CHAPTER ONE: INTRODUCTION
1.1 Background Information
Lead is a bluish soft metal with atomic number 82; atomic weight 207.19, specific gravity 11.34, melting point 327⁰C and boiling point 1740⁰C. It is the most common industrial metal that has become widespread in air, water, soil and food.1 Metals are often characterized and distinguished from non-metals by their physical properties which include the ability to conduct heat, and an electrical resistance that is directly proportional to temperature, malleability, ductility and even luster.2
Heavy metals can be classified into four major groups based on their health importance.3, 4 Copper, zinc, cobalt, chromium, manganese and iron are essentialandalso called micronutrients but are toxic when taken in excess of requirements.4, 5 Barium, lithium and zirconium are non – essential. Tin and aluminium are less toxicwhereas mercury, lead, cadmium and arsenic are highly toxic.
They are (lead, cadmium, mercury and arsenic) among the main toxic metals that remain in the environment permanently because they cannot be degraded and thus find their way into the food chain and ultimately into the tissues. They have direct physiologically toxic effects when stored or incorporated in living tissues.6
Heavy metals are kept under environmental pollutant category due to their toxic effects on plants, humans and food. Heavy metals such as lead, arsenic, cadmium and mercury are persistent, accumulate and are not metabolized in to other intermediate compounds and do not easily break down in environment.1
Poisoning by toxic chemicals can cause serious stock losses. Historically, lead and arsenic have been the most common causes of inorganic chemical poisoning in farm animals.7 The increase in industrial and agricultural processes has resulted in increased concentrations of metals in the environment. These metals are taken in by plants and consequently animals that graze on such contaminated plants and animals that drink from polluted water also accumulate such metals in their tissues.8 Some heavy metals like arsenic, cadmium and lead have been reported to have no known bio-importance in human biochemistry and physiology and consumption even at very low concentrations can be toxic.9, 10
Several organisations have pointed out the need for monitoring heavy metal concentrations in the environment because of their persistence and accumulation in the biota.11
Lead is a metabolic poison and a neurotoxin that binds to essential enzymes and several other cellular components and inactivates them.12 Toxic effects of lead are seen in haemopoeitic, nervous, gastrointestinal and renal systems.6
Lead poisoningis a medical condition caused by increased levels of the heavy metal lead in the body. Lead interferes with a variety of body processes and is toxic to many organs and tissues including the heart, bones, intestines, kidneys, reproductive and nervous systems. It interferes with the development of the nervous system and is therefore particularly toxic to children, causing potentially permanent learning and behavior disorders. Symptoms include abdominal pain, confusion, headache, anemia, irritability, and in severe cases seizures, coma, and death.13 Toxicity of lead is closely related to age, sex, route of exposure, level of intake, solubility, metal oxidation stage, retention percentage and duration of exposure, frequency of intake, absorption rate and mechanisms and efficiency of excretion.14
In a survey of lead residue in kidney and liver of slaughtered cattle in Sokoto central abattoir, Nigeria, Bala et al15 reported a prevalence rate of 100%. The presence of lead in the liver among different age groups was above the permissible limits of
0.1mg/kg set by the Food and Agricultural Organisation (FAO) but in the kidney the concentration was within the permissible level in all age groups except for 0 – 2 years of age.
