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Rodents carry a wide range of disease-causing organisms, including many species of bacteria, viruses and parasites such as protozoans and worms. They also act as vectors or reservoirs for many diseases via their ectoparasites such as fleas, ticks, lice and mites, as well as some diseases carried by mosquitoes. Rodents are thought to be responsible for more deaths than all the wars over the last 1,000 years.
Rodents have large reproductive potential which means populations can increase rapidly when conditions are favourable. So it is vital to implement measures to prevent an infestation and to eliminate them as soon as possible to protect homes and businesses.
There are over 2,200 species of rodent worldwide, of which rats and mice are the main pests in homes and businesses. Other rodents that can carry diseases and come into human contact include prairie dogs, groundhogs, ground squirrels, lemmings and voles.
The major diseases carried by rodents are described below:
Rats and mice are serious pests in all phases of food supply from farm to the consumer and in sectors such as pharmaceuticals, healthcare and property. They damage products by eating them and gnawing at them and spread diseases through their urine, faeces and particles of filth carried on their bodies.
There are many ways that we can catch a disease from rodents:
An infestation of mice or rats introduces allergens into homes or businesses. A protein found within their urine has been reported to trigger asthma and closely-related allergic conditions in susceptible people.
Mice typically urinate in micro-droplets wherever they are nesting, eating and traveling throughout the day. Mouse urine can therefore be found on thousands of surfaces throughout an infested building, with the highest concentrations usually in kitchens. Everyone in the building will then be exposed to the allergens.
Arenavirus is a genus of primitive viruses, at least eight species of which are known to cause serious diseases in humans that usually show as fever and acute haemorrhagic illness. Some such as Lassa fever have high mortality.
Each of these virus species is associated with a particular rodent species, usually in a localised geographic region. They are divided into two groups called ‘Old World’ and ‘New World’ depending on where they were discovered but they also differ genetically.
Geographic distribution by rodent vector
Transmission & treatment
Arena viruses are transmitted to humans by contact with food or items contaminated with rodent excretions or inhalation of contaminated particles, in the home, factories or agricultural areas.
Some are known to be transmitted from person to person, such as direct contact with blood or body fluid of an infected person, or infected objects such as medical equipment in a hospital.
Bartonellosis is caused by a number of species of Bartonella bacteria, several of which can be carried by rodents. The disease can be transmitted between animals by biting arthropods such as ticks, fleas, sandflies, lice and mosquitoes.
Bartonella elizabethae has been found in rats in America, Asia and Europe. Several other species that can infect humans have been found in ground squirrels and deer mice in the US and woodland rodents in Europe.
Patients with infections have shown a range of symptoms including heart inflammation (endocarditis, myocarditis) and eye disease (neuroretinitis).
Capillariasis involving rodents is caused by one species of nematode (roundworm), Capillaria hepatica. It is unusual in that the lifecycle of the nematode requires only one host and it depends on the death of the host to disseminate viable eggs.
Rodents are the main host, but it can also be other mammals, including humans.
Infection starts with ingestion of food, water or soil contaminated with environmentally conditioned eggs:
This virus is spread by ticks carried by infected rodents in the wild including: deer mouse, bushy-tailed woodrat, ground squirrel, porcupine and chipmunk. The symptoms are fever, headache and muscle pain.
Caused by Cryptosporidium species of protozoa, this is one of the most common water-borne diseases worldwide. Many rodent species have been found to carry Cryptosporidium.
It is primarily transmitted by faecal contamination of water supplies, lakes and rivers or food. It usually causes diarrhoea for a short time, but can be more serious in people with weak immune systems.
In part of its lifecycle the organism creates a hardy oocyst which can survive adverse conditions. Some strains are resistant to disinfectants and chlorine bleach.
Echinococcosis is caused by several species of the tapeworm Echinococcus. The main hosts are carnivores such as foxes, coyotes and wolves and intermediary hosts are mainly grazing animals and pigs.
In at least three species, small rodents, including mice, voles and lemmings are intermediate hosts, which can pass on the cysts of the larval stage when eaten by cats and dogs. These in turn can pass on the cysts to humans through their faeces.
Effect on the human body
After ingestion the larva hatches, burrows through the intestine wall and passes through the blood system to other organs, especially the liver and lungs where it can remain indefinitely and invade surrounding tissue.
The infection can remain without obvious symptoms for years while the infected tissue grows like a tumour.
Many species of rodent carry hantaviruses, especially voles and mice.
Humans can catch the disease through contact with rodent urine, saliva and faeces, by touch, contaminated food or drink, or from breathing in aerosolised particles.
Different species of rodent carry different viruses whose virulence varies but which show similar symptoms of flu-like conditions:
Leishmaniasis is spread by bites from female phlebotomine sandflies. Rodents, dogs and other mammals are reservoirs for the species of Leishmania protozoa that cause the disease. It is present across the world in tropical and subtropical regions.
