Medical brain notes 30

  INFECTIOUS DISEASES AND TREATMENTS INTRODUCTION The presence of virulence factors, described, allows pathogenic microorganisms to infect specific body systems and cause a vast range of diseases. A small number of such organisms are also able to cause  systemic disease, that is one affecting the whole body. A pathogen must be transmitted from a source to the patient. Direct contact between hosts is the most obvious form of transmission but coughs and sneezes (aerosols), food, water and arthropod vectors are all used by various pathogens. The long-term survival of pathogenic microorganisms also depends on them maintaining their infectivity during transmission from host to host. The sources of pathogens can be abiotic, soil, water for example, or animals or other humans. Diseases that infect animals and humans are called  zoonoses , while humans who harbor a pathogen but are symptomless are called  carriers .

  INFECTIONS OF THE SKIN The skin is a major element of the innate immune defense . It is normally colonized by a variety of microorganisms although the numbers and types vary between different areas of the body. In normal circumstances it forms an effective barrier to invading microorganisms.   Papillomaviruses can infect epidermal cells and stimulate their proliferation to form warts. Numerous herpes viruses have been described and at least eight of them, the  Human Herpes Viruses  ( HHV1-8 ), infect humans and can cause clinical disease. Largely due to historical reasons, they are also known by other names ( Table 3.1 ). For example,  Herpes simplex  viruses 1 and 2 may infect skin of the genitalia causing Herpes labialis and Herpes genitalis respectively and  Varicella zoster  causes chickenpox and shingles. Bacterial skin infections normally occur only if the normal balance between the skin environment and these organisms is disturbed. The ou...

  INFECTIONS OF THE EYES, EARS AND CENTRAL NERVOUS SYSTEM Eyelid infections generally involve the lid margins, eyelid glands or follicles causing styes or  hordeolums . They are usually associated with  Staphylococcus aureus  infections. The conjunctiva is particularly susceptible to infection.The epithelial surface enclosed by the eyelids is a warm moist enclosed environment in which microorganisms can rapidly become established. However, microorganisms must avoid being rinsed away by tears and some pathogens, such as  Chlamydia trachomatis  ( Figure 3.3 ), attach specifically to conjunctival cells. An estimated 500 million people are infected with different serotypes of  Chlamydia trachomatis , making trachoma the most significant eye infection worldwide. The disease blinds approximately 1% of infected individuals while many others suffer visual impairment.  Chlamydia trachomatis  is transmitted by contact with contaminated flies,fingers an...

  INFECTIONS OF THE RESPIRATORY SYSTEM The respiratory system is constantly exposed to inhaled microorganisms but is protected by extensive defenses. The nose filters out particles larger than 10  L m although those smaller than 5  L m may reach the bronchi and alveoli. Inaddition, there is a host of immune defenses including alveolar macrophages, secretory IgA antibodies, complement proteins, surfactant proteins, secreted defensins and lactoferrin . Despite this, infections of the respiratory tract are frequent causes of illness. The World Health Organization (WHO) has reported that many hundreds of millions of patients suffer acute infections of the lower respiratory tract worldwide.  Figure 3.6  indicates the sites of a number of respiratory diseases. Respiratory viruses are transmitted directly by aerosols or indirectly from contaminated surfaces. The first site of attack is, not surprisingly, the epithelium of the nose and throat. Indeed, the hundreds of co...

  INFECTIONS OF THE GASTROINTESTINAL TRACT All regions of the gastrointestinal tract (GIT) are subject to infection. Saliva traps and removes many pathogens and these can also be killed by stomach acid . Unfortunately new ones are constantly introduced through breathing and eating. Infections of the oral cavity ( Figure 3.6 ) differ in type and symptoms to those of the stomach and intestines. Inflammation of oral tissues caused by fungal infection,  actinomycosis , often occurs following injuries, such as an accidental bite to the lining of the mouth or fracture of the jaw. Immunosuppression resulting from viral infections, AIDS, cancer treatment or treatment with broad spectrum antibiotics can all allow the yeast  Candida albicans  ( Figure3.10 ) to invade and colonize the mucous membrane, eventually producing athick layer of yeast cells called candidiasis or thrush. Some bacteria can resist removal by saliva and become immobilized by binding to surface receptors of...

  INFECTIONS OF THE UROGENITAL SYSTEM The urinary system and the genital systems are subject to infections by specific respective pathogens ( Figure 3.16 ). Urinary tract infections (UTIs) are frequent in the developed world, with many millions of cases occurring each year. A number of factors, including diabetes mellitus , scarring, kidney stones, use of catheters or anatomical peculiarities of the urinary tract all predispose individuals to UTIs. These originate in the perianal area and move up the urethra into the bladder causing a short-lived, acute infection called cystitis.  Escherichia coli is the most common agent causing up to 80% of the relatively uncomplicated UTIs, like cystitis. Women are more prone than males to cystitis because of their relatively short urethra and the hormonal changes associated with the menstrual cycle . Cystitis is readily treated by increasing the uptake of fluids, particularly of acid drinks like cranberry juice, which causes increased flus...

  SEPSIS AND SYSTEMIC INFECTIONS A relatively small number of pathogens are able to enter the body in the lymphatic or circulatory systems and produce a generalized or systemic infection involving numerous body organs: brain, bone marrow, kidneys, liver, lungs and spleen. Local skin infections, such as with  Streptococcus pyogenes  or  Staphylococcus aureus  or infections of the GIT or urogenital system with Enterobacteriaceae can unfortunately progress to acute generalized infections within only a few days, given appropriate conditions. For example,  Streptococcuspyogenes  or  Staphylococcus aureus  secrete toxins called superantigens  that stimulate the release of cytokines from immune cells and produce an excessive inflammatory response called  systemic inflammatoryresponse syndrome  ( SIRS ) consisting of four characteristic stages. In stage I,respiration, heart rate and body temperature all increase. The leukocyte count ma...

  INVESTIGATING INFECTIOUS DISEASES To diagnose an infectious disease, two criteria must be satisfied. First, signs and symptoms compatible with the suspected infectious agent must be apparent. Secondly, the pathogen must be recovered from the infected site of the patient or there must be evidence of the pathogen being present at that site. Infectious diseases can affect any organ or system and can cause a wide variety of symptoms and signs . The clinical history and examination should aim to identify the sites of infection and the causative organism. The clinical history focuses on aspects relevant to infectious disease, such as recent travel history, food and water intake, occupational exposure, sexual activity and any use of intravenous drugs. The clinical examination involves identifying fever, skin rashes, swollen lymph nodes (lymphadenopathy) and investigations of the eyes, ears, mouth and throat. Fever is a typical symptom of infection but not all patients with fever have an...