Module 7. Milk borne diseases

Lesson 28


28.1 Introduction

The spoilage and pathogenic micro-organisms causing illnesses could come from the animal, handler, environment, water, equipment, air, and raw materials and due to poor sanitation practices.  The knowledge about sources of contamination will be helpful in preventing or reducing such spoilage causing and pathogenic microbes in milk.

Milk is, an efficient carrier for a variety of disease producing microbial agents. With mass collection and distribution of milk in industrial countries, the potential of milk for disease transmission became a widespread problem. The disease control, however, can be maintained only by constant supervision of the health of dairy animal and by adequate controls at all points from the time the milk leaves the udder until it reaches the consumer. While the problems of ensuring a safe milk supply are of different orders of magnitude in economically advanced and in developing countries, yet there are essential similarities. In both cases, where a highly mechanized system with extensive distribution services from a centralized milk plant is employed, the slightest relaxation of attention at any crucial links in the milk chain from the farm to consumer invites problems.

A second factor that is common to advanced and developing countries, is the disease causing microbes.

Such microbial agents can be conveniently classified as :

a)      Communicable disease causing microbes - viruses, rickettsiae, bacteria, protozoa, and other parasites-and/ or their toxins;

b)      Specific and non - specific sensitizing agents; and

c)      Toxic chemicals - pesticides, preservatives, drugs, radionuclides, and other substances.

28.2 Milk as Vehicle of Microbes

Milk, by virtue of possessing all sorts of nutritional factors, can serve as an excellent media for microbes, especially including pathogens.  Bacteria have the ability to utilize various milk constituents to grow and multiply. While growing at the expenses of milk constituents these microbes release certain metabolites like lactic and other organic acids, gases, enzymes, flavouring compounds, pigments, toxins etc in the system which may be useful and/or harmful, and thus, effects the quality of milk. Generally, these metabolites lead to different spoilage conditions in milk products and make these unfit for consumption. For this reason, raw milk is inherently dangerous and should not be consumed by anyone, at any time, for any reason. However, this has been a practice in past in northern parts of India, might be due to the unawareness of consumers.

Food safety experts say that pasteurization saves lives. Pasteurization has reduced food-borne illnesses from milk to one-fourth in comparison to before the technique was widely adopted in early 1900s to about 1 percent now.

Although all are vulnerable to illness from E. coli or Salmonella poisoning, children and people with weak immune systems are at higher risk from drinking raw milk. The number of infections is growing, as raw milk is gaining popularity and people are taking a risk.

Milk serves as a potential vehicle for transmission of diseases under certain circumstances. Pathogens grow and multiply to produce certain toxic metabolites and make itself an extremely vulnerable commodity from public health point of view. The microbiological health hazards arising from the consumption of contaminated milk has grown in recent past and has resulted in intensification of food hygiene programme world over. Although, the occurrence of incidences of food borne illness has been considerably reduced in developed countries, mainly due to adoption of strict microbiological quality control and sanitary practices during production, processing and distribution of milk yet,  the situation is continues to be grim in developing countries like Indian subcontinent where, such practices are impracticable even at present.

Milk spoilage is manifested by a reduction in aroma, flavor, texture and nutritional value of foods. In extreme cases the dairy products become totally unpalatable. In addition, some microbes are known to release toxins that may cause damage to health of consumers. 

28.3 Different Sources of Pathogens

A variety of pathogens may gain access to milk from a number of sources and cause different types of food borne illness. Milk and its products may carry microbes as such or their toxic metabolites called toxins to the consumers.

28.3.1 Animals

The health of dairy animals is a very important parameter because a number of diseases including brucellosis, Q-fever, salmonellosis, staphylococcal and streptococcal infections and foot and mouth disease virus may be transmitted to man through milk. The microbes causing these diseases may be transmitted to milk either directly from the udder or indirectly through the infected body discharges that may drop, splash or be blown in to milk.

28.3.2 Handlers

The diseased persons may transmit diseases like typhoid fever, scarlet fever, diphtheria, septic sore throat, infantile diarrhea by contaminated hands or by coughing, sneezing and talking during milking or subsequent handling of milk at farm level.

