26 August 2011

Rodent control

Overview: Rodents are small mammals with sharp, continuously growing incisors (teeth) in their upper and lower jaws, which must be kept short by gnawing. Rodents use their incisors to gnaw wood, break up food, excavate tunnels, and defend against predators. Examples of rodents include mice, rats, chipmunks, woodchucks, voles, squirrels, prairie dogs, and beavers.



Overview: Rodents are small mammals with sharp, continuously growing incisors (teeth) in their upper and lower jaws, which must be kept short by gnawing. Rodents use their incisors to gnaw wood, break up food, excavate tunnels, and defend against predators. Examples of rodents include mice, rats, chipmunks, woodchucks, voles, squirrels, prairie dogs, and beavers.

All rodents have a single pair of incisors at the front of each jaw. They lack canines and have a gap (diastema) between the incisors and molars. While rodents' incisors are used for gnawing and cutting, their molars are used for chewing.

Rodents are classified as mammals (warm-blooded vertebrates that feed their young with milk from mammary glands and have skin that is covered with hair). The rodent order (Rodentia) comprises 44% of all mammalian species on Earth.

The adult weight of different rodent species ranges from 10 grams (one-third of an ounce) to more than 66 kilograms (145 pounds). The pygmy jerboa of Africa and Asia is the world's smallest living rodent at about 51 millimeters (two inches) in length; the capybara of South America is the world's largest at about 50 centimeters (20 inches) in height and about 120 centimeters (almost four feet) in length.

The first rodent fossils, from the Paramyidae family, date back to the Paleocene period in North America and Eurasia, about 55 million to 60 million years ago. The largest extinct rodent on Earth is the 1,000-kilogram (2,200 pounds) Josephoartigasia monesi, a hippo-sized guinea pig; its fossils were found in South America in 2008. A close second in bulk is the extinct giant beaver (Castoroides), about the size of a bear.

Rodents inhabit all continents except Antarctica. They can be found in a wide variety of environments, from arctic tundra to rainforests, deserts, mountaintops, bogs, and swamps. They may, for example, burrow in the ground (gophers and prairie dogs), live in trees (squirrels), or live in or around water (beavers).

There are about 1,700 species of rodents around the world. About 89% of rodents fall within five families: 1) rats and mice, 2) squirrels, 3) spiny rats (nocturnal rodents with a bristly coat of flat, flexible spines in place of fur), 4) pocket mice (small American mice with pockets adjacent to their mouths, used to transport food) and kangaroo rats, and 5) jerboas (small desert-based jumping rodents) and jumping mice. Rats and mice are the most populous group, comprising 66% of all rodent families on the planet.

Rodents have an exceptional capacity to reproduce. It is possible for two mice to create more than 600 young in only six months' time, with new young produced every 21 days. This high birth rate may cause a rodent population to quickly swell beyond an area's capacity to supply food; the group's numbers then plunge, with this cyclical increasing-and-decreasing pattern repeated every three to four years.

Throughout the world, rodents are responsible for the loss of agricultural crops (such as oil palms and rice in Southeast Asia), damage to homes, and the spread of disease.

History: Rodents have a worldwide, lengthy reputation for decimating grain and food supplies. To counter the loss of valuable crops and stored foods, people have long turned to a variety of pest-control solutions; some solutions have been more successful than others. About 4,000 to 5,000 years ago, cats were used in Egyptian homes to kill mice and rats. Egyptians also made use of poison and a variety of homemade traps. Chinese citizens made mouse traps out of ceramics.

In the Middle Ages, poisonous plants such as wolfsbane, ratsbane (arsenic), hemlock, and oleander were used to eliminate rodents. In the late 1500s, Leonard Mascall published A Booke of Engines and traps to take Polcats, Buzardes, Rattes, Mice and all other kinds of Vermine and beasts whatsoever, [sic] the first-known compilation of pest-related extermination methods. Mascall included descriptions of 34 traps, of which 12 were designed for mice and rats. There were also nine recipes to create poisonous baits. One form of rodent trap was a precursor of the modern "snap" trap, although its metal springs were not strong enough to catch or kill many rodents.

