Parasitic Infections
  • Malaria

This is a mosquito borne infection which injects the parasite Plasmodium into the blood.

– There are 4 main variants of the parasite and the natural vector is the female anopheles’ mosquito

Pre-erythrocytic stage:

– Mosquito injects sporozoites into human blood

– These migrate to liver and infect hepatocytes, multiplying by mitosis

– They can stage in the liver for up to 2 years remaining immature as hypnozoites

– These differentiate into merozoites which are released from the liver into the bloodstream

 

Erythrocytic stage:

– Trophozoites infect RBCs, feeding on haemoglobin using the enzyme haem polymerase

– These form a schizont and multiply asexually, and are released when the RBC ruptures

– Some merozoites produce gametocytes instead which are uptaken by the mosquito

 

Mosquito stage:

– The mosquito ingests the gametocytes which fuse to form an oocyst

– This matures to gives sporozoites which migrate to the salivary glands and can be reinjected into human

 

General Symptoms – Cyclical Fever (according to rupture of infected RBCs), headache, splenomegaly

 

Symptoms – We can divide this into Severe and Benign Malaria

 

i) Severe – Almost always caused by Plasmodium Falciparum, the most common variant of parasite

– Gives Cyclical Fever (according to rupture of infected RBCs), headache, splenomegaly

– Temperature > 39ºC

– Metabolic acidosis

– Severe anaemia

– Hypoglycaemia

 

 ii) Uncomplicated – Due to non-Falciparum strains, and gives cyclical fever, headache + splenomegaly

P. Vivax and P. Ovale –> Gives a cyclical fever – intermittent shivering with fever every 2 days.

P. Malariae –> Gives cyclical fever every 3 days and is complicated by nephrotic syndrome

 

Complications – Infected RBCs display adhesion proteins making them stick and block small vessels

– Cerebral malaria –> Occurs when stuck RBCs break down blood brain barrier giving seizures + coma

– Blackwater fever –> Lysed RBCs release haemoglobin which gets filtered giving dark urine and AKI

– ARDS –> The anaemia and acidosis stimulate the lungs giving acute respiratory distress syndrome

– Jaundice –> Haemolytic anaemia releases unconjugated bilirubin

 

Diagnosis:

– Microscopy of thick and thin blood smear –> shows parasite on blood film

– Rapid diagnostic test (RDT) of parasite antigen

 

Treatment:

i) Uncomplicated P. falciparum –> 1st line is artemisinin combination therapy (Artemether-Lumefantrine)

ii) Severe P. Falciparum –> Artemisinin therapy with IV Artesunate

iii) Non-Falciparum –> Chloroquine (if sensitive) or artemisinin combination therapy (if resistant)

– If Ovale or Vivax, also give primaquine after to destroy hypnozoites and stop relapse

 

Prophylaxis:

– If not drug resistant or little resistance –> give chloroquine + proguanil

– If chloroquine resistance –> Mefloquine or doxycycline

 

N.B. There are some factors which give natural protection against malaria:

– Sickle-cell trait                 -G6P deficiency            – HLA-B53                   – Absence of Duffy antigens

  • Threadworm Infection

This is due to the worm Enterobius Vermicularis (pinworm), seen in children

Life Cycle:

– Children swallow eggs from the environment

– These hatch in the intestines and grown into adults in the colon

– Female worms move to perianal region to lay eggs

– Eggs are then transferred to mouth by hands and cycle repeats.

Symptoms – Most children do not experience any symptoms

– Perianal itching worse at night             

– Vulval itchiness in females

 

Diagnosis – Clinical but can use Sellotape over anus to detect eggs

 

Management – Mebendazole for child and everyone else in the house

  • Toxoplasmosis

This is an infection due to the parasite toxoplasma gondii, found in cats

Life Cycle:

– Cat acquires infection by eating cysts in infected tissue

– These then grow and release trophozoites and form tissue cysts

– Release gametes which form oocysts released in faeces

– Faeces eaten by rats and birds which then eaten by cats again

The problem is that humans acquire oocysts via GI tract (infected animals, and water) or through breaks in the skin

– The trophozoites spread to organs and develop tissue cysts giving symptoms

 

Symptoms – Most infections are asymptomatic

– Usually resembles infectious mononucleosis –> fever, lymphadenopathy, fatigue

– Meningoencephalitis –> seen in HIV, giving meningitis symptoms of fever, headache and pain on neck movement

– Imaging shows numerous ring enhancing lesions

 

Diagnosis – Antibody test + Sab-Feldman dye test (tests for antibodies stopping dye enter T. gondii)

 

Treatment – Sulfadiazine and pyrimethamine for 6 weeks

  • Schistosomiasis

A parasitic flatworm infection found in freshwater snails, common in Africa

Life Cycle:

– Eggs hatch in freshwater and release miracidia which infect snails

– They multiply and release cercariae which penetrate human skin in rivers

– Inside they migrate to the portal circulation and mature into adult worms

– Depending on the variant they then stay there or migrate elsewhere

– They lay eggs which is shed in urine and stools leading to chronic inflammation

 

Symptoms – Each variant causes different symptoms depending on its location

 

i) Schistosoma haematobium –> deposits eggs in the bladder giving irritation

– Starts as “swimmer’s itch” in legs of travellers from Africa

– Gives urinary frequency, haematuria –> bladder calcification is then a risk factor for squamous cell bladder cancer

 

ii) Schistosoma mansoni/japonicum –> worms mature in the liver giving portal vein congestion

– Can lead to progressive liver cirrhosis, and signs of portal hypertension

 

Treatment – Oral praziquantel

  • Neurocystericosis

This is caused by the worm Taenia Solium, a tapeworm found in pigs.

Life Cycle:

– Pig acquires eggs from faeces which hatch in intestines

– The larvae migrate to the pig muscle where they form cysticerci

– Humans ingest cysticerci by eating undercooked meat from infected meat

– These mature in the small intestine and release eggs in the faces restarting cycle.

 

The problem is that in humans, cysticerci can develop in organs like the brain

– This leads to CNS dysfunction giving headaches, seizures, meningitis etc.

 

Symptoms – Most common cause of epilepsy worldwide

– Headaches + Blindness + Dementia + Meningitis

 

Diagnosis – CT scan shows cysticerci in brain     

 

Treatment – Anti-epileptics + Praziquantel and Mebendazole

1. Centers for Disease Control and Prevention / Public domain

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