GI parasites Flashcards
Why do doctors need to know about parasites?
- common infections generally cause mild disease but more severe if severe infestation oro immunocompromised eg giarfdia, cryptosporidium, pin and whipworm, head lcie
- mild to severe disease in special groups e.g. strongyloides and hookworm in Aboriginal populations, E histolytica in MSM, hydatid in sheep, Toxopalsmosis in pregnant
- acute in returned travellers e.g malaria and enteric parasites; as well as refugees
- glocval: signigicant morbidity and mortlaity, often gn=eglecte,d leading to limited treatemnt optons
- ## lifecycle: where and how catch it, incubation, symptoms, diagrnosis, treatment, and prevention and individual and comminity level
List and briefly define the groups of parasites
- Protozoa – single-celled eukaryotes
-
Metazoa – multi-celled eukaryotes
- Helminths - worms
- Arthropods – insects; cause disease or serve as vectors
Where do we encounter parasites?
- Simple to complex lifecycles – determine transmission routes
- May have multiple hosts and/or environmental stage
- Many parasites are considered zoonoses (a disease of animals that can be transmitted to humans); difficult to control especially if not domesticated animal
Describe parasite hosts
- Host specificity
- range of different hosts in which a parasite can mature
- can be fairly specific to a particular host species
- Definitive Host: parasites mature into adult form; Adult undergoes sexual reproduction;
- Incidental Host: Parasite develops but does not mature sexually; may undergo asexual reproduction
- also paratenic: parasite alive and infective but no development
- incidental: intermediate; does not allow transmission to another host
- Humans may serve as
- Definitive
- Intermediate
- Incidental/Dead end
e.g. humans as definitive host for tapeworm:
- eat cyst or larval form in pork
- adult tapeworm develops in intestine
- eggs excreted in faeces
e.g. humans as intermediate host for tapeworm:
- eat egg in contaminated food or water
- larvae released and migrates to various tissues and encysts
- will only complete lifecycle if another human eats tissue with cysts
How do parasites cause disease?
- Different parasitic forms have specific tissue tropisms that dictate clinical presentation e.g., hookworm - GIT, malaria - liver/blood, pork tapeworm cysts - brain
- Whether the host is symptomatic often depends on infection density, site of infection and immune response
General consequences of parasite infections
- Nutritional stress i.e. depletion of certain nutrients, damage to absorptive surfaces in bowel
- Mechanical or physical tissue damage resulting from blockage during growth, parasite replication or migration, and pressure
- Toxicity from parasitic products or metabolites
- Immunopathological reactions - inflammation (chronic), immunodeficiency
- Secondary infections – usually bacterial
Describe protozoa in more detail
- free living unicellular eukaryotic organisms
- most protozoa live in watery environment and form hardy cysts in unfavourable conditions
- classified based on their motility
- mastigophorans/flagellates
- sarcodines or amoebae - pseudopodia
- ciliates
- apicomplexans or coccidia - immobile
Describe the types of protozoa
Examples of the Flagellates
- Characterised by the presence of flagella
- Trichomonas vaginalis: Vaginitis/urethritis
- Giardia: Diarrhoea
- Trypanosoma: African sleeping sickness
Protozoa – the Sarcodines/ Amoebae
- Amoeba – move by means of pseudopodia (+/- flagella)
- Feed mainly on other microorganisms
- Of medical significance - amoebic dysentery (diarrhoea) & liver abscess
- Entamoeba histolytica
Protozoa – the Ciliates
- Move through the action of cilia which covers their surface
- The only ciliate to parasitize humans is Balantidium coli – associated with pigs and causes diarrhoea (very uncommon)
Apicomplexans or Coccidia
- Immobile
- Protozoan with complex lifecycles
- Enzymes present in the apex of the cell digests their way into host cells
- Examples of genera of medical significance include
- Plasmodium species – malaria
- Toxoplasma gondii – lymphaenopathy/neurological/foetal infections
- Cryptosporidium - diarrhoea
- Cyclospora - diarrhoea
List the invasive forms of protozoa, and distinguish between cyst and oocyst
- Sporozoites
- Merozoites
- Trophozoite
-
Tachyzoites/Bradyzoites
All are infective forms involved in active infection and invasion of host cells.
What they are called depends on the parasite e.g. giardia and trophozoite and toxoplasma and tachyzoites.
