Occupational leptospirosis in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University, Institute of Veterinary, Animal and Biomedical Sciences, Palmerston North, New Zealand
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Date
2016
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Massey University
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Abstract
Although a decreasing trend of human notifications was observed from 2001 to
2014 (Chapter 1), the incidence of human leptospirosis in New Zealand continues to
be higher than in other high-income countries and affecting predominately people
occupationally exposed to livestock (i.e. abattoir workers and farmers). Additionally,
evidence suggests that leptospirosis may have a higher detrimental effect on
production in deer compared with beef cattle or sheep. It was also observed that
vaccination against Leptospira of not previously infected animals reduce the risk of
urinary shedding of leptospires after challenge, and that there is limited evidence
supporting or disproving that maternally derived antibodies interfere with the effect
of vaccination when animals are vaccinated at a young age.
When sero-positivity was defined as a serum microscopic agglutination test (MAT)
titre of ≥48, 6.6% of farmers (Chapter 2) and 5.1% of veterinarians (Chapter 3)
were sero-positive to at least one of five Leptospira serovars (Hardjo-bovis, Pomona,
Copenhageni, Ballum, Tarassovi). Veterinarians had a higher risk of being seropositive
when they slaughtered cattle or pigs at home or worked in a mixed animal
practice. Assisting calving of cattle or deer, farming deer alone or in combination
with cattle and/or sheep, flat terrain on farm, and abundance of wild deer on farm,
increased Leptospira sero-positivity of farmers. Apart from vaccinating farmed livestock,
increased awareness of such risk factors and the use of protective equipment
may reduce the human leptospirosis incidence in these occupational groups.
Similar to earlier observations in abattoir workers, Leptospira sero-prevalence
of farmers and veterinarians was associated with the recall of influenza-like illness
of sampled individuals. Using the estimated incidence of influenza-like illness attributable
to Leptospira infection (population attributable risk) of veterinarians
(0.05%), farmers (1.3%) and abattoir workers (2.7%), we quantified the burden of
human leptospirosis in terms of disability-adjusted life years (DALYs) and economic
cost of infection; the latter including the cost of vaccination, which is primarily used
in dairy cattle (Chapter 4). Annual DALYs were estimated to be 0.43 per 100,000
people in New Zealand, and 16.76 per 100,000 people when only considering the
occupationally-exposed population (abattoir workers, farmers, veterinarians). This
ranks leptospirosis in New Zealand’s high-risk population similar to worldwide estimates
of DALYs for rabies and dengue. The total annual cost of leptospirosis due to
human disease (i.e. treatment and absence from work); production loss in deer, beef
cattle, and sheep; and the cost for vaccinating them was estimated to be NZ$25.36
million. One third of this total was attributed to vaccination of dairy cattle. The
annual cost of human treatment and workplace absence due to severe and mild leptospirosis
was NZ$4.49 million. Total lost production cost was NZ$11.31 million,
half of which was attributable to reproductive and growth reduction in deer. No estimates
are currently available from any other country for the public health burden
and the overall economic loss including farmed livestock for this disease.
Since vaccination of livestock is currently regarded as the most effective means of
preventing human exposure, the literature on the efficacy of Leptospira vaccines for
preventing urinary shedding was systematically reviewed (Chapter 5). The metaanalysis
of vaccination trial results, using articles with sufficiently detailed data,
resulted in a pooled vaccine efficacy estimate of 82% when shedding was assessed
by culture.
The findings of this thesis contribute towards a better understanding of the public
health burden, economic cost, infection sources for humans, and the efficacy of
vaccination for reducing the risk of Leptospira urinary shedding in domestic livestock
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Keywords
Leptospirosis, Epidemiology, Veterinarians, Abattoir workers, Farmers, New Zealand, Leptospira sero-prevalence, Research Subject Categories::MEDICINE::Social medicine::Public health medicine research areas