Animal husbandry has numerous facets, and great changes in farming practices as well as an enormous expansion in the pharmaceutical industry, have changed the face of the animal-products industry over the past decades. Particularly the use of drugs has had a profound influence on animal husbandry. Drugs are not only used to curtail disease but they are also extensively used to stimulate growth, and this last factor has led to an unprecedented increase in the use of drugs.
Antibiotics
Antibiotics have had a profound effect on the agricultural sector, and today antibiotics are used extensively in animal husbandry. Antibiotics are not only administered to animals for the treatment of disease, but subtherapeutic doses of these antimicrobials are administered routinely for growth promotion in livestock and poultry production. Antibiotics are also used in wildlife and fish, and for the control of plant diseases and food spoilage. In the United States alone some 15 million pounds of antibiotics are administered to farm animals annually. The fact that animals grow faster when receiving subtherapeutic doses of antimicrobials serves as an incentive to farmers to administer these products, particularly since an added benefit is found in disease prevention. Although vehemently denied in some quarters, this practice is giving rise to a new generation of antibiotic-resistant microbes which can cause serious outbreaks of disease among humans. Infectious diseases account for millions of deaths annually, with respiratory infections, diarrhoeal diseases and tuberculosis accounting for the majority of these.
Drug-resistant bacteria have accounted for a steady increase in the incidence of human Salmonella infections,73 and in 1983 an outbreak of Salmonella infection in the Midwestern states of the USA was actually traced to the farmyard from which the disease spread.74 An epidemiologic investigation in Minnesota revealed that patients had eaten ground beef (hamburger) and that the meat had come from a farm lot where the cattle had been fed subtherapeutic quantities of chlortetracycline for growth promotion and disease prevention.
In outbreaks of gastrointestinal disease from drug-resistant bacteria, it is a common occurrence that infected patients had taken antibiotics for other diseases such as bronchitis, pharyngitis, otitis media (ear infection) or other non-diarrhoeal diseases prior to the onset of the gastrointestinal disease. This suggests that whilst patients are on antibiotics, and they consume foods infected with resistant bacteria, the destruction of the natural non-resistant intestinal bacteria offers some selective advantage to the drug-resistant varieties which then flourish and become pathogenic. The symptoms are normally diarrhoea, abdominal cramps, nausea and vomiting and in some cases chills, fever, and confusion. Furthermore, the disease is difficult to treat, as the bacteria will not respond to drugs in view of their antimicrobial resistance, and death can be the ultimate result, even in hospitals with all the necessary care
facilities.
The fact that antibiotics are widely used in hospitals, accounts for the fact that outbreaks of antimicrobial-resistant bacterial infections are largely recorded in these institutions. Bitter rows have developed over this issue between governments and trade organizations. In Germany and Denmark the antibiotic avoparcin, which farmers inject into their livestock, was banned because of concern that antibiotic resistance could spread from the farmyard to hospitals. Brussels, on the grounds that the ban could interfere with free trade, declared the ban illegal. Avoparcin is similar to other antibiotics such as vancomycin and teicoplanin which are the only drugs available that kill methicillin-resistant Staphylococcus aureus (MRSA), which is becoming more and more prevalent in hospitals. Microbiologists were dismayed that the German studies had not been able to uphold the ban.72 Sadly it is often the elderly or infants who succumb to the disease due to their weaker constitution.
