Book: pathogens Flashcards
What is the type of microorganism causing Trachoma?
Bacteria. Chlamydia trachomatis, a microorganism that has features of both a
bacterium and a virus.
What is the organism causing poliomyelitis (polio)?
Poliovirus (Enterovirus) types 1, 2 and 3 cause poliomyelitis (polio)
What are the clinical features of poliomyelitis (polio)?
The clinical features of poliomyelitis (polio):
Infection commences with fever, general malaise and headache, the majority of cases resolving after these mild symptoms, but approximately 1% proceed to paralytic disease.
The virus has a predilection for nerve cells, especially those with a motor function (the anterior horn cells of the spinal cord and the motor nuclei of the cranial nerves). These cells are destroyed and a flaccid paralysis results.
In general, the paralysis is more common in the lower part of the body, becoming less common the higher up it affects. Unilateral lameness is commoner than bilateral lameness. The severe form of bulbar poliomyelitis is generally fatal in poor countries where respirators and intensive nursing care are not available. The site of paralysis is associated with injections or operations and such procedures should be avoided if there is any suggestion of poliomyelitis.
How is poliomyelitis (polio) diagnosed?
Diagnosis of polio(-myelitis) of the disabled case is made on clinical grounds, differentiating from
the spastic paralysis of birth injury with which it is commonly confused. In polio, there will be a history of normal birth with commencement of walking, followed by a feverish illness and the development of flaccid paralysis. The paralysis is limited to well-demarcated muscle groups and there is no sensory loss.
A similar history may be given for meningitis, but the damage will be central with accompanying mental deficiency. Virus may be recovered from
throat swabs in the early stages of the illness or from rectal swabs or faeces later on. A rise in the antibody level of serological tests is not diagnostic due to the widespread use of polio vaccine.
How is polio/poliomyelitis transmitted?
Transmission is generally via the faecal–oral route, although the virus initially multiplies in the oropharynx so airborne transmission can also occur.
The virus then invades the gastrointestinal tract, where it is excreted for several weeks. A disease of low hygiene, young children (4–5 months) meet the virus with only a small proportion showing overt disease; 80–90% have an inapparent subclinical disease, 5–10% suffer from fever, headache and minor clinical signs,
with 1% only going on to paralysis. Paralysis is more common with older age, so a non-immune person going into an endemic environment is at far greater danger of developing paralytic poliomyelitis. Raising standards of hygiene will also have the same effect because it spares people from meeting the virus as young children and allows a pool of susceptibles to develop. In time, the number of non-immunes will be sufficient for an epidemic to take place. There will also be a higher proportion of paralysed cases (peak age 5–9 years), and many deaths. So, sadly, the raising of living standards will change polio from an endemic disease with a few paralysed cases to an epidemic disease of increased severity.
In epidemic poliomyelitis, where sanitation is good, pharyngeal spread becomes a more important method of transmission.
Poliovirus strains vary in their neurovirulence, with the more virulent strains having a greater tendency to spread. This could be due to a lower infective dose of the virulent virus being required to produce disease.
What is the occurrence and distribution of polio/poliomyelitis?
Poliomyelitis formerly occurred throughout the world, and was endemic in the poorer regions and epidemic in those with good sanitation, but this has changed considerably with the WHO programme of eradicating polio from the world.
The Americas, Europe, South-east Asia and Western Pacific are now free of infection. Only three countries are now endemic with polio – Nigeria, Pakistan and Afghanistan – but outbreaks occur in countries previously free of infection, such as China, resulting from these endemic sources. The fatwa declared by Islamic fundamentalists against vaccination in northern Nigeria has resulted in several outbreaks.
What are the methods of control and prevention of polio/poliomyelitis?
The main method of prevention and control is with polio vaccine. Two types of vaccine are available: the inactivated polio vaccine (IPV) (Salk) and the attenuated live vaccine (Sabin).
