Infectious Disease Flashcards
Necrotizing fasciitis
This infection is typically caused by a mixture of aerobic and anaerobic organisms leading to the necrosis of subcutaneous tissue. Signs of infection include tissues that become hot, red, and swollen, resembling severe cellulitis. Severe pain is common. Without timely treatment the area of infection becomes gangrenous. Patients present acutely ill. Diagnosis is through history and examination, supported by evidence of overwhelming infection. Treatment includes antibiotics and surgical debridement. Prognosis is poor without early aggressive treatment.
Most necrotizing fasciitis cases result from infection with group A streptococcus (Streptococcus pyogenes) or a mixture of aerobic and anaerobic bacteria (Bacteroides species). Organisms establish infection in subcutaneous tissue from an ulcer, infection elsewhere (Streptococci can arrive from a remote site of infection via the bloodstream), or after trauma. Perineal infections usually result from a complication of a recent surgery and perirectal abscesses. Patients with diabetes are also at risk for developing necrotizing fasciitis.
The primary symptom of infection is intense pain (pain out of proportion of clinical findings). Tissue is hot, red, and swollen and rapidly becomes discolored. Bullae, crepitus (resulting from soft tissue gas), and gangrene may develop. Subcutaneous tissues necrose but muscles are spared initially. Patients present acutely ill with a high fever, high heart rate, altered mental status (confusion), and low blood pressure. Patients may be bacteremic or septic. Streptococcal toxic shock syndrome may develop.
Diagnosis is made by history and examination and is supported by leukocytosis, soft-tissue gas on x-ray, positive blood cultures, and deteriorating metabolic and hemodynamic (blood pressure) status. Mortality rate is about 30%. Old age, underlying medical issues, delayed diagnosis, and insufficient surgical debridement worsen prognosis. Treatment is primarily surgical (immediate) with IV antibiotics and fluids.
Acute infectious arthritis
This is a joint infection that progresses very rapidly (within hours to days). The infection resides in synovial or periarticular tissues and is usually bacterial. In young, sexually active adults infections are usually caused by Neisseria gonorrhoeae. Symptoms include a rapid onset of pain, effusion, and restriction of range of motion, usually within a single joint. Diagnosis requires synovial fluid analysis and culture. Treatment is through IV antibiotics and drainage of the pus from the infected joints. Risk factors for infectious arthritis include: • Advanced age (>60 years) • Alcoholism • Arthrocentesis or joint surgery • Bacteremia • Cancer • Chronic illness (lung or liver disease) • Diabetes • Hemodialysis • Hemophila • History of previous joint infection • Immunodeficiency • Immunosuppressive therapy • Injection drug use • Prosthetic joint implant • RA • Risk factors for sexually transmitted diseases (multiple sexual partners, etc) • Sickle cell disease • Skin infections • SLE
Infectious organisms reach the joints by direct penetration (trauma, bites, surgery, etc), extension from an adjacent infection (osteomyelitis, abscess, infected wound, etc), or hematogenesis spread from a remote site of infection. Common microorganisms that cause these infections are classified either as gonococcal or nongonococcal in adults. Distinction is important because gonococcal infections are far less destructive to the joint. Overall, Staphylococcus aureus is the most frequent cause of infection in adults.
Osteomyelitis
inflammation and destruction of bone caused by bacteria, mycobacteria, or fungi. Causes localized bone pain and tenderness. 80% of infections result from contiguous spread or from open wounds.
Myositis
infection of the muscle that leads to muscle inflammation. Can be caused by many different microorganisms including viruses, bacteria, and helmniths.
Bacteremia
the presence of bacteria in the bloodstream
Viremia
the presence of virus in the bloodstream
Septicemia
bloodborne systemic infection. Can lead to spread of the infectious organism to other tissues, massive inflammation, septic shock, and rapid death. Associated with bacterial infections.
Streptococcus pyogenes
gram +, cocci, catalase -, beta-hemolytic, bacitracin sensitive.
