The 1st element of chain of infection, Infectious agent: Infectious agents come in many shapes and sizes. Bacteria and protozoans are microscopic one-celled organisms, while viruses are even smaller. Fungi grow like plants, and helminthes resemble worms.

Agents can be classified into Single agent (As the Gonococci causing  Gonorrhea), Multiple agents  (As the group of bacteria causing Gas gangrene) and Interacting agents  (Where one agent predispose to the other as PEM & diarrheal disease).                                                                                                  

Examples of weak organisms: Influenza virus, Measles virus, Meningococci, AIDS virus, Gonococci and Treponema pallidum.

The 2nd  element of chain of infection, Reservoir:

Reservoir of infection: 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.

Animal reservoir: Many of the diseases are transmitted from animal to animal, with humans as incidental hosts.

Animal as a reservoir of infection: Cattle (Salmonellosis, Brucellosis, T.B, T. saginata, Rift valley fever), Camel (Rift valley fever, Hydaid disease, Trypanosomiasis, Middle East respiratory syndrome), Sheep (Brucellosis, Anthrax, Fasciola hepatica, Rift valley fever), Goat (Brucellosis), Rabbit (Tularemia), Swine (Salmonellosis, Brucellosis, T.B, T. saginata, Chaga’s disease), Dog (Rabies, Echinococcus granulosus, Dipylidium caninum, H. heterophyes, Leishmaniasis,  Chaga’s disease), Cat (Toxoplasmosis, Chaga’s disease), Rodents (Murine typhus, Dermatophytoses, Salmonellosis, Plague, H. diminuta, H. nana, Chaga’s disease) and Wild animals (Leishmaniasis, Tickborne relapsing fever).                                                                                            

Environmental reservoir: 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.

Human reservoir: Diseases that are transmitted from person to person without intermediaries include the sexually transmitted diseases, measles, mumps, streptococcal infection, and many respiratory pathogens. Because humans were the only reservoir for the smallpox virus, naturally occurring smallpox was eradicated after the last human    case was identified and isolated. Human reservoir is either case or carrier.

Case is the patient showing signs and symptoms of the disease while Carrier is the person carrying the organism in the body without showing any signs or symptoms and they can infect others by passing organisms in their discharges.                                                   

Why carriers important? Apparently healthy (not known), move freely in the community, large number (more than cases) and sometimes having very long I.P.

Incubatory carrier: When an individual transmits pathogens immediately following infection but prior to developing symptoms, they are known as an incubatory carrier.

Convalescent carrier:  Humans are also capable of spreading disease following a period of illness. Typically thinking themselves cured of the disease, these individuals are known as convalescent carriers. Viral diseases such as hepatitis and poliomyelitis are frequently transmitted in this manner.

Healthy carrier: Considered to be the classic asymptomatic or passive carriers, never exhibit signs or symptoms of the disease, yet are capable of infecting others. Healthy carriers are persons who harbor an infectious agent but never fall ill or manifest any overt evidence that they are infected. This commonly happens with certain virus diseases, such as several forms of viral hepatitis and poliomyelitis, and with some bacterial diseases, including diphtheria and meningococcal meningitis.

The pathogenic organisms responsible for diseases in this category live as commensal organisms in the carrier’s respiratory and/or gastrointestinal tract, apparently coexisting without causing disease because the host is not susceptible to infection or because the organisms are not virulent. Carriers of the diphtheria bacillus, meningococcus, or other organisms in this category manifest no symptoms or signs of infection, but the organisms can be recovered from their nose, throat, or from their feces.

Animals and birds can be carriers of some human diseases. For instance, jungle yellow fever is carried by monkeys and possibly other jungle-dwelling small mammals. The rabies virus, which is almost invariably fatal when humans are infected and not protected immediately by anti – rabies vaccine, is carried by bats who seem to suffer no ill effects.                                                                                                                   

Diseases Having Healthy Carriers: Hepatitis – B, Bacillary dysentery, Poliomyelitis, Enterica.                                                                                                                        

One notorious carrier is Mary Mallon, or Typhoid Mary, who was an asymptomatic chronic carrier of Salmonella typhi. As a cook in New York City and New Jersey in the early 1900s, she unintentionally infected dozens of people until she was placed in isolation on an island in the East River, where she died 23 years later.

Is it carrier or subclinical infection? The term “subclinical” is used to focus on the absence of signs and symptoms while “ carrier” is used to focus on transmission of pathogen. The subclinical infection may or may not result in a carrier state. e.g. in case of poliomyelitis, the sub clinically infected persons may initially show a carrier state. But on the other hand, in case of positive tuberculin test (which signifies subclinical infection), the patient may not be a carrier i.e.,  he may not be actively transmitting the tubercular bacilli. The carrier state may result from a subclinical infection (as is the case with healthy carriers) or a clinical infection (as with convalescent carriers).

The subclinical term is used only when there are no signs and symptoms i.e., we can say that the infection was subclinical only after incubation period is over. But on the other hand, carrier can be incubatory (transmitting the pathogen in incubation period)/ convalescent (transmission during convalescent phase)/ healthy (transmission after convalescent phase, from subclinical infection).

No carrier for all parasitic diseases, except the healthy cyst-passers of entamoeba histolytica.

No carrier in the following diseases: Smallpox, tuberculosis & pertussis, gonorrhea, syphilis, measles, chickenpox.

No sub clinical infection in: Gonorrhea, syphilis, measles, chickenpox, smallpox.        

