Differentiating between bacterial food poisoning or due to preformed toxins is important

Food poisoning is a very common outbreak reported in India. Food poisoning is caused by eating contaminated food. Clinically, it may present as acute gastroenteritis with abdominal pain, nausea, headache, fatigue, vomiting, diarrhea and dehydration.

The two common causes of food poisoning are infectious pathogens (bacteria, viruses and parasites) and their preformed toxins. It is important to differentiate between the two.

Food poisoning due to preformed toxins presents within 6 hours and patient presents with upper gastrointestinal symptoms, predominantly vomiting, gastritis, etc. The commonest is Staph aureus, Bacillus cereus emetic toxin. On the other hand, bacterial food poisoning presents after 6 hours. The predominant symptom is diarrhea with or without vomiting.

Symptoms after 24 hours are due to pathogens that make toxin once they have been ingested. They mainly cause diarrhea that may be watery (Vibrio cholerae or E. coli) or bloody (Shiga toxin-producing E. coli).

Symptoms that occur after variable time are due to microbes that cause pathology by either damaging the epithelial cell surface or by actually invading across the intestinal epithelial cell barrier. They can produce a wide spectrum of clinical presentations from watery diarrhea (Cryptosporidium parvum, enteric viruses) to inflammatory diarrhea (Salmonella, Campylobacter, Shigella) or systemic disease (L. monocytogenes).

Toxin-induced food poisoning cases are diagnosed most often in India, especially in schools as they occur within school hours, and not bacterial food poisoning as people often do not correlate the two. Because, presently, there is no provision for microbiological testing of food samples suspected to have caused the symptoms. So, many cases of diarrheal illnesses in our country may actually be part of the spectrum of food poisoning or food-borne illness.

The CDC has published a new surveillance report, which has analysed the causes of foodborne disease outbreaks between 2009 and 2015 in the US. The researchers found 5,760 outbreaks that caused 100,939 illnesses, 5,699 hospitalizations, and 145 deaths in the US during that time.

The foods that were most often implicated in outbreaks were: Fish (17% of all outbreaks), dairy (11% of all outbreaks), chicken (10% of all outbreaks). However, some foods were more likely to cause outbreak-related illnesses: Chicken (12% of cases), pork (10% of cases), seeded vegetables (10% of cases).

When categorizing the pathogens, Norovirus accounted for 38% of the outbreaks, salmonella was responsible for 30% and shiga toxin-producing escherichia coli (STEC) was implicated in 6%. Other causes (including campylobacter, clostridium perfringens, scombroid toxin, ciguatoxin, staphylococcus aureus, vibrio parahaemolyticus and listeria monocytogenes) were all responsible for 5% or fewer outbreaks.

Of note is the definition of an outbreak in this CDC report. “An outbreak is defined as two or more cases of a similar illness that happens after people eat a common food. So if you get sick after leaving your plate of potato salad out in the heat for too long, it doesn’t qualify”.

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