Parasitic Infections

Question 1

Explain the differences between endoparasites and ectoparasites, and between protozoan and metazoan parasites.

Answer 1

Infection vs Disease

Infection: invasion by and growth of pathogenic microorganisms within the body

Disease: a disordered or incorrectly functioning organ, part, structure or system of the body resulting from the effect of genetic or developmental errors, infection, poisons, nutritional deficiency or imbalance, toxicity, or unfavorable environmental factors; illness; sickness; ailment.

Parasite; organism living in or on the host and dependent on it for nutrition - causing damage

2 different type of parasites

Endoparasites:

1. Protozoa – amoeba, coccidia, ciliate, flagellates.
2. Metazoa – roundworms, flatworms, flukes

Ectoparasites

Question 2

Endoparasites

Answer 2

Protozoa – single-celled organisms.

• Eukaryotic organisms – genome is within a nucleus, complex organelles.
• Causes of pathogenesis varies.
• Some have insect vectors.
• NO eosinophilia

Protozoa – Amoebae, Coccidia, Ciliates, Flagellates:

Metazoa – multi-cellular organisms:

• Free-living, have intermediate hosts and vectors.
• Some just inhabit the gut (geo-helminths) whilst other invade tissues.
• Eosinophilia if they invade the blood.

Question 3

Amoeba

Answer 3

Forms:

• Entamoeba histolytica – infects 10% of the world’s population – pathogenic in 10% of cases
• Entamoeba dispar – normal commensal of the GI tract.

Infection occurs by ingestion of mature cysts in food or water, or on hands.

• Epidemiology:
  
  • 3^rd most common cause of death – after schistosomiasis and malaria.
  • 90% of infections are asymptomatic.
  • Incubation period is as short as 7 days; tissue invasion occurs during first 4 months of infection.
  • Causes dysentery and amoebic liver abscess
  • Asymptomatic carriers pass cysts in the feces and the asymptomatic carriage state can persist indefinitely

Life cycle and symptoms:

• Humans are the only reservoir.
• Cysts enter small intestine and release parasites which invade epithelial cells and cause ulcers.
• Infection spreads via the venous system to liver or brain

Diagnosis:

• Wet mount (feces on a slide) – microscopy of cysts

Treatment:

• Nitroimidazole derivatives, parmomycine or diloxanide furoate ?

Question 4

Coccidia

Answer 4

Organisms and associated disease:

• Plasmodium species – malaria (VECTOR BORNE).
  
  • Facts:
    
    • 2 hosts include humans and female anopheles’ mosquito.
    • 2 stages in humans – liver and blood stages (blood-stage = starting to cause symptoms).

Different types of plasmodium: P. falciparum (most severe), P.malariaw, P.ovale, P. vivax

Symptoms – can appear as early as 7 days but the time between exposure and signs of an illness can be as long as one year

1. fever
2. headache
3. chills
4. emesis
5. myalgia.

Complications

1. severe anaemia
2. cerebral malaria.

• Treatments – different treatments for severe and uncomplicated malaria.
• Diagnosis – blood film, antigen detection tests.

Question 5

Coccidia: toxoplasma, cryptosporidium

Answer 5

Toxoplasma – toxoplasmosis – mild disease in immunocompromised – major for pregnancy!

• Toxoplasmosis gondii.
• Caused by – infected food (warm-blooded animals with cysts or cat faeces contamination), blood transfusion, organ transplantation, trans-placentally to foetus.
• Diagnosis: serological test: measuring antigen for toxo

Cryptosporidium – diarrhoea.

• Symptoms – diarrhoea, fever, nausea, emesis. Very common in HIV patients.
• Diagnosis – stool examination.
• Treatment – fluid rehydration.

Question 6

Ciliates and Flagellates

Answer 6

Ciliates

Organisms – Balantidum coli -> balantidiasis.

• Reservoir hosts – pigs, rodents, primates (infects people worldwide).
• Transmitted by feces

Symptoms:

• Most asymptomatic (problem of asymptomatic carriers)
• Immunocompromised people – persistent diarrhoea, dysentery, abdominal pain, weight loss, nausea, emesis; all leading to perforation of the colon.
• Diagnosis – stool examination.

Flagellates:

1. Organism – Giardia lamblia -> Giardiasis.
   
   • Symptoms – diarrhea.

Epidemiology: 2% of adults and 6-8% of children in developed countries and 33% in people of developing countries

Diagnosis: stool examination: cysts, trophozoites

Treatment: metronidazole/ tinidazole

Organism – Trichomonas:

• Most common curable non-viral STD
• Transmitted sexually.
• Symptoms:
  
  • Females – 10-50% asymptomatic – vaginal discharge, vulval itching, dysuria, offensive odour
  • Males – 15-50% asymptomatic – discharge and/or dysuria.
• May enhance HIV risk (enhance its transmission) and is very bad for pregnant women: detrimental outcome: preterm delivery and low birth weight
• Diagnosis and treatment – microscopy and metronidazole.

Organism – Leishmania – sand fly vector, 4 main types of Leishmania.

Question 7

List the three main types of metazoan parasites, naming important pathogens of each type of metazoa

Answer 7

Metazoa – Roundworms, Flatworms, Flukes:

• Metazoa/Helminths:
  
  • Multicellular parasites.
  • Cycles may involve insect vectors and intermediate hosts.
    
    • For most, humans are the definitive host – a few are zoonoses (acquired from animals).
  • Adult worms cannot multiply in man – the number of adults is related to the infection.
  • A large burden is found in school-aged children which has a massive effect on their development.

Worms:

1. Roundworms/Nematodes – Ascaris, hookworm, filaria, Strongyloides.
2. Flatworms/Cestodes – Taenia (tapeworms).
3. Flukes/Trematodes – Schistosoma.

