The physiology of tick-induced stress in grazing animals

Stress can be defined as a threat to homeostasis. Physiologic effects of acute stress include heightened awareness, increased heart rate, and muscle tone; all positive developments if the individual is being called upon to defend itself or escape. Chronic stress, however, can be detrimental to homeostasis. Ticks are external parasites and their blood-feeding behavior poses a significant economic burden to domestic animal agriculture worldwide. Direct negative economic effects of ticks on their hosts include loss of body weight and condition, lowered reproduction, and hide damage. These observed effects are due to a combination of a direct impact on homeostasis and lowered nutritional status via depression of intake. Additional hardships may result from disease transmission. Combating an external parasite burden may be considered a "cost of fitness" which incurs a drain on available energy. Protein-energy malnutrition has metabolic, endocrine and immune consequences especially with respect to parasitism. In this chapter, the effects of tick stress on grazing animals are discussed in relation to an interconnected series of physiological events involving multiple systems and indicators. Topics include: behavior, nutrition, metabolism, endocrine and immune factors, gastrointestinal function, and commensal bacteria. Recent research dealing with direct measurement of animal performance and metabolic indicators in cattle experiencing experimentally induced tick burdens will be discussed. Detection of tick stress via fecal analysis is presented as a means of bio-forensics.