Rodent-Borne Diseases

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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.

Rodents play a significant role in the transmission of many disease-causing pathogens to humans. In fact, rodents are thought to be responsible for more deaths than all the wars over the last 1,000 years.

Allergens and Asthma

An infestation of mice or rats also means the introduction of allergens. A protein found within their urine has been reported to trigger asthma and closely-related allergic conditions in susceptible people.

The residue from rodent urine can easily spread all over a home, with the highest concentrations usually in kitchens, and lead to asthma attacks that have the potential of being quite serious.

Because mice typically urinate in micro-droplets wherever they are nesting, eating and traveling throughout the day, mouse urine can literally be found on thousands of surfaces throughout a home. This means residents are likely exposed to rodent allergens whether they realize it or not.

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 are reservoirs for many diseases including those carried by ectoparasites such as fleas, ticks, lice and mites.

Research has shown that rats can harbor multiple infectious parasites at one time. One study found parasites that 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.

Am I at Risk for a Rodent-Transmitted Disease?

  • Inhalation or direct contact with rodent excreta (urine, feces, saliva);
  • handling of or close contact with infected rodents;
  • getting bitten or scratched;
  • consuming water or food contaminated by rodent droppings or urine.

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

In the US most cases were caused by humans breathing in aerosolized particles but there is a slight risk of catching the disease through contact with rodent urine, saliva and feces, by touch, contaminated food or drink.

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.

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. It is most commonly transmitted by the bites of infected fleas.

Rats are known reservoirs for Bartonella. Fleas found on rats can potentially vector this bacteria to humans who come into close contact with them.

Cat scratch fever

The most well-known species is B. henselae, also referred to as cat scratch fever. The bacteria can live within the cat’s blood for months, sometimes year. At one time it was thought to be a one-time disease that those who were infected would not recur. Further research has shown this not to be case. It is usually transmitted via the fleas from infected cats. However, because rats can also carry B. henselae, humans can also become infected if exposed to fleas from infected rats.

Salmonellosis

In the US, probably one of the most common diseases associated with mice is food-borne illness. Mice have a tendency to live in or frequent unsanitary places such as sewers or garbage piles, where they acquire bacteria such as salmonella, and harbor it in their digestive tracts. Mice then enter homes and contaminate food and food surfaces (kitchen counters, utensils etc.) with their infected droppings, which can lead to acute food-poisoning in humans.

Symptoms of Salmonella

Symptoms show 12 to 72 hours after infection and include:

  • diarrhea;
  • fever;
  • vomiting;
  • abdominal cramps.

Salmonella Prevention

If you think you might have salmonella, seek immediate treatment from a doctor.

Once a person is infected, salmonella is easily transmitted to other people through poor hand hygiene and poor sanitation.

The US Centers for Disease Control (CDC) recommends that to prevent risk, 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 to frozen rodents supplied by a pet feed company for feeding pet reptiles and amphibians.

Leptospirosis

Leptospirosis is an infection caused by a bacteria called Leptospira. It is found in the blood and urine of animals, which include rodents and also cattle, pigs and dogs.

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. Humans can become infected by:

  • bites by infected mice or rats;
  • ingestion of food or water that was contaminated by the urine of infected mice or rats; and
  • skin contact with water, soil or vegetation that was contaminated with the urine of mice, rats or other infected animals.

Risk

Leptospirosis is found worldwide, including the U.S. In the U.S. rodent-associated leptospirosis has been shown to be quite prevalent in some areas.

The risk of contracting leptospirosis is low for most people. However, occupations or activities that involve frequent contact with freshwater sources or moist environments have a higher risk.

Occupations & activities at higher risk

  • Farming;
  • stockyard workers;
  • vets;
  • sewer workers;
  • mine workers;
  • fish workers;
  • fishing;
  • sailing; and
  • 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;
  • diarrhea; and
  • skin rash.

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; and
  • coughing up blood.

Plague

Plague is the most notorious disease that is linked to rats in the human environment.

In the 14th century, it was better known as the “Black Death” in Europe, killing 25 million people over a 50-year period.

This disease is caused by the bacteria Yersinia pestis, which cycles between rodents and fleas. Several species of rodents, including rats, ground squirrels, prairie dogs and chipmunks are reservoirs of the plague bacteria in the wild.

