VETERINARY PUBLIC HEALTH

Chemical contamination of foodstuffs can be due to their presence in the environment, or their intentional or unintentional application somewhere along the food chain. Some chemical toxins may occur naturally. Controlling chemicals which are only unintentionally present in the food chain is obviously difficult. Foodborne Chemical Hazards may be from the following categories:

Drug Residues

This category consists of:

Veterinary Medicines.
Veterinary Medicines, after being administered to an animal, will still be present in the animal's tissues (or food products) for a variable ammount of time depending on the drug & the species amongst other things. Drugs given to treat animals intended for food production, if licienced, will have a minimum withdrawal period which must be observed before the animal can go for slaughter (for meat withdrawal). Withdrawal periods also exist for other animal products, e.g. egg withdrawal, milk withdrawal, etc...This is to allow enough time for the drug to be excreted or metabolised sufficiently so that no drug or only insignificant ammounts will be present in any food derived from that animal.
If a Veterinary Medicine is not Licienced for use in a particular species for treating a particular condition then (having observed the Cascade System) the standard withdrawal period of 7 days for meat & 28 days for milk should be observed. Some drugs are specifically prohibited from use in all food animals (e.g. Phenylbutazone, Lignocaine to name two) and some drugs are prohibited in specific types of food producing animal (e.g. Tilmicosin is prohibited in Dairy cows but not beef cows).
Veterianry Medicines whose use are widespread in food producing animals are Antimicrobial & Antiparasitic drugs. Residues of ranquilizers are also of concern.
Organic producers ofen have to observe much longer withdrawal periods and can only administer drugs when the use has been specifically approved by a vet. Use of drugs other than to treat sick animals is generally not approved (although welfare should not be compromised).
Agricultural Chemicals.
As the use of these substances is generally intentional they can be applied well in advance of harvesting in a controlled manner (this is the case for herbicides, fungicides, fertilizers & rodenticides), thus limiting their presence in feeds then given to animals. Some insecticides may be used nearer to harvest or on animals & their environments.
1. Insecticides-As the most widely used agricultural chemicals represent the greatest risk to food contamination of this category. Some are extremely environmentally stable & toxic.
  1. Chlorinated Hydrocarbons
    DDT (Dichloro-Diphenyl-Trichloroethane) is th most well known. It is very durable, accumulates in tissues & persists in the environment. Chlorinated Hydrocarbons are CNS stimulants & cause congestion of internal organs & focal centrilobular necrosis of the liver.
  2. Organophosphates
    The most widely used insecticides, are much less persistant in the environment & body tissues than Chlorinated Hydrocarbons and hence, present less of a risk. They can however, be extremely toxic to mammals causing non-specific acute signs or chronic lung congestion. They may be an Occupational Hazard.
2. Herbicides-Are not usually used on animal food crops but may sometimes be used for plants used for animal bedding causing residue problems. Some may contain Dioxins.
3. Fungicides-(e.g. dicloran & folpet) Do not usually cause residue problems. They may contain heavy metals. They are often dyed to allow identification of treated crops, however, accidental feeding of treated crops to animals or use in cereal foods has led to poisonings.
4. Fertilizers-Can cause significant environmental problems, particularly if they contaminate run-off waters. Algal blooms may result aswell as the growth of toxic algae that may contaminate shellfish.
5. Rodenticides- (e.g. Warfarin & Antu) although widely used should not contaminate foodstuffs.
Growth Promoters
Growth Promoters are used amongst other things to increase production performance of animals and not for a health benefit to the animal or consumer. Commonly used Growth Promoters are Hormone-like compounds & antimicrobials. Growth Promoters Growth Promoters are not allowed in the EU but are in the USA & other countries. This has important trade implications.
1. Hormone-like growth promoters
  These include synthetic hormones (e.g.Diethylsibestrol), natural hormones (e.g. estradiol, Progesterone, Testosterone), fungal oestrogens (Zearalenone), Beta-agonists (Trenbolone) and thyreostatics. Risk associated with these compunds can be difficult to evaluate as administered natural hormones may be dificult to distinguish from endogenous hormones in animals & people. Also synthetic hormones given to young animals may not be detectable at time of slaughter.
2. Antimicrobial Growth Promoters
  Fed are low but continuous doses in feed they have been observed to cause increased growth rates. Although not understood the mechanism of action could be via suppressing disadvantagous intestinal microflora or direct anabolic effects of the drugs themselves. Virginamycin, bacitracin, polymyxin B & sulphonamides are commonly used. There used is strongly suspected of causing Antimicrobial Resistance.

