Mice: a case study

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Mice: a case study

When the first Europeans arrived in Australia with the First Fleet, so did the house mouse (Mus domesticus). In the south-eastern Australian grain belt which stretches from the South Australia-Victoria border to the Darling Downs in southeast Queensland, mouse population levels are normally quite low. However, favourable seasonal conditions can trigger extensive breeding.

Mouse plagues now erupt in these regions on average every three years with an estimated 100,000 to 500,000 hectares of grain crops affected each year.

Mouse breeding facts

  • Mice breed in the Southern Hemisphere from August to May
  • Mice commence breeding from six to eight weeks of age
  • A mouse is pregnant for 19 days and re-mates 1-3 days after giving birth
  • A mouse normally gives birth to a litter of five to six young, but it can be as high as nine to ten during the lead up to a plague
  • One breeding pair of mice and their offspring has the potential to produce 500 mice in just 21 weeks
  • During a typical plague in southern Australia, mouse densities of 1000 per hectare are common
A plague of mice

Pest Animal Control Cooperative Research Centre

These plagues cause massive disruption to communities and losses to farmers in Australia of around $36 million annually in lost agricultural production and control.

Mouse plagues don’t just cause economic problems either. Swarms of mice invade households, hospitals, livestock pens, food storage and other facilities, causing significant damage to infrastructure. They consume large quantities of grain and contaminate grain shipments with their faeces. They pose a major threat to health and welfare, inflicting stress on humans and livestock.

Traditional controls

Mice have traditionally been controlled using trapping and powerful poisons. However, these can only provide limited control and the poisons can also kill dogs and cats, as well as native predatory animals that eat poisoned mice.

Biotechnology controls

Biotechnology is being used to develop a new approach to controlling mice through limiting their reproduction. The process is called immunocontraception and it involves fooling the body into thinking that certain proteins found on mouse egg cells are foreign. The body's immune system produces antibodies that bind on to these proteins, preventing pregnancy.

Researchers at the Pest Animal Control Cooperative Research Centre (PAC CRC), CSIRO and the University of Western Australia, have genetically modified a virus that occurs naturally in mouse populations – mouse cytomegalovirus (MCMV).

Scientists added a modified gene to the virus, which causes it to make an altered form of a protein called ZP3. In its natural form, ZP3 is a component of a jelly-like material called the zona pellucida that surrounds the eggs of all mammal species. To fertilise an egg, a sperm must first pass through the zona pellucida, but first certain proteins on the sperm’s surface must match up with complementary proteins on the surface of the egg.

The mouse’s immune system normally sees ZP3 as ‘self’, but mouse cells infected by the transgenic MCMV produce a modified form of ZP3, which the immune system sees as ‘non-self’ and attacks. Without functioning ZP3 proteins on the egg surface, fertilisation cannot occur.

Researchers have tested the transgenic MCMV on laboratory mice and captive wild mice. Results showed that all female mice infected with a large dose of the virus became infertile, with the infertility lasting for up to six months in most mice. MCMV naturally infects only the introduced house mouse, and all tests so far confirm that the modified virus does not affect native species or introduced rats.

Every mammal species has its own version of the ZP3 protein and so the virus is programmed to target only the house mouse ZP3 protein. This technique carries further insurance against the remote possibility that the contraceptive virus might infect another mammal species.

PAC CRC researchers are also investigating whether a non-infectious oral contraceptive can be developed for use in grain baits to produce an identical immune response in female mice. Unlike poisoned grain, the grain 'Pill' would have to be harmless to other mammals and birds.

Although there are clear benefits in preventing mouse plague by immunocontraception using GM viruses, biosafety needs to be taken very seriously. All research involving modified viruses is conducted in secure containment laboratories in accordance with regulations set by the Office of the Gene Technology Regulator. Rigorous safety testing and extensive community consultation would precede any decision to release a genetically modified MCMV in the Australian environment.