In view of the widespread use of antibiotics in animal husbandry, the most common sources of contamination, by this new breed of antibiotic resistant bacteria, are poultry, cattle, calves, eggs and milk. In fact, in over two thirds of US outbreaks of multiple drug-resistant Salmonella infections with a definite source, the bacteria came from food animal populations, and the transmission of resistant bacteria to man, through the consumption of food animals, is thus not a rare event.75 The World Health Organization has reported that resistant strains of Salmonella typhimurium have increased dramatically in many countries, and there was hardly any medication that was effective against the DT 104 strain of this species. In some European countries the total number of Salmonella infections has increased 20 fold in the last decade and in Britain, where DT 104 was first isolated in 1988, the number of infections from this strain increase from 300 in 1990 to 3 500 in 1996. In Germany, the percentage of infections caused by this strain rose from ten to 18% within the year 1996 alone.76
Tetracyclines are the most commonly used antibiotics in feeding operations, and these drugs commonly occur in the animal products purchased from supermarkets and other stores. Contamination of foods with antibiotics may present various health hazards and can be strongly allergenic in sensitive individuals. A study on the occurrence of antibiotic residues and drug-resistant bacteria in beef and chicken tissues purchased from supermarkets in Hermisillo, Mexico, showed that 86% of beef samples were contaminated with streptomycin whilst other antibiotics were also prevalent. Chicken breasts sampled, likewise, showed high levels of contamination which exceeded FDA tolerance limits (Table 4.6).77
Table 4.6. Levels of antibiotics found in 50 beef and 30 chicken samples. (Adapted from reference 77)
Figure 4.13. Number of antibiotics detected in beef and chicken samples. (Adapted from reference 77)
Figure 4.14. The frequency of microorganisms isolated from beef and chicken samples. (Adapted from reference 77)
In this study it was found that in more than 50% of the chicken and beef samples investigated, more than one antibiotic was present at the same time, and in some cases three or four different antibiotics had been administered to the animals simultaneously (Fig. 4.13). The frequency at which the different micro-organisms occurred in the samples varied, but a wide range of potentially pathogenic species was prevalent in both beef and chicken samples (Fig 4.14).
From table 4.7 it is evident that no penicillin was found in any of the samples investigated. The reason for this is that penicillin is no longer used subtherapeutically in these areas as it has become ineffective. Bacteria resistant to penicillin were however isolated from these tissues, showing that drug resistance persists beyond the time in which the drugs were used. Nearly all the bacteria isolated from the above mentioned tissues were resistant to penicillin, with high resistance to tetracycline and streptomycin also noted (Table 4.7).
In view of the current controversy surrounding the issue of antibiotic resistant bacterial strains, two questions seem vital at this stage:
- Is the use of subtherapeutic doses on farms responsible for the increase in resistant strains?
- What is the status regarding vancomycin resistance?
Table 4.7 Antibiotic resistance in bacteria isolated from 50 beef and 30 chicken samples. (Adapted from reference 77).
The argument by industry has been that subtherapeutic doses cannot enhance resistance, however, this does not seem logical as even low doses of antibiotics should provide a selective advantage to resistant strains. If indeed this is the case, then at least certain antibiotics should be banned for use in the animal husbandry industry. Vancomycin is a case in point, as vancomycin is the only drug that can kill methicillin-resistant Stapylococcus aureus which is causing hospital epidemics with alarming regularity.72 To examine this issue, a project was jointly undertaken by the Departments of Zoology and Microbiology at the University of the Western Cape, in which chicken, pork, beef and milk samples were tested for bacterial contamination and multiple antibiotic resistance in the greater Cape Town area. Animal bacterial samples were taken at abattoirs and at retail outlets, the rationale being that differences in resistance between the two groups of samples should pinpoint the source. Higher resistance levels in abattoir samples taken at the beginning of the slaughtering cycle would indicate that the resistance had emanated from the farm. The results for the chicken samples have thus far been evaluated, and are presented in table 4.8.
