e.g. from cattle, deer or pigs) requires clarification, as does the ability of sheep to act as maintenance hosts and the dynamics of transmission in flocks.

In the period since the publication of the "Guidelines" there has been discussion on the merits of vaccination of all livestock (dairy, beef, deer, and sheep) destined for slaughter and whether vaccination is a "viable option". Peter Davies makes the point in the paper that this suggestion is premature as clinical leptospirosis has only been sporadically reported and at the time of writing there were no vaccines registered for use with sheep. Even through, since the work reported in the paper, there are now two vaccines registered for use with sheep, little research has been done to establish the effectiveness of them in all age classes of sheep. More research needs to be done on the range of serovars that needs to be covered in any effective vaccination programme, gathering proof of the adequacy of cover (i.e. that the vaccines will reliably prevent urinary shedding), and how vaccination programmes should be designed for commercial flocks.

Dr Cord Heuer in his paper "Human Health and Leptospirosis",�presented to Sheep and Beef Cattle Veterinarians (Heuer 2006) made the point that cattle were maintenance hosts for L. hardjo, farmed deer for L. hardjo and L. pomona, and pigs for L. pomona and L. tarassovi. While sheep could be readily infected with L. hardjo and renal infection could persist for at least 13 weeks, shedding by infected cattle could occur intermittently for up to 18 months.

Cattle may remain serologically positive to leptospirosis for up to seven years. Infection with L. hardjo usually causes a sudden decrease in milk production and flaccid or atypical mastitis in cows. Where large outbreaks of leptospirosis due to serovar L. hardjo occurred, the number of herds becoming infected without clinical signs increased as the outbreak spread.

One longitudinal study of sheep in 2005 suggested that seroconversion occurred throughout the year in ewes, hoggets and lambs and that at slaughter, lambs had a lower seroprevalence than older animals because of their shorter time at risk from birth to slaughter.

Beef Cattle

Some research on leptospirosis in beef cattle was carried out in Hawkes Bay in 1998 in 1999 and reported by Matthews et al in 1999. Titres to L. hardjo were detected in all herds and 44% of all the animals tested had titres of 1:384 or greater indicating these animals had recent or current infections (Matthews 1999).

During the next year two herds were examined in more detail with the view to ascertaining at what time of year, and at what age, infection was occurring. The results indicated that infection with L. hardjo occurs after the first year of life in both sheep and cattle and that infection is probably present in all stock types.

There was a difference in the time of infection of cattle on the two farms and because young and old stock were not mixed on either farm, the reason could possibly be that one farm has lower rainfall compared with the other farm that was monitored.

The authors concluded that the data in the survey suggests that by vaccinating young stock they would be protected from contracting leptospirosis, the risk of which increases after their first year. This would also allow for the possibility of control, if not eradication, on these farms.


In 2003-2004 the number of human leptospirosis cases in Southland increased to more than double the previous rates and there were seven cases in deer slaughtermen from venison processing plants, all of whom had no other animal exposure. This indicated that deer are an important vector for the disease (Bell 2005).

Research projects commissioned by industry and conducted by researchers from Massey University and AgResearch Invermay investigated the epidemiology of leptospirosis infections in the deer herds, its national distribution, and the potential for vaccination strategies to reduce the risk to animals and humans and were reported (Wilson 2005), and Ayanegui-Alcerreca 2005). The survey included 113 non-vaccinated farms from both islands with 2,165 animals being sampled. Use was made of a questionnaire where data for a risk factor analysis was requested, such as herd composition, herd size, type of production, demographic data on deer and other species, vaccination practices and previous history of disease on the farm.

It was discovered that:

  1. 83% of herds have serological evidence of leptospirosis - serovars L. hardjo alone 65%, L. hardjo and L. pomona combined 14%, and L. pomona alone 4%.
  2. The prevalence of the infections was similar in all regions of New Zealand.
  3. Most disease was caused by serovar L. pomona with some been caused by serovar L. hardjo.
  4. When the infection is endemic in a herd the prevalence is low in deer 3 - 6 months of age and increases during the first year of life, remaining reasonably constant thereafter.
  5. With newly introduced infection - transmission may be from deer to deer or cattle to deer, or possibly from sheep to deer. Transmission can also occur from deer to other livestock species.
  6. The risk of disease appears to be higher when the serovar is L. pomona.
  7. Infection with L. hardjo tends to persist (with deer acting as the maintenance host) whereas infection with L. pomona appears to be less persistent (deer appear to be acting as the accidental host).
  8. A significant number of the deer shed leptospires in the urine.
  9. Vaccination with "Leptavoid 3" manufactured by Schering-Plough Animal Health Ltd produced a good antibody response, reduced shedding of leptospires in the urine, reduced the prevalence and severity of kidney lesions.

