AL Bio Notes_Health and Diseases (II) Communicable diseases P..1
Health and Diseases (II)
Communicable diseases
The Ch____ of Infection is a basic component of understanding the prevention and control of infection. It is a critical concept in infection control.
pathogen / / / where an pathogen normally lives and multiplies, on which it depends primarily for survival, and where it reproduces in such numbers that it can be transmitted to a susceptible host.reservoir / / / the way the causative agent gets out of the reservoir. In a person, this is often by a body fluid.
means of exit / / / a person or animal lacking effective resistance to a particular infectious agent.
mode of transmission / / / the micro-organism that causes infection such as bacteria, viruses, fungi and parasites. There must be an adequate number of pathogens to cause disease.
means of entry / / / how the microorganism enters the host. Common means / portals of entry include the mouth, nose, eyes, rashes, cuts, needlestick injuries, surgical wounds andintravascular sites.
susceptible host / / / how the pathogen moves from place to place
The occurrence and presence of all these factors and events is considered the "chain of infection". Effective infection control strategies prevent disease transmission by inter______one or more l____ in the chain of infection (CDC, 2003).
- Modes of Transmission
Invading pat______ usually gain access to the patient by different routes including a___, veh_____ (water or food), vec___ and b____ fluids.
a.Transmission of Pathogens through Air
The most common route by which infection enters the body is through a__. Most microorganisms cannot travel through the air on their own, but are carried on air-borne particles, for example d____ and d______.Dust is the major source of microorganisms in the air. Dust may remain airb____ for hours and microorganisms carried may be inh____ into the respiratorytract or settle onto wou___ of hosts. / Droplets can be expelled out from the respiratory tract during sneezing
Droplets are expelled from the r______tract by coughing, sneezing and talking. The droplets are composed mainly of sal___ and a small number of path_____organisms from the respiratory tract. The larger droplets fall onto the ground within a few seconds and are not inhaled. Small droplets evaporate rapidly and can remain air-b______for hours, and are inh___.
Air-borne pathogens attack mainly therespiratory t , causing various respiratorydiseases, e.g. pneu_____, diphtheria, meningitis, mea___, mumps, rubella, common c___ and inf______.
b.Transmission of Pathogens through Vehicle
Vehicle refers to a non-living object or material, which becomes contam______with inf______agents (e.g. virus). Examples of vehicles that can transmit diseases include water and f____, c_____ utensils, sur____ or med____ instruments.
Some bacteria(chol____), viruses (hep_____ A)and protozoa (Amoebic dysentery)use f____ or w_____ as a vehicle. They may causegastrointestinal infection. In places where proper san______facilities are lacking, w____-borne diseases can spread rapidly.
c.Transmission of Pathogens through Vector
Vectors(carrier)refer to any living organisms that transmitan infectious agent to another living organisms. A vector is usually an arth______such as a tick or mosq____. The pathogenic agents are virus, bacteria, protozoa or even a worm.
Plasmodium, a protozoan that causes m______, is transmitted from the blood of a patient to other people through the sting of the m______vector Anopheles. Dengue virus that causes d_____ fever is spread by the m______Aedes. Xenopsylla, a rat fl__, transmits a bacterium Pasteurella, causing bubonic p______ (black plague).
d.Transmission of Pathogens through Body Fluid
The pathogens can be transmitted through the d_____ transfer of infectious agentsthrough body fluids. The development of diseases depends on the qua_____ of pathogens that have gained entry into the victim, usually through sex____ contact, blood tran______or sharing of n______.
Human reservoirs and transmission of infectious agentsReservoir / Transmission Vehicle / Infectious Agent
Blood / Blood, needle stick, other contaminated equipment / Hepatitis肝炎Band C; HIV/AIDS, S. aureus金黃色葡萄球菌,
Tissue / Drainage from a wound or incision / S. aureus, E. coli大腸桿菌
.
