Medical Microbiology: Parasites, Viruses, and Fungi
Taenia solium (Pork Tapeworm)
Life Cycle
Humans can become infected by eating undercooked pork containing cysticerci (larvae). This can lead to discomfort, diarrhea, etc. Eggs and proglottids (segments) can be found in feces. Microscopic examination reveals the scolex (head) with four suckers and two rows of hooks. The gravid segment contains a uterus with 7 to 13 branches.
Cysticercosis
Cysticercosis occurs when humans ingest T. solium eggs, leading to the development of cysticerci in human tissues. This can happen through contaminated water, vegetables, or autoinfection. If cysticerci lodge in vital areas like the brain, they can cause seizures, meningitis, and even death due to inflammation and immune reactions.
Oncogenic Papillomaviruses (HPV)
Certain HPV types have oncogenic potential due to viral DNA integration into the host cell’s chromosomes. This integration disrupts the expression of the E2 protein, leading to increased expression of E6 and E7 proteins. E6 degrades p53, activates telomerase, and promotes cell growth. E7 inhibits tumor suppressor proteins like pRb, further contributing to uncontrolled cell proliferation.
Fungal Morphology: Hyphae
Fungi exist in two main forms: yeast and filamentous. Hyphae are tubular, branching filaments that can be coenocytic (multinucleated) or septate (divided by partitions). A mass of hyphae is called a mycelium. Vegetative hyphae absorb nutrients, while aerial hyphae produce conidia for fungal dispersal. Some fungi exhibit dimorphism, switching between yeast and filamentous forms.
Diagnosis of Mononucleosis
Hemogram
Lymphocytosis (60-70%) with atypical lymphocytes (30%) is observed, often appearing early in the illness.
Heterophile Antibodies
IgM antibodies against sheep, horse, and bovine red blood cells (Paul-Bunnell test) are detectable by the end of the first week and persist for several months. This test is less sensitive in children.
Serology
Antibodies to viral capsid antigen (VCA), Epstein-Barr nuclear antigen (EBNA), and early antigen (EA) can be measured. VCA IgM and negative EBNA indicate acute infection. Increasing VCA and EA titers also suggest acute infection. Detection of both VCA and EBNA indicates past infection (EBNA presence signifies cell lysis and lymphocyte control).
Diagnosis of Plasmodium (Malaria)
Direct Staining
- Thick Film: Concentrates the sample (20-30x), increasing sensitivity for low parasitemia. Requires experienced observers.
- Thin Smear: Methanol-fixed, Giemsa-stained smear allows for morphological identification and parasite quantification.
- Acridine Orange: Stains parasite nuclei, but also other cell nuclei (e.g., Howell-Jolly bodies). Can be used with QBC II capillary centrifugation.
Molecular Diagnosis
Highly sensitive PCR targeting rRNA genes and circumsporozoite protein can identify and differentiate Plasmodium species, detect low parasitemia, assess treatment efficacy, and identify drug resistance.
Immunochromatographic Tests
Detects HRP-2, pLDH, and antibodies.
Influenza
Influenza symptoms vary from asymptomatic to severe complications. The incubation period is 1-4 days, with a prodrome of headache and malaise, followed by fever, myalgia, and nonproductive cough. The illness typically lasts around 3 days, with full recovery in 7-10 days. Complications include bronchiolitis, croup, otitis media, pneumonia (primary or secondary bacterial), myositis, encephalitis, Reye’s syndrome, and Guillain-Barré syndrome.
RNA Virus Properties
Viral RNA is labile and transient, replicating in the cytoplasm. RNA viruses must encode an RNA-dependent RNA polymerase. Genome structure determines transcription and replication mechanisms. RNA viruses have a high mutation rate. Genomic structure and mRNA polarity determine protein generation and processing.
DNA Virus Properties
Viral DNA behaves similarly to cellular DNA in transcription and replication. It is not transient and can persist in infected cells, leading to persistent infections. Early genes encode enzymes, while late genes encode structural proteins. Viral genes interact with cellular transcriptional machinery. Larger DNA viruses have greater control over their replication.