Introduction

Rodents are one of the most successful groups of mammals and have been able to exploit a wide variety of habitats and environments throughout the world. Although people have largely categorised rodents as pests in urban and agricultural environments, most rodent species live in the wild with little interaction with humans. A small minority of rodent species have adapted to live in close association with humans, using our agriculture and waste as their own food resources and finding our buildings can make good rodent homes. The close proximity of these rodents to human habitation means that they can spread and transmit diseases which affect people. The ability of rodents to carry and vector diseases is encouraged by their habits of utilising our waste and sewage, their mobility and the similar physiologies that humans and rodents share. Rodents are able to vector more than 60 known diseases, and the list grows as more research on zoonosis continues.

Plague is perhaps the most well-known rodent-vectored disease, due to the enormous impact it had on the human population of Medieval Europe when it was referred to as the Black Death. Although plague still exists and kills many people in many parts of the world, other rodent transmitted diseases such as Lassa Fever and Leptospirosis are potentially more serious in the numbers of people infected. There is growing concern from many experts around the world that zoonotic diseases are emerging and re-emerging, particularly in developing countries that lack the resources to investigate, manage and treat a number of diseases and the underlying reasons for their spread and persistence. See the Publications and Links to find out more about rodents and the diseases they carry.

Potential socio-economic factors that may affect zoonosis transmission, particularly for developing country situations
• general hygiene at household and community levels
• eating rats, frequently handling rats
• food preparation methods, eating uncooked fruits and vegetables
• food storage location and accessibility level, ranging from inside dwelling to rodent-proofed store
• drinking water storage
• source of drinking water
• source of washing water - bathing, clothes, kitchen utensils
• proximity to sewage
• proximity to open water
• proximity to agricultural land - vegetable or staple crops
• proximity to bush or fallow land
• amount of rodent harbourage inside home - notably nesting in thatched roofs, burrows in mud walls, roof or wall voids, behind furniture
• amount of rodent harbourage outside relatively near home
• density and quality of housing
• occupation - paddy fields, slaughter houses, animal husbandry, activities that may increase cuts to hands and feet
• wealth and education levels
• gender and age

Leptospirosis

Leptospirosis is a bacterial disease caused by a number of pathogenic species of the genus Leptospira. Pathogenic leptospires live in the kidney of host animals and are excreted in the urine over a few days or for a long time up to lifelong. They may be irregularly shed in the urine as opposed to being continuously shed. Different serovars of leptospires are often associated with, or adapted to, different animal hosts. Adapted species may cause no or only minor clinical problems for the host. However, when the leptospires find their way into another animal species, they may cause severe disease. There are, therefore, a range of leptospires in the environment that range in clinical effects from benign to acute. Hence, leptospires that are harmful to humans may be benign to rodents. Other wild and domestic animals can contract and excrete leptospires and will suffer the same range of symptoms ranging from benign to acute depending on the species of leptospira infecting them. Some species of leptospira are known to be more harmful than others, and many of these harmful species are known to be adapted to rodent hosts.

The severity of the disease will be a factor of the person's general health and age and their extent of exposure to the bacteria as well as the species of leptospira they have contracted. As with many bacterial infections, the old and those in poor health or nutrition will be most susceptible. Leptospirosis in children is less frequently reported but that may at least partly be due to different clinical manifestations in children (meningitis has been mentioned as dominant clinical sign) leading to misdiagnosis. Symptoms in humans can range from benign, to a mild cold or flu, to severe febrile disorder, to kidney and/or liver failure, to internal haemorrhaging and death (5 to 20 %, but fatalities of 50% have been reported). It is likely that many cases go unreported or are confused with other diseases such as malaria. Most people probably recover naturally, but even relatively mild cases may cause loss of income due to debilitation as the recovery period can be prolonged.

For survival outside the host, the bacteria require a warm, moist and neutral, slightly alkaline environment. Under optimal circumstances they can survive up to several months. In a dry, too hot (40°C) or too cold (< 10°C) and /or pH-hostile environment, leptospires are killed quickly (few hours) Typically, they are transmitted through contaminated water. It is feasible that leptospirosis could be transmitted through exposure to contaminated moist soil, a water-borne aerosol or contaminated food. The bacteria enter the body in the ways that most bacteria do, through wounds, cuts, mouth, eye, nose. It is unknown which transmission route is most common, but this is likely to be a factor specific to the situation. Working or washing in potentially contaminated water will be a high risk factor as will collecting household water from poorly covered wells. Poor storage of water at the household level, proximity of open sewage, proximity of livestock and low levels of general household hygiene may all contribute to disease transmission. Leptospirosis infection rates may be seasonal under some circumstances related to rainfall patterns and the amount of standing water available.

