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Rodents are the most diverse order of the mammals, with over 2,200 species. They inhabit all continents except Antarctica, and are adapted to a wide range of habitats.
Rodents are social animals and have large reproductive potential which means populations can increase rapidly when conditions are favourable.
A few species, especially rats and mice, have adapted well to the human environment and the benefits it offers to them for food and shelter. They have become serious pests in all phases of food supply and to human health due to these close encounters with the food we eat and the places we live and work.
As we humans take over and expand our activities into the remaining natural environment we come into closer contact with more species of rodents and more diseases.
Other well-known rodents that can carry diseases and come into human contact include prairie dogs, groundhogs, ground squirrels, lemmings and voles.
Rodents carry a wide range of disease-causing organisms, including many species of bacteria, viruses, protozoa and helminths (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.
In fact, rodents are thought to be responsible for more deaths than all the wars over the last 1,000 years.
Rodents can also carry several parasites and diseases at the same time. A study of rats on farms in the UK found 13 zoonotic (infect humans) parasites and 10 non-zoonotic parasites, with some rats having nine zoonotic parasites at the same time. Many of these had rarely or never previously been investigated in wild rats (eg Cryptosporidium, Pasteurella, Listeria, Yersinia, Coxiella and Hantavirus), showing that the threat to human health is greater than previously thought.
The common, well-known and dangerous diseases associated with rodents are described below, classified according to the major animal groups.
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 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.
An unusual source of Salmonella infection was recorded in the US in 2014. An outbreak of Salmonella Typhimurium was traced by the US Centers for Disease Control (CDC) to frozen rodents supplied by a pet feed company for feeding pet reptiles and amphibians.
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.
Typhoid can be treated with antibiotics and vaccines are available to give protection from infection.
Leptospirosis is an infection caused by species of Leptospira bacteria. It is caught from the urine of infected animals, which include 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 do not only enter the body through the mouth, they can also enter through the skin, especially if broken by a scratch or cut, and the mucus membranes of the eyes, nose and mouth.
Leptospirosis occurs throughout temperate and tropical zones, but is more common in tropical and subtropical areas where the temperature and humidity are more favourable for its growth.
The risk of catching it is low for most people. However, occupations or activities that have contact with animals or freshwater sources have a higher risk.
Occupations & activities at higher risk
Symptoms of Leptospirosis
Symptoms of Leptospirosis show in around 7-14 days and can include mild to severe flu-like symptoms including:
It can be treated with antibiotics.
In about 10% of Leptospirosis cases a more serious form develops, called Weil’s disease. This can result in organ failure, internal bleeding and death.
Symptoms of Weil’s disease
How to treat Weil’s disease
It needs urgent treatment in a hospital where ventilator, dialysis treatment, and intravenous antibiotics and fluids can be given.
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:
Both infections can be treated with antibiotics.
Reports of rat-bite fever are rare in Europe and North America, but as reporting is not required it may be under-reported.
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 Russia, the main species is thought to be the marmot which lives in the Steppes.
In the western US, several species of rodents are now known to carry the bacteria — even causing instances of colony collapse in the prairie dog.
The California Department of Public Health has a plague surveillance programme that tests wild rodents for the disease. It produces a map of ‘plague positive’ rodents tested at sites where they are likely to interact with people (eg campsites).
Transmission to humans
The symptoms that can occur depend on how the disease was transmitted:
The plague is treatable with antibiotics.
It is important to obtain diagnosis and treatment rapidly as death can occur rapidly. In bubonic plague death can occur in less than two weeks.
With septicemic plague death can occur before symptoms appear, and with pneumonic plague all untreated patients die! The potential causes such as flea bites and visits to endemic areas should be relayed the doctor.
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.
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 bacteria 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.
Among mosquitoes, Aedes, Culex, and Anopheles species are known to carry the disease.
Among the 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.
How can I catch tularemia?
The Tularemia bacteria can enter the human body via the skin, eyes, mouth, throat or lungs. This can occur through:
There are no known cases of human to human transmission (which is actually seen as an advantage in biological warfare in restricting infection to the target population) or of the direct transmission from one human to another by arthropods (fleas, ticks, mosquitoes, biting flies).
However, due to the very small number of bacteria needed to cause an infection, it is one of the most infectious diseases known.
The symptoms differ depending on the route of infection, but all produce a fever:
Symptoms last several weeks and can easily be mistaken for other diseases as Tularemia is relatively rare.
Tularemia responds to a range of antibiotics. Left untreated it can spread to multiple organs including lung, spleen, liver, lymphatic system.
Bartonellosis is caused by a number of species of Bartonella bacteria, several of which can be carried by rodents and which cause a wide range of symptoms.
The disease can be transmitted between animals by biting arthropods such as ticks, fleas, sandflies, lice and mosquitoes.
The most well-known species is B. Quintana which was the cause of trench fever during the First World War and spread by the body louse. This species is not known to have an animal reservoir, however. Cat scratch disease is also caused by several Bartonella species.
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 these infections have shown symptoms of heart inflammation (endocarditis, myocarditis) and eye disease (neuroretinitis).
Treatment is with antibiotics.
Many species of rodent carry hantaviruses, especially voles and mice.
Different species carry different viruses whose virulence varies but which show similar symptoms of flu-like conditions.
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.
A severe infection is caused by the Hantaan virus which occurs in China, the Democratic People’s Republic of Korea, the Republic of Korea and far eastern Russia. This is carried by the striped field mouse.
In Europe the main carrier is the bank vole, which hosts the Puumala virus, the cause of a relatively mild form of haemorrhagic fever with renal syndrome (HFRS). Finland, countries of the former Yugoslavia, Sweden, Belgium, France, Germany, Greece and the Netherlands report significant numbers of cases annually.
The Dobrava virus, which causes a severe form of HFRS, is present in southern Europe, carried by the yellow-necked mouse. The milder Saaremaa virus is also carried by the striped field mouse in Estonia and nearby in Russia.
In North America many species of hantavirus have been identified in rodents. The most important of these is Sine Nombre virus which is carried by deer mice in Canada, Mexico and the US. This causes Hantavirus pulmonary syndrome, which has a high fatality rate.
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
There is no vaccine or specific treatment for these diseases and their biology is not well understood. They 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.
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.
Risk to human health
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.
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:
The lifecycle of Hymenolepis nana. Source : Wikimedia Commons. CDC https://commons.wikimedia.org/wiki/File:H_nana_LifeCycle.gif
Rat tapeworm symptoms
Light infections may not produce any symptoms. Severe infections can cause:
Infection may have no damaging effect on adults, but is more likely to cause serious medical problems in children.
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.
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.
The lifecycle of Capillaria hepatica (also called Calodium hepaticum)
Source: Wikimedia Commons: CDC https://commons.wikimedia.org/wiki/File:Calodium_hepaticum_lifecycle.gif
The adult nematodes feed on the liver, slowly causing loss of liver function, inflammation (hepatitis) and abnormal fibrous tissue production as the liver responds to the death of the adults and the presence of eggs.
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 the parasite 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 bush meat, can also spread infection as preserving meat by smoking and drying does not kill roundworms effectively.
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
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