prevention and control of microbial diseases

PREVENTION AND CONTROL OF MICROBIAL DISEASES

DR. AMADIN A. OLOTU

LECTURER/CONSULTANT CLINICAL MICROBIOLOGIST

BOWEN UNIVERSITY/BOWEN UNIVERSITY TEACHING HOSPITAL OGBOMOSO

Outline

• Introduction

• Definitions

• Chain of Infection

• Elements of the Chain of Infection

• Prevention and Control of Microbial Diseases

Introduction

• Microbial diseases are a major cause of morbidity (illness) and mortality (deaths) worldwide

• They are responsible for a significant number of mortalities across low, lower-middle, upper-middle and high income countries.

• In 2019, lower respiratory tract infections accounted for the fourth leading course of death globally according to WHO.

• In low and lower-middle countries microbial diseases account for an even greater percentage of morbidity and mortality.

• The prevention and control of microbial diseases is a continuous, arduous task that faces health workers, public health officials and governments across the world.

Definitions

• Microbial is an adjective from the noun microbe.

• A microbes is an organism so small that it can only be seen with the aid of a microscope

• Microbe is a synonym for microorganism.

• A disease is any harmful deviation from the normal structural or functional state of an organism, generally associated with certain signs and symptoms and differing in nature from physical injury. A diseased organism commonly exhibits signs or symptoms indicative of its abnormal state.

Definitions

• Microbial diseases obviously denotes diseases caused by microbes, however not all microbes cause disease.

• To cause disease a microbe must be able to invade a host and cause an infection. Such microbes are called pathogens.

• Invasion - The process whereby bacteria, animal parasites, fungi, and viruses enter host cells or tissues and spread in the body.

Definitions

• Pathogen – An agent, a microorganism or acellular entity, capable of causing disease.

• Nonpathogen - A microorganism or acellular entity that does not cause disease; may be part of the normal flora.

• Opportunistic pathogen - An agent capable of causing disease only when the host’s resistance is impaired (for example in immunosuppression) may be part of the normal flora.

Definitions

• Infection - Invasion of the body by agents, organisms or acellular entities that have the potential to cause disease and the multiplication of such agents within the body usually accompanied by a host reaction. Multiplication of the bacteria that are part of the normal flora of the gastrointestinal tract, skin, etc, is generally not considered an infection; on the other hand, multiplication of pathogenic bacteria (eg, Salmonella species)—even if the person is asymptomatic—is deemed an infection.

• Asymptomatic infection – Infection without signs and symptoms

• Symptomatic infection – Infection with signs and symptoms

Definitions

• Infectious agents are organisms or acellular entities capable of producing inapparent infection or clinically manifest disease in hosts and include bacteria, fungi, parasites, viruses, and prions.

• An infectious disease is an infection that results in clinically manifest disease. They are also referred to as communicable diseases because they can spread from one person to another

• Infectious diseases may also be due to the toxic product of an infectious agent, such as the toxin produced by Bacillus cereus that causes food poisoning or the toxin produced by Clostridium botulinum that causes botulism also as food poisoning.

• From all the previous definitions it is apparent that microbial diseases are synonymous with infectious diseases and are used interchangeably.

• Also evident is the fact that the microbes we are interested in, are those that are infectious agents or pathogens

Chain of Infection

• Infectious diseases result from the interaction of a pathogen, its modes or routes of transmission and a host.

• Infection occurs when the pathogen or infectious agent leaves its reservoir through a portal of exit, is conveyed by some mode or route of transmission, and enters through an appropriate portal of entry to infect a susceptible host.

• This sequence is called the chain of infection.

The chain of

infection has 3

main parts or

elements. An

infectious agent

or a pathogen

such as an

amoeba or a virus

from a reservoir

such as a human.

The mode of

transmission

which can include

direct contact,

droplets, a vector

such as a

mosquito, a

vehicle such as

food, or the

airborne route.