Plague

Plague is a bacterial disease caused by the organism Yersinia pestis. Although there is only one bacterial species, it's life cycle and ecology are relatively complex. There is a good deal of information available about plague epidemiology, but it is far from a complete picture due to its complexity. In addition to a number of small mammals (not all rodents) acting as carriers and vectors of plague, there are a number of flea species that vector the disease. Normally an animal becomes infected through flea bites and fleas will be responsible for transmitting the disease among different animals and humans. Some species of flea and rodent are more likely to be involved in vectoring the disease and have become partially adapted to the disease, these animals do not become noticeably ill. Plague persists in the wild, circulating in certain species of fleas and animal without any noticeable effect or human cases occurring (quiescent plague). Outbreaks of plague appear to be related to interactions between different rodent species, particularly among those species that are tolerant and susceptible to plague. Commensal rodents (associated with man) such as Rattus rattus may pick up plague from interactions with wild rodents (or their fleas) and spread the disease in urban areas. Aspects of flea ecology will also be important, and flea species that confine themselves to rodent burrows may be the true 'reservoir' of the disease. Plague is well known to re-emerge in an area after long periods of quiesence. This recently happened in Algeria in 2003, 50 years after the last case was reported in the country. Because of a lack of surveillance in these situations, it is not always well-established whether plague had gone locally extinct and was subsequently reintroduced, or whether plague had been circulating in the local enviornment all this time at very low levels, waiting for favourable outbreak conditions to arise.

Plague is a blood-borne pathogen which manifests itself in two main ways, termed bubonic plague and pneumonic plague (with septicaemic, pharyngeal and meningitic manifestations less commonly reported). Plague has very high mortality rates if left untreated, but fortunately symptoms are usually quite distinct leading to proper identification and treatment. Bubonic plague is relatively more common than pneumonic plague. Bubonic plague results in a series of spots on the skin and the distinct swelling of the lymph glands. The disease progresses relatively slowly over a few days usually allowing treatment to occur in time. If left untreated, severe cases of infection can arise when the bacteria move to the lungs. This results in coughing out a fine aerosol of infected blood that can then be inhaled by others, resulting in the pneumonic form of plague transmission. Pneumonic plague is fortunately rare as its progress is rapid, leading to death in approximately 48 hours unless treatment is sought. Where plague is endemic, most people even in isolated areas will be aware of plague and its symptoms and treatment. What is not known fully is whether, and to what extent, sub-clinical or sub-lethal cases of plague occur and whether it is possible to build immunity to plague. To date there is no evidence that human civilisation is more immune to plague due to previous global pandemics.

Besides the traditional route of disease transmission via flea bites, there will also be other ways in which plague transmission may occur. Plague bacteria can survive for relatively long periods of time in the environment (several days) and disease transmission has been known to occur through handling infected blankets or pelts of infected animals. Handling or eating rodents will increase the risk of exposure, and theoretically infection could occur through the normal entry points for bacteria (cuts or wounds in skin, mouth, eye, nose). There are recorded instances of pharyngeal plague, presumably contracted by eating poorly cooked infected meat (often camel or rodent meat).

Plague outbreaks appear to be strongly seasonal and cyclical. In the African context, plague outbreaks appear to be associated with the late dry season. This is probably related to reduced food resources for rodents, their migration to urban areas in search of food and the subsequent mass mortality of starving rodents shedding their fleas in urban areas. The cyclical nature may be due to changes in the population dynamics of rodents with higher numbers of rodents in some years, but the reasons why some years are worse for plague than others is not really understood.

Toxoplasmosis

Toxoplasmosis is a protozoan disease caused by the organism Toxoplasma gondii. The main host is the domestic cat where the organism is able to sexually reproduce. Cysts are shed in cat faeces which are then consumed by other animals. When other animals eat them, the cysts do not enter a sexually reproductive stage but migrate around the body asexually dividing. These are eventually attacked by the animal's immune system and become encapsulated, meaning they are alive but dormant. The cycle is completed when another cat eats the animal with encapsulated cysts which then wake up and infect the cat where they can sexually reproduce. Humans and other animals can be exposed to the disease by eating infected animals (e.g. rodents, pigs) or food/water that has been contaminated by infected cat faeces. Research has shown that rodents with toxoplasmosis are more easily predated upon by cats (so that the parasite completes its lifecycle). This is not surprising as many parasites affect the behaviour and physiology of their host to promote its survival. As with cysts of trichinosis, adequate cooking should readily kill any toxoplasmosis cysts in meat.

Many people contract toxoplasmosis without realising it. The worst case symptoms are usually similar to a very bad case of the flu. Severe cases would be debilitating for a few weeks, but cases of death are unheard of in normal healthy individuals as long as any associated dysentery is treated. Life-long immunity occurs after a single exposure so although no illness will derive from future exposure episodes, the cyst load may marginally increase with each subsequent exposure. If contracted for the first time during pregnancy, the disease will either cause miscarriage or severe congenital deformity. Besides congenital risk, the main concern with toxoplasmosis is immuno-compromised individuals (e.g. HIV/AIDS) that were previously exposed to toxoplasmosis. These patients can have cerebral re-activation where the dormant cysts in the body come alive again. As the immune system is unable to re-encapsulate the cysts, the person dies as the cysts continue to multiply and migrate around the brain.

The disease is unlikely to persist in an area where there are no cats. It is likely that disease incidence is only minimally affected by climate, but it could be affected by the susceptibility of cysts to desiccation and periods of drought. Hygiene standards and eating rats will contribute to the prevalence rate.