RADIOBIOLOGY AND ANIMAL HEALTH: THE VETERINARY PERPECTIVE

Veterinary medicine as a profession apart from directly handling problem pertaining to animal health also indirectly caters for human health. Considering the important position man occupies in the food chain as a final consumer and man’s quest to maintain the human population in the best standard of health, the effect of radiation and radioactive substances on domestic animal and several activities of man must always check man’s excesses in this nuclear age. The effect of radiation on domestic animals which range from physiological disturbances and disability to death could be accessed in the veterinary contest from two perspectives: economic and public health. From the economic point of view, these are losses in productivity which could be measured in terms of milk yield, carcass weight and fertility. Milk, meat and other animal products could be contaminated very heavily rendering them unfit for human consumption and consequently unsellable. This does not take into account economic expends accruing from treatment and maintenance of such exposed animals. The second and more critical effect of exposures of animal to radiation is public health consequences which includes the transfer of fission products to man especially long half-lives isotopes like Cessium 137 and Strontium 90 which will go down to man via animals or animal productS causing man’s genetic mutation, neoplasia and death.
SOURCES OF RADIATION OF VETERINARY IMPORTANCE
Ever since the discovery of ionizing radiations and other radioactive substances, radiations are extensively used in the society. The source of radiation of veterinary importance can be broadly divided into natural and artificial resources. The cosmic rays from space and the natural occurring radioactive materials in the environment constitute the natural sources which could not pose much danger to animals. The artificial sources which are the main sources of dangerous radiation include the X-rays and radioactive nuclides used in medical practice for diagnostic and therapeutic purposes. Also included in this group are radioactive fallout from nuclear explosion, accident involving radioactive materials and radioactive waste arising from nuclear energy operation. Radionuclides that can enter the biosystem include those of Iodine 131, Barium 140, Strontium 89 and 90, Cesium 137. Those of higher biological significance are Strontium 90 and Cesium 137 because they are usually produced in large quantity and have long half -lives and therefore can persist in the food web.
EFFECTS OF RADIATION ON THE CELL. The effect of radiation on the cell and organism are due mainly to three aspects of radiation mainly blast, heat and irradiation. Irradiation causes more of the mortality. Radiation disturbs the normal organization like presenting additional stress to the cells. The exposure of the cells to radiation sets up a chain of events which occurs in three stages namely: physical stage, chemical stage and physiological stage.
 The Physical Stage: During this stage, energy absorption occurs and important macromolecules and other cell component become ionized and changed to radical. The Chemical stage: Starts with reaction of activated molecules with one another and with normal cell constituents leading to macromolecular changes. These changes terminate when chemical stability is restored.
The Physiological stage: This stage is characterized by the translation of the biochemical changes into metabolic disturbances. In summary, the mechanism of action of radiation on cells involves the ionizing radiation giving discrete and relatively large amounts of energy to only a few atoms within the irradiated material. This energy activates molecules which readily take part in chemical reactions resulting in damage, random selection of cells and permanent chemical changes in others. These changes could lead to the death of the cell, loss of proliferative ability of the cells, development of abnormal set of chromosomes and abnormal transmission of genetic information. It may also lead to irreparable damage to certain critical structures; the integrity of which is essential to the cell and whose damage could give rise to metabolic disability. The most important target of radiation in the cell is the genetic material deoxyribonucleic acid (DNA).
Effects of the Radiation on General Animal Body
The deleterious effect of radiation on the cell is reflected on the organ systems and the general animal body. In terms of duration of exposure and effect of radiation to the animal; the effects of radiation could be considered from three viewpoints namely: Acute, Subacute and chronic syndromes.
Acute Syndrome: This result from somatic dose received from intense direct exposures. The average lethal dose for most domestic animals is 500 roentgen for 24 hours and it should be noted that any level of irradiation likely to cause observable injury to animals is considerably greater than the dose which will tender their products unfit for human consumption. On exposing the animal to this dose of ionizing radiation, the immediate response is radiation sickness characterized by anorexia, vomiting, profound lethargy which could manifest as acute irritation of alimentary tract leading to intense diarrhea.  The ultimate effect is either death due to dehydration and salt depletion or in some cases the recovery of the animal. After a period of about one week, profound depression of the bone marrow activity occurs leading to lymphopaenia and depressed granulocyte and platelet counts. This permits bacteria invasion of the system from the alimentary tract. Impairment of the clotting mechanism and antibody production which are other consequences also facilitate bacteria proliferation and septicaemia. Thrombocytopaenic haemorrhages result in animal and progressive gut wall necrosis without inflammation. There is the depression of germinal epithelium leading to infertility. In addition to these, shedding of hairs, skin ulcerations, degenerative changes in the lens occur with the latter leading to blindness in some cases. Mostly those of haematopoietic origin characterize this stage.
Sub acute syndrome: This form occurs after about four weeks of exposure and most deaths occurs during this period. There is fever, knuckling of the fetlocks, swelling of legs, development of malaria, dysentery and tenesmus. Anorexia, marked thirst, weakness, recumbency and hyperirritability characterize this stage. Severe anaemia and septicaemia are also observed. After this period, there is a long period of convalescence coupled with weight loss, alopecia, lenticular defects and sterility. If the animal however recovers, it tends to produce mutant offspring. Incessant occurrences of tumours also appear in this group of animals. In pregnant animals, foetal death and resorption, organ defects, decreased survival of young born alive, reduced growth rate of surviving young ones may occur.  
Chronic syndrome: This results from chronic exposure to ionizing radiation. In the event of contamination of pasture by radioactive fallouts, high level of irradiation in pasture will give effects similar to those of direct irradiation with medium lethal dose. Lower levels of exposure will give effects which could be assessed from two viewpoints-The genetic effect which is its ability to induce mutations in a population of animals and also cause leukemia. The intensity of this action depends on the radioactive material formed, its solubility and half- life. In leaf contamination, radio-iodine and radiostrontium are of importance. Radioiodine damages the thyroid glands and the gonads while exposure to radiostrontium leads to the destruction of the bone tissue, decrease of platelet production, decrease of white blood cell production and absolute depression of erythropoiesis which results in slow red blood cell counts and haematocrit level. In soil contamination and absorption by plants, radiostrontium and radiocesium are of importance. It should be noted that radioiodine and radiostrontium can be excreted in milk and could have deleterious effects to human and animals after the consumption of contaminated milk. In addition to this, chronic exposure to gamma and mixed neutron gamma radiation can lead to lenticular opacities.
Conclusion: The biological effects of radiation can be summarized as follows:
(a) Genetic effects which include chromosomal aberrations and gene mutations. (b)leukemia and general tumour induction (c) Death. When these effects of ionizing radiation occur in animals, the consumption of the animal or its products by man may have dangerous consequences. The ability to secret radioiodine and radiostrontium in milk is important in this respect. In addition, radioactive materials which causes radiation injury apart from its economic consequences, may serve as a potential reservoir of radioactive materials which could be passed to man in meat, milk and other products- a problem that is of public health importance. In view of these facts, third world countries like Nigeria should resist the temptation of allowing their countries to be used as dumping ground for radioactive wastes from developed countries. It should be understood that even after the direct effect of ionizing radiation on domestic animals, they could still act as reservoir and transport hosts for dangerous radioactive materials which will ultimately affect man.