28.3.3 Environment

Dairy farm environment may also introduce pathogens in to milk products at different stages of production and processing. Some common air borne pathogens are like Group A Streptococci, Corynebacterium diptheriae, Mycobacterium tuberculosis, Coxiella burnetii and some viruses of respiratory origin. Contaminated water, fodder and unclean vessels and containers used for handling milk and other unhygienic conditions at farm and plant may significantly contribute to pathogens and spoilage causing micro-organisms in milk.

28.4 Different Terms Used in Milk Borne Infections

28.4.1 Outbreak

According to Communicable Disease Centre, an outbreak of food-borne disease is defined as an incident in which two or more persons experience a similar illness usually gastro-intestinal after ingestion of a common food.

28.4.2 Etiology

Etiology of a food-borne disease is the confirmation or identification of the causative agent of the disease through laboratory diagnosis.

28.4.3 Epidemiology

Epidemiology of a food-borne disease is a systematic approach to locate the cause and mode of transmission of the disease, so that corrective measures can be applied.

28.5 Investigation of Milk Borne Diseases

There exists a systematic monitoring and surveillance system to investigate the causes of food-borne illness in developed countries but, there is lack of adequate investigating system in developing countries and as a result, no follow-up action is taken to avoid reoccurrence. Generally gastro intestinal disorders are perhaps the greatest single cause of morbidity in developing countries.

For investigation of an outbreak the following steps are to be followed:

·     A detailed description of gastro-intestinal cases should be made.

·     A record of food eaten and a common source of infection should be identified, if large number of individuals is involved.

·     History of previous illness of personnel handling milk should be traced.

·     Evidence of enteric disorders, scratches, wounds, sores, pyogenic infections or other evidence of sepsis should be looked for and swabs should be taken.

·     Sanitary facilities and practices used in plant should be recorded.

·     A detailed veterinary record of animals should be obtained with particular attention to recent cases of mastitis.

·    Pooled milk samples from one or several animals should be taken aseptically, immediately cooled and held cool until delivered for examination.

After identification of the suspected animals carrying the causative microorganism, the individual samples should be obtained.

28.6  Diseases from Animal to Man and Milk to Man

28.6.1 Tuberculosis

The causative microorganism is Mycobacterium tuberculosis. German physician Robert Koch (1843-1910) revealed the micro-organism, Mycobacterium tuberculosis from contaminated raw milk, and its role in infant mortality.
Koch also reported that another strain, M. bovis, was responsible for tuberculosis in cows, and that it was species specific and believed that cow strain would not infect humans. Milk borne tuberculosis is directly or indirectly related to consumption of raw milk from infected dairy herds. The tuberculosis traceable to raw milk was the result of external contamination or lesions in the udders of cows racked with bovine tuberculosis. The milk buckets, too, were easily contaminated by workers.

There are two types of tuberculosis, pulmonary and non pulmonary type. Pulmonary is caused by human type of microorganisms that affects mainly respiratory tract. Bovine type bacillus cause non pulmonary tuberculosis. Tuberculosis of cattle is produced by Mycobacterium bovis. Avian type of the microorganism may cause both types of tuberculosis.

·         Mycobacterium tuberculosis – Human

·         Mycobacterium bovis – Cattle and human

·         Mycobacterium avium – Birds, swine but rarely human

Human type tuberculosis bacilli may gain access to milk from milkers and other handlers. It causes human type tuberculosis in cattle. This cannot be immediately noticed and may give tuberculin negative test but after 2 – 3 months, this test will be positive. Such suspected animals are usually held under observation and rested periodically. If the reaction disappears, these are restored to their normal status in herd. Such cattle may excrete bacilli in their milk from apparently normal udders.

Milch animals other than cattle are also affected with tuberculosis mainly by bovine type. Buffaloes and goats are less frequently affected by tuberculosis. Bovine type infection in man appears to be practically nonexistent, in spite of a considerable proportion of cows being infected. It may mainly be attributed to the habit of boiling milk before consumption. Sour milk may kill human and bovine tuberculosis bacilli within 18 – 24 h

Avian type tuberculosis bacilli also cause natural infections in cattle. Human infection with avian type bacilli is quite rare. Symptoms

Tuberculosis is characterized by the onset of paranchymal pulmonary infiltration recognizable by X-ray examination, pleurisy, followed by advanced stage that is accompanied by cough, fever, and fatigue and weight loss. Incubation period is 4 – 6 weeks from infection to demonstrable primary lesion. Prevention and control

·       Animals should be subjected to tuberculin test.

