Drug application after appearance of clinical signs may eliminate or damage pathogens/parasite and lead to recovery of health in the patient.
Chemotherapy (CT) and chemoprophylaxis (CP) are experimental sciences dealing with drugs that selectively inhibit or destroy parasites or other pathogens. Thus, attention to selective toxicity of antiparasitic drugs is central to CP and CT, and useful chemotherapeutic agents should affect the parasite more adversely than the host. Until now, there is no completely safe drug. Knowledge of sources of drugs (pharmacognosy), action and fate in the body (pharmacodynamics), use in the treatment of disease (therapeutics), adverse effects (toxicology) and contraindications is a must to evaluate potential risks as well as possible benefits of drugs to the patient's well-being.
Claims made by the drug manufacturer for a drug in treatment of target animals consumed by humans (poultry, bovines, equines) must have been investigated and evaluated in the target animal, besides toxicological studies in rodents (rat, mouse) and other species. The latter are required to determine the ADI. The acceptable daily intake is the dose of a drug residue in edible tissues, like meat, various organs, fat, etc., that during the entire lifetime of a person seems to be without obvious risk to health on the basis of all toxicological data known at the time. The ADI for humans may be determined by applying a safety factor of 1:100, or a safety factor of at least 1:1000 in case of a teratogenic drug. Therapeutic claims made by the manufacturer must coincide with safety and tissue residue data for the drug approved by government regulatory agencies. The preslaughter withdrawal time refers to the interval from the time an animal is removed from medication until the permitted time of slaughter. During this interval the residue of toxicological concern will reach a safe concentration as defined by the MRL (maximum residue limit). Recent workshops give guidance to the analytical approaches. There may be three types of tolerances in defining allowable concentrations of drug residues at the time of slaughter up to the time of consumption by humans. (1) The finite tolerance (measurable amount of drug residue permitted in food), (2) the negligible tolerance (insignificant amounts of residue, i.e., a small fraction of maximum ADI), or (3) the zero tolerance (no residue is permitted in feed or food because of extreme toxicity or because the drug is carcinogenic).
Drug can be broadly defined as any chemical compound that affects living processes and is used in the diagnosis, prevention, treatment (cure) of disease(s), or for controlling or improving any physiological or pathological disorder or for relief of pain in animals and humans. A drug may have various names: one or more chemical names depending on rules of chemical nomenclature used: (1) Usually one international nonproprietary name (INN), (2) one or more nonproprietary names, e.g. in different countries, and (3) usually several proprietary names or brand names. Products are galenic, pharmaceutical, and/or medicinal preparations of drugs, e.g. various dosage forms. These may be tablets, pills, capsules, sustained-release boli, or several liquid preparations, e.g. mixtures, or emulsions for oral administration. Dosage forms for injections are ampules, multi-dose vials, large-volume capped bottles, and implants that may be hard, sterile pellets inserted under skin. The parenteral administration of such preparations may be done either by subcutaneous (s.c.), intramuscular (i.m.), intravenous (i.v.), intraperitoneal (i.p.), or intrathecal (i.e., into the subarachnoid space) injection. For application to skin surface several external preparations of drugs may be used such as liniments, lotions, ointments, creams, dusting powders, or aerosols, e.g. topical insecticides.
The administration of chemotherapeutic or chemoprophylactic agents can only be regarded as a suitable measure when individual cases or parts of the population or herds are treated under controlled conditions after a diagnosis has been made. Parasitism in the field is often a multifactorial problem. Thus, methods of control must include epizootiological/epidemiological and therefore ecological and economic aspects as well as consideration of the development of resistance to drugs by the parasites. Drug application is important but not the only control measure in a large-scale, complex control strategy. The aim of each of the measures that can be taken is to reduce or even completely eliminate the parasite population in its environment. However, the tasks, solutions, and objectives of a certain strategy require profound knowledge of the biology of the parasites and the epizootiology or epidemiology of parasitism. Thus, when planning and implementing programs of this type, account must be taken of the many different interactions between host and parasite on the one hand (mode of transmission and susceptibility of the population) and those between parasite and/or host and biotype on the other (e.g., pathogen reservoir). Knowledge of the environmental factors such as temperature, humidity, soil structure, and prevailing weather conditions which are critical within the infection chain are of practical importance for forecasting parasitic risk in certain areas.
