Pharmacology

Once a diagnosis has been made and medical treatment is deemed necessary, safe and effective pharmacologic agents that exert the appropriate actions should be selected. A dosing regimen should be individualized for each patient. In addition to the route, which often is based on drug availability or convenience, a number of factors should be considered when designing a dosing regimen. These include host considerations that may alter the response to or disposition of the drugs. Adjustment in route, dose, or interval may be indicated based on host and drug factors. For antimicrobial drugs, microbial factors, including resistance, also should be considered. Finally, particularly for food animals, public health, environmental implications, and regulatory constraints must be considered.

See also Principles of Therapy of Cardiovascular Disease in Animals and Management and Fluid Therapy.

Also see Principles of Therapy of GI Disease in Animals.

Also see Ophthalmology.

Also see Principles of Topical Therapy in Animals.

Drugs that affect skeletal muscle function fall into several therapeutic categories. Some are used during surgical procedures to produce paralysis (neuromuscular blocking agents); others reduce spasticity (skeletal muscle relaxants) associated with various neurologic and musculoskeletal conditions. In addition, several therapeutic agents influence metabolic and other processes in skeletal muscle, including the nutrients required for normal muscle function used to prevent or mitigate degenerative muscular conditions. For example, selenium and vitamin E are used to prevent or treat muscular dystrophies such as white muscle disease (see Nutritional Myodegeneration). The steroidal, nonsteroidal, and various other anti-inflammatory agents (eg, dimethyl sulfoxide) are also commonly used to treat acute and chronic inflammatory conditions involving skeletal muscle. Anabolic steroids promote muscle growth and development and are administered in selected cases in which serious muscle deterioration has developed as a complication of a primary disease syndrome.

Drugs used to modify or treat disorders of the nervous system fall into several categories: anticonvulsants or antiepileptic drugs (AEDs), tranquilizers, sedatives, analgesics, and psychotropic agents. Also see Principles of Therapy of Neurologic Disease, discussed in Analgesic Pharmacology, and Principles of Pharmacologic and Natural Treatment for Behavioral Problems.

Also see Principles of Therapy for Reproductive Disorders in Animals.

Also see Principles of Therapy of Respiratory Disease.

Also see Principles of Therapy of Urinary Disease.

Treatment with any chemotherapeutic agent involves an understanding of the complex interrelationships among the host animal, the infecting pathogen, and the drug, the interactions of which comprise the chemotherapeutic triangle. The advent of resistance places further emphasis on consideration of these factors when selecting a dosing regimen. That the FDA struggles to identify the most reasonable way to approve antimicrobials while maintaining high standards of proof of efficacy and safety as well as clinician flexibility in the design of dosing regimens is a testament to the complex host, drug, and microbial interactions associated with infections.

Many highly effective and selective anthelmintics are available, but such compounds must be used correctly, judiciously, and with consideration of the parasite/host interaction to obtain a favorable clinical response, accomplish good control, and minimize selection for anthelmintic resistance. Any decrease or increase of the recommended dose rate must always be discouraged. Underdosing is likely to result in lowered efficacy and possibly increased pressure for development of resistance. Overdosing may result in toxicity without necessarily increasing product efficacy.

β-Lactam antibiotics, named after the active chemical component of the drug (the 4-membered β-lactam ring), include the 6-membered ring–structured penicillins, monobactams, and carbapenems; and the 7-membered ring–structured cephalosporins and cephamycins. In addition to their chemical structure, the major difference between these two subclasses of β-lactams is their susceptibility to β-lactamase destruction, with the cephalosporins, in general, being more resistant.

Pathogenic fungi affecting animals are eukaryotes, generally existing as either filamentous molds (hyphal forms) or intracellular yeasts. Fungal organisms are characterized by a low invasiveness and virulence. Factors that contribute to fungal infection include necrotic tissue, a moist environment, and immunosuppression. Fungal infections can be primarily superficial and irritating (eg, dermatophytosis) or systemic and life threatening (eg, blastomycosis, cryptococcosis, histoplasmosis, coccidioidomycosis). See also Dermatophytosis and see Fungal Infections.) Clinically relevant dimorphic fungi grow as yeast-like forms in a host but as molds in vitro at room temperature; they include Candida spp, Blastomyces dermatitidis, Coccidioides immitis, Histoplasma capsulatum, Sporothrix schenckii, and Rhinosporidium.

Inflammation is the complex pathophysiologic response of vascularized tissue to injury. The injury may result from various stimuli, including thermal, chemical, or physical damage; ischemia; infectious agents; antigen-antibody interactions; and other biologic processes. After tissue injury, the process of tissue healing includes three distinct phases: an inflammatory phase, a repair phase, and a remodeling phase. The desired outcome of the inflammatory response is isolation and elimination of the injurious agent to prepare for the repair of tissue damage at the site of injury and restoration of function. Finally, new tissue formed during the repair phase (eg, scar tissue) may be remodeled over several months.

Antineoplastic chemotherapy is an important component of small animal practice and is routinely used for selected tumors of horses and cattle. Effective use of antineoplastic chemotherapy depends on an understanding of basic principles of cancer biology, drug actions, toxicities, and drug handling safety.

Antiseptics and disinfectants are nonselective, anti-infective agents that are applied topically. Their activity ranges from simply reducing the number of microorganisms to within safe limits of public health interpretations (sanitization), to destroying all microorganisms (sterilization) on the applied surface. In general, antiseptics are applied on tissues to suppress or prevent microbial infection. Disinfectants are germicidal compounds usually applied to inanimate surfaces. Sometimes the same compound may act as an antiseptic and a disinfectant, depending on the drug concentration, conditions of exposure, number of organisms, etc. To achieve maximal efficiency, it is essential to use the proper concentration of the drug for the purpose intended. The logic that “if a little is good, twice as much is better” is not only uneconomical but often has toxicologic implications.

The conventional approach to control of viral diseases is to develop effective vaccines, but this is not always possible. The objective of antiviral activity is to eradicate the virus while minimally impacting the host and to prevent further viral invasion. However, because of their method of replication, viruses present a greater therapeutic challenge than do bacteria.

Arthropod parasites (ectoparasites) are major causes of livestock production losses throughout the world. In addition, many arthropod species can act as vectors of disease agents for both animals and people. Treatment with various parasiticides to reduce or eliminate ectoparasites is often required to maintain health and to prevent economic loss in food animals. Some ectoparasiticides were derived from pesticides used to protect crops. The choice and use of ectoparasiticides depend to a large extent on husbandry and management practices, as well as on the type of ectoparasite causing the infestation. Endectocides are capable of killing both internal and external parasites. Accurate identification of the parasite or correct diagnosis based on clinical signs is necessary for selection of the appropriate parasiticide. The selected agent can be administered or applied directly to the animal, or introduced into the environment to reduce the arthropod population to a level that is no longer of economic or health consequence.

Achieving increased efficiency of conversion of feed into human food products of high quality, without posing any significant risk to the consumer, is an important goal of livestock producers worldwide. The physiologic mechanisms involved in converting feed into muscle, fat, and bone by animals are increasingly being elucidated. Recently, consumer concerns about additives for food production have focused on animal safety, organoleptic quality, and the potential human health hazards of the food we eat.

The adaptive immune system responds to microbial invasion by producing protective antibodies or cell-mediated immunity, or both. Appropriate administration of specific antigens, as in a vaccine, can result in effective, persistent immunity to infection. Microbial molecules also stimulate innate immune responses. These innate responses also promote adaptive immunity and can be used in the form of adjuvants to significantly increase responses to vaccines.