Part Xill Health and Diseases



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.

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|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 and |

| | | |intravascular 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).

A. 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 |Droplets can be expelled out from the respiratory tract during |

|microorganisms cannot travel through the air on their own, but are carried on |sneezing |

|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 respiratory tract or | |

|settle onto wou___ of hosts. | |

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 the respiratory t , causing various respiratory diseases, 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 cause gastrointestinal 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 transmit an 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 agents through 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 agents |

|Reservoir |Transmission Vehicle |Infectious Agent |

|Blood |Blood, needle stick, other contaminated |Hepatitis肝炎B and C; HIV/AIDS, S. aureus金黃色葡萄球菌, |

| |equipment | |

|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 |

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 |nose and throat. |air-borne: inhaling droplets / |hand-washing |

| |viruses |sneezing, scratchy throat, |particles of respiratory secretions |Wear a mask |

| | |coughing and headache. |transferring infectious secretions to |virus-killing disinfectant |

| | | |eyes or nose; |vaccination is impractical |

|Influenza (flu) |Influenza viruses:|upper respiratory tract, and |air-_______ resp________ drop____ of |Avoid close contact with the sick |

| |type A and type B.|rarely also the lungs. More |coughs and sneezes. |Stay at home when you are sick; |

| |Type A virus tends|severe than common cold , with |touches respiratory droplets on an |Cover the mouth and nose when coughing or |

| |to cause more |additional symptoms of rapidly |object and then touches his m_____ or |sneezing; |

| |severe illness |rising fever, chills, and body |n____ |Wash the hands often |

| |than type B |and muscle aches, complications | |Avoid touching the eyes, nose or mouth |

| | |can lead to death | |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. |

Q. 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?

|Avian influenza|influenza virus |patients developed symptoms of |does not seem to spread from person |See notes later |

|(Bird flu) |H5NI. |fever, sore throat, cough and, in |to-person. | |

| | |some fatal cases, severe |All the causes seem to have been acquired| |

| | |respiratory distress secondary to |from close contact with infected birds. | |

| | |viral pneumonia. | | |

|Cholera |bacterium Vibrio |cholera toxin that causes increased|W____-borne disease. |Improve san________ |

| |cholerae. |secretion of w____ and chloride |conditions of poor san________, |Provide clean w_____ sources |

| | |ions in the intestine to induce |The infection is acquired by ingesting |Foods and water must be well-c_____ |

| | |massive dia_______. Without |cont_________ food or water. |before eaten |

| | |treatment, death can occur within | | |

| | |hours due to deh_________. | | |

|Malaria |Protoctist |invades the liver cells first and |malarial parasite enters the human host |Drugs and insecticides seem |

| |Plasmodium |then the red blood cells. |when an infected mosquito bite a human. |ineffective because of r_________ of |

| | | |only transmitted by female Anopheles |Plasmodium and mosquitoes. |

| | | |mosquitoes |destruction of the b_______ grounds |

| | | | |of mosquito |

| | | | |Personal p________ against mosquito |

| | | | |b___ |

|Hepatitis B |hepatitis B virus |L____. 10% of those infected |Hepatitis B virus can be found in the |V____________ |

| |(HBV). |develop a chronic infection. They |b_____, saliva, semen and other body |Ster__________ of the donated blood |

| | |are car____ and can spread the |fluids of an infected person. |Con_____ to reduce the transmission |

| | |disease. |It is spread by direct contact with |through sex. |

| | | |infected b____ f_____. |Avoid s________ personal-care items, |

| | |chronic hepatitis B could lead to | |such as razors and toothbrushes |

| | |cirr_____and liver c______. | | |

|Hepatitis A |hepatitis A virus |Although some patients become |HAV is found in f______ of infected | |

| |(HAV) |acutely and desperately sick from |persons. | |

| | |this infection, most people |C___________ foods or water | |

| | |tolerate it well and fully recover.|from an infected mother to her baby | |

| | |No chronic infection occurs with |during birth. | |

| | |this virus. | | |

|STDs |bacteria and |infect the skin and mu___ linings |S______ contact |no vac____ to prevent sexually |

|syphilis, |viruses. |of the vag___, rect__, uret___, |sharing of n_______. |transmitted diseases. |

|gonorrhoea, | |cer___, eyes, mouth and throat. |from an infected mother to her baby |Some STDS, like gonorrhoea, can be |

|genital herpes | | |during p________ or nur____. |cured. However, many of them, like |

|and genital | | | |genital herpes and syphilis, can only|

|warts. | | | |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’?