Respiratory tract / Droplets from sneezing or coughing / Influenza viruses, Strep spp.鏈球菌, S. aureus
Gastrointestinal tract / Vomitus, feces, bile, saliva / Hepatitis A, Shigella spp痢疾, Salmonella spp沙門氏菌
Urinary tract / Urine / E. coli, enterococci腸球菌
Reproductive tract and genitalia / Urine and semen / N. gonorrhoeae淋病, T. pallidum梅毒螺旋菌, Herpes simplex virus type 2疱疹, Hepatitis B
- Some common communicable diseases and their transmission routes
Diseases / Causative agent / Areas affected & symptoms / Mode of transmission / Preventive measures
and reasons
Common Cold / > 200 types of viruses / nose and throat.
sneezing, scratchy throat,coughing and headache. / air-borne: inhaling droplets / particles of respiratory secretions
transferring infectious secretions to eyes or nose; / hand-washing
Wear a mask
virus-killing disinfectant
vaccination is impractical
Influenza (flu) / Influenza viruses:
type A and type B. Type A virus tends to cause more severe illness than type B / upper respiratory tract, and rarely also the lungs. More severe than common cold , with additional symptoms of rapidly rising fever, chills, and body and muscle aches,complications can lead to death / air-______resp______drop____ of coughs and sneezes.
touches respiratory droplets on an object and then touches his m_____ or n____ / Avoid close contact with the sick
Stay at home when you are sick;
Cover the mouth and nose when coughing or sneezing;
Wash the hands often
Avoid touching the eyes, nose or mouth with hands.
Avoid close contact with the sick
Stay at home when you are sick;
Cover the mouth and nose when coughing or sneezing;
Wash the hands often
Avoid touching the eyes, nose or mouth with hands.
- One could be vaccinated to reduce the chance of Influenza infection. Who should be vaccinated and what factors determine the effectiveness of the vaccine? Why should the vaccines be different from years to years?
All the causes seem to have been acquired from close contact with infected birds. / See notes later
Cholera / bacteriumVibrio cholerae. / cholera toxin that causes increased secretion of w____ and chloride ions in the intestine to induce massive dia______. Without treatment, death can occur within hours due to deh______. / W____-borne disease.
conditions of poor san______,
The infection is acquired by ingesting cont______food or water. / Improve san______
Provide cleanw_____ sources
Foods and water must be well-c_____ before eaten
Malaria / Protoctist
Plasmodium / invades the liver cells first and then the redblood cells. / malarial parasite enters the human host when an infected mosquito bite a human.
only transmitted byfemale Anopheles mosquitoes / Drugs and insecticides seem ineffective because of r______of Plasmodium and mosquitoes.
destruction of the b______grounds of mosquito
Personalp______against mosquito b___
Hepatitis B / hepatitis B virus (HBV). / L____. 10% of those infected develop a chronicinfection. They are car____ and can spread the disease.
chronic hepatitis B could lead to cirr_____and liver c______. / Hepatitis B virus can be found in the b_____, saliva, semen and other body fluids of an infected person.
It is spread by direct contact with infected b____ f_____. / V______
Ster______ of the donated blood
Con_____ to reduce the transmission through sex.
Avoid s______personal-care items, such as razors and toothbrushes
Hepatitis A / hepatitis A virus (HAV) / Although some patients become acutely and desperately sick from this infection, most people tolerate it well and fully recover. No chronic infection occurs with this virus. / HAV is found in f______of infected persons.
C______foods or water
from an infected mother to her baby during birth.
STDs
syphilis, gonorrhoea, genital herpes and genital warts. / bacteria and viruses. / infect the skin and mu___linings of the vag___, rect__, uret___, cer___, eyes, mouth and throat. / S______ contact
sharing of n______.
from an infected mother to her baby during p______or nur____. / no vac____ to prevent sexually transmitted diseases.
Some STDS, like gonorrhoea, can be cured. However, many of them, like genital herpes and syphilis, can only be controlled.
Reference readings on Avian Flu
Reading 1.The disease in birds: Impact and control measures (WHO)
Avian influenza is an infectious disease of birds caused by type A strains of the in______virus. The disease, which was first identified in Italy more than 100 years ago, occurs world___.
All birds are thought to be susceptible to infection with avian influenza. Infection causes a wide spectrum of symptoms in birds, ranging from mild illness to a highly contagious and rapidly fatal disease resulting in severe epidemics.