In order for the chemotherapeutic agent administered to have its optimum effect against the parasites, the time of treatment must be adapted to the development cycle of the parasite. An example of this is the metaphylaxis often practiced in veterinary medicine when the aim is to prevent the outbreak of disease in animals already infected, i.e., to kill off certain parasitic developmental stages in the host before serious damage occurs. The correct timing of the treatment in relation to the course of the infection is therefore critical for the success or failure of drug metaphylaxis.
As is well known, the acquired and often species- or even life-cycle-stage-specific immunity (immune responses) of the host plays an important role in the elimination of parasites. It would be ideal if CT and immunoprophylaxis had a synergistic effect. However, drugs can suppress the development of immunity and the immune response of the host may not materialize. Thus, the lack of an immune response may result from starting treatment at the incorrect time in relation to the course of the infection. This may be true for instance when treatment and infection take place at the same time, or the treatment starts shortly after the infection, as a result of which the parasites are immediately eliminated (without antigenic effect).
In modern livestock farming in which large numbers of animals are reared and fattened in a confined space, continuous drug application must often be performed via the feed. This is because other preventive measures, such as methods for maintaining good hygiene and disinfection, have either not yet been developed or are as yet inadequate (such as reliable immunoprophylaxis) against the high risk of parasitism that exists with this form of farming. Thus, without prophylactic medication, the rapidly increasing pressure of infection would lead to an outbreak of disease in a herd. Consequently, in the fattening of poultry feed additives such as anticoccidiostats can control outbreaks of coccidiosis and therefore serious financial losses can be reduced to a minimum during the fattening period.
A few examples clearly show that prophylactic or therapeutic agents must not be used in a stereotyped fashion. Often their use must be adapted to varying relationships between the parasite and host populations and their environment; only then will a treatment campaign have the desired success. Flexible handling of the chemotherapeutics also considerably slows down the development and therefore the spread of drug-resistant parasites. Before the start of every course of treatment adverse effects of the chemotherapeutic agent must be taken into account in the treatment plan. They should be reduced to a minimum by varying the dosage regimen and the treatment intervals. Anaphylactic reaction to the drug used or the intrinsic toxicity of the drug including its toxic metabolites may induce a severe and life-threatening risk to a patient. This may be applied also to severe allergic reactions caused through toxic metabolic products of endoparasites released after the drug has killed parasites.
Acanthocephalacidal Drugs, Antidiarrhoeal and Antitrichomoniasis Drugs, Babesiacidal Drugs, Cestodocidal Drugs, Coccidiocidal Drugs, DNA-Synthesis-Affecting Drugs I, DNA-Synthesis-Affecting Drugs II, DNA-Synthesis-Affecting Drugs III, DNA-Synthesis-Affecting Drugs IV, DNA-Synthesis-Affecting Drugs V, Drugs against Sarcocystosis, Drugs against Microsporidiosis, Ectoparasitocidal Drugs, Energy-Metabolism-Disturbing Drugs, Hem(oglobin) Interaction, Inhibitory-Neurotransmission-Affecting Drugs, Leishmaniacidal Drugs, Malariacidal Drugs, Microtubule-Function-Affecting Drugs, Myxosporidiacidal Drugs, Nematocidal Drugs, Animals, Nematocidal Drugs, Man, Opportunistic Agents, Treatment of Opportunistic Agents, Theileriacidal Drugs, Trematodocidal Drugs, Trypanocidal Drugs, Animals, Trypanocidal Drugs, Man, Vaccination Against Protozoa