·       Animal suffering with tuberculosis should be isolated.

·       Proper heat treatment of milk. The traditional habit of boiling every lot of milk before consumption in India is good, in combating the incidence of tuberculosis.

·      Overcrowding of animals must be avoided and living conditions must be improved

·      Tuberculosis patients should be prohibited from handling cattle as well as milk.

·      Proper disinfection should be followed.

28.6.2 Brucellosis

It is one of the most common milk-borne diseases. Brucellosis, also called Bang's disease, Crimean fever, Gibraltar fever, Malta fever, Maltese fever, Mediterranean fever, rock fever, or undulant fever, is a highly contagious zoonosis caused by the ingestion of unsterilized milk or meat from infected animals or close contact with their secretions. Brucella spp. are small, Gram-negative, non-motile, non-spore-forming, rod shaped (coccobacilli) bacteria. This function as facultative intracellular parasites causing chronic disease that usually persists for life. Symptoms include profuse sweating and joint and muscle pain. Malta fever, the disease now called brucellosis first came to the attention in 1850s in Malta during the Crimean War. In cattle, this disease is also known as contagious abortion and infectious abortion.

The popular name undulant fever originates from the characteristic undulance (or ‘wave-like’ nature) of fever, that rises and falls over weeks in untreated patients. In 20th century, this name, along with brucellosis, gradually replaced the 19th century names Mediterranean fever and Malta fever. Brucellosis in humans is usually associated with the consumption of unpasteurized milk and soft cheeses made from milk of infected animals, primarily with Brucella melitensis and with occupational exposure of workers, veterinarians and slaughterhouse workers. Some vaccines used in livestock, B. abortus, also cause disease in humans, if accidentally injected. Brucellosis induces fevers, sweating, weakness, anaemia, headaches, depression and muscular and body pain.

The main symptoms are like muscular pain and sweating and the duration of disease can vary from a few weeks to months or years. In the first stage of disease, septicaemia occurs and leads to undulant fevers, sweating and migratory arthralgia. Blood tests characteristically reveal leukopenia and anemia, and demonstrate positive Bengal Rose and Huddleston reactions. During episodes of Malta fever, melitococcemia (presence of brucellae in blood) can usually be demonstrated by means of blood culture in tryptose medium. If untreated, the disease can become chronic. The focalizations of brucellosis occur usually in bones and joints and spondylodiscitis of lumbar spine accompanied by sacroiliitis is very characteristic of this disease. Diagnosis of brucellosis

·       Blood cultures in tryptose broth, bone marrow cultures. The growth of brucellae is extremely slow and the culture poses a risk to lab personnel due to high infectivity of brucellae.

·       Demonstration of antibodies against the agent either with, ELISA or 2-mercaptoethanol assay for IgM antibodies associated with chronic disease

·       Histologic evidence of granulomatous hepatitis

·       The preferential erosion of antero-superior corner of lumbar vertebrae and marked osteophytosis are suspicious of brucellic spondylitis. Prevention and control

·         Segregation of infected herd to avoid cross infection and if possible the infected animals should be slaughtered.

·         Herds should be properly vaccinated.

·         Adequate heat treatment should be given to milk for the destruction of causative microbial agents.

28.6.3 Diphtheria

Diphtheria is caused by only toxigenic strains of Corynebacterium diphtheriae. Rarely, a diphtheria-like illness is caused by a toxigenic strain of C. ulcerans or C. pseudotuberculosis. C. diphtheriae has three biotypes: gravis, intermedius, and mitis. The gravis biotype is associated with the most severe disease, but any strain may be toxigenic. All clinical isolates of C. diphtheriae should be tested for toxigenicity. Nontoxigenic strains can cause sore throat and other invasive infections, and are associated with endocarditis. Illness

Classic diphtheria is an upper-respiratory tract infections characterized by sore throat, low-grade fever, and an adherent pseudomembrane of the tonsil(s), pharynx, and/or nose. The disease can involve almost any mucous membrane. For clinical purposes, diphtheria can be classified according to the site of the infection: Anterior nasal diphtheria

