Downer pigs are a problem for packers and producers. While actual levels of pigs who are unable to move when they arrive at U.S. packing plants are typically reported at 0.25 percent to 0.75 percent, levels as high as 10 percent have been seen for individual loads of hogs.
The incidence of downers has been on the rise — from 0.08 percent to 0.30 percent during the past 10 years. Stressful handling appears to be a factor, triggering cases of downer pigs.
While genetic predisposition has been investigated, the stress gene is not a prerequisite for the condition, notes Harold Gonyou, Prairie Swine Centere, Saskatchewan, Canada. Statistics show 90 percent of dead pigs arriving at Canadian packing plants do not carry the stress gene. High blood-lactate and ammonia levels, low blood pH, blotchy skin, open-mouth breathing, vocalizations, muscle tremors and a refusal to walk are typical symptoms. Those behavioral and physiological symptoms are characteristic of a hyperthermic animal under stress, which results in a typical fight/flight response and attempts to dissipate heat, he notes.
Rapid energy release from the muscle and liver causes a build up of lactate and ammonia in the blood. This increase causes metabolic acidosis, which may be involved in the animal’s refusal to move.
Gonyou’s study attempted to shift the animal’s acid/base balance by increasing dietary electrolytes. The goal was to increase the blood’s buffering capacity and reduce the risk of acidosis during stressful handling.
Stressful handling was based on handling groups of animals, with the understanding that social stress (unfamiliar animals) and crowding would represent typical moving experiences. Electric prods were used as they are considered a stress source in aggressive handling.
Two studies, using a total of 336 pigs, were conducted to determine if altering pigs’ acid/base balance through diet manipulation (electrolytes) would affect the animal’s response to stressful handling. Gonyou also evaluated what caused stressful handling situations.
Study 1 had a downer rate of 38 percent. Results show a drop in blood pH and an increase in lactate, ammonia, glucose and glycerol levels in the blood as well as an increase in rectal temperature in downer pigs compared to non-downers. Those changes are indicative of rapid energy mobilization in downer pigs as a response to aggressive handling.
Group-run pigs had lower blood oxygen and carbon dioxide levels and higher blood glycerol post-handling than pigs handled individually. A higher proportion of downers in group-handled pigs (54 percent) suggests that handling pigs in groups was more likely to induce downers. Altering the diet’s acid/base balance did not affect pigs’ responses to aggressive handling in either study. The high-electrolyte diet raised the pre-handling blood pH in the first study, but this did not prevent the typical physiological responses to handling and did not affect the incidence of downers.
In the second study, downer rates of 2 percent, 15 percent and 34 percent occurred for pigs handled gently, aggressively but not prodded and aggressively with electric prods, respectively.
Among the aggressively handled pigs, using an electric prod caused a high metabolic response to handling. Pigs handled aggressively but not prodded had higher blood lactate and glycerol levels after handling than gently handled pigs. This suggests that aggressive handling, even without using a prod, may contribute to the downer response. But, Gonyou emphasizes, electric prods exacerbate this response.
The bottom line:
Use of electric prods during aggressive handling contributes to the incidence of downer pigs.
Aggressive handling can result in the metabolic response associated with downer pigs.
Increasing the dietary electrolyte balance was not effective in reducing pigs’ metabolic response to aggressive handling nor the incidence of downers.