In another study of metal composition of livers and kidneys of cattle from Southern Nigeria, Iwegbue16 reported levels of various metals were generally low and within international statutory safe limits.16 However, Nwude et al17 in a study of metal quantification in cattle: a case study of cattle slaughtered at Ota abattoir, Nigeria observed that the level of six metals quantified were higher than the WHO standards and gross contamination of cattle could be inferred; and the levels of metals in cattle parts could be used as biomarkers of metal pollution, though highly influenced by other factors.17
In a study of outbreak of fatal childhood lead poisoning related to artisanal gold mining in Zamfara State, Doyeema et al18 reported that venous blood lead samples were obtained from 59% (204 of 345) of children < 5 years of age. All blood samples indicated lead poisoning, blood Lead level (BLL) ≥ 10 μg/dl. In 97% of children, BLLs were ≥ 45 μg/dl which is the CDC-recommended threshold for initiating chelation therapy.19 Eighty-five percent of blood samples exceeded 65μg/dl, the maximum detection limit of the Lead Care II instrument.18
1.2 Problem Statement
Lead poisoning accounts for about 0.6% of global burden of diseases.20 Lead has been impacting the health of humankind since the Romans began mining it 2500 years ago, and despite early knowledge of its harmful effects, exposure to lead from a wide variety of sources persists to this day.21
Lead bio-accumulates in many organs of the body after ingestion, it adversely affects many organs and systems which results in conditions such as anaemia, kidney damage, high blood pressure, impaired hearing and mental retardation22 and elevated levels can cause abortion in female animals and humans.22 Consumption of carcass containing lead can cause lead poisoning as a result of bioaccumulation in body tissues, and this varies with individual and the duration of lead exposure.23, 24
In an assessment of metal levels in fresh milk from cows grazed around Challawa Industrial Estate, Kano, Nigeria, Ogabiela et al25reported that the concentrations were very high as the metals of samples exceeded the World Health Organisation permissible limits.25 Another study by Okoye and Ugwu26 on the impact of environmental cadmium, lead, copper and zinc on quality of goat meat in Nigeria reported high levels of cadmium, copper and zinc in the liver, kidney and muscles of goats.26 However, lead levels determined were relatively within safe limits. Akan et al27 in a study of heavy metals in the liver, kidney and meat of cattle, sheep, goat and chicken from Kasuwan Shanu, Maiduguri, Nigeria reported that the concentrations of all metals were within the tolerance limits with the exception of chromium and lead which were higher than standard limits.27 Nwude et al28 also reported varying levels of heavy metals in different seasons indicative of the effect of season as a factor in accumulation of heavy metals by animals.28
Environmental levels of lead have increased more than a thousand fold over the past three centuries as a result of human activity. The greatest increase occurred between the years 1950 and 2000 and reflected increasing world wide use of leaded gasoline. Lead is released into the air in the form of metal fumes or suspended particles from fuel combustion or smelting and disposal of wastes, however, most of the lead poisoning is from leaded gasoline.6
Dioka et al29 and Kamala and Kumar30 reported that some of these waste materials may contain some heavy metals such as lead and others that are dangerous to human and animal health.29, 30
Results of a recent study conducted in Macedonia indicate that Lead concentration in animal tissues is dependent on sampling locality, the organ and animal species. The concentrations of lead in the liver and kidney tissues taken from industrial area were higher as compared to other localities from which samples of tissues were taken for analysis.31
It is evident from available information that lead poisoning in man could result from consumption meat and milk of poisoned cattle, camel, sheep, goats and birds. Man can also be poisoned by lead through mining activities as is obtained in Zamfara State due to illegal, informal or unsafe mining of lead. Environmental poisoning is another risk for human especially from leaded petrol, diesel and even water.
The current study will provide information on lead poisoning in cattle, camel, sheep and goats as a risk for consumers of these various meats. This will serve as a guide to the public.
1.3 Justification of the Study
From a public health point of view, consumption of lead-contaminated animal products from lead-poisoned animals poses a food safety concern especially in children less than five years of age. Lead concentrations in the liver and kidneys are usually the highest and can be detected within few days before it is redistributed for storage in bones.
The primary route of lead exposure for humans is through ingestion of contaminated foods. The recent outbreak of lead poisoning and other heavy metals amongst local miners in Zamfara State calls for investigations of animals in that locality and other locations as they may have ingested the heavy metals. According to Doyeema et al18prevalence of lead poisoning is very high in Zamfara State. Over 400 children were reported to have died as a result of the lead poisoning outbreak in Zamfara State; about 2000 children are currently on treatment. In 2010, WHO stated that over 4000 children are at risk of lead poisoning and this is entirely preventable.
The habitats of animals are continually being polluted with lead metal as a result of indiscriminate dumping of waste materials on the land and in water bodies, illegal mining of ores, painting of animal‟s houses, and the use of tetra ethyl lead as an antiknocking additive to improve the quality of petrol in Nigeria and many other developing countries.