The results clearly show that a large proportion of the bacterial strains showed multiple antibiotic resistance, and in most cases the bacteria from the abattoir samples had a higher resistance to antibiotics than the retail samples thus indicating that the resistance route came via the farm. The very high resistance levels displayed by most of the bacteria is certainly unsettling. Staphylococci were resistant to tetracycline and oxacillin but the percentage of abattoir isolates that displayed simultaneous resistance to both tetracycline and oxacillin was nearly double that of retail samples (69.6% versus 39.4%).Gram positive bacteria (Staphylococcus) are susceptible to vancomycin, and although resistance of Staphylococcus to vancomycin as well as methicillin was not very high, even the 7% resistance recorded in retail samples is cause for concern, considering that this is the only drug that can kill methicillin-resistant Staphylococcus aureus.78
Producers are required to observe a withdrawal period after administering antibiotics, prior to marketing their product. It is, however, not feasible to monitor all the meat that goes to market, and studies on swine have shown that producers do not adhere to the specified withdrawal times.79 Antibiotic residues are even found in carcasses of cattle with no record of antibiotic treatment.80 Even if they should adhere to these withdrawal times, it is doubtful whether this will be of much benefit, considering the fact that resistance has been maintained over years to antibiotics that are no longer in use. Besides the antibiotic problem, contamination of carcasses with antimicrobial agents and other dangerous compounds such as heavy metals, organochlorine compounds, organophosphorous compounds and growth stimulants, is now so widespread that methods are being devised to routinely monitor these contaminants in the interest of human safety.81
Table 4.8. Percentage resistance of various retail and abattoir chicken isolates to six antibiotics. (From reference 78)
Additional growth promoters
Besides the addition of antimicrobials to promote growth, animal feed is also routinely spiked with hormonal growth promoters of which some may be carcinogenic. Hormone residues that have been isolated from beef include trenbolone acetate, zeranol and stilbene diethylstilboestrol (DES), oestradiol, dienoestrol, hexaestrol, 17α-ethynyloestradiol, ketosteroids, testosterone, progesterone and progesterone acetate, which are all used as anabolic steroids to promote weight increase. To promote lean meat production, animals are fed β-agonists, a group of drugs that convert fats to fatty acids and stimulate the formation of proteins, to promote rapid weight gain. In addition in some countries growth hormone is administered and even genetically-engineered hormones are used, such as PST, which is used
to promote lean meat production in pigs. Some of these growth promoters, such as clenbuterol, are banned, but there is a healthy black market trade in these growth-promoting drugs as shown by the clenbuterol scandal in 1996 when German authorities found that the drug was being used on more than 40 calf fattening farms in Nordrhein-Westfalen and consequently prohibited the slaughter of 2400 calves and arrested a veterinary drug dealer. In a further case around the county of Gütersloh, calves were found with the banned antibiotic Chloramphenicol.82
Farm animals today are often treated as commodities, like inanimate consumer goods. They are frequently housed in unhealthy environments and fed virtually anything that will promote growth and increase profits, even though the long-term effect on the health of the animals or the human consumer is not known. In large chicken hatcheries the chicks never see sunlight, but are subjected to low-intensity light for close on 24 hours per day. The lights are switched off only for approximately 15 min each day so that the chickens can get used to darkness, lest they panic during a power failure and cause production losses. Animals are cramped together to limit their movements and energy expenditure, because growth and mass increase are the paramount criteria that are taken into account when designing these facilities. New breeds of chickens are selected for growth performance with virtual disregard for all other parameters. In the past, a free range chicken could be assumed to consume approximately 17kg of feed to grow to a market mass of 1.5kg. Today, some breeds utilise only 3.5kg to achieve the same mass increase, and this in only six weeks. The chickens are geared for rapid growth, but their other systems are severely compromised. The cardiovascular system, internal organs and immune systems are poorly developed so that extreme care must be taken not to induce stress or exposure to infectious bacteria lest they die before reaching market size.
Farm animals are fed carcass meal, fish meal, edible plastic, sewage, petro-chemical residues and excrement. On some farms veritable food chains have been set up where chicken manure, from battery chickens, is fed to the sheep and cattle, and dead chicks and unhatched eggs in turn are the feed items used in the piggery. In the chicken industry, the slaughter offal such as entrails, legs and heads are often dried, ground and recycled as feed , thus effectively turning the chickens into cannibals. Moreover, the chickens are routinely manipulated with a host of growth stimulating, antimicrobial and digestibility enhancing drugs.