A very basic trap using a helical (coiled) spring was first created in the 1600s; it was not coiled tightly enough, however, to be fully effective at trapping rodents. In the 1800s, manufacturing improvements allowed for smaller, tighter coils and the creation of stronger traps.

It has been reported that Irish citizens quoted rhymes to bothersome rats in an attempt to scare them away. Ancient Greeks wrote notes to marauding rats asking them to leave the area; the notes stated that the rodents would be killed if they continued to damage local crops. These "rat letters" were weighted down with stones and left in pest-damaged fields; the writing was turned face-up for easier rodent perusal.

Another extermination method in England in the 1600s involved a "survival of the fittest" battle among mice placed in a cage. The ultimate-survivor mouse would be released so that it could return to its colony, where it would supposedly frighten and scatter the group's members and kill any mouse that remained.

In 1885, the U.S. Department of Agriculture (USDA) first became involved in the handling of wildlife-related damage. Operating as the Division of Economic Ornithology and Mammalogy, the group was tasked with teaching farmers about mammals and birds that may affect their crop yields.

In 1905, the Division was renamed the Bureau of Biological Survey, which eventually became the U.S. Fish and Wildlife Service. In the same year, the U.S. government created a special laboratory in New Mexico to investigate methods of controlling rodents and predatory animals.

Unusual (and ineffective) rodent-control methods continued into the 20th century. In 1909, it was suggested that rodents that ate from a mixture of plaster of Paris and sugar and then drank from a nearby water source would "stiffen" as the plaster hardened inside of them. A 1990 experiment, however, suggested that this proposed rodent-control technique may be ineffective.

On March 2, 1931, the Animal Damage Control (ADC) Act was passed by the U.S. government. Under this regulation, the federal government, in cooperation with state and local authorities, was authorized to handle damages caused by wildlife on U.S. soil.

In 1940, the Denver Wildlife Research Laboratory (DWRL) was created under the newly formed U.S. Fish and Wildlife Service.

In 1948, blood thinners were first suggested for use in rat extermination in the United States.

In 1959, the U.S. Congress changed the DWRL's name to the Denver Wildlife Research Center (DWRC). The DWRC was subsequently asked to examine the effects of pesticides on wildlife. A pesticide is any chemical that is used to control pests, which may include insects, animals, or plants. A rodenticide is a form of pesticide that is used to control rats, mice, and other rodents. In some cases, regulatory agencies also include birds, bats, and fish in this category.

During the 1960s, the U.S. Animal Damage Control program came under fire by environmental activists for their use of poisons for wildlife extermination purposes.

Ultrasonic and electromagnetic rodent-control devices were introduced in the 1970s; manufacturers claimed that the products' magnetic fields or extremely high-pitched sounds could drive rodents away from homes and businesses. The ultrasonic products had limited success, and the electromagnetic items were deemed ineffective.

The USDA first registered naphthalene as a pest-control agent in 1948, aimed at eliminating small rodents. In the 1970s, the recommended naphthalene-mothball "dose" for attics and other indoor locations was five pounds per 2,000 cubic feet of space. Expected uses for the product included moth and small-rodent control. Due to the toxic nature of naphthalene to humans and pets, and its questionable ability to repel rodents, however, the U.S. Environmental Protection Agency (EPA) has not registered it for use as a rodenticide; in fact, its use is no longer recommended within any human dwelling.

In 1985, the U.S. Congress placed the U.S. Animal Damage Control program under the control of the USDA.


Preventing rodent infestations in buildings: Rodents are attracted to areas with easily accessible food supplies, water, and shelter. Rodent infestations in buildings may be prevented by plugging holes inside and outside a building's walls. Recommendations to block rodent access include placing steel wool in visible holes and patching any gaps in walls with wood, drywall, or other applicable materials; covering or plugging any areas that may be attractive nesting sites; and carefully storing or removing food sources that may attract mice, rats, or other rodents.