Fifteen subtypes of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses potentially circulating in bird populations. To date, all outbreaks of the highly pathogenic form have been caused by influenza A viruses of subtypes H5 & H7. Migratory waterfowl–notably wild ducks – are the natural r______ of avian flu viruses, and these birds are also the most resistant to infection. Domestic poultry, including chickens, are particularly susceptible to epid_____ of rapidly fatal influenza.
Direct or indirect contact of domestic flocks with wild mig water has been implicated as a frequent cause of epidemics. L___ bird markets have also been important in the spread of epidemics.
Q.Why do you think ‘domestic poultry are particularly susceptible to epidemics of rapidly fatal influenza’?
- Suggest a precautionary measure to prevent avian flu in domestic poultry.
Recent research has shown that viruses of low pathogenicity can, after circulation for sometimes short periods in a poultry population,mutate into highly p______viruses. During a 1983–1984 epidemic in the US, the H5N2 virus initially caused low mortality, but within six months became highly pathogenic, with a mortality approaching 90%. Control of the outbreak required destruction of more than 17 million birds at a cost of nearly US$ 65 million. During a 1999–2001 epidemic in Italy, the H7N1 virus, initially of low pathogenicity, mutated within 9 months to a highly pathogenic form. More than 13 million birds died or were destroyed.
The quarantining of infected f and destruction of inf or potentially exp flocks are standard control measures aimed at preventing spread to other farms and eventual establishment of the virus in a country’s poultry population. Apart from being highlycontagious, avian influenza viruses are readily transmitted from farm to farm by mechanical means, such as by contaminated equipment, vehicles, feed, cages, or clothing. Highly pathogenic viruses can survive for long periods in the environment, especially when temperatures are low.Stringentsanitary measures on farms can, however, confer some degree of protection. In the absence of prompt control measures backed by good surveillance, epidemics can last for years. For example, an epidemic of H5N2 avian influenza, which began in Mexico in 1992, started with low pathogenicity, evolved to the highly fatal form, and was not controlled until 1995.
- What are the necessary measures to contain the virus, and why these measures are sometimes difficult to be implemented?
A constantly mutating virus: two consequences
All type A influenza viruses, including those that regularly cause seasonal epidemics of influenza in humans, are genetically labile /unstable and well adapted to elude host defenses. Influenza viruses lack mechanisms for the “proofreading” and repair of errors that occur during replication. As a result of these uncorrected errors, the genetic composition of the viruses changes as they replicate in humans and animals, and the existing strain is replaced with anew antigenic variant with increased virulence. These constant, permanent and usually small changes in the antigenic composition of influenza A viruses are known as antigenic “drift”.
Q.Virulence is the ability of an organism, in this case avian influenza virus, to cause disease. What changes would cause the increase in virulence of a virus?
Q.How would the crowded conditions in Poultry farms aggravate the problem of genetic drift?
The tendency of influenza viruses to undergo frequent and permanent antigenic changes necessitates constantmonitoringof theglobal influenza situation andannualadjustments in the composition of influenza vaccines. Both activities have been a cornerstone of theWHO Global Influenza Programme since its inception in 1947.
Influenza viruses have a second characteristic of great public health concern: influenza A viruses, including subtypes from different species, canswap or “reassort” genetic materials and merge. This reassortment process known as antigenic “shift”,results in anovel subtypedifferent from both parent viruses. As populations will have no immunity to the new subtype, and as no existing vaccines can confer protection.
This reassortment process has historically resulted in highly lethal pandemics. For this to happen, the novel subtype needs to have genes from human influenza viruses that make it readily transmissible from person to person for a sustainable period.
- What are the two genetic characteristics of the avian flu viruses that cause great health concern?
- What is the necessary condition for the avian flu to be able to become transmissible from ‘human to human”?
Conditions favourable for the emergence of the novel virulent birds’ flu have long been thought to involve humans living in close proximity to domestic poultry and pigs.Because pigs are susceptible to infection withboth avian and mammalian viruses, including human strains, they can serve as a “mixing vessel” for the scrambling of genetic material from human and avian viruses, resulting in the emergence of a novel subtype. Recent events, however, have identified a second possible mechanism. Evidence is mounting that, for at least some of the 15 avian influenza virus subtypes circulating in bird populations, humans themselves can serve as the “mixing vessel”.
