Atypical pneumonia

The term "atypical" means that a causative bacterium cannot be isolated on routine media in the diagnostic laboratory or that the disease does not resemble pneumococcal pneumonia.Mycoplasma pneumonia is the most common type of atypical pneumonia. Other atypical pneumonias are

1. Legionnaires' disease, 2. Q fever, : Coxiella burnetii3. Psittacosis: Chlamydia psittaci4. Viral pneumonias such as influenza.

The clinical picture in atypical pneumonia is dominated by constitutional symptoms, such as fever and headache, rather than respiratory symptoms.

What do you know about mycoplasma pneumonia?Its incubation is 2-3 weeks and it is usually seen in children and young adults. Reinfection can occur in older patients with detectable M. pneumoniae antibody.

What are the extrapulmonary manifestations of mycoplasma pneumonia?During M. pneumoniae infection, autoantibodies are produced against red cells (cold agglutinins: IgM autoantibodies against type O red blood cells that agglutinate these cells at 4°C but not at 37°C.) and brain, lung, and liver cells. These antibodies may be involved in some of the extrapulmonary manifestations of infection.

1. AIHA2. Myocarditis, 3. Pericarditis, Cardiac arrhythmias4. Neurological manifestations involving both CNS (meningoencephalitis) and PNS (GBS).5. Arthralgia and arthritis. 6. Hepatitis, 7. Glomerulonephritis.8. Non-specific rash, erythema multiforme, Stevens–Johnson sy, erythema nodosum, 9. Disseminated intravascular coagulation (DIC).10. Raynaud's phenomemon

Legionella pneumophila Mycoplasma pneumoniae Chlamydia pneumoniaeDisease Legionnaires' disease

("atypical" pneumonia)."Atypical" pneumonia. Atypical pneumonia.

Characteristics Gram-negative rods, but stain poorly with standard Gram stain. Require increased iron and cysteine for growth in culture.

Smallest free-living organisms. Not seen on Gram-stained smear because they have no cell wall, so dyes are not retained. The only bacteria with cholesterol in cell membrane. Can be cultured in vitro.Pathogen only for humans

Obligate intracellular parasites. Not seen on Gram-stained smear. Exists as inactive elementary body extracellularly and as metabolically active, dividing reticulate body intracellularly.

Habitat Environmental water sources. (cooling systems in hotels, hospitals.)

Human respiratory tract

Transmission Via aerosol from the water source. Person-to-person transmission does NOT occur.

Via respiratory droplets. Respiratory aerosol.

Pathogenesis Aside from endotoxin, no toxins, enzymes, or virulence factors are

No exotoxins produced. No endotoxin because there is no cell wall.

No toxins or virulence factors known.

known. Predisposing factors include

1. being older than 55 years of age,

2. smoking 3. Having a high

alcohol intake.Immunosuppressed patients, e.g., renal transplant recipients, are highly susceptible. The organism replicates intracellularly, therefore cell-mediated immunity is an important host defense. Smoking damages alveolar macrophages, which explains why it predisposes to pneumonia.

Produces hydrogen peroxide, which may damage the respiratory tract.

Laboratory Diagnosis

Microscopy with silver impregnation stain or fluorescent antibody. Culture on charcoal yeast extract agar containing increased amounts of iron and cysteine. Urinary antigen provides rapid diagnosis. Diagnosis can be made serologically by detecting rise in antibody titer in patient's serum.hyponatraemia, ↑ liver enzymes,hypoalbuminaemia ↑ creatine kinase.

Gram stain not useful. Can be cultured on special bacteriologic media but takes at least 10 days to grow, which is too long to be clinically useful. Positive cold-agglutinin test is presumptive evidence. /nonspecificComplement fixation test for antibodies to Mycoplasma pneumoniae is more specific. (4fold or greater rise in specific antibody titer)

Serologic tests for antibody in patient's serum.

Treatment Azithromycin or erythromycin. Rifampin can be added in severe cases.

Erythromycin or tetracycline. A tetracycline, such as doxycycline.

Prevention No vaccine or prophylactic drug is available.Middle to old age. Some features more common, e.g. headache, confusion, malaise, myalgia, high fever and vomiting and diarrhoea. Smoking, corticosteroids, diabetes, chronic kidney disease increase risk.

Children and young adults. Epidemics occur every 3–4 years, usually in autumn.

Young to middle-aged. Large-scale epidemics or sporadic; often mild, self-limiting disease. Headaches and a longer duration of symptoms before hospital admission. Usually diagnosed on serology

Davidson's Principles and Practice of Medicine 21st EdBaliga 250 cases in internal medicineReview of Medical Microbiology & Immunology - 10th Ed