Identifying the problem: It is possible for mice, rats, or other rodents to infest a building without being seen. Experts agree that it may be impossible to estimate the actual number of rodents involved in any single infestation. The best way to verify an infestation is to look for evidence: droppings under sinks, in cupboards, or around food sources; piles of nesting material (such as shredded newspaper or stuffing from pillows) in hidden corners; gnawed or damaged wood around food sources or entryways; or a musty or stale odor in out-of-the-way parts of the building.

Trapping: Rodents may be removed from a building or killed through the use of traps. A variety of trapping methods are available, depending on the intended result. Spring-loaded "snap" traps are the most common means of killing rodents. "Live" traps are designed to capture rodents in a cage, but not kill the animals; rodents caught in live traps are either released in new surroundings or removed from the container and killed separately. Glue boards are another trapping method; these devices use sticky glue pads to catch and immobilize rodents who walk over them.

Rodenticide s : Rodenticides are pesticides (chemical agents) created to kill rodents. If used improperly, however, they may also harm humans, pets, other animals, and the environment.

Rodenticides are divided into three main classes: first-generation anticoagulants (blood thinners), second-generation anticoagulants, and non-anticoagulants. First-generation anticoagulants cause death by interfering with the blood-clotting process. Warfarin and coumafuryl are in this category. Warfarin was first developed in 1945 for use as a rat poison and was then adapted in 1954 for therapeutic use as a blood thinner in humans.

Second-generation anticoagulants are extremely toxic to humans and animals. They remain in tissues of affected animals long after death and may injure or kill animals that feed on the carcasses of contaminated creatures. Second-generation anticoagulants were developed around 1990 to combat the increasing problem of rodent resistance to the first-generation variety. Second-generation anticoagulants, such as brodifacoum and bromadiolone, work well at doses that are much smaller than those of first-generation rodenticides.

Non-anticoagulant rodenticides cause death through respiratory failure. Metal phosphide-based baits combine with digestive acids in an affected animal's stomach to release toxic gas, and rodenticides kill through ingestion of toxic doses of vitamin D3. Large doses of vitamin D cause death by raising blood calcium levels, which leads to the calcification of various internal organs. A lethal vitamin D3 dose for humans would be 440,000 400iu-RDA pills.

Rodenticides are usually sold as baits (pellets) or tracking powder. Tracking powder is picked up on the feet and fur of rodents who walk through the substance. The powder is then swallowed by animals when they groom themselves, resulting in death within two weeks. Tracking powders may also be lethal to exposed cats and other pets that regularly perform grooming actions.

In May 2008, the U.S. Environmental Protection Society (EPA) released its Risk Mitigation Decision for Ten Rodenticides. The intent of this measure is to reduce children's exposures to pesticides; lower the risks to "non-target wildlife," such as hawks, owls, skunks, deer, coyotes, and foxes; and reduce harm to the environment. As of January 2007, the EPA has required that all consumer-use rodenticides be sold in containers that are resistant to tampering by children and dogs. These consumer products may not be sold as loose baits (pellets or other forms). Regulations were also enacted on the minimum weight of rodenticide packages that may be sold for use around agricultural buildings and for use in professional rodent-control applications. These restrictions were intended to keep large quantities of toxic rodenticides out of the hands of inexperienced users and away from unintended victims.

The 10 rodenticides targeted by the EPA for strict control measures are brodifacoum, bromadiolone, bromethalin, chlorophacinone, cholecalciferol, difenacoum, difethialone, diphacinone, warfarin, and zinc phosphide.

Integrated pest management: Integrated pest management (IPM) is also known as ecologically based pest management (EBPM). In contrast to the liberal use of pesticides to control rodent populations, IPM takes a more environmentally friendly approach to the problem. IPM attempts to minimize damage by using the least-expensive rodent-control methods that will cause the least amount of harm to people, the land, and the overall environment.