- Distinguish between antigenic ‘Drift’ and antigenic ‘Shift’. What are their genetic bases and what are the consequences?
- Why it is recommended that pig farms should be separated from poultry farms?
- The Human body can become a ‘mixing vessel’ for reassortment of different strain of influenza viruses. What are the conditions favourable to such an event?
Evidence suggests that domestic quailcan also become mixingvessels for avian influenza viruses because they have the potential to become infected by multiple avian influenza subtypes. One report proposes that the simultaneous infection of domestic quail with influenza subtypes from geese, ducks, and quail provided the opportunity for genetic shift in the quail virus resulting in increased virulence in new hosts, including poultry and humans (Webby and Webster, 2001). This is believed to have been the source for avian influenza viruses that infected humans inHong Kong in 1997 and 1999.
There is concern that humans could assume a role similar to swine and domestic quail by becoming the "vessel" for mixing avian and human influenza genes. This would provide the opportunity for a genetic shift in the avian influenza virus to a virus that is highly virulent and transmissible to and among humans, possibly sparking the next influenza pandemic.
- Why is it necessary for persons at high risk of exposure to infected poultry to be vaccinated against currently circulating human influenza strains?
Human infection with avian influenza viruses: a timeline
Avian influenza viruses do not normally infect species other than birds and pigs. The first documented infection of humans with an avian influenza virus occurred inHong Kong in 1997, when the H5N1 strain caused severe respiratory disease in 18 humans, of whom 6 died. The infection of humans coincided with an epidemic of highly pathogenic avian influenza, caused by the same strain, in Hong Kong’s poultry population.
Extensive investigation of that outbreak determined that close contact with live infected poultry was the source of human infection. Studies at the genetic level further determined that the virus had jumped directly from birds to humans. Limited transmission to health care workers occurred, but did not cause severe disease.
Rapid destruction – within three days – ofHong Kong’s entire poultry population, estimated at around 1.5 million birds, reduced opportunities for further direct transmission to humans, and may have averted a pandemic.
That event alarmed public health authorities, as it marked the first time that an avian influenza virus was transmitted directly to humans and caused severe illness with high mortality. Alarm mounted again in February 2003, when an outbreak of H5N1 avian influenza in Hong Kong caused 2 cases and 1 death in members of a family who had recently travelled to southern China. Another child in the family died during that visit, but the cause of death is not known.
Two other avian influenza viruses have recently caused illness in humans. An outbreak of highly pathogenic H7N7 avian influenza, which began in the Netherlands in February 2003, caused the death of one veterinarian two months later, and mild illness in 83 other humans. Mild cases of avian influenza H9N2 in children occurred in Hong Kong in 1999 (two cases) and in mid-December 2003 (one case). H9N2 is not highly pathogenic in birds.
Why H5N1 is of particular concern
Of the 15 avian influenza virus subtypes, H5N1 is of particular concern for several reasons. H5N1 mutates rapidly and has a documented propensity to acquire genes from viruses infecting other animal species.Its ability to cause severe disease in humans has now been documented on two occasions. In addition, laboratory studies have demonstrated that isolates from this virus have ahigh pathogenicityand can cause severe disease in humans. Birds that survive infection excrete virus for at least 10 days, orally and in faeces,thus facilitating further spread at live poultry markets and by migratory birds.
Q.What features of H5N1 make it a particular concern?
The epidemic of highly pathogenic avian influenza caused by H5N1, which began in mid-December 2003 in the Republic of Korea and is now being seen in other Asian countries, is therefore of particular public health concern. H5N1 variants demonstrated a capacity to directly infect humans in 1997, and have done so again in Viet Nam in January 2004. The spread of infection in birds increases the opportunities for direct infection of humans.If more humans become infected over time, the likelihood also increases that humans, if concurrently infected with human and avian influenza strains, could serve as the “mixing vessel”.