Anterior nasal diphtheria usually presents with mucopurulent discharge from nose that may be bloody and a white pseudomembrane on nasal septum. Pharyngeal and tonsillar diphtheria

Pharyngeal and tonsillar diphtheria, the most common type of infection, initially presents with malaise, sore throat, anorexia, and low-grade fever. Within a few days, a bluish-white pseudomembrane forms on one or both tonsils that can extend to the tonsillar pillars, uvula, soft palate, pharynx and nasopharynx. Over time, the pseudomembrane evolves, assuming a dirty gray color with areas of green or black necrosis surrounded by a minimal amount of erythema. Attempts to remove the pseudomembrane cause bleeding. With severe disease patients can develop edema of the anterior neck. If a significant amount of toxin is absorbed into the blood stream, patients may develop pallor, rapid pulse, coma and death. The differential diagnosis of diphtheria includes streptococcal pharyngitis, viral pharyngitis, Vincent's angina, infectious mononucleosis, oral syphilis and candidiasis. Laryngeal diphtheria

If the infection involves larynx, it may occur either as an extension of pharyngeal form, or as laryngeal involvement alone. Patients can present with fever, hoarseness and a barking cough. The pseudomembrane can cause potentially fatal airway obstruction. Cutaneous diphtheria

Cutaneous diphtheria, caused by either toxigenic or nontoxigenic strains of C. diphtheriae, is usually mild, typically consisting of non-distinctive sores or shallow ulcers, and rarely causes toxic complications. The disease may present as a scaling rash or as clearly demarcated ulcers. A chronic skin lesion may harbor C. diphtheria along with other micro-organisms. Skin infections with C. diphtheria are common in tropical climates, and this is likely the reason for high levels of natural immunity among local populations in these regions. Reservoir

Infected humans are the reservoir. Modes of transmission

Diphtheria is transmitted from person to person through respiratory droplets or less commonly, through contact with discharge from skin lesions. Historically, raw milk and fomites were known to have served as vehicles. Incubation period

The incubation period is usually 2–5 days (range 1–10 days). Communicability

Persons are communicable for up to 4 days after treatment with effective antibiotics has been initiated. Untreated persons generally shed bacteria from the respiratory tract or from skin lesions for 2–4 weeks after infection. A chronic carrier state is rare, but known to exist, and such a carrier may shed micro-organisms for 6 months or more. Prevention and control

·         adequate heat treatment of milk.

·         Infected person should not be allowed to handle milk and milk products.

·         Unhygienic practices like sneezing and coughing by the dairy persons should be avoided.

·         Proper vaccination of individuals against disease is an efficient prophylactic measure.

28.6.4 Q – Fever

Q-fever is caused by Coxiella burnetti. Raw milk is commonly implicated as a vehicle for transmission of disease. Coxiella burnetti is more heat resistant than Mycobacterium tuberculosis. It can survive pasteurization, if the specified temperature is not maintained and also freezing temperatures. In view of the considerable heat resistance of this microbe the time-temperature combination used in pasteurization is selected on the basis of heat inactivation of this microorganism. It has been found to be viable for 2 years at –20˚C and resist 0.5 per cent formalin and 1 per cent phenol. It has also been observed to remain viable for 25 days in rennet cheese, 42 days in cottage cheese, whereas in yoghurt it is killed within 24 hours due to acidity. All these features make Coxilla burnetti a micro-organism of great public health significance for its pathogenic potentials.

Individuals, who have frequent direct contact with animals, including veterinarians, meat workers, and sheep and dairy farmers, are at higher risk. Q fever is rarely fatal. Most patients get Q fever by coming in contact with animals infected with the Q fever bacteria, their tissues, or fluids. Transmission may occur through breathing contaminated air or dust from an area with a large concentration of animals. Tissues from animals giving birth pose a particular risk. People can also become infected indirectly from animals through contaminated materials like wool, straw, and fertilizer. There is a risk of Q fever from consumption of contaminated raw milk. Sheep, cattle, goats, cats, dogs, some wild animals like bobcats and rodents, birds, and ticks carry the bacteria. Most infected animals do not show signs of illness, but Q fever may sometimes cause abortion.