Environmental Chemical Hazards

Often from an industrial source they include:

Heavy Metals
(e.g. Lead, Arsenic, Mercury, Cadmium, Fluorine &Selenium)
1. Lead-Lead can occur in animals grazing near smelting plants or from animals licking lead based paints or old car batteries. Lead then accumulates in the bones. Acute poisoning may result in high levels in the liver & kidneys. Chronic exposure may cause softening & cavitation of the CNS.
2. Arsenic-Contamination of food animals with arsenic is typically due to exposure to arsenical herbicides, rodenticides or insecticides. Its accumulation in the liver & kidneys can lead to fatty degeneration. It also accumulates in the bones.
3. Mercury-Rare cases of mercury poisoning have been due to animals being fed with grain treated with mercury-based anti-fungicides. The result is enlarged internal organs & petechiation). Inorganic mercury is also stored in the liver & kidneys, whereas organic mercury is more widely distributed. See Minamata disease
4. Cadmium-An increasing problem, high cadmium levels occur in animals grazing pasture irregated with aerobically digested sludge. Contamination of shell fish & fish also occur as cadmium may contaminate water. It accumulates in the bidy tissues & may cause kidney failure. Prolonged accumulation is required to reach toxic levels.

Food Additives

This refers to chemicals deliberately added to foods in a controlled manor & at a level that is judged as posing no significant health risk. However, uncertain circulmstances some food additives may present a health risk. They may be curing agents, smoke compunds, antioxidants, preservatives or agents improve sensory qualities.

Additives to improve Sensory Qualities
There are many food additives for food that do not offer any food safety benefits & are added to improve the sensory qualities (Organoleptic qualities) of the food. Possible health risks of these additives are poorly defined but some associations have been put forward (e.g. hyperactivity in children).
Organoleptic qualities that they may afffect are colour (e.g. dyes), emulsifiers, sweetners (e.g. saccharin, acesulfame k, aspartame) & flavour enhancers (e.g. Sodium Glutamate).
Nitrites & Nitrates
Nitrogen Dioxide reacts with myoglobin in meat to give it an attractive, bright red colour. Nitrates helps to inhibit the growth of clostridial spores in cured meats (e.g. sausages) & provide the cured flavour. Nitrites are toxic if concentrations are too high (so levels are monitored), it may combine with the blood pigment Haemoglobin to form nitrosohaemoglobin. In addition, the nitrites may react with amines in the meat to form Nitrosamine compounds that are carcinogenic. This small risk is seen to be outweighed by that health benefits of inhibiting C.botulinum. Nitrites levels of tobacco smoke & some vegetables (e.g. spinach) are many times higher than in cured meats. See Preservation Methods.
Polyphosphates
Polyphosphates are added to meats to increase the meat proteins ability to bind water. This improves texture & increases the weight of the meat. THey could potentially cause a disturbance to the bodies calcium/phosphorus balance due to excess phosphorous intake. Children would be particuarly affected by this due to their rapid bone growth.
Smoke
Smoke flavours are used to improve the organoleptic properties of some foods. Some compounds from smoke have antimicrobial properties (e.g.organic acids & phenols) and some smoke compounds are carcinogenic. Polycyclic aromatic hydrocarbons, such as 3,4 benzpyrene, are carcinogens produced when smoking at over 300◦C. A liquid smoke product is frequently used that has the toxic compunds removed but retains the compounds that improve organoleptic qualities.
Plastic Packaging Compounds
Vinyl chloride monomers, that are carcinogenic, are used to produce Polyvinyl Chloride (PVC) plastic packages, that are safe/non-carcinogenic. However, in some circumstances the toxic monomers may still be present & could potentially transfer to the food. Other plastics, not intended for microwave use, if microwaved may react with fats in the food to produce toxic substances.

Mycotoxins

Some fungi produce toxins (Mycotoxins). If they grow in animal feeds any toxins then consumed may be present in products that come from these animals. Toxin producing fungi can also grow on some foods after harvest.
Mycotoxins of concern are Aflatoxins produced by Aspergillus flavus & A. parasiticus. They are carcinogenic, affecting the liver. Ochratoxins produced by Penecillium spp. & some Aspergillus moulds and associated with pigs, affect the kidney.

Marine Toxins

Algae can produce toxins that may then accumulate in filter-feeding shellfish. Use of Phosphate-based fertilizers on farms may lead to algal blooms in costal waters (which may be seen as a red tide) leading to toxin production. These toxins may cause Paralytic Shellfish Poisoning (PSP) & other similar poisonings. Algal toxins are not destroyed by cooking. The clinical signs of PSP are numbness & tingling of the mouth & lips. Cranial nerves may be affected. Weakness of the limbs & paralysis progress & in some cases death may occur due respiratory failure due to paralysis of the respiratory muscles.

Plant Toxins

Some mushrooms & plants produce toxins. Examples are phytohaemagglutinin found in red kidney beans & deastroyed by heating & grayanotoxin found in honey produced from rhododendrons.

Toxic Amines

Toxic biogenic amnines, such as histamine & tyramine, are produced by the decarboxylation of free amino acids in any food. This can be caused by excessive consumption of amines or a deficiency of detoxifying enzymes (e.g. Monoamine Oxidase) in an individual. In the case of Scombroid poisoning Histadine in fish is converted into Histamine by Spoilage Bacteria before the fish is consumed.

This syptoms of Histamine intoxification are analygous to an allergic reaction i.e. rash, oedema & hypotension. Tyramine causes a rise in blood pressure possibly associated with migranes. Ripened cheeses, fermented sausages, red wine & chocolate are implicated as sources of tyramine poisoning.

References

See References