Fish Microbiology and Quality Assurance

Unit 6 - Biological hazards in foods

6.5.1. Protozoan parasites
The important protozoan parasites involved in human illness are members of the genus Giardia, Entamoeba, Toxoplasma, Sarcocystis and Cryptosporidium.

Giardia Sp
The flagellated protozoan Giardia lamblia exists naturally in aquatic environment and causes human illness called giardiasis.
Characters

• The Giardia cells (trophozoites) produce cysts which infect humans through water and food. T rophozoites are characterized by the presence of eight flagella arising on the ventral surface near the paired nuclei.
  
• The cysts are pear shaped, with the size ranging from 8-20 µm in length and 5-12 µm in width.
  
• The excystation of ingested cysts in upper small intestine releases the trophozoites which invade intestinal wall and bile duct.
  
• Growth is generally favoured by high levels of bile juice in the duodenum and upper jejunum.

Symptoms of illness

• Giardiasis is highly contagiouis.
  
• The disease symptoms appear after a incubation period of 6-13 days, cysts appear in stools in 3-4 weeks, and symptoms may last for months to a year or more.
  
• The leading symptoms of infection are diarrhea, fatigue, abdominal cramps, fever, nausea, vomiting and weight loss.
  
• The minimum dose required for infection in humans is 10 cysts or less.

Control measures

• Avoiding sewage contamination of natural waters.
• Avoiding consumption of sewage contaminated water and fish harvested from sewage polluted waters.

Entamoeba histolytica
This protozoan parasite is responsible for amebiasis or amoebic dysentery and is transmitted mainly through fecal-oral route, and also through water, food handlers and foods.
Characters

• Entamoeba histolytica is an motile aerotolerant anaerobe, trophozoites are of 10-60 µm in size and lack mitochondria, cysts non motile and are of 10-20 µm in size.
  
• Trophozoites do not persist in the environment but cysyts can survive in sewage sludge for up to 3 months and transmitted through water and food.
• The trophozoites in intestine adhere to host cell glycoprotein, cause abscesses in intestinal mucosal cells and ulcers in colon, multiply by binary fission in large intestine, encysts in ileum, and produce enterotoxic protein.
  

Disease symptoms

• After ingestion of the parasite
• The symptoms of disease occur after 2-4 weeks of incubation of ingested parasite and symptoms persist for several months.
  
• The common symptoms include mucus and blood in stool, abdominal pain, fever, severe diarrhea, and vomiting and weight loss.
  
• Affected persons can be treated using amebicidal drugs such as metronidazole and chloroquine.

Toxoplasma gondii
The obligate intracellural protozoan, Toxoplasma gondii, is responsible for causing a disease calld toxoplasmosis.
Characters

• Domestic and wild cats are the definitive hosts for this parasite and serve as primary source of human infection.
  
• Oocysts are transmitted from cat to cat and infect all other animals, and survive over a year in warm, moist environment.
  
• Clinical symptoms in humans are caused by as few as 100 oocytes. Encysted form can remain latent in humans.

Disease symptoms

• Toxoplasmosis is regarded as universal infection and symptomless in most individuals.
  
• When severe, symptoms of fever with rash, headache, muscle ache and pain and swelling of lymph nodes is noticed.

Control measures

• Avoiding environmental contamination with cat feces.
• Avoiding consumption of meat and meat products containing viable tissue cysts.
• Heating food above 600c which destroys cysts.

Sarcocystis sp

• Sarcocystis sp cause sarcocystosis in humans.
  
• Cattle and pigs serve as intermediate host while humans are definitive hosts.
  
• Species involved in disease include Sarcocystis hominis which is associated with cattle, and Sarcocystis suihomonis, associated with pigs.
• Infection occurs through consumption of infected porcine and bovin meat, contaminated food and water.
• Sarcocysts in humans transform to bradyzoites which penetrate small intestine, reproduce sexually resulting in sporocysts and pass out through feces.
  
• The sporocysts when ingested by pigs or bovines release sporozoites and spread throughout the body.
  
• These multiply asexually in skeletal and cardiac muscle producing sarcocysts.
  
• Sarcocysts are visible to naked eye as they reach a size of about 1cm.
  

Disease symptoms

• Symptoms occur within 3-6 hr of infection and consist of nausea, stomachache, and diarrhea.