Protozoan Stages – cyst forms
- Cyst – many protozoa can secrete a resistant covering and enter a resting stage
- Oocyst – formed after gamete union in coccidians (Apicomplexans)
Provide an overview of protozoa lifecycles
- In general
- Protozoal gastrointestinal infections are characterised by a very low infective dose i.e. few cysts/oocyts required to cause infection
- Protozoa replicate to very high numbers in the GIT
i. Intracellularly
ii. Extracellularly - Large numbers of cysts/oocysts are shed in the faeces
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Descriebge giardia
- G. lamblia =
- G. intestinalis =
- G. duodenalis
- Giardia has a wide host range in which it can complete its entire lifecycle
- Common
- Transmission from contaminated water or directly such as in childcare centres
Pathophysiology of Giardia infection
- Excystation occurs in response to exposure to stomach acid, releasing four trophozoites (TZ)
- TZ colonise the small intestine
- TZ undergo rapid asexual reproduction
- Encystation occurs as TZ transit through the large intestine
Pathophysiology of Giardia infection – Encystation
- Flattening of the villi and inflammation of the mucosa is observed
- Pathophysiological mechanisms potentially including
- Direct damage to the intestinal brush border and mucosa
- Alteration of absorption
- Alteration of small intestinal function
- Induction of a host immune response resulting in secretion of fluid and damage to the gut
Symptoms
- range from
- asymptomatic
- chronic diarrhoea
- epigastric pain
- nausea and vomiting
- bloating and flatulence
- malabsorption
- weight loss
- symptoms: 6-15 days after exposure
- usually last several days but are self limiting
Treatment of Giardiasis
- Treatment
- Metronidazole
- Tinidazole
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Describe entamodba
- Several species
- Entamoeba histolytica causes invasive disease
- Other species considered non-pathogenic
- Travellers, MSM
Pathophysiology of Entamoeba histolytica Infection
- Excystation occurs in the small intestine
- The trophozoites replicate in the large intestine and may then invade through bowel wall
- Encystation occurs in large intestine and excreted in faeces
- mediates extensive local necrosis: attaches to local enterocytes, pridces cytotoxin that lyses enterocytes, neutrophils and other immune cells; causes flask shaped ulcerations
- invasion to deeper mucosa facilitates spread to extraintestinal sites, most commonly liver
Symptoms and Treatment of Amoebiasis
- Symptoms vary from
- Luminal amoebiasis – asymptomatic
- Invasive intestinal amoebiasis - dysentery, colitis, appendicitis
- Invasive extraintestinal amoebiasis - liver abscess, peritonitis
- Treatment
- Metronidazole (trophozoites)
- Iodoquinol, paromomycin, or diloxanide furoate (cysts)
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Describe cryptosporidium
- thick walled oocyst ingested, sporulated thick walled oocyst excreted, contaminates water and food
- Note – resistant to chlorination. Outbreaks can be associated with water supply, swimming pools
Pathophysiology of Cryptosporidium infection
- goes through several lifecycle stages
- asexual and sexual cycles
Cryptosporidium Infections - Symptoms
- Immunocompetent adults - self-limiting gastroenteritis lasting 1-2 weeks most commonly characterized by watery diarrhea
- Immunocompromised populations – potentially disseminated disease and/or life threatening symptoms
- Cholera-like illness
- Chronic diarrheal illness
- Intermittent diarrheal illness
- Transient diarrheal illness
Extremely difficult to treat
- Nitazoxanide – need to import to Australia under special access scheme
Provide an overview of helminths
- Multicellular “worms”
- Bilaterally symmetrical
- Head
- Tail
- Tissues – usually three distinct layers
i. Ectoderm
ii. Mesoderm
iii. Endoderm
- Undergo different life stages in addition to the adult or worm stage including
- Larvae - usually several larval developmental stages
- Eggs
Helminths – Trematodes - Flukes
- All flukes require molluscs (snails, clams) as the first intermediate hosts
- Tissue flukes
- Lung flukes – Paragonimus westermani
- Liver flukes – Fasciola hepatica, Clonorchis sinensis
- Intestinal flukes – Fasciolopsis buski
- Blood flukes
- Schistosoma sp
Helminths – Cestodes - Tapeworms
- Echinococcus granulosa & multilocularis (hydatid)
- Taenia solium & saginata (pig/cattle tapeworm)
- Diphyllobothrium latum (fish tapeworm)
- Diphylidium caninum (dog tapeworm)
- Hymenolepsis nana (dwarf tapeworm)
Helminths – Flatworms - Cestodes
- General structure
- Head (scolex) contains cup-shaped suckers and a crown of hooklets
- Flat, ribbon-like body comprised of proglottids – contain male and female reproductive organs (hermaphrodite)
- Food is absorbed through the body wall
- Complex life cycles involving intermediate hosts
Describe the types of helminth infections
- Blood-tissue
i. Adult worms live in the blood, lymph, subcutaneous, and connective tissue
ii. Produce larval worms called microfilariae that are transmitted by mosquitos or biting flies- Intestinal infections
Blood & tissue nematodes - tropics
- Wuchereria bancrofti (worldwide/SE Asia/Pacific)– elephantiasis
- Onchocerca volvulus (Africa/South America) – river blindness
- Loa loa (Africa) - African eye worm
Intestinal nematodes
- Extremely common - cosmopolitan & tropics
- Enterobius vermicularis (pinworm)
- Trichuris trichiura (whipworm)
- Ancyclostoma sp & Necator americanus (hookworm)
- Strongyloides stercoralis
- Ascaris lumbricoides
Describe pinworm
- worldwide distribution
- young children
- asymptomatic
- perianal itching
- sleep disruption
- migration into vagina
- abdominal pain
- appendicitis
