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.