The susceptible

host which has

multiple portals

of entry such as

the mouth or a syringeSource: Centers for Disease Control and Prevention. Principles of epidemiology, 2nd ed. Atlanta: U.S. Department of Health and Human Services;1992.

• The pathogen or infectious agent – a very important part of the chain of infection

• When a pathogen causes an infectious disease, the resulting illness may be asymptomatic or mild, but is sometimes very severe. This variation may be due to host factors or to virulence factors possessed by the organism.

• People get infections when microorganisms overpower our host defenses, that is, when the balance between the organism and the host shifts in favor of the organism. The organism or its products are then present in sufficient amount to induce various symptoms, such as fever and inflammation, which we interpret as those of an infectious disease.

Elements of the Chain of Infection

• From the pathogen's perspective, the two critical determinants in overpowering the host are the number of organisms to which the host, or person, is exposed and the virulence of these organisms. Clearly, the greater the number of organisms, the greater is the likelihood of infection. It is important to realize, however, that a small number of highly virulent organisms can cause disease just as a large number of less virulent organisms can.

• The virulence of an organism is determined by its ability to produce various virulence factors



• Reservoir - The reservoir of an infectious agent is the habitat in which the agent normally lives, grows, and multiplies.

• Reservoirs include humans, animals, and the environment.

• The reservoir may or may not be the source from which an agent is transferred to a host. For example, the reservoir of Clostridium botulinum is soil, but the source of most botulism infections is improperly canned food containing C. botulinum spores.

• Many common infectious diseases have human reservoirs. Diseases that are transmitted from person to person without intermediaries include the sexually transmitted diseases, measles, mumps, streptococcal infection, and many respiratory pathogens


• Human reservoirs may or may not show the effects of illness.

• Animal reservoirs. Humans are also subject to diseases that have animal reservoirs. Many of these diseases are transmitted from animal to animal, with humans as incidental hosts. The term zoonosis refers to an infectious disease that is transmissible under natural conditions from vertebrate animals to humans

• Environmental reservoirs. Plants, soil, and water in the environment are also reservoirs for some infectious agents. Many fungal agents, such as those that cause histoplasmosis, live and multiply in the soil. Outbreaks of Legionnaires disease are often traced to water supplies in cooling towers and evaporative condensers, reservoirs for the causative organism Legionella pneumophila


• Portal of exit is the path by which a pathogen leaves its host. The portal of exit usually corresponds to the site where the pathogen is localized. For example, influenza viruses and Mycobacterium tuberculosis exit the respiratory tract, schistosomes through urine, cholera vibrios in feces, Sarcoptes scabiei in scabies skin lesions, andenterovirus 70, a cause of hemorrhagic conjunctivitis, in conjunctival secretions

• Some bloodborne agents can exit by crossing the placenta from mother to fetus (rubella, syphilis, toxoplasmosis), while others exit through cuts or needles in the skin (hepatitis B) or blood-sucking arthropods (malaria)

• Modes (Routes) of transmissionAn infectious agent may be transmitted from its natural reservoir to a susceptible host in different ways. There are different classifications for modes of transmission. Here is one classification:

• Direct transmissionDirect contactDroplet spread

• Indirect transmissionAirborneVehicleborneVectorborne (mechanical or biologic)


• In direct transmission, an infectious agent is transferred from a reservoir to a susceptible host by direct contact or droplet spread.

• Direct contact occurs through skin-to-skin contact, kissing, and sexual intercourse. Direct contact also refers to contact with soil or vegetation harboring infectious organisms. Thus, infectious mononucleosis (“kissing disease”) and gonorrhea are spread from person to person by direct contact. Hookworm is spread by direct contact with contaminated soil.Droplet spread refers to spray with relatively large, short-range aerosols produced by sneezing, coughing, or even talking. Droplet spread is classified as direct because transmission is by direct spray over a few feet, before the droplets fall to the ground. Pertussis and meningococcal infection are examples of diseases transmitted from an infectious patient to a susceptible host by droplet spread.


• Indirect transmission refers to the transfer of an infectious agent from a reservoir to a host by suspended air particles, inanimate objects (vehicles), or animate intermediaries (vectors).