IPM methods can be used to control rodent infestations on farms, in homes, and in the workplace. IPM relies on a set of four decisions that are made in advance by people anticipating a rodent influx. These four steps are 1) setting thresholds to take action (for example, deciding how many rodents must be spotted before acting), 2) properly identifying the rodents (mice, rats, etc.) to ensure that the correct removal or pesticide method is chosen, 3) managing the threatened area in advance to keep the rodent population from developing into an infestation (for example, storing foods properly or selecting more rodent-resistant crops on farms), and 4) using control methods (pesticides, for example) that are specific to the rodents in question. The least-risky methods should be employed first and evaluated for effectiveness before using sprayed pesticides as a last resort.

Difficulties eliminating rodents: Many rodents use ultrasonic communication to alert others within their colony to danger. Both rats and mice emit "danger" calls at 22 kilohertz, out of hearing range for humans and predators.

Researchers have studied the intelligence and communication abilities of rodents (specifically prairie dogs). Examples of information conveyed to fellow prairie dogs includes the size of the predator that is stalking a colony, how fast the predator is moving, what kind of animal is involved, and how serious the threat is perceived to be.

Rodents are very adaptable in their habitats and are able to live in fields, farms, homes, outbuildings, and other locations as long as there is adequate food, shelter, and water.

It may be difficult to prevent rodent access to buildings; for example, mice are able to move through holes that are no larger than a nickel, and rats may squeeze through openings of less than one inch.

Rodents are prolific breeders, making them difficult to eradicate. It is possible for two mice to create more than 600 young in only six months' time, with new young produced every 21 days.

Rats are known to be neophobic, or very cautious in approaching any new items in their environment. As a result, it may take many days before a rat is willing to approach a trap or feed on rodenticide-laced bait.

Some strains of rats and mice have started to develop resistance to the first-generation warfarin-based rodenticides; as a result, these warfarin pesticides are being phased out.

Some rodenticides attempt to take advantage of established rodent behavior. For example, most rodents in buildings prefer to travel along walls for safety rather than move across open areas; placing tracking powder or other baits along this route may have more success than random rodenticide placement.

Disadvantages to rodent eradication: Not all rodent-eradication programs have successful outcomes. The prairie dog eradication program in the United States provides a good example. Starting in 1900, the U.S. government encouraged the destruction of prairie dog towns in an effort to free up more grasslands for grazing cattle in the western states. Poison bait was distributed to many towns, recreational prairie dog "shoots" were established, and, in some cases, gasoline was poured into prairie dog burrows and set on fire. This extermination campaign continued for almost 80 years, reducing the prairie dog population from an estimated five billion animals in the United States in the 1800s to the near extinction of some prairie dog species by the 1980s. Research in the 1990s revealed that, far from being destructive pests, prairie dogs actually improved the quality of grass for grazing cattle and stimulated the grasses' growth over time.

Rodent eradication may also negatively affect other animals. An unintentional side effect of prairie dog poisoning was the death of thousands of other prairie animals (such as foxes and golden eagles) that fed on the poisoned animals.


Rodents have a worldwide presence, inhabiting all continents except Antarctica. There are about 1,700 species of rodents, with rats and mice making up two-thirds of the rodent population. With their sharp incisors, they are uniquely suited to gnawing into and through food storage containers in agricultural, business, and home settings.

Rodents not only cause damage within buildings, but are also responsible for the devastation of crops in many developing countries. Under the right conditions (with adequate food stores, shelter, and water), rodents are able to eat 10% of their weight in food per day. In February 2008, rat infestations in farming fields of Bangladesh affected the crops of almost 150,000 people. The government was forced to supply food to at least 15,000 citizens whose fields had been overrun by rats. In January 2008, more than 84,000 families in one Indian state experienced damage to their homes and lost much of their crops, feed, and produce from a severe rat infestation.

