Transmission And Maintenance Of Infection PDF

TRANSMISSION AND MAINTENANCE OF INFECTION Edited by: Krestel Joy v. Isla, DVM, MIH  TRANSMISSION AND MAINTENANCE OF INFECTION  Infectious disease is the result of the invasion of a host by a pathogenic organism.  The continued survival of infectious agents, w...

TRANSMISSION AND MAINTENANCE OF INFECTION Edited by: Krestel Joy v. Isla, DVM, MIH  TRANSMISSION AND MAINTENANCE OF INFECTION  Infectious disease is the result of the invasion of a host by a pathogenic organism.  The continued survival of infectious agents, with or without the induction of disease, depends on their successful transmission to a susceptible host, the instigation of an infection therein and replication of the agent to maintain the cycle of infection.  TRANSMISSION AND MAINTENANCE OF INFECTION The complete cycle of an infectious agent is its life history (life cycle).  TRANSMISSION AND MAINTENANCE OF INFECTION  A knowledge of the life history of an infectious agent is essential when selecting the most appropriate control technique.  This involves knowledge of transmission and maintenance of infection and the ecological conditions that favor the survival and transmission of infectious agents.  TRANSMISSION AND MAINTENANCE OF INFECTION BASIC MECHANISMS OF TRANSMISSION OF INFECTIOUS AGENTS  TRANSMISSION AND MAINTENANCE OF INFECTION HORIZONTAL TRANSMISSION  It is the transmission of infection from one segment of a population to another.  TRANSMISSION AND MAINTENANCE OF INFECTION  Direct transmission occurs when a susceptible host contacts an infection, either by contact with an infected host or by contact with the latter’s infected discharges.  TRANSMISSION AND MAINTENANCE OF INFECTION  Indirect transmission involves an intermediate vehicle, living or inanimate, that transmits infection between infected and susceptible hosts.  TRANSMISSION AND MAINTENANCE OF INFECTION  HOST  a plant, animal or arthropod that is capable of being infected with, and therefore giving sustenance to, an infectious agent.   TYPES OF HOST  Definitive host - a parasitological term describing a host in which an organism undergoes it sexual phase of reproduction (e.g. Taenia pisiformis in dogs). TRANSMISSION AND MAINTENANCE OF INFECTION  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Final host - a term used in a more general sense as a synonym for definitive host. Both ‘final’ and ‘definitive’ imply ‘end of the line’ or termination of the dynamic process.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Primary (natural) host - an animal that maintains an infection in the infection’s endemic area. Since an infectious agent frequently depends upon a primary host for its long-term existence, the host is also called a maintenance host.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Secondary host - a species that additionally is involved in the life cycle of an agent, especially outside typical endemic areas. A secondary host can sometimes act as a maintenance host.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Paratenic host - a host in which an agent is transferred mechanically, without further development. This term is exclusive to helminthology, and could be considered to have its entomological analogue in the term mechanical vector.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Intermediate host - an animal in which an infectious agent undergoes some development, frequently with reproduction. This term is parasitological in origin.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Amplifier host - an animal which, because of temporally associated changes in population dynamics that produce a sudden increase in the host population size, may suddenly increase the amount of infectious agent.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Amplifier host - Multiplication of the agent occurs in this type of host. This term is commonly used in relation to virus diseases.  TRANSMISSION AND MAINTENANCE OF INFECTION  MAINTENANCE AND AMPLIFIER HOSTS OF JAPANESE ENCEPHALITIS VIRUS IN JAPAN  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Hibernating host - an animal in which an agent is held, probably without replication, in a state of ‘suspended animation’.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Incidental (dead-end or accidental) host - one that does not usually transmit an infectious agent to other animals. ‘Final’ and ‘definitive’ can be applied validly to this type of host.