Prion diseases
Prions cause a group of animal and human neurodegenerative diseases which are now classified together because of their etiology and pathogenesis. The infectious agent is not a virus, nor does it contain DNA sequences, it is a brain protein which has undergone modification. Prion proteins are thus thought to exist in two forms, the benign Prion protein (PrPc)and the infectious ‘scrapie form’ (PrPSc).83,84 The normal Prion protein consists of strands of amino acids twirled into helices whereas in the infectious form the amino acid strands are flattened into β-sheets which can cause transmissible dementias. Transmissible dementias are degenerative conditions associated with neuronal loss, and neuronal vacuolation or spongiform (spongy) changes. Furthermore, the changes are accompanied by the accumulation of the abnormal proteinase-resistant prion protein known as PrPSc which has undergone posttranscriptural changes, and the ensuing disease is thus termed a prion disease .
Prion disease is becoming a high-profile public health issue, particularly since the Bovine Spongiform Encephalitis (BSE) or “mad cow’s disease” epidemic rocked Britain. Beginning in 1986, this previously unknown prion disease decimated the beef industry in Britain and it has been proposed that some 160 000 cattle were affected. The infectious agent
was probably transferred to the animals by routinely feeding them meat and bone meal dietary supplements. Carcass meal is predominantly prepared from animals that have died of disease, or animals that have been condemned at the abattoir as unfit for human consumption. The carcass meal and excrement is heat-sterilized to kill the bacteria, but recently chemical sterilization has become the method of choice in many countries, as it is cheaper. Since 1988 the feeding of dietary protein supplements derived from sheep or cattle offal has been banned in Britain and it is argued, that this step has brought the epidemic under control. The incidence of reported cases has indeed declined since the peak was reached in 1992 (figure 4.15).
Figure 4.15. The number of bovine spongiform encephalopathy cases reported in Great Britain during the epidemic. (Adapted from references 83, 85).
Spongiform encephalopathies are transmissible dementias, and occur in man as Creutzfeldt-Jacob disease (CJD), Gerstman-Straussler syndrome, and kuru. In other animals it occurs as scrapie in sheep and transmissible mink encephalopathy in ranch mink. In the UK alone some 75 000 people per year die demented, of which 50% have Alzheimer’s disease and 2% have Creutzfeldt-Jacob disease.86 The latter disease is characterized by a sudden onset of disease, with rapid progression through dementia and death within a year.87 Economic pressures led to initial denials that consumption of BSE contaminated beef could lead to Creutzfeldt-Jacob disease, but in 1996, after a decade of ministerial denials the British Government reluctantly admitted the link between the two.88,89 Since this time, European governments have reacted strongly, banning British beef90 and even slaughtering thousands of head of cattle to restore consumer confidence. In Switzerland the government subsidized the slaughter of 230 000 cows born before 1 December 1990 to restore faith in Swiss beef.91
Prior to the admission that there was a link between BSE and CJD it was argued, that cross species contamination was not possible and that one could not contract the disease from eating contaminated food. However, instances of exotic animals in captivity with transmissible dementias have been linked to consumption of meat and bone meal, and instances in domestic cats in the UK are assumed to be due to BSE-infected offal in cat food.92 In early 1996 it was recognized that the new variant of CJD that was affecting 12 young persons in the UK had been transmitted via BSE contaminated beef. The experimental transmission of the disease by inoculating macaques (Old World monkeys) with BSE infected brain homogenate proved that cross species transmission was possible,93,94 and it is now even well established that transmission occurs between infected cows and their calves thus raising questions about the future of the epidemic.95,96,97
