Rodent-borne diseases

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.

Apart from rats and mice, other well-known rodents that can carry diseases and come into human contact include prairie dogs, groundhogs, ground squirrels, lemmings and voles.

In fact, rodents are thought to be responsible for more deaths than all the wars over the last 1,000 years.

Disease-causing organisms

Rodents carry a wide range of disease-causing organisms, including many species of bacteria, viruses, protozoa and helminths (worms).

Parasites

Rodents can also carry several parasites and diseases at the same time.

Rodents 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.

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.

How Can I Catch A Rodent Disease?

• inhalation or direct contact with rodent excreta (urine, faeces, saliva);
• handling or inhaling microorganism particles aerosolized from hay, woodpiles or other materials contaminated with infectious rodent urine;
• particles aerosolized by sweeping rodent infested spaces;
• handling of infected rodents by hunters or other people;
• bites from rodents — microorganisms carried in saliva can infect both humans and other rodents;
• scratches from rodents;
• drinking contaminated water or eating contaminated food;
• rodents acting as sources for infecting ectoparasites (ticks, fleas, mites, lice) with various pathogens;
• dogs, cats and foxes (especially urban) eating rodents and then catching parasites such as tapeworms that can be passed on to humans by them; and
• rodents can also act as reservoirs for various flying-insect-borne diseases.

Salmonellosis

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:

• Salmonella enterica, which has six subspecies and 2500 serovars, is the main cause of salmonellosis in humans and other mammals. A few of the serovars are important disease agents in humans, mainly occurring in subspecies I.
• S. bongori mainly occurs in reptiles, but can infect humans through contact with pets.

Symptoms of Salmonella

Symptoms show 12 to 72 hours after infection and include:

• diarrhoea
• fever
• vomiting
• abdominal cramps

Salmonella Treatment

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.

Typhoid
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
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

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:

• direct contact with urine or other animal body fluids (except saliva) of infected animals;
• contact with soil, water or food contaminated with the urine of infected animals.

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.

Risk

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

• farming;
• abattoir workers;
• vets;
• sewer workers;
• mine workers;
• fish workers;
• fishing;
• sailing;
• swimming

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.

It can be treated with antibiotics.

Weil’s disease

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

• jaundice;
• swollen ankles, feet or hands;
• chest pain;
• symptoms of meningitis or encephalitis, such as headaches, vomiting and seizures;
• coughing up blood.

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

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.

• Streptobacillus: 3-10 days after infection symptoms are fever, vomiting, muscle pain, headache, joint pain, rash.
• Spirillum: 7-21 days after infection symptoms can include: repeated fever, ulcer at the bite wound, swelling around the wound, swollen lymph nodes, rash.

In addition, more serious complications can include:

• heart infections;
• meningitis (brain infection);
• pneumonia (lung infection);
• abscesses in internal organs.

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.

Plague

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.

Geographic distribution

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

1. Flea bites: rats and other rodents can carry infected fleas — as can dogs and cats. When the host dies from the disease the fleas seek alternative hosts to feed on. This causes the bubonic or septicemic plague.
2. Contaminated animals: handling tissue or fluid of an infected animal. This can result in bubonic or septicemic plague. Cats (and other carnivores) can also catch the plague by eating infected rodents and pass it on to humans.
3. Infectious particles: when plague infection reaches the lungs coughing produces infected air borne particles that can be breathed by people in close proximity in and cause pneumonic plague.

Plague symptoms

The symptoms that can occur depend on how the disease was transmitted:

1. Bubonic plague: the most common sign is swollen and painful lymph nodes (buboes) where the bacteria multiply and can spread from if not treated. There is also sudden onset of fever conditions, extreme weakness.
2. Septicemic plague: fever conditions, extreme weakness, diarrhea, delirium, abdominal pain, shock and bleeding in the skin and other organs. The skin and other tissues can turn black and die especially on fingers, toes and the nose.
3. Pneumonic plague: fever conditions, shock and pneumonia, causing breathing difficulty, chest pain, cough and bloody mucous.