Question 8

Roundworms/Nematodes

Answer 8

Ascariasis:

Life cycle:

• Live in the small intestine and eggs are passed with faeces.
• After infective eggs are swallowed, larvae hatch and invade the intestinal mucosa. They are carried via the portal and then systemic circulation to the lungs.
• The larvae mature in the lungs (about 2 weeks), penetrates the alveolar walls, ascends and is then swallowed.
• When they reach the small intestine, they develop into adults (live for 1-2 years).

Symptoms

1. often asymptomatic but can cause abdominal pain or intestinal obstruction
2. Adults feed on SI contents -> malnourishment.
3. Penetration of lungs can cause Loeffler’s pneumonia – pools of blood clog the lungs.

Diagnosis – stool examination.

• Treatment – albendazole or mebendazole.

Hookworm:

• Stuck in the stomach and can suck ?
• Causes iron-deficient anaemia because it causes localised bleeding.

Life cycle:

• Larvae can go through the skin
• They are carried through the circulatory system to the heart and then lungs.
• Penetrate the alveoli and ascend to be swallowed and reach the SI to mature.
• In the SI, they attach to the lumen and cause localised bleeding.

Symptoms

1. mostly asymptomatic
2. iron deficient anemia, cardiac complications, local skin manifestations, respiratory symptoms.

Diagnosis – stool examination.

• Treatment – albendazole or mebendazole.

Whipworm – Trichuris Trichiura:

Life cycle:

• Eggs hatch in SI and release larvae to mature in colon.
• Adults live in the caecum and ascending colon and are fixed in this location where it produces eggs.
• The worms leave open wounds that causes inflammation of the intestinal wall -> it will leave open wounds, in a few cases rectal prolapse

Symptoms – bloody diarrhoea, anaemia (severe vitamin and mineral loss).

Diagnosis – stool examination.

• Treatment – albendazole or mebendazole.

Lymphatic filariasis, Filaria:

• Two main types – Brugia malayi, Wucheria bancrofti

Symptoms – Cause elephantiasis when they block the lymphatic systems.

• Both are transmitted by a mosquito
• Microfilariae are found in peripheral blood.
  
  • During the day, they are present in deep veins and at night they migrate to the peripheral circulation.
• Diagnosis – blood smear or antigen detection with an immunochromatic test (CARD) or ELISA

Loiasis – Loa loa:

• These can get into the eye and is confined to Africa.
• Adults can live for 4-12 years.
• Life cycle:
  
  • Females migrate through the SC tissues and may cross the front of the eye.
  • Microfilariae circulate in the blood from where it can be picked up by the Chrysops. In the gut of the fly, the larvae mature into the infective third stage larvae.
  • These larvae infect the next host when chrysops takes a blood meal and they mature into adults.

Question 9

Flatworms/ cestodes

Answer 9

Tapeworm – Taenia:

Humans are the only definitive hosts for:

• Taenia solium, Taenia asiatica – PIG.
• Taenia saginata – BEEF.

Symptoms – most people are asymptomatic.

• T. saginata (up to 10 meters) often experience more symptoms – abdominal pain, loss of appetite, weight loss, upset stomach (white segment in stool)
• T. solium (up to 3 meters) causes cysticercosis – most common cause of acquired epilepsy worldwide.

Diagnosis – tapeworm segments in stool or identification of eggs in stool.

Treatment – Praziquantel.

Question 10

Flukes/ Trematodes

Answer 10

Schistosomiasis:

The snails allow the parasitic transmission

3 main types:

• Schistosoma mansoni. Eggs to SI.
• Schistosoma haematobium. Eggs to bladder and ureter.
• Schistosoma japonicum. Eggs to SI.

Life cycle:

• Eggs eliminated in water via faeces or urine. These hatch and release miricidia, which penetrates the snail. The snails release cercariae that penetrate the skin.
• They then migrate to different tissues and transform into adults and females which reside in venules. Eggs move progressively towards the lumen of the SI (mansoni and japnicum) or the bladder and ureters (haematobium).

Symptoms:

• Days – rash/itchy skin – most people are asymptomatic early on.
• Months – fever, chills, cough, myalgia.
  
  • Repeated infection in children can cause anaemia, malnutrition and learning difficulties.

Children who are repeatedly infected can develop malnutrition learning disability (liver)

Diagnosis – stool or urine examination.

Treatment – praziquantel.

Question 11

Name two ectoparasites, explain the symptoms they cause, and how they are diagnosed.

Answer 11

Scabies – Sarcoptes scabei and Lice:

• Symptoms – rash and burrows presence.
• Treatment – scabicides.
• Lice:
  
  • Types of lice:
    
    • Pediculus humanis corporis – body louse.
    • Pediculus humanis capitis – head louse.
    • Pthirus pubis – crab louse of pubic area.
• Life cycle:
  
  • Exist in 3 stages – eggs, nymphs, adults.
  • Transmission is by direct contact.

Question 12

Explain what is meant by a vector and list two specific examples, naming the diseases they transmit.

Answer 12

Parasite trasmitted by mosquito

Attachment and phagocytosis

Leishmania promastigotes:

They swim in the direction of the flahhelum

Sandflies are found mainly in southern Europe, Asia, Africa, Australia, central and south America

Appearance and behavior

• small
• hairy
• they hop around before setting down to bite
• unless mosquitos, their attack is silent

Major forms:

Visceral leishmaniasis: affects internal organs

Kala azar

Vector – an organism that transmits a disease or parasite from one organism to another.

• Snail – transmits schistosomiasis (Fluke, Metazoa, endoparasite).
• Female anopheles’ mosquito – transmits malaria (Coccidia, protozoa, endoparasite).
• Chrysops – transmits Loiasis (Roundworm, Metazoa, endoparasite).