• Airborne transmission occurs when infectious agents are carried by dust or droplet nuclei suspended in air.Airborne dust includes material that has settled on surfaces and become resuspended by air currents as well as infectious particles blown from the soil by the wind.

• Droplet nuclei are dried residue of less than 5 microns in size. In contrast to droplets that fall to the ground within a few feet, droplet nuclei may remain suspended in the air for long periods of time and may be blown over great distances. Measles, for example, has occurred in children who came into a physician’s office after a child with measles had left, because the measles virus remained suspended in the air


• Vehicles that may indirectly transmit an infectious agent include food, water, biologic products (blood), and fomites (inanimate objects such as handkerchiefs, bedding, or surgical scalpels). • A vehicle may passively carry a pathogen — as food or water may carry hepatitis A

virus. • Alternatively, the vehicle may provide an environment in which the agent grows,

multiplies, or produces toxin — as improperly canned foods provide an environment that supports production of botulinum toxin by Clostridium botulinum .

• Vectors such as mosquitoes, fleas, and ticks may carry an infectious agent through purely mechanical means or may support growth or changes in the agent. • Examples of mechanical transmission are flies carrying Shigella on their appendages

and fleas carrying Yersinia pestis , the causative agent of plague, in their gut. • In biologic transmission, the causative agent of malaria or guinea worm disease

undergoes maturation in an intermediate host before it can be transmitted to humans



• Portal of entryThe portal of entry refers to the manner in which a pathogen enters a susceptible host.

• The portal of entry must provide access to tissues in which the pathogen can multiply or a toxin can act.

• Examples of portals of entry include: respiratory tract (eg infuenzavirus and Mycobacterium tuberculosis), intact skin (eg Strongyloidesstercoralis and hookworms), fecooral or gastrointestinal tract (egCampylobacter jejuni and Vibrio cholera) and genitourinary tract (egChlamydia trachomatis and Trichomonas vaginalis).

• HostAn important link in the chain of infection is a susceptible host. Susceptibility of a host depends on genetic or constitutional factors, specific immunity, and nonspecific factors that affect an individual’s ability to resist infection or to limit pathogenicity.

• An individual’s genetic makeup may either increase or decrease susceptibility. For example, persons with sickle cell trait seem to be at least partially protected from a particular type of malaria.



• Specific immunity refers to protective antibodies that are directed against a specific agent.

• Such antibodies may develop in response to infection, vaccine, or toxoid (toxin that has been deactivated but retains its capacity to stimulate production of toxin antibodies) or may be acquired by transplacental transfer from mother to fetus or by injection of antitoxin or immune globulin.

• Nonspecific factors that defend against infection include the skin, mucous membranes, gastric acidity, cilia in the respiratory tract, the cough reflex, and nonspecific immune response.

• Factors that may increase susceptibility to infection by disrupting host defenses include malnutrition, alcoholism, and disease or therapy that impairs the nonspecific immune response.

Prevention and Control of Microbial Diseases

• Control of microbial diseases or infectious diseases refers to the actions and programmes aimed at reducing new infections (disease incidence), reducing infections in the community at any given point in time (disease prevalence), mitigating, reducing or eliminating the long term effects of infections or completely eradicating the disease.

• Thus control of infectious diseases can be categorized into three• Primary prevention of infectious diseases

• Secondary prevention of infectious diseases

• Tertiary prevention of infectious diseases


• Primary prevention of infectious disease involves measures aimed at preventing the occurrence of infectious diseases or reducing their risk factors.

• These are measures instituted to reduce the incidence of infectious disease.

• Primary prevention protects health through individual and community-wide measures, including such actions as maintaining good nutritional status, keeping physically fit, immunizing against infectious diseases, providing safe water, and ensuring the proper disposal of faeces.


• Secondary prevention of infectious disease involves measures aimed at curing or resolving infectious diseases.

• These measures are directed at shortening the duration of infectious disease and thus reducing the prevalence.