Rodent invasions also have monetary impacts. According to the World Health Organization, for example, a rat infestation from 1978 to 1985 in Budapest, Hungary, caused crop and home damages ranging from $6.4 million to $8.5 million per year. In Vietnam, 15% to 30% of agricultural losses have been reported in some areas in the last decade, with occasional individual farm losses running as high as 100%. A survey of rodent damage to Cuban food-storage warehouses determined a significant cost/benefit ratio between 1:22 and 1:51 when good hygiene and rodent-control methods were put in use. Throughout the world, the total amount of crops lost to rodent infestations each year could provide enough food for 200 million people.

Organizations interested in the challenges presented by rodent infestations and subsequent rodent-control techniques include International Institute of Biological Control (IIBC), International Pest Control Association (IPCA), National Pest Management Association, National Pest Technicians Association (NPTA), Rodenticide Resistance Action Committee (RRAC), Rodenticide Resistance Action Group (RRAG), Section of Biological Control of the International Union of Biological Sciences (IUBC), U.S. Centers for Disease Control and Prevention (CDC), U.S. Department of Agriculture (USDA), U.S. Environmental Protection Agency (EPA), U.S. National Wildlife Disease Program (NWDP), U.S. National Wildlife Research Center (NWRC), and the World Health Organization (WHO).


Disease transmission: Around the world, rodents are currently responsible for the spread of 35 different diseases to humans. Disease transmission may occur through the bites of infected animals, by ingestion of water or food contaminated by rodent droppings, or by the breathing of "aerosolized" germs arising from rodent urine or feces that have been disturbed. Rats and mice produce substantial amounts of potentially infectious urine and feces. A single rat, for example, is able to generate 5.5 liters of urine per year, a 100-rat colony may produce 1.5 million droppings per year, and 100 mice are even more prolific, with about three million droppings per year.

According to the U.S. Centers for Disease Control and Prevention (CDC), some diseases may be directly transmitted from rodents to human. In the case of rodent-borne diseases, direct transmission may occur when an infected rodent bites a human or when contaminated water or food or rodent urine or feces enter a person's body through the mouth, nose, eyes, or an open sore or cut. Examples of directly transmitted diseases include hantavirus, hemorrhagic fever with renal syndrome, Lassa fever, leptospirosis, lymphocytic choriomeningitis (LCM), Omsk hemorrhagic fever, plague, rat-bite fever, salmonellosis, South American arenaviruses (including Argentine, Bolivian, Sabia-associated, and Venezuelan hemorrhagic fevers), and tularemia.

Diseases may also be indirectly transmitted from rodents to humans. Indirect disease transmission occurs through "third-party" contact. For example, ticks, mites, or fleas may feed on infected rodents and then bite humans, transferring the infection. Some indirectly transmitted rodent-to-human diseases include babesiosis (transmitted by ticks; treated with antibiotics), California serogroup viruses (including LaCrosse encephalitis; transmitted by mosquitoes; no defined treatment), Colorado tick fever (transmitted by ticks; no defined treatment), cutaneous Leishmaniasis (transmitted by sand flies; treated with miltefosine), human granulocytic anaplasmosis (transmitted by ticks; treated with antibiotics), Lyme disease (transmitted by ticks; treated with antibiotics), Murine typhus (transmitted by lice or fleas; treated with antibiotics), Omsk hemorrhagic fever (transmitted by ticks or contact with waste products of an infected muskrat; treatment is primarily supportive, through hydration and electrolyte balance), Powassan encephalitis (transmitted by ticks; no defined treatment), scrub typhus (transmitted by mites; treated with antibiotics), rickettsialpox (transmitted by mites; treated with antibiotics), relapsing fever (transmitted by lice or ticks; treated with antibiotics), Rocky Mountain spotted fever (transmitted by ticks; treated with antibiotics), Sylvatic typhus (transmitted by lice; treated with antibiotics), and Western equine encephalitis (transmitted by mosquitoes; no defined treatment). These diseases are not contagious. Both rodent droppings and urine appear equally risky to human health.