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Link host - a host that forms a link between other host species.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Reservoir host - one in which an infectious agent normally lives and multiplies, and therefore is a common source of infection to other animals. Reservoir host may be primary or secondary hosts.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF HOST  Reservoir host - ‘Reservoir’ is also used to refer to any substance that is a common source of infection (e.g. soil, water).  TRANSMISSION AND MAINTENANCE OF INFECTION  VECTOR  an animate transmitter of infectious agents.  By common usage, vectors are defined as invertebrate animals (usually arthropods) that transmit infectious agents to vertebrates. Inanimate carriers are called ‘fomites’ (singular: fomes, from the Greek meaning ‘tinder’).  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF VECTOR  Mechanical Vector - an animal (usually arthropod) that physically carries an infectious agent to its primary or secondary host. The infectious agent neither multiplies nor develops in the mechanical vector.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF VECTOR  Biological vector - a vector (usually arthropod) in which an infectious agent undergoes either a necessary part of its life cycle, or multiplication, before transmission to the natural or secondary host.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF VECTOR  Biological vector - Biological vectors are frequently either definitive or intermediate hosts. Three types of biological transmission occur:  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF VECTOR  Biological vector A. Developmental transmission – with an essential phase of development occurring in the vector.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF VECTOR  Biological vector B. Propagative transmission – when the agent multiplies in the vector.  TRANSMISSION AND MAINTENANCE OF INFECTION  TYPES OF VECTOR  Biological vector C. Cyclopropagative transmission – combination of a and b.  TRANSMISSION AND MAINTENANCE OF INFECTION  FACTORS ASSOCIATED WITH THE SPREAD OF INFECTION  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF HOSTS  a host’s susceptibility and infectiousness determine its ability to transmit infection.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF HOSTS  Susceptibility within a species may vary markedly and may be associated with selection of genetically resistant animals following exposure to an infectious agent.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF HOSTS  ‘Infectiousness’ refers to the:  a) duration of the period when an animal is infective.  b) the relative amount of infectious agent that an animal can transmit.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF HOSTS  An animal is not infectious as soon as it is infected – a period of time elapses between an infection and the shedding of the agent; this is a parasite’s prepatent period, a virus’ eclipse phase, and a bacterium’s latent period.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF HOSTS  In contrast, the incubation period is the period of time between infection and the development of clinical signs.  Thus inapparent infections have no incubation period but have a prepatent period.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF HOSTS  The generation time is the period between infection and maximum infectiousness.  The time between infection and availability of an infectious agent in an arthropod vector is the agent’s extrinsic incubation period.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF HOSTS  For transmission to occur between a vertebrate host and an arthropod vector, an infectious agent must be present at a minimum concentration in the vertebrate host’s circulation.  This is the threshold level.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF PATHOGENS  Infectivity refers to the amount of an organism that is required to initiate an infection. The infectivity of different organisms varies considerably.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF PATHOGENS  When an agent is capable of infecting more than one host, its infectivity for different hosts is often quite different.