It has been argued, that what happened in Britain is but the tip of the iceberg, and that major epidemics could erupt worldwide. Thousands of Europeans could be unknowingly infected with the disease and could die, and even in the US some researchers feel that the conditions which led to the UK outbreak can lead to similar events in the USA.98. Statistics already suggest, that BSE is now Europe-wide and by May 1996 Switzerland had reported 211 cases of BSE, Eire 125, France 18, Portugal 30, whilst a total of 71 706 tonnes of British meat and bone meal and 33 424 breeding bovines have been exported to EU member states from 1985 to 1990.99 Even if infection of new animals ceases, it has been predicted that between 15 000 and 24 000 new cases of BSE would occur between 1996 and 1999.100
Mycotoxins
Regulations regarding animal feeds are not as stringent as those regarding food for human consumption, and feed that is contaminated with mycotoxins can be a further source of disease if fed to farm animals. Mycotoxins are toxins produced by fungi, and contaminated foodstuffs is a major problem in most tropical and subtropical countries. Aflotoxins are fungal toxins produced by Aspergillus flavus, A. parasiticus and Penicillium puberculum, and constitute a serious world-wide problem. Aflotoxins contaminate peanuts, nuts, rice, grains, soya beans, peas and sorghum seeds, all of which can end up as animal feed. Aflotoxins AFM and AFM2 are hydroxylated metabolites of the aflotoxin AFB and AFB2, which are the most potent liver carcinogens in rats, producing 100% incidence of liver tumours in rats at a dosage of 95µg/kg. These aflotoxins have been found in liver, milk, blood and kidneys of animals fed aflotoxin-contaminated feeds.12 Orchatoxin-A, another potent carcinogen, is produced by Penicillium, and has been found in cereals and meat products at levels of 1035µg/kg.
Contamination of the animals does not end at the farmyard, but continues in the slaughterhouses of the world. Abattoirs are concerned with speed rather than the well-being of the animals, and stress and terror are endemic.74 Carcasses are contaminated by faecal bacteria when they come into contact with ruptured intestines or fall on the ground. Because of contamination with the faecal bacteria Yersinia enterocolitica, Campylobacter spp., Salmonellae and Aeromonas hydrophila, as well as residues of veterinary drugs and mycotoxins, it has even been suggested that carcasses be decontaminated by radiation to combat the health hazards.101
Genetic engineering
Genetically engineered livestock is expected to revolutionize the agricultural industry. It is envisaged that animals can be made to grow faster and to incorporate changes in fat composition so as to be more suitable for human dietary needs. Gene transfers have been carried out on laboratory animals, but have also recently been extended to domestic animals. Most studies have centred around growth-regulation, as studies in mice have shown that gene transfers of bovine, ovine, rat or human growth-hormone genes can almost double the rate of growth in mice. Transgenic farm animals already include chickens, cows, pigs, sheep, rabbits and fish. Human, Bovine and rat growth-hormone genes have been transferred to pigs, but the mortality rates in these animals is high. Leaner meat has been produced in transgenic pigs, but at a price.
Transgenic animals suffer from a variety of pathological changes which shorten their lifespan. In pigs it was found that the animals were lethargic, and suffered from lameness, uncoordinated gait, protruding eyeballs and thickened skin. Moreover, they suffered from gastric ulceration, severe synovitis, degenerative joint disease, pericarditis and endocarditis, cardiomegaly, parakeratosis, nephritis and pneumonia.102 The risk of disease from contaminated animal products today is indeed something to consider. In the past the major concerns revolved around infectious diseases and parasitic and viral infections. This risk still exists today, but additional risk factors have been added through modern animal husbandry.
Modern technology has also had its effect on plant foods, with gene transfers being currently in vogue. The long term effects of these manipulations are also not known and these products may also be viewed with some degree of scepticism. However, man has to eat, and if a choice has to be made between manipulated plant or animal foods, the consumption of plant foods would certainly be associated with lower risks of contracting diseases. A diet free from animal products, and concentrating on a variety of fresh whole foods will supply excellent nutrition and will at the same time protect against many of the modern ills associated with Western diets. Such a diet need not, and indeed should not, be less attractive than a diet based on animal products. A little ingenuity with emphasis on variety will provide an exciting, healthy lifestyle.