Plague in the US

In the Western US, two types of plague occur: urban and sylvatic plague. Urban plague cycles between commensal rats (species that live in close association with humans) and their fleas (primarily the oriental rat flea). Human cases of urban plague are extremely rare. More common is sylvatic plague, which is maintained between wild rodents and rabbits, and their fleas. Humans become infected when they come into close enough contact to be bitten by the fleas of these infected wild rodents or rabbits, or by handling these animals and coming into contact with their bodily fluids.

Transmission to humans

  1. Bites from fleas: rats and other rodents can carry infected fleas — as can cats.
  2. Contact with infected animals: handling tissue or fluid of an infected animal. Cats (and other carnivores) can also acquire plague-causing bacteria by eating infected rodents.
  3. Inhalation of infectious particles: when plague infection reaches the lungs coughing produces infected air borne particles that can be breathed by people in close proximity.

Clinical Forms of Plague

  • Bubonic plague: This is the most common form, acquired by the bite of an infected flea. The bacteria travel through the bloodstream and concentrate within lymph glands. Signs include swollen and painful lymph nodes (called buboes) in the groin and under the armpits. There is accompanied by a sudden onset of fever and extreme weakness.
  • Septicemic plague: When bubonic plague is untreated, the bacteria will continue to spread throughout the body, leading to this deadly form. Symptoms include fever, extreme weakness, diarrhea, delirium, abdominal pain, and shock. Bleeding occurs under the skin, turning it black. This is the reason why plague was referred to as the “Black Death”.
  • Pneumonic plague: This form is known to be the most dangerous, because the bacteria is concentrated in the lungs and can spread by coughing. Symptoms include fever, difficulty breathing, chest pain, bloody mucous, pneumonia and shock.

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 feces 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 feces 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); and
  • abscesses in internal organs.

Reports of rat-bite fever are rare in North America. However, many experts believe that rat-bite fever is under-reported, and that numbers of actual human cases are much higher than what the literature shows.

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 several species of ticks and fleas, though the level of infection varies, so the significance each plays in human infection is not well understood.

Biting flies

When it comes to the transmission, horse flies (Tabanus spp. and Chrysozona spp.) and deer flies (Chrysops spp.) are the most common insect vectors of tularemia-causing bacteria.

Am I at Risk for 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; and
  • handling infected wild animals or animal meat.

Tularemia Symptoms

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: CDC, Wikimedia Commons

Rickettsial Diseases

Rickettsia are a specialized type of bacteria that are parasitic of cells within vertebrates and arthropods. They are generally transmitted from rodents to humans via ticks, mites, fleas and lice.

Rickettsialpox

A non-fatal and rare disease with a number of symptoms such as:

  • headaches;
  • rash; and
  • fever.

The cause of this disease is the rickettsial bacterium known as Rickettsia akari which is usually transmitted via the house mouse mite.

The diseases is often misdiagnosed as chicken pox because it bears similarities to that illness.

Murine Typhus

Caused by the bacterium Rickettsia typhi, this disease is transmitted from fleas found on rats and other rodents. The Norway rat and the roof rat are the predominant rodent reservoirs.

Interestingly, murine typhus is not transmitted directly through the bites of fleas. Rather, infections occur as a result of scratching itchy flea bites, when infected flea feces are rubbed into the bite wound or breaks in skin and enter the bloodstream.

Symptoms of murine typhus include fever, chills, headache, and a rash. The rash usually appears about five days after symptoms start occurring and will last about a week.

Although relatively rare in the United States, cities near seaports in warmer climates could see the risk increase.

Toxoplasmosis

Toxoplasmosis is an infection caused by the protozoan Toxoplasma gondii.

In the US, the CDC estimates that up to 22% of the population have been infected. Cats are the main host of T. gondii, acquiring this protozoan by eating infected rodents or contaminated meat. Humans can serve as an intermediate host for T gondii. Although a human infection is usually mild and self-limiting, toxoplasmosis can have devastating and /or deadly effects on unborn fetuses.

Humans most commonly acquire toxoplasmosis infections through contact with infected cat feces. As such, pregnant women are advised to use extreme caution around cat litter boxes during pregnancy.

Lymphocytic Choriomeningitis (LCM)

This one of the most common viruses, widely reported worldwide that are part of the Arenavirus genus of primitive viruses.

LCM infection has a wide range of symptoms from asymptomatic to cases that produce mild meningitis. A typical infection involves fever, headache, and muscle pains that resemble typical flu and other common viral ailments. Thus, this virus may affect more people than is actually known as the illness might be diagnosed as something else.

Transmission

Arenaviruses such as LCM and their biology is not well understood, but 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.