Health officials are seeking the reasons behind a spike in leptospirosis cases in Northland.
The region normally has about four cases a year but the 15 cases in 2016 — the highest number in 20 years — has led to warning farmers and possum trappers to take care around animals.
Leptospirosis is a potentially fatal disease that can cause flu-like symptoms and, in severe cases, bleeding from the lungs, meningitis or multiple organ failure.
Dr Tanya Quin, a GP locum who with her husband runs a dairy farm at Okaihau, knew of some of the cases and approached Northland's medical officer of health, Dr Virginia McLaughlin, to find out what was going on.
There are more than 200 strains or serovars of leptospirosis in the world and only a handful cause problems in New Zealand. While usually associated with cattle in this country, the disease can also be spread by pigs, possums, rats, mice and hedgehogs.
The disease had mainly affected farmers and meat workers, but examination of the 2016 Northland leptospirosis victims found other occupations infected and unexpected high rodent involvement. The cause of the spike was difficult to pinpoint and there was speculation about various factors that could be contributing to it.
Dr Quin considered talking to farmers directly about what they had observed could help unravel the reasons behind the sudden increase in infections so, with a Northland District Health Board public health official, Dr Laupepa Va'a, she interviewed more than 130 farmers during the Northland Field Days at Dargaville in March last year.
"We found that despite some knowledge gaps about the range of symptoms and the importance of rodents in transmission of the disease, knowledge of livestock vaccination was good and its importance in risk reduction understood," Dr Quin said.
"Essentially there was no change in farmers' practice around vaccination and rodent control measures, which did not explain the increase in Northland leptospirosis numbers."
Dr Quin and Dr Va'a produced a study included in the International Leptospirosis Society conference at Palmerston North from November 27-December 1 last year.
They collaborated with Massey University veterinary epidemiologist Dr Jackie Benschop, who in a keynote address told the conference it was of critical concern there had been 91 cases of leptospirosis in New Zealand in the first half of last year — 39 of them in the Waikato, which had also topped the regional incidence in 2016.
Increasing diagnostic use of more sensitive polymerase chain reaction (PCR) leptospirosis testing in many regions did not explain the large increase in numbers with the disease seen in district health boards where PCR was not in use, such as Northland, she said.
Most of the cases of leptospirosis in Northland in 2016 were due to serovars not in the livestock vaccinations.
'Something has changed'
Dr Quin said this did not mean the vaccinations were not working. There was good evidence that vaccinating stock had significantly reduced human cases in New Zealand and it should expand and continue.
"What it meant is that something has changed in the animal pathways of two serovars that would previously be considered more prevalent in the wildlife species, like rodents."
Were infected rodents directly affecting humans? For a lot of the Northland cases that was likely and would explain non- farming individuals being affected, but why? Could it relate to rodent populations, especially with the sudden peak? Dr Quin said the field days interviews were equivocal around the topic of observed rodent populations on farms.
"To me, there was enough doubt to further investigate how we track and understand our rodent populations not just in regard to our forest biodiversity monitoring but expanding this to the risks they pose in terms of disease transmission to humans and other animals," she said.
"This raises the question of wildlife transmission to livestock as a potential source of infection in humans. Is this changing? Marie Money, of Massey University, last year found serovars in wildlife species are commonly found in livestock in the same environment, supporting further assessment of transmission pathways between and within species."
Massey University veterinary researchers have joined the Farmer Leptospirosis Action Group (FLAG), which has representatives from Rural Women New Zealand, DairyNZ, the Deer Farmers' Association, Beef + Lamb New Zealand, Federated Farmers and the New Zealand Veterinary Association.
With funding from the government's Sustainable Farming Fund they aim to gain greater understanding of leptospirosis and its effect on the New Zealand agricultural industry.
Funding is now being sought for research that investigates animal, human and environmental targets together. Dr Quin, who is honorary secretary and treasurer of the Northland faulty of the Royal New Zealand College of General Practitioners, said she looked forward to bringing that research to Northland.