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF PATHOGENS  Virulence also affects transmission and can change. Repeated passage through the same species tends to increase virulence for that species but simultaneously lowers the virulence for the original natural hosts.  TRANSMISSION AND MAINTENANCE OF INFECTION  CHARACTERISTICS OF PATHOGENS  Stability is the length of time for which an organism can remain infective outside its host.  TRANSMISSION AND MAINTENANCE OF INFECTION  EFFECTIVE CONTACT  describes the conditions under which infection is likely to occur. For a particular infection, it depends on stability of the organism and the routes by which the organism leaves an infected host and enters a susceptible one.  TRANSMISSION AND MAINTENANCE OF INFECTION  EFFECTIVE CONTACT  Effective contact may be very short or potentially of many years’ duration. The duration of infectiousness determines the number of susceptible that can be infected by an infected animal.  TRANSMISSION AND MAINTENANCE OF INFECTION  EFFECTIVE CONTACT  Behavior, which may be changed during infection, can also affect the likelihood of effective contact.  TRANSMISSION AND MAINTENANCE OF INFECTION ROUTES OF INFECTION  TRANSMISSION AND MAINTENANCE OF INFECTION  ROUTES OF INFECTION  Body surfaces as sites of horizontal transmission and shedding of infectious agents.  TRANSMISSION AND MAINTENANCE OF INFECTION  ROUTES OF INFECTION  Oral route – one of the more common routes of entry especially in relation to enteric organisms.  TRANSMISSION AND MAINTENANCE OF INFECTION  ROUTES OF INFECTION  Respiratory route - agents that enter through this route are usually associated with other organic matter in the form of droplets or dust.  TRANSMISSION AND MAINTENANCE OF INFECTION  ROUTES OF INFECTION  Particles of a diameter of 5 nm or greater do not reach the alveoli and therefore initially cause infection only of the upper respiratory tract.  TRANSMISSION AND MAINTENANCE OF INFECTION  ROUTES OF INFECTION  Infections spread through the respiratory route are more likely to occur where population densities are high and ventilation is poor.  TRANSMISSION AND MAINTENANCE OF INFECTION  ROUTES OF INFECTION  Infection via skin, cornea and mucous membrane – transmission via skin is percutaneous (Latin: per = through, across, cut = skin).  TRANSMISSION AND MAINTENANCE OF INFECTION  METHODS OF HORIZONTAL TRANSMISSION  Ingestion may occur via mechanical vehicle or by ingestion of intermediate host.  TRANSMISSION AND MAINTENANCE OF INFECTION  Aerial transmission involves airborne transmission of infectious agents via contaminated air. ‘Aerosol’ transmission is a type of airborne transmission via an aerosol which is defined variously as:  TRANSMISSION AND MAINTENANCE OF INFECTION  (1) any solution in the form of fine spray in which the droplets approximate colloidal size (1-100 nm) and  TRANSMISSION AND MAINTENANCE OF INFECTION  (2) finely divided virus particles hanging or floating in the air.  TRANSMISSION AND MAINTENANCE OF INFECTION  Contact - transmission without vectors and without participation of an external medium.  TRANSMISSION AND MAINTENANCE OF INFECTION  Inoculation - introduction into the body, by puncture of the skin or through wounds, of infectious agents.  TRANSMISSION AND MAINTENANCE OF INFECTION  Iatrogenic transmission - ‘iatrogenic’ literally means ‘created by doctor’, thus transmitted during surgical and medical practice through dirty instruments or contaminated prophylactic or therapeutic measures.  TRANSMISSION AND MAINTENANCE OF INFECTION Coitus - diseases transmitted only through this are called venereal, from Latin venerues (sexual love).  TRANSMISSION AND MAINTENANCE OF INFECTION VERTICAL TRANSMISSION  It is the transmission of infection from one generation to the next by infection of the embryo or fetus while in utero or in ovo. Some consider transmission by milk as vertical. There are two types of vertical transmission: hereditary and congenital.  TRANSMISSION AND MAINTENANCE OF INFECTION  Hereditarily transmitted diseases: carried within the genome of either parent.  TRANSMISSION AND MAINTENANCE OF INFECTION  Congenitally transmitted diseases - literally means ‘present at birth’; by common usage it refers to diseases acquired in utero or in ovo, rather than inherited. Alternatively, infection which is inapparent and continuous after birth (innate infection) can occur.  