Plague treatment

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.

Hantavirus

Many species of rodent carry hantaviruses, especially voles and mice.

Symptoms

Different species carry different virus whose virulence varies but which show similar symptoms of flu-like conditions.

Transmission

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.

Asia

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.

Europe

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.

America

In 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.

Tularemia

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.

Vectors

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 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.

Mosquitoes

Among mosquitoes, Aedes, Culex, and Anopheles species are known to carry the disease.

Biting flies

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:

• contact with infected pets;
• inhaling contaminated dust or aerosols;
• eating contaminated food;
• drinking contaminated water;
• handling infected wild animals and meat.

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.

Tularemia Symptoms

The symptoms differ depending on the route of infection, but all produce a fever:

• ulceroglandular: results from a bite or after handling infected animals and is the most common form. A skin ulcer appears at the site of infection and lymph glands in armpit or groin swell;
• glandular: lymph glands swell following a bite, but there is no ulcer;
• pneumonic: results from breathing dust or aerosols, which can cause cough, chest pain, and difficulty breathing. This is the most serious form of infection;
• occuloglandular: caused by infection through the eye resulting in inflammation of the eye and swelling of lymph glands near the ear.
• oropharyngeal: this occurs after eating consuming contaminated food or water. Symptoms include sore throat, mouth ulcers, tonsillitis, and swelling of lymph glands in the neck.

Symptoms last several weeks and can easily be mistaken for other diseases as Tularemia is relatively rare.

Tularemia Treatment

Tularemia responds to a range of antibiotics. Left untreated it can spread to multiple organs including lung, spleen, liver, lymphatic system.

Bartonellosis

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.

Trench fever

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.

Geographic distribution

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.

Arenaviruses

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

• Lymphocytic choriomeningitis: house mouse — worldwide;
• Lassa fever: Natal multimammate mouse— West Africa;
• Lujo hemorrhagic fever: rodent vector unknown but assumed (discovered in 2008) — South Africa, Zambia;
• Argentine haemorrhagic fever: drylands vesper mouse — Argentina;
• Bolivian haemorrhagic fever: large vesper mouse — Bolivia;
• Chapare haemorrhagic fever: rodent vector is unknown — Bolivia;
• Sabiá-associated haemorrhagic fever: rodent vector is unknown — Brazil; 
• Venezuelan haemorrhagic fever: short-tailed cane mouse — Venezuela

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

Toxoplasmosis is a very common infection caused by the protozoan Toxoplasma gondii.

In the US the CDC estimates that up to 22% of the population have been infected, while in the UK the NHS estimates that over 350,000 people could be infected.

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.

Rat tapeworm

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.

Transmission

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:

Rat tapeworm symptoms

Light infections may not produce any symptoms. Severe infections can cause:

• abdominal pain
• enteritis
• diarrhoea
• loss of appetite
• restlessness
• irritability
• restless sleep
• anal and nasal itching

Infection may have no damaging effect on adults, but is more likely to cause serious medical problems in children.

Echinococcosis

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.

Echinococcosis symptoms

The infection can remain without obvious symptoms for years while the infected tissue grows like a tumour.

• Liver infections can cause abdominal pain and in the lungs chest pain, cough and bloody mucus.
• If the infected tissue ruptures it can cause fever, skin rash, increase in white blood cells and anaphylactic shock in response to the large numbers of larvae released into the body.

Capillariasis

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.

Lifecycle

Infection starts with ingestion of food, water or soil contaminated with “environmentally conditioned” eggs.

• The eggs hatch into first stage larvae in the intestines where they bore through the intestinal wall into the blood system and the liver.
• In the liver the larvae mature into adults in 18-21 days and then lay eggs in the liver tissue. These cannot mature into larvae until they have spent time in the environment, which is usually on the death of the animal or if the rodent is eaten by a predator or scavenger.
• It the rodent is eaten, the eggs do not hatch but are passed out into the environment in the faeces of the predator. The eggs then require 4-5 weeks to develop but can remain viable for several months.

Symptoms

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.

Bibliography

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