• Secondary prevention corrects deviations from good health through individual and community-wide measures, including such actions as screening that result in early detection of disease, prompt antibiotic treatment, and ensuring adequate nutrition.

• It should be noted that such control efforts in secondary prevention in a group of infected individuals may also result in primary prevention in uninfected people. • For example, prompt and specific drug therapy for tuberculosis patients resulting in sputum

conversion to culture-negative status renders them no longer a source of infection to others and treatment of HIV-positive pregnant women reduces transmission of HIV to their newborns.


• Tertiary prevention of infectious disease involves measures directed at reducing or even eliminating long-term complications of infectious disease.

• Tertiary prevention prevents, reduces or eliminates disabilities, minimizes suffering, and promotes adjustment to permanent disabilities through such actions as providing care, orthotic devices and even rehabilitative surgery for victims of leprosy, counselling, vocational training, and prevention of opportunistic infections. • The prevention of opportunistic infections in HIV infection can be considered

as tertiary prevention


• An good understanding of the Chain of infection, aids in the design of appropriate and effective control measures.

• Breaking the chain of infection at any point would interrupt the transmission of infection or halt an ongoing infection.

• Thus controls measures are directed at the elements or parts of the chain of infection.


• Control measures directed at the host include:

• Active immunization• vaccination is the process of administration of an antigen• immunization is the development of a specific immune response• Active immunization is the development of an immune response triggered by the

administration of an antigen.

• Passive immunization • Passive immunization is a temporary immunity in a host due to the protection provided by

antibody produced in another host.

• Chemoprophylaxis• Chemoprophylaxis is the prevention of infection or its progression to clinically manifest

disease through the administration of antimicrobial substances.

• Behavioural change (eg sexual, eating, personal hygiene and work), reverse isolation, barriers, and general improvement in host resistance


• Control measures directed at vectors include:

• Chemical controls eg use of insecticides or molluscicides

• Environmental eg removal of breeding sites

• Biological controls eg use of natural predators such as fish that feed on mosquito larvae


• Control measures directed at infected humans include:

• Hospital infection control,

• Chemotherapy • Use of antimicrobial agents to treat infections

• Isolation • the ‘separation, for a period at least equal to the period of communicability, of infected persons or animals

from others, in such places and under such conditions as to prevent or limit the direct or indirect transmission of the infectious agent from those infected to those who are susceptible to infection or who may spread the agent to others’

• Quarantine • the limitation of activities (which may or may not include limitation of movement) for well persons or animals

who have been exposed (or are considered to be at high risk of exposure) to a case of communicable disease during its period of communicability to prevent disease transmission during the incubation period if infection should occur

• Restriction of activities

• Behavioural change


• A zoonosis is any infectious disease that is naturally transmissible from vertebrate animals to humans. There are many zoonoses.

• Therefore control measures directed at animals include:

• Active immunization

• Restriction • Limitation of movement

• Reduction• Killing

• Chemoprophylaxis

• Chemotherapy


• Control measures directed at the environment include:

• Provision of safe water

• Proper disposal of faeces

• Food sanitation

• Design of facilities and equipment


• Control measures directed at the agent include:• measures that remove the infectious agents from the environment or

inactivate the agents such as:• Cleaning

• The removal of visible soiling organic or inorganic material from objects and surfaces, by manual or mechanical means through the use of water with detergents or enzymatic products.

• Disinfection• A process that results in the destruction or elimination of many or all pathogenic

microorganisms, except bacterial spores, on inanimate objects.

• Sterilization• A process that results in the destruction or elimination all forms of microbial life. It is

an absolute term.

BIBLIOGRAPHY AND REFERENCES

• Mandel, Douglas and Bennetts’ principles and practice of infectious diseases. 9th ed.Philadelphia. Churchhill Livingstone Elsevier

• Jawetz, Melnink and Adelberg’s medical microbiology. 28th edition

• Oxford Textbook of Global Public Health 6th edition

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prevention and control of microbial diseases

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