L48: Medically Important Parasites

Question 1

Parasitism

Answer 1

• Reciprocal association in which a species depends upon another for its existence
• not necessarily a disease

Question 2

Classification of parasite

Answer 2

1. Obligatory / Facultative
2. Temporary / Permanent
3. Ectoparasite / Endoparasite

Definitive host: harbours adult/sexual stages of parasite
Intermediate host: harbours intermediate/larval stages of parasite

Question 3

Diagnosis of parasitic infection

Answer 3

1. Microscopic examination
2. Serology
3. Culture
4. PCR

Question 4

Prevention of fungal infections

Answer 4

1. Reduce number of vector (mosquitoes)
2. Avoid exposure
3. Chemoprophylaxis (anti-malarials)
4. Vaccines

Question 5

Classification of parasites

Answer 5

1. Protozoa
   - Unicellular
   —> lumen-dwelling: Entamoeba histolytica, Giardia lamblia
   —> blood and tissue: Plasmodium, Bayesian, Toxoplasma
2. Helminths
   - Nematodes (round worms, 線蟲)
   —> intestinal: hookworms
   —> blood and tissue: filariae
   - Trematodes (flukes, 吸蟲)
   —> lung: Paragonimus spp.
   —> liver: Clonorchis sinensis (Chinese liver fluke —> very common in HK)
   —> intestinal: Fasciolopsis buski
   —> blood: Schistosoma spp.
   - Cestodes (tapeworms, 絛蟲)
   —> intestinal: Taenia solium
   —> tissue: larval stage of cestodes (Taenia solium)
3. Arthropods (insects (3 pairs of legs), arachnids (4 pairs of legs))
   - Spiders, scorpions: envenoming
   - Ectoparasites: lice, fleas, maggots, ticks, mites
   - Vectors for infectious agents: mosquitoes, tsetse flies

Question 6

5 plasmodium species in Malaria

Answer 6

1. Plasmodium vivax: worldwide
2. Plasmodium falciparum: tropics (MOST DANGEROUS)
   (95%: vivax + falciparum)
3. Plasmodium malariae: sporadic
4. Plasmodium ovale: West Africa
5. Plasmodium knowlesi: Southeast Asia, Malaysia

Question 7

Transmission of malaria

Answer 7

1. Female Anopheles mosquitoes
2. Blood transfusion
3. Contaminated needle
4. Congenital

Question 8

Life cycle of Plasmodium

Answer 8

1. Infection: Sporozoites (from mosquito) enter hepatocytes
2. Pre-erythrocytic/exoerythrocytic cycle: Merozoites replicate within hepatocytes and rupture into blood)
3. Erythrocytic cycle: invasion into RBC
   - Schizogony: Merozoites in RBC
   - Gametogony: male/female Gametocytes (ingested by Anopheles mosquito)
4. Secondary schizogony: Hypnozoites in hepatocytes (P. vivax and P. ovale)
5. Within mosquito
   —> Zygote
   —> Ookinete (penetrate stomach wall of mosquito)
   —> Oocyst
   —> Sporogony: formation of Sporozoitesu

Question 9

Pathogenesis of malaria

Answer 9

1. Destruction of RBC
   - Decreased deformability
   - Infection by parasites
   - Clearance in splenomegaly
2. Haemolysis —> Iron depletion, Anaemia, Haemoglobinuria
3. Microvascular obstruction (adhesion of infected RBC to vessel by P. falciparum —> may obstruct brain vessels: dangerous)
4. Nephrotic syndrome (immune complex deposition in chronic infection by P. malariae)
5. Inflammatory cytokines

Question 10

Signs and symptoms of malaria

Answer 10

1. Paroxysms / periodic fever pattern
   - chills and rigors for 1-2 hours, then spiking fever in next few hours
   - sweating and defervescence: rapid drop in body temp
   - P. malariae: 72 hour cycle; others: 48 hour cycle
   - Severe symptoms: Cerebral malaria, anaemia, renal failure, shock
2. Relapse: recurrence of symptoms after initial COMPLETE clearing of parasitaemia: re-invasion of bloodstream by hypnozoites (P. vivax and P. ovale)
3. Recrudescence: recurrence of symptoms after initial INCOMPLETE clearing of parasitaemia (only reduced to low level)

Question 11

Diagnosis of malaria

Answer 11

• Consider malaria in ALL cases of unexplained fever after seventh day of stay in endemic area
  1. History (travel history, fever pattern, blood transfusion)
  2. Specimen (blood anticoagulated with EDTA)
  3. Blood films stained with Giemsa, Wright, Field’s stain (1 set of negative blood smear does not rule out malaria)
  4. Antigen detection, PCR

Question 12

Treatment of malaria

Answer 12

Principles:
- Supressive therapy / chemoprophylaxis: destroy asexual erythrocytic stage to prevent symptoms
- Clinical cure: eliminate asexual erythrocytic stage during acute attack
- Radical cure: eliminate gametocytes and hypnozoites (P. vivax and P. ovale)

Antimalarial agents:
- Blood schizonticide: Chloroquine, Mefloquine, Atovaquone-proguanil
- Tissue schizonticide (kill hypnozoites): Primaquine

Question 13

Prophylaxis of malaria

Answer 13

1. Chemoprophylaxis
   - Start before going to endemic area
   - Continue regularly
   - Continued after returning
2. Vector control
3. Avoid exposure (insect repellent)
4. Vaccine