Of the diseases directly and indirectly transmitted from rodents to humans, the following are the most deadly: hantavirus (50% death rate, even when treated aggressively), hemorrhagic fever with renal syndrome (5-15% death rate), leptospirosis (about 10% death rate), and South American arenaviruses (up to a 30% death rate).

Rodents responsible for the direct and/or indirect transmission of diseases to humans include deer mice, cotton rats, rice rats, white-footed mice, striped field mice, brown or Norway rats, bank voles, yellow-necked field mice, multimammate rats, house mice, muskrats, voles, rock squirrels, prairie dogs, wood rats, fox squirrels and other species of ground squirrels and chipmunks, cane rats, drylands vesper mice, and beavers.

An estimated 2-5 million animal bites occur in the United States each year. Children who are five years old or younger are the most frequent victims of rodent bites. These bites tend to occur during the night, on children's faces or hands. Wild rodents may bite if they are frightened or handled. About 85-90% of bites are caused by dogs, 5-10% are caused by cats, and 2-3% are caused by rodents. Of all rodent species, rats cause the most bites.

The Black Death: There is a long-held belief that the Black Death (plague) of the Middle Ages was spread, starting around 1347, by fleas infected with the bacterium Yersinia pestis. These fleas were carried by rats throughout Europe and Asia. While some scholars still accept this theory of the origins of the plague's spread through Europe, the belief has been challenged since 2003 by several biological science experts. All of the Yersinia pestis dissenters hold several beliefs in common.

Based on its symptoms and broad reach, the Black Death of the Middle Ages was a hemorrhagic plague, not a bubonic plague as commonly believed. This hemorrhagic plague was spread by direct person-to-person or aerosolized contacts, often before the contagious person was aware of the illness. Records from the 1300s showed that a 40-day quarantine of infected persons effectively slowed, and in some cases stopped, the spread of plague in some affected towns. Bubonic plague, however, has a much shorter infectious period, with death occurring within 3-5 days.

The mortality rate from hemorrhagic plague approached 100%, consistent with its deadly effect in the Europe of the Middle Ages. Bubonic plague, however, has a lesser mortality rate of 30% to 50%, matching the results of later plague outbreaks (such as the plague epidemic that started in Hong Kong at the end of the 1800s).

According to a 2004 study, DNA specific for Yersinia pestis was not found in the teeth of 61 13th-to-17th-century plague victims buried in five European cemeteries. This study directly contradicted a report by an earlier research team that Yersinia pestis DNA was retrieved from a much smaller sample of plague victims' teeth (five teeth). The study attributed the competing teams' result to environmental bacterial contamination of the DNA in question.

Dangers of rodenticides to humans and animals: A pesticide is any chemical that is used to control pests; pests may include insects, animals, or plants. A rodenticide is a form of pesticide: it is any chemical used to control rats, mice, and other rodents.

Rodenticides may pose a danger to non-target animals and humans. Many rodenticides are highly hazardous blood thinners that cause death through internal hemorrhage. Rodents that eat some of these poisons are able to feed for several days on the bait before succumbing, due to the delayed action of the poison; this feeding practice allows the toxic material to accumulate to high levels in the rodents' bodies by the time of death. Birds of prey (for example, hawks or owls) may receive toxic doses of rodenticide through ingestion of a single day's diet of dying or dead poisoned mice.

Rodenticides that come in the form of powder (for example, coumatetralyl) may harm humans (especially children) and pets if the powder is spread or tracked through living areas of a home. Rodent baits are sometimes attractive to pets, which may die as a result of feeding from bait trays.

In the United States, the U.S. Environmental Protection Agency (EPA) has passed restrictions on the sale and use of four out of the 10 rodenticides that are considered to be the most hazardous to wildlife, pets, and children. These rodenticides are the "second-generation" blood-thinning products brodifacoum, bromadiolone, difenacoum, and difethialone. No consumer rodenticide products may include these four second-generation blood thinners. Loose bait, such as rat pellets, may not be sold for home use, and "consumer-sized" bait may be sold only as preloaded bait stations.

