Rodents running riot in supermarkets – do we need to worry?

In January and February 2024, the public of Aotearoa New Zealand (NZ) were captivated by graphic images of rodents running around supermarkets in Dunedin, Christchurch and Auckland.¹ The purpose of this Briefing is to look at the ecology of rodents in our country, examine the evidence linking rats to human health, and examine the need to expand our rodent control efforts.

Rodents and humans have lived together for thousands of years.² It is likely that rodents were one of the original reasons why cats were domesticated.³ The close relationship that can occur between many rodent species and humans is described as being commensal² which comes from a Latin word which means dining at the same table.⁴

Four species of rodents have been introduced into NZ, the house mouse (Mus musculus), and three species of rats, the kiore (Rattus exulans), brown rat (R. norvegicus), and black rat (R. rattus). While kiore are mostly restricted to a few offshore islands, the other three species are distributed across the main islands, with all four species having commensal relationships with humans.⁵ See Appendix One for more information on the ecology of rodents in NZ. 

Supermarkets – a rodent utopia?

Rodents can harm human health in a large number of ways – see Table 1. In particular rodents contaminate food and the environment with their saliva, urine and faeces which can carry pathogenic microbes. For example, black rats excrete an average of 52 faecal pellets per 24 hours⁶ and during active shedding one rat faecal pellet may contain up to 1.0 x 10⁸Salmonella colony-forming units.⁷ 

Supermarkets are very attractive for rodents because of the large amount of food and places to hide. For every rodent seen during the day it is estimated that there are 20 to 50 individuals that are unseen.⁸ Contaminated ready-to-eat foods carry a particularly high risk to human health. Bacterial pathogens such as non-typhoidal Salmonellae can grow on contaminated foods above 5.2°C but most serovars are killed instantly at temperatures above 75°C.⁹ Rodent body parts found in food that has been heated such as bread or canned produce are likely to be lower risk and in fact are a source of human food in some parts of the world.¹⁰

It is usually difficult to pinpoint rodents as the cause of a foodborne illness and there is a lack of evidence on the scale of this source of infection in NZ. While finding evidence of rodent activity in human food can be very upsetting, it is likely that it causes only a small part of the total enteric disease burden. For example, Campylobacter contamination of fresh poultry meat is estimated to cause 54 000 cases, 548 hospitalisations, and 28 deaths per annum in NZ.¹¹ Despite this, it is clear that all businesses involved in the food supply chain require pest management strategies combined with stringent food safety standards and a zero tolerance to rodent sightings. It is likely that climate change with warmer weather and more extreme weather events will increase rodent populations and increase the adverse impacts on humans.^{12 13 14}

Rodent urine and leptospirosis

The most common reported illness linked to rodents in NZ is leptospirosis due to L. borgpetersenii sv Ballum and L. interrogans sv. Copenhageni. 

Leptospirosis is an illness which can vary from having no symptoms to a severe illness that may be life-threatening. In addition to a severe acute illness with two thirds of notified patients hospitalised, up to 50% report a prolonged recovery with associated social, financial, and mental health consequences.^{15 16} Both mild and severe leptospirosis are highly underdiagnosed.¹⁷

Mice and rats are the maintenance hosts for L. borgpetersenii sv. Ballum and L. interrogans sv. Copenhageni, respectively. Once infected rodents excrete Leptospira in their urine for their remainder of their lives. On average mice urine contains 3.1x10³ and rat urine contains 5.7 x 10⁶Leptospira cells per millilitre.¹⁸ Contact with rodent urine such as farmers handling rodent-contaminated animal feed, people working in pest control, and the general population dealing with rodents in their homes are at risk.¹⁹ Pet rodents are also increasingly popular. While there is no direct evidence linking a pet rodent to a case of leptospirosis in NZ, there are overseas reports suggesting a possible relationship.^{20 21}  Appendix Two has extra detail on rodents and leptospirosis.

Table 1: Summary of the potential harms to human health from rats and mice in NZ (adapted and updated from:²²; more detailed evidence provided in Appendix Three) 

Expanding rodent control in Aotearoa New Zealand?

NZ has been successful in eradicating rodents (and other predatory mammals) from approximately a third of its offshore islands, although these islands were typically small and uninhabited, and account for just 10% of offshore island area.²³ While some recent success has been reported from Miramar Peninsula,²⁴ expanding rodent management to the majority of mainland NZ, and achieving the current 2050 predator free goal, may require the use of different approaches or technologies.

Author details

Dr Jonathan Jarman, Retired Public Health Medicine Specialist and former Medical Officer of Health

Dr Chris Niebuhr, Researcher - Wildlife Ecologist, Manaaki Whenua – Landcare Research

Prof Nick Wilson, Professor of Public Health, University of Otago, Wellington. 

Prof Jackie Benschop, Professor of Veterinary Public Health, Massey University

Images credit: Grant Morriss, Manaaki Whenua – Landcare Research

Appendix One: Rat ecology in NZ – extra details

Although brown rats are the subject of most world-wide research on urban rats, the most common urban rat in NZ is actually the black rat.²⁵ While brown rats tend to be only found near water or foodstuffs, black rats are more widespread within NZ. This species possesses stronger climbing abilities, which allow them to better take advantage of three-dimensional spaces, including vegetation and building ceilings.²⁶

Appendix Two: Leptospirosis and rodents – extra details

Leptospirosis is one of the most common zoonotic bacterial diseases in the world. Infection generally occurs when humans have direct contact with contaminated animal urine, fluids or body tissue, or through indirect contact with contaminated soil or freshwater.²⁷ The pathogen gains entry through cuts and abrasions on skin and through mucous membranes such as conjunctival or oral surfaces.²⁸ 

A New Zealand 2019-2022 study found that people notified with leptospirosis were 2.2 (95% CI: 0.9-5.3) times more likely to report seeing evidence of rodents (eg, droppings, gnawing) in the months before they became ill than controls were in the month before interview. This estimate is adjusted for sex, rurality, age, season, and occupation. In 2023, 17 L. Ballum cases were reported and four people indicated possible contact with rats.²⁹ 

Our farmed ruminants (sheep, cattle and deer) can also be exposed to rodent urine and develop antibodies to the rodent strains. This means they have been infected and may shed these strains as well acting as bridge for infecting humans.  

Appendix Three: Harms to human health from rodents

Supplementary Table 1: Detailed summary of the potential harms to human health from rats and mice in NZ (adapted and updated from:²²)