There are several forms of the disease, causing variously, skin sores, chronic ulcers, mucosal infections, and infections of the spleen, liver, bone marrow and lymph nodes, producing fever and anaemia.
Leptospirosis is an infection caused by species of Leptospira bacteria. It is caught from the urine of rats, other rodents and also cattle, pigs and dogs.
Humans can become infected by:
The bacteria live inside the animal’s kidneys and are passed out in urine. They can survive for weeks or months in soil or water.
The bacteria can infect the body through the mouth, through the skin if broken by a scratch or cut, and the mucus membranes of the eyes, nose and mouth.
Leptospirosis occurs throughout temperate and tropical zones. Occupations or activities that have contact with animals or freshwater sources have a higher risk.
Occupations and activities at risk of leptospirosis:
Symptoms of Leptospirosis
Symptoms of Leptospirosis show in around 7-14 days and can include mild to severe flu-like symptoms including: headache, chills, muscle pain, nausea, vomiting, redness of the eyes, diarrhoea, and skin rash.
Weil’s disease is a more serious form of leptospirosis that develops in about 10% of cases. This can result in organ failure, internal bleeding and death if not treated promptly.
Symptoms of Weil’s disease:
The plague is the classic disease that is linked to rats in the human environment, causing many epidemics through history and wiping out large proportions of populations. It spread along the ancient land and sea trade routes and into urban environments with their dense human populations.
The disease is caused by the bacteria Yersinia pestis, which cycles between rodents and fleas. Several species of rodents are long-term reservoirs of the plague bacteria in the wild. In central Asia the main species is thought to be the marmot which has a wide distribution. In North America, several species of rodents are now known to carry the bacteria. Recent outbreaks have been reported by WHO in China, Peru, Democratic Republic of the Congo and most recently in Madagascar.
How is plague transmitted to humans?
What are the symptoms of plague?
The symptoms that can occur depend on how the disease was transmitted:
The main hosts of rabies are dogs, wild carnivores and bats. Rodents can contract rabies from these sources and potentially pass the virus onto people handling them. The risk of rabies infection from rodents is therefore believed to be very low.
Rat-bite fever is caused by two bacteria Streptobacillus moniliformis and Spirillum minus.
In infected rodents the bacteria are present in rat faeces and urine and secretions from the mouth, nose and eyes.
It is usually caused by a bite or scratch from an infected rat or other rodents such as mice, squirrels and gerbils. It can also be caught by handling infected animals and ingesting food or drink contaminated with rodent faeces or urine.
Rat-bite fever symptoms
Symptoms of rat-bite fever differ between the two bacteria.
In addition, more serious complications can include: heart infections, meningitis (brain infection), pneumonia (lung infection), abscesses in internal organs.
There are two types of rat tapeworm, Hymenolepis nana and H. diminuta. Both species use a beetle (eg a flour beetle) as the main secondary host and are found in warm climates worldwide.
H. nana is the most common as, unusually for helminths, it can have a complete lifecycle in human intestines and spread from person to person through eggs in faeces. It attaches to the intestine wall and absorbs nutrients through the cells lining the intestine.
People can become infected through ingesting food or water contaminated with beetle or rat faeces or via hand contact with contaminated products, then ingesting from the hands.
The lifecycle is illustrated in a diagram produced by the US CDC:
What are the symptoms of rat tapeworm infection?
Light infections may produce no symptoms. Severe infections can cause:
Infection may have no damaging effect on adults, but is more likely to cause serious medical problems in children.
Tick borne encephalitis is a disease of the central nervous system caused by a virus in the Flaviviridae family. Small rodents are the main hosts for immature castor bean ticks and taiga ticks that carry the virus, which can infect a range of mammals and birds. It is not passed from person to person but can be caught from unpasteurized milk.
Most cases of tick-borne encephalitis occur in the Russian Federation, but it has been reported in a band across the northern hemisphere, stretching from Italy to Sweden in the west to the Republic of Korea in the east. There are three types: European/ Western; Far Eastern; and Siberian. The risk is highest in forested areas and from April to November.
Symptoms are high fever, severe headache, nausea, vomiting and back pain. About one third of cases are more severe and can lead to paralysis or death.
Toxoplasmosis is a very common infection caused by the protozoan Toxoplasma gondii.
The main host is the domestic cat, but rodents and other small animals are intermediate hosts, passing on the parasite when eaten by cats. Contamination from cat faeces is then a means of human infection.
Raw meat and vegetables are also routes of infection.
Symptoms of toxoplasmosis
In most people there are no symptoms, but pregnant women and people with weak immune systems are at risk. It can cause miscarriage, stillbirth or other health complications to foetuses.
Some cases produce flu-like symptoms with swollen lymph nodes and severe toxoplasmosis can cause damage to the brain, eyes or other organs.
Typhus is used to describe a number of diseases that are caused by Rickettsia species of bacteria. They are carried by many insects and arachnids, such as ticks, fleas, lice and mites that are ectoparasites of rodents.