Q. 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.

Q. 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 a new 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 constant monitoring of the global influenza situation and annual adjustments in the composition of influenza vaccines. Both activities have been a cornerstone of the WHO 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, can swap or “reassort” genetic materials and merge. This reassortment process known as antigenic “shift”,results in a novel subtype different 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.

Q. What are the two genetic characteristics of the avian flu viruses that cause great health concern?

Q. 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 with both 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”.

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Q. Distinguish between antigenic ‘Drift’ and antigenic ‘Shift’. What are their genetic bases and what are the consequences?

Q. Why it is recommended that pig farms should be separated from poultry farms?

Q.

Q.

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 in Hong 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 – of Hong 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 a high pathogenicity and 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”.

Q. Why there is such a great concern over the human body possibly becoming a ‘mixing vessel’?

Influenza pandemics: can they be averted?

Based on historical patterns, influenza pandemics can be expected to occur, on average, three to four times each century when new virus subtypes emerge and are readily transmitted from person to person. However, the occurrence of influenza pandemics is unpredictable. In the 20th century, the great influenza pandemic of 1918–1919, which caused an estimated 40 to 50 million deaths worldwide, was followed by pandemics in 1957–1958 and 1968–1969.

Experts agree that another influenza pandemic is inevitable and possibly imminent. Most influenza experts also agree that the prompt culling of Hong Kong’s entire poultry population in 1997 probably averted a pandemic.

Q. Compare and contrast ‘ endemic, epidemic’ and ‘pandemic’ with respect to contagious diseases.

Q. What measures should be taken to avoid the birds’ flu to become a pandemic?

Clinical course and treatment of human cases of H5N1 avian influenza

Published information about the clinical course of human infection with H5N1 avian influenza is limited to studies of cases in the 1997 Hong Kong outbreak. In that outbreak, patients developed symptoms of fever, sore throat, cough and, in several of the fatal cases, severe respiratory distress secondary to viral pneumonia.

Tests for diagnosing all influenza strains of animals and humans are rapid and reliable. Many laboratories in the WHO global influenza network have the necessary high-security facilities and reagents for performing these tests as well as considerable experience. Rapid bedside tests for the diagnosis of human influenza are also available, but do not have the precision of the more extensive laboratory testing that is currently needed to fully understand the most recent cases and determine whether human infection is spreading, either directly from birds or from person to person.

Antiviral drugs, some of which can be used for both treatment and prevention, are clinically effective against influenza A virus strains in otherwise healthy adults and children, but have some limitations. Some of these drugs are also expensive and supplies are limited. Experience in the production of influenza vaccines is also considerable, particularly as vaccine composition changes each year to match changes in circulating virus due to antigenic drift. However, at least four months would be needed to produce a new vaccine, in significant quantities, capable of conferring protection against a new virus subtype.

Q. What are the good news and bad news concerning our efforts to prevent the avian flu to becoming a pandemic?

Reading 2. Views invited on ways to minimise human contact with live poultry (HKSAR release)

The Government has invited the public to give their views on strategic approaches that would help achieve the long-term vision of sustaining "zero infection and transmission" of avian influenza through minimising human contact with live poultry.

Launching the consultation exercise at a press conference today (April 2), the Secretary for Health, Welfare and Food, Dr Yeoh Eng-kiong, said: "Public health has always been a primary concern of the Government, which is committed to protecting people from infection by avian influenza."

The World Health Organisation noted the recent outbreaks of avian influenza in poultry across Asia were unprecedented and the human infections in Vietnam and Thailand had aroused great concern over the risk of avian influenza viruses jumping across species to infect humans.

"Chances are high that avian influenza would become endemic in poultry in the region. Hong Kong will be susceptible to avian influenza outbreaks again in the future unless additional preventive measure are adopted," Dr Yeoh said.

He pointed out that the vaccines currently given to chickens did not confer protection from all kinds of avian influenza virus nor could it eliminate the possibility of virus reassortment and mutation. "There remains a risk that humans might be infected by avian influenza," he said, adding the retail outlets that sell live poultry posed the greatest risk.

To minimise such risk, Dr Yeoh outlined the following two major strategic approaches to achieve the policy objective:

Approach A: All live poultry will be slaughtered and undergo a chilling process in a centralized slaughterhouse. Retail outlets would only be allowed to sell chilled poultry at a temperature between zero and four degrees Celsius ("cold chain approach").

Approach B: All live poultry will be slaughtered at regional slaughtering hubs. Consumers would have a choice of purchasing either chilled chickens or freshly slaughtered chickens that would not have to undergo a chilling process. (freshly slaughtered chicken approach)

In the long term, the Government intends to remove existing poultry farms from populated areas and locate them in more remote areas to further keep live poultry from humans and facilitate management.