Causes necrotizing fasciitis
White Blood Cell Count
Measures the number of leucocytes per milliliter of blood.
WBC – cells of the immune system defending the body against both infections and foreign materials.
Normal range = 4,500 to 11,000 cells/ml
You get high counts during infections, inflammatory diseases, autoimmune systemic diseases, leukemia, and emotional and physical stress.
High lymphocyte count in virus infections, high neutrophil count in bacterial infections (especially banded immature)
C-Reactive Protein
Produced in the liver and is present in circulation at low levels normally. Involved in the promotion of the immune system through the activation of the complement cascade.
Normal: ≤ 1mg/dL
Elevated in bacterial infections, inflammation conditions, acute rheumatic fever, acute rheumatoid arthritis, inflammatory bowel disease, and others.
synovial fluid
foul smell means obligate anaerobe infection
the more cloudy, the more infected
Acute Infectious Arthritis
Rapid onset
Personal history
Pain
Range of motion restriction (single joint)
Synovial fluid analysis and culture
Positive blood cultures
caused by Staphylococcus aureus or Neisseria gonorrhoeae
Staphylococcus aureus
tricuspid valve endocarditis frequently affects IV drug users. Patients with granulomatous disease are vulnerable. Leading cause of osteomyelitis in children and adults.
Clinical presentation: Local = skin/subcutaneous: impetigo, cellulitis, folliculitis, furuncles, carbuncles. Respiratory: pneumonia with cavitations. Systemic = acute endocarditis, meningitis, osteomyelitis, septic arthritis.
Pathology: Bacteria colonize skin (following breach) or nasopharynx (following intubation or viral infection) → overgrow and evade host defenses using protein A, coagulase, hemolysins, and leukocidins. Neutrophils localize to site of infection → purulent abscesses form → skin/subcutaneous infections or pneumonia results. Deeper invasion into the bloodstream relies on the hyaluronidase, staphylokinase, and lipase virulence factors.
Diagnosis: blood culture for Gram (+) clusters, catalase (+), coagulase (+).
Treatment: antibiotic treatment is with either penicillinase-resistant penicillins or vancomycin
Neisseria gonorrhoeae
Obligate human pathogen. Virulence factors include specialized pili (allows attachment to mucosal surface, provides antigenic variation, prevents phagocytosis; endotoxin; capsule; and IgA protease. Higher incidence of infection with menstruation or IUD. Most common cause of septic arthritis in sexually active people. Antigenic variation prevents immunity allowing recurrent infections.
Clinical presentation: Local infection = (genital tract or anorectal infections) either asymptomatic, urethritis, dysuria (men), cervicitis (women), opthalmia neonatorum. Systemic = septic arthritis. Complications = Pelvic Inflammatory Disease (PID), ectopic pregnancy, sterility, Fitz-Hugh-Curtis Syndrome
Pathology: Colonization begins when bacterial pili attach to mucosal cells of the urethra and vagina. Evades mucosal IgA antibodies via IgA protease. Endocytosed by immune cells and kills ciliated cells. Together this leads to inflammatory response leading to urethritis (men) and cervicitis (women). In women, the infection can progress to the uterus, fallopian tubes, and ovaries (PID) which can lead to an increased risk for ectopic pregnancies. From the fallopian tubes the bacteria can spill into the peritoneal cavity causing peritonitis. This can lead to an infection of the liver capsule (Fitz-Hugh-Curtis Syndrome).
The bacteria can also invade the submucosa and enter into the bloodstream where it can collect in synovial fluid causing septic arthritis.
In neonates, it can inoculate the conjunctiva during passage through the birth canal causing opthalmia neonatorum à risk for blindness.
Diagnosis: Gram (-) diplococci within PMNs, metabolizes glucose but not maltose, selectively grows on Thayer-Martin media.
Treatment: Ceftriaxone (+ doxycycline for probable concurrent Chlamydia infections), prophylactic erythromycin eye drops for neonates. Vaccine development difficult.
Osteomyelitis
Staphylococcus aureus
Salmonella typhi
Pasteurella multocida