Typhus and related diseases caused by Rickettsia include:
Trichinellosis is caused mainly by eating undercooked meat infected with the Trichinella nematode worm. Rats and other rodents play a major role in maintaining the presence of this parasitic roundworm in the environment by being sources of food and infection for carnivores and omnivores. T. spiralis and T. britovi are the main roundworm species causing infection.
Domestic and wild pigs are particularly important for human infection worldwide, being an important food source — and they will eat raw meat, including rodents that can carry the nematode.
In developing countries eating undercooked pork is a major problem. Thailand, for example, has a spicy salad dish called larb, which contains ground pork that is often undercooked. In rural areas restaurants like to use wild pig as an ingredient, which makes it more likely that the dish will contain roundworm.
Trichinellosis is rare in developed countries but it is re-emerging as a problem in developing countries due to relaxation or failure of regulatory systems. Trade in meat products, including illegal trade in bushmeat, can also spread infection as preserving meat by smoking and drying does not kill roundworms effectively.
Lifecycle of the Trichinella roundworm
When the larvae enter the stomach of an animal or human they are released from their cysts by the action of stomach acid and the protein enzyme pepsin. The larvae invade the intestine wall where they mature into adult worms over about four weeks. After another week the adults produce fresh larvae that migrate into muscles and turn into cysts. The cycle is continued when the animal is eaten.
Tularemia is caused by the bacteria, Francisella tularensis, which has several strains that vary in virulence and geographical range.
Taxonomically it is classified in the group of primitive intracellular bacteria that includes Listeria, Legionella, Brucella, Coxiella and Rickettsia. It is in an isolated branch of primitive bacteria, having only one other species in the family Francisellaceae: F. philomiragia. However, genetic analysis may lead to new species being classified.
It is present in a wide geographic band across the whole northern hemisphere.
Tularemia infects or is carried by a large number of mammals and arthropods:
Rodents: The rodent reservoirs of Tularemia include voles, mice, rats, muskrats, beavers, ground squirrels, lemmings and hamsters. Rabbits and hares are also common carriers of the disease. Outbreaks in humans correlate to peaks in populations of rodents and hares.
Ticks & fleas: The bacterium has been found in many species of tick and flea, though the level of infection varies, so the significance each plays in human infection is not well understood.
Mosquitoes: Aedes, Culex, and Anopheles species are known to carry the disease.
Biting flies: true horse flies (Tabanus spp. and Chrysozona spp.) and deer flies (Chrysops spp.) can pick up the disease from the reservoir animals and spread infection between animals.
The Tularemia bacteria can enter the human body via the skin, eyes, mouth, throat or lungs. This can occur through:
Due to the very small number of bacteria needed to cause an infection, it is one of the most infectious diseases known.
Symptoms of tularemia are flu-like, including a fever, headache and nausea. These symptoms are accompanied by an ulcerated lesion at the site of inoculation, if the infection was acquired by a bite or through a break in the skin. Additional signs include swollen lymph nodes, pneumonia and a rash.
A tularemia lesion on a hand. Source: Wikimedia commons: CDC
Rodents can carry Salmonella bacteria that cause illness in both humans and pets. Infection occurs by consumption of food or water contaminated with rodent faeces.
The most common source of infection is by food contaminated with the faeces of farm animals. Genetic studies of Salmonella show that it is extremely complex and as a result has a complex classification. There are two species recognised and many sub-species and sub-types, called serovars:
Symptoms of Salmonella
Symptoms of Salmonella show 12 to 72 hours after infection and include:
Most people recover in a few days without treatment other than replacement of fluid lost by the body. Once a person is infected, the disease is easily transmitted to other people through poor hand hygiene and poor sanitation.
The UK NHS recommends that you clean toilet seats, toilet bowls, flush handles, taps and wash hand basins after use with detergent and hot water, followed by a household disinfectant.
One strain of Salmonella, S. Typhi, causes more severe infection and spreads from the intestines to the blood and lymphatic system and then to other body sites.
Typhoid fever (full name Salmonella enterica subsp. enterica serovar Typhi) is endemic in many developing countries where poor hygiene is widespread, affecting 27 million people a year, especially children.
Humans are the only animal infected by this strain so it is unlikely to be transmitted by rats unless they come into direct contact with human faeces, for example in sewer systems.
Bonnefoy X, Kampen H, Sweeney K. Public Health Significance of Urban Pests. WHO, Copenhagen, 2008
US CDC: www.cdc.gov
UK NHS: www.nhs.uk
Epidemic Typhus Associated with Flying Squirrels — United States. www.cdc.gov/mmwr/preview/mmwrhtml/00001177.htm
Medscape: Rickettsialpox http://emedicine.medscape.com/article/227956-overview
The Center for Food Security and Public Health, Iowa State University http://www.cfsph.iastate.edu/DiseaseInfo/factsheets.php