Human pathogenic bacteria in raw milk

Dairy animals, an important sector of livestock, play a vital role in the economy of rural population. It can be gauged from the fact that 30 to 35 million rural populations are engaged in raising dairy animals. Every rural family has two to three cattle/buffalo and five to six sheep/goat. Every family is driving 30 to 40 percent of their income from these animals. Dairy animals include cattle, buffaloes, sheep, goats and camels which are major source of milk, mutton, wool, hair, bones, fats, blood, hides and skins.
Pakistan has 29.6 million cattle, 27.3 million buffaloes, 26.5million sheep and 53.8 million goats. These animals have great potential for enhanced milk production but infectious diseases are main obstacles to further development of the animals. These animals are a source of human pathogens and environment pollution. These pathogens may not cause any disease in animals but may dwell commensally in the intestines of the animals that may act as a source of pathogens of zoonotic importance.
Escherichia coli (E. coli) is a normal inhabitant of the intestinal tracts of vertebrates, including man.
The strains of E. coli are composed of cell wall (Somatic antigen: “O”), capsular (capsular antigen: “K”) or Flaggella (Flaggelar antigen: “H”). Comparison of these antigens by serological methods has shown that there are 146 different O antigens, 91 K antigens, 49 H antigens and their classification (especially O and K antigens) is used as the method of classifying strains of E. coli into serotypes. Some of the serotypes are pathogenic for calves, some are for poultry, some are for sheep/goats, and others are for human beings.
Under certain favourable conditions the number of these organisms in the intestines of the susceptible host undergoes a marked and rapid increase and may cause disease that is associated with definite signs of ill health and sometimes death. E. coli is readily grown on ordinary laboratory media without the addition of enrichment additives such as blood, serum, ascetic fluid, glucose etc. Serotype E. coli O 157 is shiga toxin producing. This serotype is either sorbitol fermenter or sorbitol non-fermenter and hence can be differentiated from other E. coli on the basis of white colored colony on sorbitol MacConkey’s agar.
E. coli normally played a beneficial role in intestine by competing with harmful bacteria. Cattle, sheep, goats and other ruminants (e.g. water buffaloes) are natural reservoirs of STEC. Animals’ dung containing STEC or their products such as meat or milk or their byproducts contaminated with the dung may be a major source of this food-borne pathogen. In farming animals’ manure is traditionally used as fertilizers which may cause infections to the handlers of the manure. Similarly, milk and meat may serve as an important vehicle for dissemination of the pathogenic bacteria of animal origin like E. coli O157 to end consumers or humans.
Cattle have been considered as major reservoir of pathogenic E. coli and fecal contamination the animal meat during its handling, slaughtering and processing. Traditional ways of slaughtering and processing of meat in Pakistan are so complex that they could increase the prevalence of E. coli O157 to many folds. E. coli O157:H7 was first identified as a cause of illness in 1982 during an outbreak of severe bloody diarrhea that was traced to contaminated hamburgers. In meat, if E. coli is present, it will cause economical losses to the farmers because recent advancements in World Trade Organization (WTO) have to be followed.
Higher Education Commission, Islamabad, Pakistan funded a Rs 2.2 million research project “Isolation and Molecular Characterization of Shiga Toxin producing Escherichia coli O157”. Under the project, cultural and molecular techniques (PCR) for rapid characterization of the pathogen in meat, eggs, milk or their byproducts have been established in WTO-Quality Control Laboratory, UVAS, Lahore. All the factors affecting the efficacy of the technique have been explored. The technique is being used for testing quality of animals or their byproducts. More than 45 postgraduates, researchers, and faculty members have been trained in the technique in the Laboratory. The writer is serving as a lecturer at University of Veterinary and Animal Sciences, Lahore.
DVM, M. Sc (Hons) Microbiology