infectious disease ,introduction

Infectious diseases lec 1 ا.م.د. حسن سالم الجميلي
Infectious diseases are disorders that are caused by organisms, usually microscopic in size, such as bacteria, viruses, fungi, or parasites that are passed, directly or indirectly, from one person to another. Humans can also become infected following exposure to an infected animal that harbors a pathogenic organism that is capable of infecting humans.
Infectious diseases are a leading cause of death worldwide, particularly in low income countries, especially in young children.
Three infectious diseases were ranked in the top ten causes of death globally in the most recent survey by the World Health Organization. They are lower respiratory infections (3.1 million deaths), HIV/AIDS (1.5 million deaths), and diarrheal diseases (1.5 million deaths). In low income countries, malaria and tuberculosis accounted for an additional two of the major ten causes of death.
The leading killer due to a single infectious agent is HIV/AIDS, followed by tuberculosis and malaria. Lower respiratory infections (including pneumonia) and diarrheal diseases are caused by a variety of infectious agents.
Measles
The WHO has set the objective of eradicating measles globally using the live attenuated vaccine. However, vaccination of more than 95% of the population is required to prevent outbreaks. Natural illness produces
life-long immunity.
Clinical features
Infection is by respiratory droplets with an incubation period of 6–19 days. A prodromal illness, 1–3 days before the rash, occurs, with upper respiratory symptoms, conjunctivitis and the presence of the pathognomonic Koplik’s spots, small white spots surrounded by erythema on the buccal mucosa. As natural antibody develops, the maculopapular rash appears, spreading from the face to the extremities. Generalised lymphadenopathy and diarrhea are common. Complications are more common in older children and adults, and include otitis media, bacterial pneumonia, transient hepatitis and clinical encephalitis . A rare late complication is subacute sclerosing panencephalitis (SSPE), which occurs up to 7 years after infection.
Diagnosis is clinical and by detection of antibody (serum IgM or salivary IgM). Measles is a serious disease in the malnourished, vitamin-deficient or immunocompromised, . In tuberculosis infection, measles suppresses cell mediated immunity and may exacerbate disease; for this reason, measles vaccination should be deferred until after commencing antituberculous treatment. Measles does not cause congenital malformation but may be more severe in pregnant women . Death usually results from a bacterial superinfection, occurring as complication of measles, most often pneumonia, diarrhoeal disease or noma/ cancrum oris, a gangrenous stomatitis. Death may also result from complications of measles encephalitis.
Management and prevention
Normal immunoglobulin attenuates the disease in the immunocompromised (regardless of vaccination status) and in non-immune pregnant women, but must be given within 6 days of exposure. Vaccination can be used in outbreaks and vitamin A may improve the outcome in uncomplicated disease. Antibiotic therapy is reserved for
bacterial complications. All children aged 12–15 months should receive measles vaccination (as combined measles, mumps and rubella (MMR), a live attenuated vaccine), and a further MMR dose at age 4 years. Rubella (German measles).
Clinical features
Rubella is spread by respiratory droplet, with infectivity from up to 10 days before to 2 weeks after the onset of the rash. The incubation period is 15–20 days. clinical features may include fever, maculopapular rash spreading from the face, and lymphadenopathy. Complications
are rare but include thrombocytopenia and hepatitis. Encephalitis and haemorrhage are occasionally reported. In adults, arthritis involving hands or knees is relatively common, especially in women. If transplacental infection takes place in the first trimester or later, persistence of the virus is likely and severe congenital disease may result . Even if normal at birth, the infant has an increased incidence of other diseases developing later, e.g. diabetes mellitus.


