Historically rodents have played a profound role in the transmission of diseases to humans.
The “Black Death” (Bubonic Plague), which claimed more than 25 million lives in 14th Century Europe, is perhaps the most documented case history of rats and disease. The plague bacterium (Yersinia Pestis) was transmitted amongst rats and from rats to humans by the bite of the Oriental Rat Flea. Even today incidence of plague has not been entirely wiped out, but closer understanding of the mechanism involved has seen a significant reduction in the occurrence of this dreaded disease.
Among the many diseases transmitted to humans by rodents, some of which we will outline below, perhaps the most insidious and widespread problem involves the distribution of food-poisoning organisims (e.g. Salmonella bacteria). Unfortunately, the nature of disease organisms of this type is such that it will always be difficult to pinpoint, without doubt, the cause of disease transmission. It is very likely that rats and mice play a significant role in the transmission of such diseases, particularly those which are gastro-intestinal and somewhat difficult to track back.
Rodents can pose a serious threat to human health and can transmit disease to humans by a variety of means, including:
- Contamination of food or utensils with rodent urine or faeces.
- Contamination by direct contact with urine or faeces, where bacteria enter the skin through small scratches.
- Indirect contamination via blood-sucking insects (i.e. ectoparasites such as fleas).
- Indirect contamination via pets to humans.
- Contamination by directly biting humans.
- Indirect contamination by being eaten by an intermediate carrier.
The presence of rats the mice in buildings pose a serious threat to human health. The disease threat alone is justifiable cause for concern and for the implementation of sound control procedures.
1. Contamination of food or utensils with rodent urine or faeces
Mouse Typhoid / Duck Egg Disease – bacteria (Salmonella Typhimurium) spread from mice to humans via the faeces of an infected mouse, with a single mouse dropping having the potential to contain 100,000 salmonella cells. This bacteria has been around for a long time and has now found new ways to reach human victims. This includes passing the strain from rodents onto chickens and consequently their eggs. The infection of chickens and their eggs have been aided by an increase in large commercial animal husbandry of chickens, food distribution and eating habits. Do to the large spread of the salmonella bacteria by this means egg farmers are beginning to instituted “biosecurity” practices starting with rodent-proofing their sheds.
Salmonella typhi, is the most dangerous of the salmonella bacteria as it causes typhoid fever, a frequently fatal bloodstream infection. symptoms: vomiting, stomach pain, diarrhea and fever.
Lymphocytic Choriomeningitis – is a virus that remains dormant in mice and is spread via the faeces of an infected mouse. If the faeces of the infected mouse has contact with human food or utensils the virus will then be passed on to any human who consequently ingests the virus. People who contract the virus will suffer from a mild form of meningitis. This condition is endemic is Australia.
Meningitis is the inflammation of the meninges (the soft tissue membranes which cover the brain and spinal cord underneath the skull and spinal column). The symptoms may include a headache, nausea or vomiting, fever, general malaise, neck stiffness, an aversion to bright lights, joint aches and pains, muscle aches, drowsiness or confusion, and also may include a rash, sore throat, stomach pains and diarrhoea. These symptoms may occur in any order and may not all be present at the same time or during the course of the illness. The symptoms of viral meningitis are usually milder than those associated with bacterial meningitis and may begin with flu like symptoms. However, the symptoms of viral meningitis can also present in the same manner as bacterial meningitis, with the latter potentially able to cause death within hours. Consequently it is important that people seek medical attention as soon as possible, if they are concerned that they may have meningitis.
Most cases of viral meningitis are usually mild and most people make an excellent recovery. However on rare occasions viral meningitis can be life threatening or cause lasting after effects, particularly if people have problems with their immune system. Viral meningitis is more common in children however it can occur in any age group.
2. Contamination by direct contact with urine or faeces, where bacteria enter the skin through small scratches
Leptospirosis (Weil’s disease / Infectious Jaundice) – Leptospirosis is caused by infection with Leptospira bacteria. The infection passes from rats to humans. The bacteria can live for many years within the animal’s kidneys and can be passed out in their urine into water, soil, vegetation and mud, where they can survive for several months. Often, infected animals show no outward signs of illness.
Leptospirosis can be passed on to humans through direct contact with the bodily fluids or tissues of an infected animal (such as urine) or through contact with soil or water contaminated with infected urine. The bacteria usually get into the human body through a cut or small abrasion in the skin. They can also pass through mucous membranes such as the skin that lines the mouth, the nose and the front of the eye. The bacteria may also be inhaled and enter the body via the lungs. Once inside the body, Leptospira have the ability to pass via the bloodstream and the lymph system to the internal organs.
In most cases, leptospirosis causes a mild illness, that is expressed as flu-like symptoms including high temperature and chills, headache, nausea, vomiting, diarrhoea and loss of appetite. patients can also experience conjunctivitis, joint and muscle pains (particularly in the calves and lower back) and fatigue.
The majority of people then recover as their immune system clears the infection. The main treatment for leptospirosis is antibiotics and most people make a full recovery. However, in some people, a second stage of the illness then develops (Weil’s disease). Patients who develop Weil’s disease will present with diarrhoea, skin rash, muscle pains, abdominal pain and jaundice (yellowing of the skin and the whites of the eyes).