Examples of diseases having contact carriers: Diphtheria, Cholera, Enterica, Shigellosis. That is why school pupils, school personnel and food handlers are considered as contact carriers and require to be segregated (to be examined daily for temperature, general condition and early manifestations. Examination to be continued for the usual I.P. in addition to exclusion from school or work & not allowed to return except after (3) negative consecutive samples).

Examples of carriers of long infectivity period ( for years): Convalescent carrier of enterica, Convalescent carrier of EL-Tor vibrios, Healthy carrier of hepatitis – B.

The 3rd element of chain of infection, Portal of exit:

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, and enterovirus 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).

Carried organisms according to portal of exit: Respiratory discharges (Contain all droplet infection agents), Feces (Contain all food borne infection agents), Vomitus (Contains Vibrios of cholera), Urine (Contains Salmonella typhi & paratyphi), Skin discharge (Contains Staph., Strept.  , Herpes Zoster, Variola, Varicella virus),  Blood (Contains hepatitis viruses B , C and Aids H. I. V. ) and  Breast milk (Contains Cytomegalo virus).                                                                                            

The 4th  element of chain of infection, Mode of transmission: Four basic modes: Respiratory or droplet, Food borne, Arthropod borne, Contact. & Two  occasional modes: Injection, In utero or congenital.

Droplet nuclei & Dust: Droplet nuclei are minute residues of fine droplets after evaporation of water.  It remains suspended in air for long periods and is created in situations as hot environment allowing evaporation of droplets as abattoirs, microbiology laboratories, mortuaries and devices for changing liquids to mists while dust refers to big droplets & respiratory discharges falling on the floor or any surface and drying. Will be evoluted as dust when disturbed by wind, after sweeping, shaking or beating carpets, beddings, etc.

Vector is an arthropod that can transmit communicable disease either mechanically or biologically.                                                                                                                                               

Vector reservoir: The vector acts as a reservoir of infection and help to maintain infection in the community.     

Vectors Acting as Vector Reservoir: Ticks passing borrelia in its  generations, trans ovarian transmission, thus considered as a reservoir for relapsing fever, Aedes egypti remains infective for some time thus considered as a reservoir for yellow fever.

Anterior station infection is a way of arthropod borne infection. The arthropod transmits organisms to the victim by biting intact skin.

Posterior station infection: The arthropod transmits the organism to the victim by their feces coming on abraded skin.         

How blood sucking insects spread infection: The insect takes a blood meal from a reservoir host (the insect become infected). After extrinsic or extracorporeal I.P, the insect becomes infective.  Insect will Transmit infection to a new host by: anterior station or posterior station infection. Act as a vector reservoir by Trans-ovarian transmission or Remaining infective for some time.                                             

Contact infection: Contact infection occurs through undamaged surfaces (As conjunctiva leading to infective conjunctivitis, Skin causing Infective skin diseases, Infective exanthema, Leptospirosis, Parasitic diseases as schistosomiasis, ankylostomiasis & strongyloidiasis,etc.) or Damaged surfaces (As wounds: wound sepsis, puerperal sepsis, erysipelas, tetanus, gas gangrene), Biting vector (As in  malaria), Injection infection (As in  Aids, viral Hepatitis, pyogenic infection).

In utero infection: Infections acquired in utero or during the birth process are a significant cause of fetal and neonatal mortality and an important contributor to early and later childhood morbidity. The infected newborn infant may show abnormal growth, developmental anomalies, or multiple clinical and laboratory abnormalities. Exposure to infection in the tomb increases the risk to autism.

Vertical transmission is transmission of infection from mother to baby. it can be: Trans placental (As for hepatitis B & C, AIDS, CMV and syphilis), Through birth canal During labor (As for ophthalmia neonatorum and herpes simplex) or Through lactation (As hepatitis B & C, CMV and AIDS).

Injection infection: According to the CDC, there have been nearly 50 disease outbreaks linked to unsafe injection practices. These outbreaks have included transmissions of hepatitis B and C, as well as bacterial infections. One Needle, One Syringe, Only One Time. It could save life. Safe injection practices are steps—such as not using the same needle or syringe on more than one patient—that healthcare providers should follow when they give injections.

As a healthcare provider watch for potentially unsafe practices such as using the same syringe to administer medication to more than one person.  Also make sure that the syringe being used to flush out IV line is brand new—and hasn’t been used to flush out another person’s line. Finally, don’t use insulin pens and other injection equipment containing multiple doses of medication on more than one person.

The 5th   element of chain of infection, Portal of entry: The 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. Often, infectious agents use the same portal to enter a new host that they used to exit the source host.  For example, influenza virus exits the respiratory tract of the source host and enters the respiratory tract of the new host. In contrast, many pathogens that cause gastroenteritis follow a so-called “fecal-oral” route because they exit the source host in feces, are carried on inadequately washed hands to a vehicle such as food, water, or utensil, and enter a new host through the mouth. Other portals of entry include the skin (hookworm), mucous membranes (syphilis), and blood (hepatitis B, human immunodeficiency virus).

The 6th   element of chain of infection, Susceptible host: The final link in the chain of infection is a susceptible host. Susceptibility of a host depends on: Genetic or constitutional factors (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) and Specific immunity (Refers to protective antibodies that are directed against a specific agent. Such antibodies may develop in response to infection, vaccine, or toxoid or may be acquired by trans placental transfer from mother to fetus or by injection of antitoxin or immune globulin).                                                                                                                   

Nonspecific factors that affect an individual’s ability to resist infection or to limit pathogenicity: Nonspecific factors 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.

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