Diagnosis
Laboratory confirmation of rubella is required if there has been contact with a pregnant woman. This is achieved either by detection of rubella IgM in serum. In the exposed pregnant woman, absence of rubella-specific IgG confirms the potential for congenital infection.
Prevention
All children should be immunised with MMR . In view of the risks of congenital rubella syndrome, all women of child-bearing age should also
be tested for rubella and vaccinated if seronegative.
Mumps
Mumps is a systemic viral infection characterised by swelling of the parotid glands. Infection is endemic worldwide and peaks at 5–9 years of age. Vaccination has reduced the incidence in children but incomplete
coverage and waning immunity with time have led to outbreaks in young adults. Infection is spread by respiratory droplets.
Clinical features
The median incubation period is 19 days, with a range of 15–24 days. Classical tender parotid enlargement, which is bilateral in 75%, follows a prodrome of pyrexia and headache . Meningitis complicates
up to 10% of cases. The CSF reveals a lymphocytic pleocytosis or, less commonly, neutrophils. Rare complications include encephalitis, transient hearing loss, labyrinthitis, electrocardiographic abnormalities, pancreatitis and arthritis. Approximately 25% of post-pubertal males with mumps develop epididymo-orchitis but, although testicular
atrophy occurs, sterility is unlikely. Oophoritis is less common. Abortion may occur if infection takes place in the first trimester of pregnancy. Complications may occur in the absence of parotitis.
Diagnosis
The diagnosis is usually clinical. In atypical presentations without parotitis, serology for mumps-specific IgM or IgG seroconversion (four-fold rise in IgG convalescent titre) confirms the diagnosis. Virus can also be cultured from urine in the first week of infection or detected
by PCR in urine, saliva or CSF.
Management and prevention
Treatment is with analgesia. There is no evidence that corticosteroids are of value for orchitis. Mumps vaccine is one of the components of the combined MMR vaccine



Chickenpox (varicella)
Varicella zoster virus (VZV) is a dermotropic and neurotropic virus that produces primary infection, usually in childhood, which may reactivate in later life. VZV is spread by aerosol and direct contact. It is highlyinfectious to non-immune individuals. Disease in children is usually well tolerated. Manifestations are more severe in adults, pregnant women and the immunocompromised.
Clinical features
The incubation period is 11–20 days, after which a vesicular eruption begins, often on mucosal surfaces first, followed by rapid dissemination in a centripetal distribution (most dense on trunk and sparse on limbs). New lesions occur every 2–4 days and each crop is associated with fever. The rash progresses from small pink macules to vesicles and pustules within 24 hours. Infectivity lasts up to 4 days
before the lesions appear until the last vesicles crust over. Due to intense itching, secondary bacterial infection from scratching is the most common complication of primary chickenpox. Self-limiting cerebellar
ataxia and encephalitis are rare complications. Adults, pregnant women and the immunocompromised are at increased risk of visceral involvement, which presents as pneumonitis, hepatitis or encephalitis.
Pneumonitis can be fatal and is more likely to occur in smokers. Maternal infection in early pregnancy carries a 3% risk of neonatal damage with developmental abnormalities of eyes, CNS and limbs. Chickenpox within 5 days of delivery leads to severe neonatal varicella with visceral involvement and haemorrhage.
Diagnosis
Diagnosis is primarily clinical, by recognition of the rash. If necessary, this can be confirmed by detection of antigen (direct immunofluorescence) or DNA (PCR) of aspirated vesicular fluid. Serology is used to identify seronegative individuals at risk of infection.
Management and prevention
The benefits of antivirals for uncomplicated primary VZV infection in children are marginal and treatment is not required . Antivirals are, however, used for uncomplicated chickenpox when the patient presents within 24–48 hours of onset of vesicles, in all patients
with complications, and in those who are immunocompromised, including pregnant women, regardless of duration of vesicles .More severe disease, particularly in immunocompromised hosts, requires initial parenteral therapy. Immunocompromised patients may have prolonged viral shedding and may require prolonged treatment until all lesions crust over. Human VZ immunoglobulin (VZIG) is used to attenuate infection in people who have had significant contact with VZV, are susceptible to infection (i.e. have no history of chickenpox or shingles and are seronegative for VZV IgG) and are at risk of severe disease (e.g. immunocompromised, steroid-treated or pregnant. Ideally, VZIG should be given within 7 days of exposure, but it may attenuate disease even if given up to 10 days afterwards. Susceptible contacts who develop severe chickenpox after receiving VZIG should be treated with aciclovir
. Children receive one dose after 1 year of age and a second dose at 4–6 years of age; seronegative adults receive two doses at least 1 month apart. The vaccine may also be used prior to planned iatrogenic immunosuppression, e.g. before transplant.