Risks to humans of leptospirosis from sheep
New Zealand Veterinary Association
24 September 2008
Risks to humans of leptospirosis from sheep confirmed
Meat workers are exposed to leptospira from handling sheep carcases, a large scale study by Massey University researchers has confirmed.
The study, reported in the New Zealand Veterinary Journal, was initiated after an increase in reports of the numbers of meat workers with leptospirosis in 2002/03. A number of the cases occurred in meat workers employed in a sheep-only abattoir.
The research was conducted at a North Island sheep-only abattoir that accepted sheep for slaughter from various areas of the country.
"Although we suspected this was the case, it was important to establish the facts and this study certainly shows us that there is a problem here," says New Zealand Veterinary Association member David West, who was one of the investigators.
The animals examined came from 89 farms and 11 different districts.
"Overall we found that about half the sampled lines and sheep farms we looked at showed evidence of leptospiral infection," says Professor West.
"The danger from animals with leptospirosis is that the bacteria lodge in the kidneys and get passed in the urine. This poses quite a risk to meat workers when they are handling sheep carcases. From our results we calculated that 13 in every 1000 sheep slaughtered were potentially passing infective leptospires."
Leptospirosis is a re-emerging disease around the world, and one that is almost always acquired through contact with animals and animal products. In New Zealand it is the most frequently notified occupational disease caught from animals.
"The worrying thing is that the official notification rate tends to decline but it is only the tip of the iceberg. The true number of cases in humans is potentially very high but has not been demonstrated," says Professor West.
He says research in New Zealand in the 1970s led to a better understanding of the disease in livestock, and widespread vaccination of cattle and pigs resulted in declining numbers of human cases.
"Dairy and pig farm workers used to be the highest risk group but now we are seeing many more cases in the meat processing industry and in meat workers employed in sheep-only abattoirs. In the past we did not consider sheep a high risk for leptospirosis but it looks as if this is changing."
Another study conducted at a sheep-only abattoir in the Hawke's Bay found 9.5 percent of meat workers had had prior infection. Massey researchers estimate that the risk of a new infection over the course of a single season may be as high as three percent.
"Surprisingly, the infection rate was highest at slaughter positions prior to evisceration. Keeping cattle or pigs at home, slaughtering of cattle and occasional floods of home properties all added to the risk."
Sheep are not routinely vaccinated against leptospirosis and the efficacy of vaccination of sheep in New Zealand farming conditions has not yet been established, according to Professor West.
Although this study has confirmed that sheep are potential sources of leptospirosis for meat workers, he cautions that the results are from a single abattoir and further studies are needed to find if this represents a true picture for the whole country.
"Much more research is needed to define the risks to meat workers and others who work with sheep and it is too soon to draw conclusions or introduce interventions that may not be effective. We need to keep an open mind and do more work to find sources of infection to sheep and more about how the infection is passed in sheep flocks."