The Salk vaccine is given by intramuscular injection, inducing a high level of immunity that is not antagonized by inhibitory factors in the gut, but is expensive to produce because it contains many organisms. The Sabin vaccine is administered orally (oral polio vaccine, OPV), making it easier and cheaper, as well as producing intestinal immunity which can block infection with wild strains of poliovirus.
Multiplication of the OPV virus in the intestine makes it very useful in preventing epidemics and allows it to spread to non-vaccinated persons in conditions of poor hygiene, so protecting them as well.
Unfortunately, the inhibiting action of antibodies in breast milk and colonization of the gut by other enteroviruses can reduce its effectiveness. Increasing the dosage and telling mothers not to breastfeed for at least an hour after administration can help.
Because there are three strains of the poliovirus, the vaccine should be given on three separate occasions, separated by periods of at least 1 month, to ensure that immunity develops to each of the strains. Polio vaccine is conveniently administered at the same time as diphtheria, tetanus and pertussis (DTP). Where there is a high risk of poliovirus and importation or the transmission potential is high, then a first dose should be given soon after birth.
In countries nearing eradication, a monovalent polio vaccine has been found to be more effective than the trivalent one, with type 3 virus predominating in the Indian subcontinent and type 1 in the remaining endemic parts of the world.
A bivalent oral polio vaccine (bOPV) containing just type 1 and 3 viruses gives a higher rate of protection and is easier to administer. Unfortunately, the use of monovalent and bivalent vaccines has led to the development of vaccine-derived poliovirus (VDPV) outbreaks, mainly in Africa and the Indian subcontinent. This has been a particular problem in the immunodeficient, leading to the risk that they will develop paralytic disease, while also being reservoirs for the spread of poliovirus. Where incomplete vaccination programmes are occurring, then the full triple vaccine should be used. VDPV can also develop after prolonged replication of OPV, reacquiring neurovirulence and transmissible characteristics of wild poliovirus (WPV). This is associated with low vaccine coverage and particularly Sabin 2 vaccine. IPV can be used to prevent this happening, but it is less effective at producing intestinal mucosal immunity. Both vaccines can be used together, especially where the incidence of paralytic polio has remained high.
WHO now recommends that one dose of IPV should be added to the routine three-dose OPV schedule, to be given from 14 weeks of age. WHO is in the process of moving to a strategy of bivalent rather than trivalent OPV with the addition of IPV.
Many countries where polio has been eradicated now use IPV in their routine vaccination programmes. A high level of vaccination must be maintained to produce ‘herd immunity’ as there is still a risk of introduced cases from parts of the world where wild virus is still circulating.
Schoolchildren and adults who have received a full course of childhood vaccinations should have booster doses every 10 years. Maintenance of vaccination coverage should continue even in countries now free of infection and is essential for travellers going to parts of the world where polio has not yet been eradicated.
The long-term aim of prevention should be to raise standards of hygiene with the provision of water supplies and sanitation, but this must proceed at the same time as an adequate vaccination programme.
What is the treatment of polio/poliomyelitis?
There is no specific treatment for the acute stage, but rest and the avoidance of physical activity are beneficial. Specific supportive measures can
be given to those with disabilities.
What is the surveillance in place for polio/poliomyelitis?
Surveillance developed for poliomyelitis eradication looks for cases of acute flaccid paralysis (AFP) in children under 15 years of age. These are investigated by stool examination, inquiry and search for other cases in the area. Remedial measures are carried out around the case, vaccinating all contacts.
What are the clinical features of trachoma?
Clinical features. Commencing as a keratoconjunctivitis, the first sign is red eye. There may be irritation and discharge but this is passed off as a self-limiting infection. A follicular infiltration of the conjunctiva then takes place, particularly in the upper lid. Blood vessels grow into the periphery of the eye, forming pannus.
