Rodent-borne diseases

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.

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 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.

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.

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

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.

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.

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).

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.

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.

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.

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.

• 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

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.

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.

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:

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 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.

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.

• 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 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 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.

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.

Bonnefoy X, Kampen H, Sweeney K. Public Health Significance of Urban Pests. WHO, Copenhagen, 2008

WHO. A global brief on vector-borne diseases, WHO, Geneva, 2014
http://www.who.int/campaigns/world-health-day/2014/global-brief/en/

US CDC: www.cdc.gov

Wikipedia: https://en.wikipedia.org

UK NHS: www.nhs.uk

PARA-SITE http://parasite.org.au/

Webster JP, Macdonald DW (1995). Parasites of wild brown rats (Rattus norvegicus) on UK farms. Parasitology, 111:247–255. doi:10.1017/S0031182000081804.

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

WHO guidelines on Tularemia, 2007
http://apps.who.int/iris/handle/10665/43793

The Center for Food Security and Public Health, Iowa State University
http://www.cfsph.iastate.edu/DiseaseInfo/factsheets.php