Milk borne diseases

Milk borne diseases are any diseases caused by consumption of milk or dairy products infected or contaminated by pathogens. Milk-borne diseases are recurrent foodborne illnesses—between 1993 and 2012, over 120 outbreaks related to raw milk were recorded in the US, with approximately 1,900 illnesses and 140 hospitalisations. With rich nutrients essential for growth and development such as proteins, lipids, carbohydrates, and vitamins in milk, pathogenic microorganisms are well nourished and are capable of rapid cell division and extensive population growth in this favourable environment. Common pathogens include bacteria, viruses, fungi, and parasites, and among them, bacterial infection is the leading cause of milk-borne diseases. Despite the modern popularity of pasteurisation, the risk of contamination cannot be eliminated. Infection can turn milk into an optimal vehicle of disease transmission by contamination in dairy farms, cross-contamination in milk processing plants, and post-pasteurisation recontamination. Signs and symptoms of milk-borne diseases depend on the amount of pathogen ingested, incubation period, and individual host susceptibility, age, and pre-existing medical conditions. Generally, milk borne diseases are not life-threatening, and taking medications like antibiotics and over-the-counter drugs helps relieve symptoms. Typical clinical signs include fever and mild gastrointestinal disturbance, such as diarrhoea, nausea, vomiting, and abdominal pain. Nevertheless, severe complications can be fatal and are often observed in young children, aged individuals, and immunocompromised patients.

History In the 19th-century United States, the urban milk supply was regularly diluted and contaminated by dairymen, dealers, and retailers. The public health harms from milkborne diseases prompted localities in the U.S. to enact minimum quality standards for milk and hire milk inspectors. According to a 2025 study, this was associated with a substantial decline in deaths from diarrheal diseases and typhoid.

Common routes of infection and contamination

There are three major routes of infection and contamination of milk:

Contamination in dairy farms – Milk-producing livestock can be infected by ingesting contaminated water and feed, and bacteria and/ or viruses are excreted by the mammary glands in milk. Poor hygiene in dairy farms can result in either contaminating raw milk during milking or contaminating the bulk tank milk during storage. Without pasteurisation, pathogens remain in milk with a high infectious risk. On some dairy farms (e.g., family farms), farm owners and farmers have the tradition of consuming raw milk rather than pasteurised milk. However, after the popularisation of pasteurisation, most dairy products available in the market are pasteurised to minimise the risk of contamination. Cross-contamination in milk processing plants – To derive a diversified variety of dairy products from milk, raw milk is sent to factories, and those with poor standards of hygiene pose a danger to the safety of dairy products. Pathogens from machines, packaging, and any materials found in the manufacturing site can enter and contaminate milk. Post-pasteurisation recontamination – Some thermophilic bacteria or bacteria with high resistance to high temperatures are capable of thriving in pasteurisation and hence can recontaminate the pasteurised milk or dairy products. For instance, L. monocytogenes can survive in high temperatures and grow extensively in the post-pasteurisation process. Moreover, certain bacterial species can secrete toxins with high heat stability that are harmful to humans.

Common bacterial pathogens

Salmonella

Salmonella can survive within 5.5 °C to 45 °C, with high sensitivity to acid, and is more commonly found in unprocessed milk. Owing to the sensitivity to pH, Salmonella have different survival rates in different dairy products like cheese under different storage temperatures. In ripening Cheddar cheese, they can survive for several months at 13 °C but most fail to survive for more than 36 days in Domiati cheese. Most Salmonella strains are pathogenic, especially S. enterica subsp. enterica which accounts for 99% of human infections and can bring about salmonellosis. Salmonellosis is caused by infection with Salmonella with a rapid onset of disease 12 to 36 hours after consumption of contaminated food. It can be clinically classified into three types: enteric fever (also Typhoid fever), gastroenteritis and sepsis. Enteric fever usually has a 7-14 day incubation with mild symptoms like malaise and headache. In rare cases, the body temperature of the patients can surge up to 40 °C, rendering them delirious. Gastroenteritis has a much shorter incubation period than enteric fever (usually 3 to 72 hours) and shows common gastrointestinal symptoms characterised by watery faeces with an unpleasant, strong odour as well as blood and mucus. Sepsis can lead to serious complications in various organs, such as arthritis. A recent case of a large-scale salmonellosis outbreak was reported in Iwamizawa, Japan, in 2011 because of contamination in school meal processing facilities, affecting over 1,000 students and school staff at nine local Japanese schools. The majority of affected individuals had acute diarrhoea, and 13 of them were hospitalised.

Campylobacter The preponderance of reported milk-borne diseases arises from Campylobacter, most notably the strains C. jejuni and C. coli. Campylobacter is implicated in more than 80% of reported American disease outbreaks from raw milk from 2007 to 2012. Aside from the US, the UK also recorded around 59,000 confirmed cases of campylobacteriosis triggered by raw milk consumption in 2016. As thermophilic strains, C. jejuni and C. coli can grow between 37 °C and 42 °C and they have a high biological activity rate inside host animals. C. jejuni, the predominant pathogenic strain, is found to have a noteworthy genetic variation that allows them to develop diversified phenotypes, for example high resistance to temperature fluctuations during pasteurisation and anti-bacterial agents in animal hosts, and improve their adaptability to changing environments in dairy products.

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