Introduction to Opportunistic Infection

• PICC stands for Peripherally Inserted Central Catheter, inserted only under imaging-guidance
• Galactomannan (GM) is a polysaccharide antigen that exists primarily in the cell walls of Aspergillus species
• Beta-d-glucan (BD Glucan) is a panfungal marker which is a useful adjuvant to the diagnosis of invasive candidiasis/invasive fungal disease
• In the case presented, a 72-year-old patient post chemotherapy for lung small cell carcinoma via a PICC line presents with fever, abdominal tenderness, and low white cell count, indicating a possible infection
• Investigations to send to the microbiology lab for the patient include blood cultures and other relevant tests
• Opportunistic pathogens are organisms of low intrinsic virulence that may cause serious infections in immunocompromised patients or when a foreign body is present
• Features of opportunist pathogens include low pathogenicity, adaptability to host and circumstances, difficulty in eradication, and variable clinical presentation
• Opportunistic pathogens may come from endogenous sources like the microbiome (GIT, skin) or exogenous sources like the environment or healthcare staff
• Immunocompromised patients are those compromised by their underlying condition, while specific immunocompromised patients have a recognized deficiency of immune parameters
• Opportunistic infections are an important concern due to the increased number of immunocompromised patients, causing significant morbidity and mortality, often being polymicrobial and complex
• Common opportunistic bacteria include S. epidermidis, S. aureus, E. coli, Enterococci, P. aeruginosa, Listeria, and mycobacteria
• Common opportunistic viruses include Herpes simplex, Varicella-zoster, Cytomegalovirus, Polyomaviruses, Parvovirus B19, and Hepatitis B
• Common opportunistic fungi include Candida spp., Aspergillus fumigatus, Cryptococcus neoformans, and Pneumocystis jirovecii
• Common opportunistic protozoa include Toxoplasma gondii and Cryptosporidium parvum
• Neutropenia, defined as a low neutrophil count, makes patients vulnerable to infections caused by opportunistic pathogens
• Immunosuppressive treatments causing neutropenia, like myelosuppressive chemotherapy, increase the risk of infections by opportunistic pathogens
• Neutropenic sepsis can be caused by various pathogens like Gram-positive cocci, Gram-negative bacilli, and fungi, affecting different sites of infection
• Infections related to IV lines, like the PICC line in the case, are commonly caused by Coagulase-negative staphylococci and managed with source control and appropriate antibiotics
• Neutropaenic enterocolitis, more common in neutropaenic patients with haematological malignancies, is usually due to a combination of chemotherapy-induced mucosal injury, profound neutropenia, and impaired host defenses
• Haematological malignancies are usually due to a combination of chemotherapy-induced mucosal injury, profound neutropenia, and impaired host defenses to invasion by bowel organisms, leading to necrosis of various layers of the bowel wall
• Pathogens involved in haematological malignancies include gram-negative bacilli, gram-positive cocci, anaerobes like Clostridium septicum, and Candida spp, with polymicrobial infection being frequent
• Clinical presentation of haematological malignancies includes fever, abdominal distension, tenderness, nausea, vomiting, and/or diarrhea, with bacteremia or fungemia being common
• Candida invasive infections are a major disease of immunocompromised hosts, with risk factors including immunosuppression, intravenous catheters, parenteral nutrition, antibiotic therapy, post-operative GI surgery, and diabetes mellitus
• Diagnosis of Candida invasive infections involves clinical suspicion, fever with no response to broad-spectrum antibiotics, laboratory tests, and treatment includes antifungal agents like azoles and echinocandins
• Aspergillus invasive infections, with A. fumigatus being the most common, have poor prognosis and risk factors include prolonged neutropenia and high-dose steroids
• Diagnosis of Aspergillus infections involves imaging like CXR/CT thorax, galactomannan antigen testing, and treatment with antifungal agents like azoles and amphotericin B
• Pneumocystis (carinii) jirovecii infections have declined with prophylaxis, with risk factors including HIV, high-dose steroids, and transplantation, and treatment involves high-dose IV cotrimoxazole
• Cryptococcus neoformans infections are diagnosed through antigen detection in blood and CSF, culture, and treatment includes Amphotericin B
• Investigations for patients with suspected infections include history, examination, blood cultures, microbiologic testing, radiology like CT scans, and treatment of neutropenic fever involves stages with specific antibiotics and antifungals
• Possible causes of ongoing pyrexia in patients include occult abscess, antibiotic-resistant organisms, intravascular catheter infections, viral infections like CMV, HSV, VZV, respiratory viruses, protozoal infections, and invasive fungal diseases
• Prevention of infections in immunocompromised patients involves high standards of asepsis and antisepsis, patient and carer education, appropriate prophylaxis, vaccination, antimicrobial prophylaxis, and protective isolation
• Summary: The population susceptible to opportunistic infections is increasing, with predisposing conditions including neutropenia, HIV/AIDS, transplants, burns, and the presence of foreign bodies, requiring repeated assessments and often multiple antimicrobial agents