3. Indirect contamination via blood-sucking insects (such as fleas)
Murine Typhus Fever / Endemic Typhus – is caused by the bacteria Rickettsia typhi and is transmitted by the fleas that infest rats. While rat fleas are the most common carrier of the bacteria it can also be transmitted by the cat fleas and mouse fleas. The fleas remain unaffected by the infection, however human infection occurs because of flea fecal contamination of the bites on human skin. The rickettsia typhi colonisation is maintained for its entire lifecycle by a range of host species including rats, cats, possums. The rats can develop the infection, and help spread the infection to other fleas, this then multiplies the number of fleas carrying the bacteria that can then infect humans.
Murine typhus is an under-recognized entity, as it is often confused with viral illnesses. Most people who are infected do not realize that they have been bitten by fleas. Symptoms of endemic typhus include headache, fever, muscle pain, joint pain, nausea and vomiting. 50% of patients develop a discrete rash six days after the onset of signs and up to 45% will develop neurological signs such as confusion, stupor, seizures or imbalance.
Endemic typhus is highly treatable with antibiotics and most people will have a full recovery, but death may occur in the elderly, severely disabled or patients with a depressed immune system.
4. Indirect contamination via pets to humans
Favus / Tinea Favosa – is a skin disease that is caused by a fungus. Favus usually spreads through direct contact with sick persons or infected objects, or through contact with infected animals such as cats, rats, or mice. Children are especially susceptible.
In most cases the scalp is affected, although the smooth skin and nails are other possible sites of infection. Crusty, yellow, compact, cup-shaped formations, which are called ‘scutula’. The scutula will have an unpleasant mousy odor, surround a central hair, and consist of pure cultures of the fungus.
Hair easily falls out from the affected areas and is lackluster, ashy gray, and dry. The scutula leave scars from which hair cannot grow back. Scutula also can appear on smooth skin, but does not scar. Affected nails are usually thickened, nodular, yellow, and brittle.
5. Contamination by directly biting humans
Rat-bite Fever / Relapsing Fever – is caused by bacteria (Spirillum Minus or Streptobacillus Moniliformis) transmitted by rodents, which is passed from rodent to human via the rodent’s urine or mucous secretions. It is a rare disease spread by infected rodents and can be caused by two specific types of bacteria. Some cases are diagnosed after patients were exposed to the urine or bodily secretions from the mouth nose or eyes of the infected rodent, it can also be transmitted through food or water that is contaminated with rat feces or urine. Though the majority of cases occur when a rodent bites a human. Household pets such as dogs or cats that are exposed to these animals can also carry the disease and infect humans. If a person is bitten by a rodent, it is important to quickly wash and cleanse the wound area thoroughly with antiseptic solution to reduce the risk of infection.
Symptoms of rat-bite fever (RBF) will be different for every person and will be different depending on the type of rat bite fever that a person is infected with. RBF symptoms are visually seen in most cases and include inflammation around the open sore and on the hands and feet, ulcers and a purple or red rash that spreads around the area. There is a lot of crossover between the systems of the two strains of RBF, however the streptobacillary strain is known for causes chills, fever, vomiting, headaches, swollen joints, back pain, and muscle aches, it is important to note that this strain of the disease can be fatal if untreated in 20% of cases due to malignant endocarditis, meningoencephalitis or septic shock. The distinctive symptoms associated with spirillary RBF include swollen or inflamed lymph nodes in the neck, groin and underarms.
6. Indirect contamination by being eaten by an intermediate carrier
Trichinosis – is a parasitic disease caused by eating raw or uncooked pork that is infected with the larvae of a species of roundworm (Trichinella Spiralis). The pig becomes infected by this parasite as a result of being feed food that is contaminated with infectious cysts in raw meat (of other pigs or rodents). Humans then become infected when they eat raw or undercooked infected pork. After humans ingest the cysts from infected undercooked meat, the pepsin and hydrochloric acid in the stomach help to free the larvae in the cysts in the stomach. The larvae then migrate to the small intestine, where they molt four times before becoming adults. Once 30-34 hours have passed since originally ingested, the adults mate, and within five days produce larvae. The worms can only reproduce for a limited time because the immune system will eventually expel them from the small intestine. The larvae then use their piercing mouthpart, called the “stylet”, to pass through the intestinal mucosa and enter the lymphatic vessels, and then enter the bloodstream. The larvae travel by capillaries to various organs, such as the retina, myocardium, or lymph nodes; however, only larvae that migrate to skeletal muscle cells survive and encyst. The larval host cell becomes a nurse cell in which the larvae will be encapsulated.
The number of cases has decreased because of legislation prohibiting the feeding of raw meat garbage to hogs, increased commercial and home freezing of pork, and the public awareness of the danger of eating raw or undercooked pork products. However, it is often brought it to Australia by migrants.
Knowing the risks that come with having a rodent problem it is important that you can quickly identify the presence of rodents so that they can be eliminated from your home or business.