However, it is at the late stages of the disease, when it is non-infectious, that scarring, particularly of the upper eyelid, turns the eyelashes inwards to rub on the eye, a condition called trichiasis.
This constant rubbing of the eyeball, aided by the dryness of the conjunctiva, damages the cornea, leading to scarring and finally blindness. Trachoma is often further complicated by secondary infection.
How is trachoma diagnosed?
Diagnosis of trachoma is usually made on clinical grounds, but can be confirmed by finding the characteristic inclusion bodies in scrapings taken from the conjunctiva.
How is trachoma transmitted?
Trachoma is a disease of poor sanitary conditions where a combination of close contact and dirty conditions encourages transmission.
Within the family unit, transmission is from child to child or by flies (mainly M. sorbens) that are attracted to the discharges around the eyes. Cycles of reinfection and recrudescence continue to damage the eye and lead to blindness at school age. The usual method of wiping away secretions with hands, towels or clothing, which is then used by the adult on other children or themselves, is a typical pattern of transmission.
What is the occurrence and distribution of trachoma?
Trachoma is found mainly in the dry regions of the world, especially Africa, South America and the extensive semidesert regions of Asia. A disease of antiquity, it was first described by the
ancient Egyptians.
In endemic areas, 80–90% of children are infected by the age of 3 years. In conditions of improved sanitation, there is a natural cycle lasting until 11 years of age, with little residual damage. Females develop trachoma and blindness as adults more commonly than males, because they are directly concerned with looking after children. The chance of acquiring infection is increased by large families with short birth intervals, as there are more children of a young age living in close proximity.
Since the introduction of the WHO elimination programme, there has been a reduction of trachoma cases from an estimated 360 million in 1985 to 41 million in 2009. Half the global burden of active trachoma is concentrated in Ethiopia, Guinea, India, Nigeria and South Sudan. It is hoped that active trachoma can be eliminated by 2020.
What is done for the control and prevention of trachoma?
The use of water to wash away secretions, the wearing of clean clothes and keeping the surroundings clean are the most effective methods. Face washing has been shown to reduce the risk of developing trachoma, so regular daily face washing should be encouraged. Long-term preventive measures are to improve sanitation and provide water supplies.
Flies proliferate in rubbish and excrement, reaching their maximum numbers during the dry sunny period of the year. The damp, moist conditions in open pit latrines may be more important in encouraging fly breeding than non use of latrines. Any flushing mechanism or improved latrine will discourage flies.
A strategy for a control programme is as follows:
• Conduct a survey to find the worst affected areas.
• Give mass treatment.
• Conduct health education through schools, stressing regular face washing.
• Provide back-up services.
WHO has launched a programme for the global elimination of trachoma by 2020 and given it the acronym of SAFE. This stands for: Surgery for trichiasis Antibiotics Facial cleanliness Environmental improvement.
What is the treatment for trachoma?
Mass treatment is preferable, as the majority of the population in an infected area will have trachoma. This is given easily in schools, but is better done in the home, where the main transmission takes place. A single dose of azithromycin (20 mg/kg) is better than topical tetracycline, and one dose a year may be sufficient to eliminate the blinding propensity of trachoma. Mothers can be taught to treat all children in the household regularly.
Preventing blindness once scarring and trichiasis have developed is very easily done by a simple operation that a medical assistant can be trained to do. This involves cutting through the scarred conjunctiva of the upper lid and everting it so that the eyelashes no longer rub on the cornea.
What kind of surveillance is done for trachoma?
After the initial survey, follow-up surveys should be conducted at
regular intervals. This is most easily done in primary schools.
What microorganism causes gastroenteritis?
Strains of enterotoxigenic, enteropathogenic and enteroaggregative Escherichia coli, as well as enteric viruses, are the main organisms. Rotavirus and Campylobacter are major causes. Norovirus is a common cause of epidemics.
What are the clinical features of gastroenteritis?