Ayanegui-Alcerreca MA et al 2005 in their paper "Deer leptospirosis vaccination: A preliminary report" report that there was a significant enhanced calf survival to weaning from 88% to 97% in the vaccinated herds. The conclusions they draw from this preliminary research are that vaccination with this product will provide significant protection for deer, reduce the risk of transmission of leptospirosis from deer to people, and provide a production response in some situations. A 3.7% increase in growth rate of young stock has been recorded in some trials (J. Moffat pers comm.)

The suggestion is made (Wilson et al 2005) that the deer industry must now consider the implications of the new found knowledge and assess its options which include the possibility of a nationally coordinated vaccination strategy similar to those adopted by the dairy cattle and pig industries. The paper proposed the establishment of a working party comprising industry stakeholder representatives to advise industry, and determine strategy is deemed appropriate by the industry.

Brown (2005) comments on the implementation of risk management programs in a number of the deer slaughter premises and the use of in-house risk assessment matrices to assess hazards to employees and quantify the associated risks based on the likelihood, severity, and consequence of harm. When applied to the leptospirosis hazard, in the absence of control measures, the risk score was a high "serious harm". With controls in place including eye, face and wound protection, as well as certain hygiene procedures, the score reduced from 270 to 60 which became a "minor moderate" harm. If known infected stock lines could be identified it would be possible to further reduce the harm levels to workers by employing additional control measures during processing of those stock.

Brown comments that the economic impact of leptospirosis can be significant to both the company and the affected worker. A North Island plant estimated that each case costs the company in excess of $5,000. the time delay to confirm the diagnosis results in delayed ACC payments to the victim and financial stress in a number of cases. ACC levies to the meat industry have increased by 60% in the past two years. Leptospirosis has no doubt contributed greatly to the escalation of premiums to the industry (Brown 2005).

Meat Industry

In the period since 2001 further the work has been done evaluating different types of Personal Protective Equipment (PPE) and related matters with the view to isolating and minimising risk to employees in the meat industry of contracting leptospirosis. One outcome of the evaluations was the realisation of how advanced the anti-fogging technology had become over the two years since the publication of the Guidelines. Fogging was essentially non-existent in the newest masks and spectacles, leading to the conclusion that this major impediment to their use had been eliminated (Dowd 2003).

Some features were identified for modification such as with the Goggle and Face Shield the bottom of the face shield hit the workers chest when performing some aspects of the gutting task, and the goggles dug uncomfortably into the bridge of the nose when worn for longer than five minutes. Specially designed eye glasses were rated best for comfort with particular advantages in the ease of head movement, light weight and the degree of facial cooling (Dowd 2003a).

Discussions between representatives of Asure and the Department of Labour in 2005 acknowledged that (Smith 2005):

  1. Meat inspectors were at risk of contracting leptospirosis through the hands and therefore Asure's hazard management policy would include the use of protective gloves in high risk areas.
  2. The exposure of meat inspectors, relative to other workers on the processing chain (due to meat inspectors' location on the chain and the nature of the work), meant that the risk of infection from splashing and aerosol factors was probably lower and therefore this supported Asure's proposal to reduce nose and mouth protection, but retain eye protection.
  3. It was incumbent on Asure as the employer to upgrade protection levels as new information and/or technology became available (i.e. take all practicable steps).

Additionally it was agreed that a number of observational exercises would be undertaken to gather data on how often meat inspectors touched their face, nose and mouth areas while carrying out inspections - to provide information on the potential for hand to mouth and nose transmission. Another exercise would record the frequency of face splashes and the number of times protective glasses and exposed faces were splashed.

A draft document on leptospirosis and PPE, providing a rationale for proposed changes to staff, would be prepared and circulated with the request for feedback and discussion on the proposed changes.

Separate initiatives would be the provision of information to GPs with the view to fast tracking the diagnosis and treatment of suspected leptospirosis and the investigation of blood testing facilities with the view to accessing quick diagnostic tests.

Finally a comprehensive employee education program would be developed covering all aspects of leptospirosis, the personal hygiene practices, and protective measures and this would be included in the induction program for new staff and the annual refresher training.

Meat companies have placed particular emphasis on the use of gloves, both Kevlar to protect against cuts, and disposable waterproof gloves, to protect against contact with contaminated urine. Initiatives taken by one company include:

During induction training staff are given a wallet card to indicate to the doctor they work in the meat processing industry. Emphasis is placed on the requirement for washing after any urine contact and the reporting of any splashes.

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