Microbial Diseases

This chapter covers selected human diseases caused by microorganisms (bacteria, viruses and fungi), the basic concepts of immunology and vaccination that protect against them, and everyday applications of microorganisms in the dairy, beverage and water treatment industries.

Typhoid

• Causative agent: the bacterium Salmonella typhi.
• Transmission: the faeco-oral route, through food or water contaminated with the faeces or urine of an infected person or of an asymptomatic carrier. The bacteria enter the small intestine, multiply, and spread into the bloodstream and other organs.
• Symptoms: sustained high fever (often 39 to 40 degrees C), severe headache, weakness, abdominal pain, constipation or diarrhoea, and loss of appetite; severe, untreated cases can cause intestinal perforation and haemorrhage.
• Diagnosis: the Widal test, which detects antibodies against Salmonella antigens in the patient's serum.
• Prevention and control: safe drinking water and sanitation, proper sewage disposal, food hygiene, hand washing, and typhoid vaccination; antibiotics are used for treatment. The historical case of "Typhoid Mary" (Mary Mallon), an asymptomatic carrier who unknowingly infected many people through food she prepared, illustrates the danger carriers pose in transmission.

Tuberculosis (TB)

• Causative agent: the bacterium Mycobacterium tuberculosis (the tubercle bacillus), most often affecting the lungs (pulmonary TB) but able to infect other organs (extra-pulmonary TB).
• Transmission: airborne, through droplets released when an infected person coughs, sneezes or talks.
• Symptoms: persistent cough, often with blood stained sputum in advanced disease, low grade fever, night sweats, chest pain, fatigue and weight loss.
• Diagnosis: sputum microscopy, chest X-ray, and skin/blood tests for prior exposure.
• Prevention and control: BCG (Bacillus Calmette Guerin) vaccination in infancy, early detection and completion of a full course of anti-tubercular antibiotics (a long, multi-drug regimen, since incomplete treatment encourages drug resistant strains), good ventilation, and covering the mouth while coughing.

Cholera

• Causative agent: the bacterium Vibrio cholerae, a comma shaped (curved) bacillus.
• Transmission: faeco-oral, through water or food contaminated with the bacterium, particularly where sanitation is poor.
• Symptoms: sudden onset of profuse, watery diarrhoea (classically described as rice water stools), vomiting and rapid dehydration, which can be fatal within hours if untreated.
• Treatment: prompt oral rehydration therapy (ORS) to replace lost fluid and electrolytes, plus antibiotics in moderate to severe cases.
• Prevention and control: safe drinking water, proper sewage and waste disposal, food hygiene, and cholera vaccination in high risk situations.