Second-generation rodenticides may be sold only to professional exterminators in quantities that are greater than 16 pounds of bait per package.

The EPA recommends that all pesticides and rodenticides be stored in locked areas that are not accessible to children or pets. The EPA also believes that the best way to manage rodents is to prevent infestations from the start, rather than relying on rodenticides. The EPA's suggestions involve sealing rodent entry points into homes and buildings, removing trash, cleaning up food and water sources, and using trapping methods rather than chemical extermination.

Environmental effects of rodents: Rodents are also responsible for the devastation of crops in many developing countries. Rat infestations in farming fields of Bangladesh in February 2008 affected the crops of almost 150,000 people; the government had to supply food to at least 15,000 citizens whose fields had been overrun by rats. In January 2008, more than 84,000 families in one Indian state experienced damage to their homes and lost much of their crops, feed, and produce from a severe rat infestation.

Prairie dogs create extensive tunnel systems and burrows in pasture lands. Researchers estimate that the Texas Panhandle was home to a prairie dog town that extended for 25,000 square miles and held about 400 million of these rodents. While some ranchers claimed that horses and cattle broke their legs in the numerous surface holes, these claims were never substantiated. Instead, prairie dog tunnels have been acknowledged as valuable ecological contributors, helping to channel rain water back to the water table.


General: Integrated pest management (IPM) programs have led to alternative methods of rodent control that are more environmentally friendly. These methods are called ecologically based pest management (EBPM) systems. The goal of EBPM is to manage pests of all types, including rodents, instead of solely eliminating them. EBPM programs are based on the tenets of safety for the people who use these methods and for those who benefit from them (agricultural workers, as well as food consumers), cost-effectiveness, and long-term success without the rodents' development of resistance to the methods used. Some examples of EBPM practices in rice-producing areas include reducing the width of paths between rice fields to 30 centimeters (11 inches) or less to make it harder for foraging rodents to burrow and collecting grain that is scattered during harvest.

In Zambia and Ethiopia, researchers are using agroforestry to combat mole rats that decimate cassava crops. Agroforestry combines agriculture with forestry, using properly placed trees to encourage the expansion of animal habitats, fight soil erosion, and discourage agricultural pests. Researchers have found that Tephrosia vogelii bushes planted around the edges of crop-producing fields prevent rodents from entering the area. Tephrosia vogelii leaves contain a natural pesticide.

Rodent control through the use of birds of prey and other rodent predators is being tested in several countries. In Kenya and Tanzania, for example, perches and nesting boxes for owls and buzzards have been installed near crop-producing farms in the hopes that these predators will reduce the rodent population in the area.

Biological rodent control: In Thailand, scientists are experimenting with the use of a parasite (Sarcocystis singaporensis) as a method of biological rodent control. S. singaporensis was first discovered by Zaman and Colley in Singapore in 1975 in spores found in the feces of a reticulated python (the natural host of this parasite). The life cycle of S. singaporensis is dependent on the predator-prey relationship between rodents and reticulated pythons in Southeast Asia: 1) S. singaporensis reproduces in the intestinal tracts of reticulated pythons, and the spores of this parasite are then excreted in python feces; reticulated pythons are neither infected by, nor become immune to, this parasite. 2) Rats ingest the parasite spores by drinking water that has been contaminated with python feces or eating invertebrates that have ingested the snake feces. 3) Rats that have ingested S. singaporensis spores do not normally die from the resultant low-level infection; the spores cause cysts to develop in rats' muscles, which may make them slower-moving and easier for predators to catch. S. singaporensis also lowers the fertility rate of infected rats. Only rats that have ingested a large number of spores face death within two weeks from resultant pneumonia. 4) Reticulated pythons prey on the slower-moving infected rats, allowing S. singaporensis to reproduce in the pythons' intestinal tracts, thereby continuing the life cycle of the parasite.

