The recent discovery of at least 20 rats at a Dunedin Countdown and video footage of a mouse on food at a Christchurch Countdown deli has prompted the Ministry for Primary Industries to launch an investigation into pest management at all Woolworths New Zealand stores.
Understandably, consumers are concerned about the health risks when rats run rampant in our supermarket aisles.
New Zealand’s Predator Free 2050 programme, introduced by the National government in 2016, has been widely promoted as providing benefits for the conservation of native species.
To date this has been successful in the major reduction or eradication of rats in some areas thanks to the dedicated efforts of Predator Free 2050, conservation workers and volunteers.
It has been less promoted as reducing risks to human health by removing a crucial link in the sometimes complex chains of infection that can result in human disease outbreaks.
So what are the risks and how do these come about?
The two kinds of rat common here and globally like to ‘‘ride on human coat-tails’’, i.e. they benefit greatly from the shelter and food our living habits, food waste and farming practices provide for them.
The recent supermarket sightings appear to be associated with a warm season and abundant natural food supplies (‘‘bush tucker’’ from the heavy fruiting of vegetation), as occurs in some years and which have contributed to unusually high numbers this summer.
What is less widely appreciated is that, in addition to direct contamination, these rats can also be carriers of pathogens acquired from other animals (zoonotic infections) which they can pass on as a vector (carrier) or become a reservoir (source of re-infection after a low season of cool or dry conditions) in linked events that can lead to an outbreak of human infections.
The 2016 Havelock North outbreak of campylobacteriosis, for example, is believed to have come from water contaminated by sheep, but there is a high chance that the sheep acquired it from a reservoir of pathogens present in rats.
There are three kinds of rats and one kind of mouse resident in New Zealand. The Polynesian rat, or kiore, was brought from the Pacific Islands by Māori. These small rats spread widely in the past and were often trapped for food.
They also had a considerable impact on native birds, reptiles and insects well before Europeans arrived, because these lacked any defence against them. However, they have been displaced by more recent arrivals and are now confined to a few offshore islands where they are protected as a cultural item.
The Norway rat (sewer rat, lab rat) probably came with whalers and sealers in the late 1700s. These large rats tend to live on the ground and are good burrowers and swimmers. They occur mostly in built up areas.
The black (bush) rat has arrived since then, probably on shipping and perhaps repeatedly. These are excellent climbers and are especially widespread around town margins, on farms and in the bush, where they can build up to very high numbers in some years. These bush rats also suppress mice, which would have also come here as stowaways on early shipping.
Norway and black rats can pick up a range of infections directly from their association with humans or livestock.
It is less well known that they can also play a major role in maintaining and spreading several animal or waterborne pathogens which can then be passed on to humans.
In New Zealand this includes leptospirosis (from livestock), toxoplasma (from cats), murine typhus (typically spread by cat or rat fleas), giardia, cryptosporidium and campylobacter (water-borne) and salmonella infections.
There is also a concern on farms regarding the spread of yersinia infection in deer and potentially trichinella in pigs (a concern for export markets).
Rats are also a major concern as potential reservoirs for mosquito-borne virus diseases such as Ross River virus from Australia and tick-borne Lyme disease from the United States, although we do not have suitable ticks at present.
New Zealand is especially vulnerable to the establishment of mosquito-borne diseases because of the combination of warming climate, vector mosquitoes already here or frequently intercepted by biosecurity agencies at airports and seaports, as well as the arrival of overseas travellers infected elsewhere.
The high rat numbers tend to be associated with mass fruiting of native vegetation, which happens from time to time and as seems to be the case this summer.
On these occasions conservation becomes a major concern when this food runs out and the rats, and other introduced predators, shift to feeding on native fauna.
At present this only happens in some years and is known to be associated with the El Nino/La Nina weather pattern, which provides for some predictability. This pattern will become disrupted by the warming climate as such food bonus years become more frequent, also making prediction and precautions more difficult.
National success on Predator Free 2050 would save money on rodent (and possum) control and greatly ease public concern over the need for poisons such as 1080, or food contamination in supermarkets as well as reducing health costs involved in dealing with outbreaks of infection.
It would also greatly reduce infestation problems in food storage facilities, around dumps and on farms.
Success, however, would require that more attention be given to biosecurity surveillance at international shipping ports and of recreational boats, to prevent reinfestation. — Newsroom
• Dr Mary McIntyre is a retired public health researcher from the University of Otago, Wellington.