Profuse, watery diarrhoea with occasional vomiting, but despite the fluid nature of the stools, faecal material is always present. There is never the rice-water stool characteristic of cholera.
Water and electrolytes are lost which, in the young child, may be sufficient to cause dehydration and ionic imbalance, leading to death. Normally, a self-limiting condition but in unhygienic surroundings, or where babies’ bottles are used, repeated infections occur, leading to chronic loss of nutrients and subsequent malnutrition. A serious infection in neonates, mortality decreases with age until in adults it is just a passing inconvenience (travellers’ diarrhoea).
How is the diagnosis of gastroenteritis made?
Diagnosis of gastroenteritis is made on clinical criteria unless laboratory facilities sufficient to
identify viral infections are available. Specific DNA probes are likely to be the
most appropriate method of identifying causative organisms in developing
countries if they can be made cheap enough
How is gastroenteritis transmitted?
Epidemics of gastroenteritis occur in families or groups of children sharing similar
surroundings. Infection is often seasonal, the beginning of the rains heralding an
outbreak. This would suggest transmission by water, and simple control
measures such as boiling of water can stop the epidemic. Improperly sterilized
babies’ bottles or their contents are a common method of infecting the neonate.
What is the occurence and distribution of gastroenteritis?
Gastroenteritis is found throughout the world,
especially in developing countries and in conditions of poor hygiene. It is
particularly common where bottle-feeding has been recently introduced, such as
by unscrupulous infant-feed companies. A seasonal distribution suggests
contamination of the water supply.
What are methods of control and prevention of gastroenteritis?
Control and prevention of gastroenteritis are by the following:
• promotion of breastfeeding;
• use of oral rehydration solution (ORS) in the community;
• improvement in water supply and sanitation;
• promoting personal and domestic hygiene;
• vaccination (rotavirus and other vaccines, e.g. measles); and
• fly control (Box 7.1).
Breastfeeding not only provides a sterile milk formula in the correct
proportions (in contrast to the often-contaminated bottle) but also promotes
lactobacilli and contains lactoferrins and lysozymes. Promoting breastfeeding
and the administration of ORS solution in the community are the main control
strategies. Improvement in water supplies and sanitation, with the promotion of
personal hygiene, are long-term measures.
Rotavirus vaccination should now become part of the routine childhood
vaccination programme (see Section 8.2). The oral cholera vaccine WC/rBS has
been shown to be about 60% effective against enterotoxigenic E. coli so might
have some place in control, although its protective effect in infants is
considerably less than in adults. Preventing other childhood infections by
vaccination, especially those associated with gastrointestinal disease, such as
polio and measles, can reduce the severity of gastroenteritis.
What is the treatment of gastroenteritis?
Treatment of gastroenteritis is by the replacement of fluid and electrolytes using oral rehydration solution in the
moderately dehydrated and intravenous replacement in the severely dehydrated. A naturally available rehydration solution is the fluid from a green coconut.
A 7-month-old coconut has been found to be the most suitable. Rice water made
from a handful of rice boiled in a saucepan of water until it dissolves, plus the
appropriate amount of salt for the volume of water, makes a simple rehydration
solution. Carrot water can also be used.
If mothers are taught how to make up these solutions, then they can treat
their own children as soon as they start to get diarrhoea. The mother should use a
cup and a spoon and sit with her child, giving small quantities of fluid at
frequent intervals. Severe dehydration can usually be prevented by primary care
from the mother.
There is no need to use an antibiotic or an antispasmodic, both of which are
contraindicated. Lactobacilli, which inhibit E. coli, colonize the gut in the
breastfed infant. Live yoghurt (curd) contains lactobacilli and can be quite
effective, especially in adults, in reducing the severity and duration of diarrhoea.
What is done in terms of surveillance for gastroenteritis?
In countries with a seasonal rainfall pattern, gastroenteritis
outbreaks often start with the beginning of the rains, so monitoring the weather
can provide early warning of an impending outbreak.