Starting in 1993, scientists in Germany and Thailand began jointly developing a biological rodent-control method using reticulated pythons as "incubators" for S. singaporensis. No other snakes could be used for this process, as reticulated pythons are the natural host of S. singaporensis. Mass production of spores in reticulated pythons of Thailand is possible due to the prevalence of tourist-related snake farms throughout the country. Parasites are harvested from the feces of these pythons and used to create contaminated food pellets, which are then spread in farmed fields for rodents to ingest. One python can create enough deadly spores to kill up to 200,000 rats.

Contaminated food pellets containing large quantities of S. singaporensis spores are lethal only to rats. Other animals such as cats, dogs, and chickens, as well as humans, are not harmed by the spores and do not develop any S. singaporensis-related infections; they will also not be harmed by feeding on other animals that have ingested the spores.

In 2001, a trial of S. singaporensis-containing bait was performed in a rice field north of Bangkok, and the results were compared both to a rice field treated with traditional chemical rodenticides and to a third field where there was no rodent-control intervention. Results showed that 5.5% of the untreated rice crop was damaged by rats, while only 1.2% of the chemically treated crops suffered damage and 0.4% of the S. singaporensis-baited rice was destroyed.

Other future forms of rodent control: A research team in Australia is working on a mouse contraceptive that is delivered to the rodents through infection with a bioengineered virus. In one research study, infected wild mice produced no offspring for more than 200 days; the control group, in contrast, birthed about 300 young. There is debate whether this form of pest control would be accepted by the general population due to fears that an altered virus may harm species beyond the original rodent-control intentions of its creators.

An Arizona biotech company has been testing a poison-free rodent-control process in the rice fields of Australia. The product prematurely ages the reproductive systems of rats, causing them to become sterile. It has been suggested that this drug would be nonhazardous to humans because it is metabolized quickly in rodents that eat it; however, there has been no discussion to date about possible harm from this product if it leached into water supplies and soil. The company responsible for testing this process believes that it may prove beneficial in countries where rice is the primary crop. Thirteen countries, many in Southeast Asia, provide more than 80% of the planet's supply of rice.


This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (

  • Cohn SK Jr. Epidemiology of the Black Death and successive waves of plague. Med Hist Suppl. 2008;(27):74-100. View abstract
  • Duncan CJ, Scott S. What caused the Black Death? Postgrad Med J. 2005 May;81(955):315-20. View abstract
  • Forbes I. Rodent control in London's sewers. J R Soc Health. 1990 Feb;110(1):5-9. View abstract
  • Gilbert MT, Cuccui J, White W, et al. Absence of Yersinia pestis-specific DNA in human teeth from five European excavations of putative plague victims. Microbiology. 2004 Feb;150(Pt 2):341-54. View abstract
  • J©kel T, Khoprasert Y, Sorger I, et al. Sarcosporidiasis in rodents from Thailand. J Wild Dis 1997 Oct;33(4):860-7. View abstract
  • Meerburg BG, Brom FW, Kijlstra A. The ethics of rodent control. Pest Manag Sci. 2008 Dec;64(12):1205-11. View abstract
  • Mohan Rao A. Preventive measures for leptospirosis: rodent control. Indian J Med Microbiol. 2006 Oct;24(4):325-8. Review. View abstract
  • Natural Standard: The Authority on Integrative Medicine.
  • U.S. Centers for Disease Control and Prevention.
  • U.S. Department of Agriculture (USDA).
  • U.S. Environmental Protection Agency (EPA).

Copyright © 2011 Natural Standard (

Read Health24’s Comments Policy

Comment on this story
Comments have been closed for this article.

Live healthier

Exercise benefits for seniors »

Working out in the concrete jungle Even a little exercise may help prevent dementia Here’s an unexpected way to boost your memory: running

Seniors who exercise recover more quickly from injury or illness

When sedentary older adults got into an exercise routine, it curbed their risk of suffering a disabling injury or illness and helped them recover if anything did happen to them.