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.