TRANSMISSION AND MAINTENANCE OF INFECTION  METHODS OF VERTICAL TRANSMISSION  TRANSMISSION AND MAINTENANCE OF INFECTION  Germinative transmission - involves either infection of the superficial layers of the ovary, or infection of the ovum itself.  Example: chicken leukosis virus  TRANSMISSION AND MAINTENANCE OF INFECTION  Transmission to the embryo - via the placenta (transplacentally) for viruses or via the fetal circulation, through the placenta, to the fetus for most microbes.  TRANSMISSION AND MAINTENANCE OF INFECTION  Ascending infection - transmitted from the lower genital canal to the amnion and placenta  (e.g. some Staph., Strep.)  TRANSMISSION AND MAINTENANCE OF INFECTION  Infection at parturition - acquired from the lower genital canal at birth.  TRANSMISSION AND MAINTENANCE OF INFECTION  IMMUNOLOGICAL STATUS AND VERTICAL TRANSMISSION  Immune tolerance of microbial antigens by the fetus van be detrimental in post- natal life, because ‘non-self’ antigens are then recognized as ‘self’.  TRANSMISSION AND MAINTENANCE OF INFECTION  IMMUNOLOGICAL STATUS AND VERTICAL TRANSMISSION  TRANSMISSION AND MAINTENANCE OF INFECTION  The result is a lack of protective immune response, sometimes with the development of a carrier state.  TRANSMISSION AND MAINTENANCE OF INFECTION  Immune tolerance can occasionally be advantageous when infections have clinical and pathological effects mediated by the immune response.  TRANSMISSION AND MAINTENANCE OF INFECTION MAINTENANCE OF INFECTION  HAZARDS TO INFECTIOUS AGENTS  The environment within the host –  the host has its natural defense mechanisms: surface-active chemicals, specific reactive cells, phagocytes and humoral antibodies.  TRANSMISSION AND MAINTENANCE OF INFECTION  HAZARDS TO INFECTIOUS AGENTS  Parasites have evolved strategies such as acid- resistant helminth cuticles and intracellular mode of life.  Some bacteria have capsules to protect them from phagocytosis.  Many parasitic nematodes have more fecundity than their free-living counterparts.  TRANSMISSION AND MAINTENANCE OF INFECTION  HAZARDS TO INFECTIOUS AGENTS  The external environment - desiccation and UV light are the two main hazards. Many agents are partially protected from desiccation by being discharged in moist carriers such as feces and urine.  TRANSMISSION AND MAINTENANCE OF INFECTION  HAZARDS TO INFECTIOUS AGENTS  They may also persist by being shed into favorable surroundings. Some agents are resistant to desiccation and can survive for long periods in dry infected scab material.  TRANSMISSION AND MAINTENANCE OF INFECTION  MAINTENANCE STRATEGIES  Avoidance of a stage in the external environment - some agents avoid transfer via the environment.  TRANSMISSION AND MAINTENANCE OF INFECTION  There are four main methods: vertical transmission, venereal transmission, vector transmission, and by sarcophagi or flesh-eating, e.g. T.spiralis.  TRANSMISSION AND MAINTENANCE OF INFECTION  Development of resistance forms - some bacteria form spores which can survive adverse temperatures and chemicals and may survive in the environment for decades. Fungi may also produce spores.  TRANSMISSION AND MAINTENANCE OF INFECTION  Some helminthes and protozoa form resistant shells (cysts). Thick-shelled helminth eggs can resist the external environment and may overwinter on pasture.  TRANSMISSION AND MAINTENANCE OF INFECTION  ‘Rapidly-in, rapidly out’ strategy - some agents enter the host, replicate and leave very quickly, before the host has time to mount an immune response.  TRANSMISSION AND MAINTENANCE OF INFECTION  Many viruses of the upper respiratory tract can do this within 24 hours. The strategy requires a continuous supply of susceptible hosts.  TRANSMISSION AND MAINTENANCE OF INFECTION  Persistence within the host – persistence occurs because the host’s defense mechanisms fail to eliminate agents.  TRANSMISSION AND MAINTENANCE OF INFECTION  This failure may be because microorganisms adapt to the host’s phagocytic cells or develop strategies for avoiding the host’s response, including antigenic variation, intracellular parasitism, multiplication in sites inaccessible to the host’s immune response, and induction of ineffective antibodies  TRANSMISSION AND MAINTENANCE OF INFECTION  Extension of host range - many infectious agents can infect more than one host; their number exceeds that of one-host agents. In man, 80% of infectious agents are zoonotic.  TRANSMISSION AND MAINTENANCE OF INFECTION  Extension of host range is facilitated by the presence of various hosts in one area.