Microparasitology Final Review: Helminths and Bacteria

Cestodes

Tapeworms (flat, segmented) with no digestive tract that absorbs nutrients.

Trematodes

Flukes (flat, leaf-like) that are a type of helminth.

Nematodes

Roundworms (cylindrical, unsegmented) that are parasitic worms.

Scolex

The head of a cestode, equipped with suckers/hooks.

Strobila

The body of a cestode, consisting of proglottids.

Proglottids

Segments of the strobila in cestodes, which can be immature, mature, or gravid.

Diphyllobothrium latum

A key species of cestode with operculated eggs, infective stage is plerocercoid, sourced from raw freshwater fish.

Taenia spp.

Includes T. solium (pork), which causes cysticercosis (larval brain infection).

saginata

Intestinal taeniasis

Egg of saginata

3 hooklets, radial striations

Infective stage of saginata

Cysticercus

Hymenolepis nana

Dwarf tapeworm

Hymenolepis nana characteristics

No need for an intermediate host

Egg of Hymenolepis nana

Polar filaments + thickenings

Hymenolepis diminuta

No hooks

Egg of Hymenolepis diminuta

No polar filaments

Dipylidium caninum

Found in dogs/cats

Dipylidium caninum characteristics

Double-pored proglottids

Eggs of Dipylidium caninum

In packets (5-30)

Echinococcus granulosus

Smallest tapeworm

Echinococcus granulosus disease

Causes hydatid cyst (mainly in the liver)

Egg of Echinococcus granulosus

Similar to Taenia (not diagnostic)

Trematodes

Flat, leaf-shaped

Trematodes reproductive system

Hermaphroditic (except Schistosoma)

Trematodes digestive system

Incomplete digestive system

Trematodes intermediate host

Require snail as first intermediate host

Infective stage of Trematodes

Metacercaria

Egg of Trematodes

Usually operculated

Lifecycle of Trematodes

Egg → Miracidium → Sporocyst → Redia → Cercaria → Metacercaria → Adult

Mnemonic for Trematodes lifecycle

My Sister Rides Cool Motorcycles Always

Key species of Trematodes

Fasciola hepatica/gigantica

Fasciola hepatica

Sheep liver fluke

Fasciola hepatica characteristics

Cephalic cone on adult

Eggs of Fasciola hepatica

Are immature

Clonorchis sinensis / Opisthorchis

Liver flukes

Eggs of Clonorchis sinensis / Opisthorchis

Operculated, with small knob

Clonorchis sinensis / Opisthorchis disease

May lead to bile duct cancer (cholangiocarcinoma)

Nematodes

Cylindrical, unsegmented

Nematodes digestive system

Complete digestive tract

Nematodes reproductive system

Separate sexes

Nematodes sensory structures

Amphids (head sensors), some have phasmids (tail)

Ascaris lumbricoides

Largest intestinal roundworm

Infective stage of Ascaris lumbricoides

Embryonated egg

Ascaris lumbricoides migration

Can migrate to lungs (causes VLM)

Trichuris trichiura

Whipworm

Egg of Trichuris trichiura

Lemon-shaped with bipolar plugs

Trichuris trichiura disease

Can cause rectal prolapse

Enterobius vermicularis

Pinworm

Egg of Enterobius vermicularis

D-shaped

Diagnosis of Enterobius vermicularis

Diagnosed by Scotch tape test

Symptoms of Enterobius vermicularis

Itchy anus (especially at night)

Capillaria philippinensis

From raw freshwater fish

Egg of Capillaria philippinensis

Peanut-shaped with flat plugs

Capillaria philippinensis disease

Causes malabsorption + weight loss

Hookworms

Infective: Filariform larvae through skin

Hookworms disease

Causes iron deficiency anemia

Strongyloides stercoralis

Penetrates skin

Strongyloides stercoralis characteristics

Larvae already developed inside egg

Strongyloides stercoralis affected population

Affects immunocompromised

Trichinella spiralis

From undercooked pork

Trichinella spiralis characteristics

Larvae encyst in muscle

Trichinella spiralis disease

Causes Trichinosis

Angiostrongylus cantonensis

Rat lungworm

Angiostrongylus cantonensis disease

Causes eosinophilic meningitis

Angiostrongylus cantonensis source

From snails/slugs

Dracunculus medinensis

Acquired via water

Infective stage of Dracunculus medinensis

L3 larva

Dracunculus medinensis disease

Adult causes painful ulcers

Mycobacteria

Acid-fast (due to mycolic acid)

Mycobacteria staining method

Ziehl-Neelsen stain

Mycobacteria growth media

Grows slowly on LJ or Middlebrook media

Mycobacterium tuberculosis

Causes tuberculosis

Transmission of Mycobacterium tuberculosis

Transmitted by droplets

Mycobacterium tuberculosis pathology

Causes granulomas with caseation necrosis

Detection of Mycobacterium tuberculosis

Detected by Mantoux test, AFB stain

Niacin & Nitrate tests for Mycobacterium tuberculosis

(+)

BCG vaccine

Derived from M. tuberculosis

Runyon Classification (NTMs)

Classification of non-tuberculous mycobacteria based on pigment production and growth rate.

Photochromogens

Pigment in light (e.g., M. kansasii).

Scotochromogens

Pigment in dark or light (e.g., M. gordonae).

Non-photochromogens

No pigment (e.g., M. avium).

Rapid growers

Grow in less than 7 days (e.g., M. fortuitum).

M. leprae (Leprosy)

Affects skin and peripheral nerves.

Tuberculoid leprosy

Characterized by few lesions and strong immunity.

Lepromatous leprosy

Characterized by many lesions, deformity, and poor immunity.

Spirochetes

Helically coiled bacteria with axial filaments.

Leptospira interrogans

Causes Leptospirosis; severe form is Weil's disease.

Borrelia burgdorferi

Causes Lyme disease; vector is Ixodes tick.

Borrelia recurrentis

Causes relapsing fever, louse-borne.

Treponema pallidum

Causes syphilis; stages include primary chancre, secondary rash + condylomata lata, tertiary gummas + CNS damage.

Neisseria gonorrhoeae

GNDC in PMNs; causes gonorrhea and neonatal conjunctivitis.

Neisseria meningitidis

GNDC, capsulated; causes meningitis and petechiae.

Moraxella catarrhalis

GNDC; causes otitis media and sinusitis.

BAP

Used for hemolysis pattern (alpha, beta, gamma).

CAP

Used for fastidious organisms (e.g., Neisseria).

MAC

Used for lactose fermenters.

MSA

Used for Staphylococcus (mannitol fermenters).