Q. What are the advantages of the "cold chain approach"?

Approach (A) could reduce close contact between the general public and live poultry would be reduced to a greater extent.

The capital cost of building a central slaughterhouse would be lower than building several regional slaughterhouses as required under the "freshly slaughtered chicken approach"

In the interim, the Government would put in place the following immediate and medium-term improvement measures to reduce the potential threat of avian influence to humans:

|locality |measures |reasons |

|At the |reducing the number of live chickens in each poultry stall in the retail markets; | |

|retail |reducing the number of live poultry stalls in retail markets through a voluntary buy-out package and | |

|outlets |redesigning the stall layout to provide for segregation between customers and live poultry; | |

| |increasing the number of rest days per month; | |

| |reviewing the transportation system for the conveyance of live poultry. | |

| |planning to separate the wholesale markets for local and Mainland chickens; | |

| |live quails are separated from live chicken from farm to retail outlets | |

|At the |further strengthening the biosecurity of local poultry farms to the highest standard and refusing to | |

|poultry |renew the licences of those who fail to meet the new standards. e.g. bird-proof facility | |

|farm |A vaccination programme for all chicken in the local farm; | |

|poultry |Imported chicken must come from registered farms with health certificate | |

|import |agreement with mainland that all export chicken to HKare vaccinated | |

| |inspection of imported poultry for infection | |

Acknowledging that either long-term approach would have an impact on the live poultry trade, the HKSAR would formulate a financial package to offer some form of financial assistance to traders affected.

He urged members of the public to give their views and comments on the strategic approaches. Copies of the consultation document and leaflet "Prevention of Avian Influenza: Long Term Direction to Minimise the Risk of Human Infection" can be downloaded from the Health, Welfare and Food Bureau's website : .hk.

Hong Kong has experienced four highly pathogenic H5N1 avian influenza outbreaks in poultry since 1997. Human infection of H5N1 was detected in the 1997 outbreak in which 18 people developed the disease, and six subsequently died.

On avian influenza, the Government spent a total of $246 million for compensation and ex-gratia allowances to the live poultry traders, waived rental charges amounting to $27 million and incurred operational expenses of $45 million to control previous outbreaks since 1997. In 2002-03, the recurrent expenditure on the preventive and surveillance programme was about $39 million. Ends/Friday, April 2, 2004

|[pic] |[pic] |

| |A Chinese boy waits for veterinarians to vaccinate his chicken |

| |near a bird flu-affected duck farm in the Guangxi region. (File |

| |photo) (Reuters/China Photo) |

| | |

| | |

| | |

Q. What make it difficult for some Asian countries to implement effective measures to reduce the risks of avian flu?

End

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◆ Elderly and people considered as "high risk" for influenza-related complications should be vaccinated.

The effectiveness of influenza vaccine depends on 1) the immunocompetence of the vaccine recipient and 2) the degree of similarity between the virus in the influenza vaccine and that in the body.

Vaccines made available for influenza peak seasons of each year are targeted at the types of flu viruses that are prevalent.. Monitoring of outbreak of influenza is made around the world all the year round to find the prevalent influenza viruses that are likely to cause a pandemic each year.

The high density of poultry in confined quarters provides an excellent opportunity for the virus to spread quickly. Some strains of HPAI have the ability to cause 90% mortality in domestic farmed fowl. Infected fowl can become virus pumps producing and shedding large quantities of infectious virus that contaminate the local environment.

Genetic drifts are small changes in the virus genetic material, known as "genetic drift", may result in the appearance of a virus strain that is highly pathogenic to poultry.

Genetic shift : Increased virulence may occur when genes from two different influenza strains reassort during co-infection in a single host, expanding the range of animals the virus can infect. A classic example of a viral mixing vessel for this gene reassortment is swine. If a swine influenza virus and an avian influenza virus simultaneously infect a pig, the two different influenza viruses can swap genetic material as they both replicate in the pig host. This is an example of genetic shift. If the avian influenza virus acquires genes required for mammalian infection and transmission, the new avian influenza virus may gain the unique ability to spread easily between mammals, possibly including humans.

There are situations that provide an ideal opportunity for individuals to become simultaneously infected with multiple influenza virus strains.

Human contact with live bird markets that house numerous bird species. Co-located poultry (backyard flocks) and swine, and high density poultry operations are ideal for potential genetic shift, allowing the virus to jump species barrier.

/349F_”—ÄÆÇ< = > ? @ if human are concurrently infected with human and avian influenza strains, could serve as the “mixing vessel”.for the emergence of a novel subtype with sufficient human genes to be easily transmitted from person to person. Such an event would mark the start of an influenza pandemic.

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