Only about one-half of all people infected with C. burnetii show signs of illness. For patients who become ill, the first symptoms of Q fever resemble flu and may include fever, chills, sweats, headache, and weakness. Q fever may rarely progress to affect liver, nervous system, or heart valve. Q fever is diagnosed by identifying the bacteria in tissues or through a blood test that detects antibody to the micro-organism. Patients with mild transient illness usually do not require treatment. Placenta, other birth products, and aborted fetuses should be disposed of immediately. Seek veterinary assistance, if animals have reproductive or other health problems. Sources

Mostly human infection is by inhalation of infected dust of the fecal matter. Infected cattle continue to excrete the microorganisms in milk for a long time. Symptoms

High fever, headache, weakness, malaise, severe sweating and virus like pneumonia. Prevention and control

·      Adequate heating of milk and cream

·      Calving sheds should be away from the milking sheds and dairy

·      Animals should be properly vaccinated

·      Survey for determining the prevalence of infection in an area should be carried out

28.6.5 Viral infections

The rapid growth of the frozen and convenience foods has given rise to increased concern about the possible role of such foods in the dissemination of viral infections. Although, food is rarely reported a vehicle for viral distribution. A few breakouts of poliomyelitis have been traced to raw milk. Personal contact and mechanical distribution by flies are believed to be the usual routes of infection with enteric viruses.

Outbreaks of infectious hepatitis have occurred in which foods appear to be the most probable means of transmission. Epidemiological evidence suggests that ingestion of raw clams or oysters taken from sewage polluted waters could be a mean by which infectious hepatitis is spread. A number of factors can influence the importance of any food as a vehicle for the spread of a virus. Among these are the opportunities for contamination with infectious virus, the ability of a virus to survive and remain infectious under conditions of handling and storage, the extent of adsorption to food, the effect of ingredients like flavourings and preservatives, and competition from other contaminants. However, unlike bacteria, viruses will not multiply in the foods. They will either survive at a constant level or die out. Since some frozen foods are subject to a great deal of handling in manufacture, are kept frozen, and require little or no cooking in the home, they would appear to be likely sources of viral distribution, should they become contaminated during preparation.

28.6.6 Enteroviruses

These are a group of viruses that can cause severe epidemics of diarrhea in infants and children on ingestion of contaminated milk. Among these most common human pathogens are polio and coxsackie viruses. Milk and its products are commonly contaminated with enteroviruses mainly through fecal contamination. Unheated milk contaminated after pasteurization play a significant role in the transmission of disease, especially in developing countries. However, polio virus can be inactivated in milk by pasteurization. Compared to polio virus, coxsackie viruses are more resistant to heat treatment

28.6.7 Poliomyelitis

Like many other diarrheal diseases, poliomyelitis occurs commonly.

28.6.8 Sources

·      Person to person contact is the main mode of spread of viruses

·      Flies may also serve as a vector for the spread of the disease

·      Fecal contamination of water and milk Symptoms

First, there is a minor disease associated with viremia characterized by gastro intestinal disturbance. Then headache, fever, muscle stiffness and paralysis associated with cell destruction in central nervous system. Prevention and control

·      Proper pasteurization of milk

·      Hygienic measures should be strictly followed to avoid fecal contamination

·      Infected persons should not be allowed to handle milk.

·      Immunization is also effective in preventing disease.

·      Proper vaccination of the children confers protection against disease.

28.6.9 Infectious hepatitis

Among various viral diseases, infectious hepatitis is considered as one of the most serious viral disease for which milk may be important vehicle for transmission. The illness is also known as hepatitis A and is caused by Hepatitis A virus, whereas hepatitis B is not transmitted through milk. Sources

·         Since the disease spreads through person to person contact, hence handlers can be an important source

·         Defective water supply in a milk plant may also introduce the virus to milk

·         Polluted environment in milk plant may also contaminate milk Symptoms

Nausea, vomiting, lethargy, abdominal pain, diarrhea, fever, chills, anorexia, lassitude, sore throat, bile in urine and jaundice. Prevention and control

·         Proper sanitary conditions should be maintained during production and processing of milk

·         Infected persons should not be allowed to handle milk

·         Fecal contamination of water should be avoided

·         Adequate heat treatment of milk is likely to inactivate the virus