The Direct Relationship Between Animal Health, Food Safety Outcomes

Many groups in society, including politicians, activists, scientists, and stakeholders, are advocating significant changes to livestock production practices.
calendar icon 8 January 2013
clock icon 7 minute read

These changes include modification of stocking densities, limitations on antimicrobial use, and requirements for outdoor “experiences.” Such changes may affect animal health, productivity, and food quality. Simultaneously, many consumers are demanding virtually risk-free food at least cost, and they believe that food safety should be addressed on-farm as well as during processing. It is critical that decision makers understand the relationship between animal health and food safety, which is a complex association requiring careful evaluation of many variables.


The objectives of this paper are to (1) discuss the quantifiable impact animal health has on public health risk due to foodborne illness from meat, milk, eggs, and poultry; (2) identify the factors that impact animal health; and (3) highlight specific research needs. This paper will focus on direct and indirect impacts that animal health may have on public health.

Pressures to Change Livestock Rearing Methods

There are many pressures and trends to change the way livestock are raised. If these changes affect animal health, they may well also affect public health. Figure 1 shows the relationship between animal health and public health. Various policy changes may negatively impact animal health, resulting in more marginally or not visibly ill pigs, which tips the scales toward reduced public health. These proposed changes and their consequences need to be considered carefully. Trends include

  • Sustainability (social, environmental, economic),
  • Local production,
  • Economies of scale,
  • Housing,
  • Antibiotic use, and
  • Animal welfare.
Conceptual model of the relationship between animal health and public health. As on-farm animal health improves (fewer subclinically ill pigs), the incidence of foodborne human illness may decrease (better human public health).

According to a recent National Academy of Sciences report on sustainability, “U.S. farmers are under pressure to satisfy multiple demands” (NRC 2010). These pressures will change the way livestock are raised, impacting practices such as housing, feeding, and location of production.

There is a significant push for locally raised products, with programs such as “Know Your Farmer, Know Your Food” (USDA 2011). In every country, economics is a leading consideration in the drive toward consolidation and intensification of production methods (FAO 2004). Animal and human health impacts of shifting from low-cost outdoor housing to controlled environments with higher stocking densities need to be evaluated. The European Union has severely curtailed the use of disease-preventive antibiotics in food-animal production (Casewell et al. 2003; Ferber 2003) and has begun to legislate how animals are housed (e.g., banning of confinement housing such as gestation stalls for pigs). Subsequent to these bans, treatment of pigs for clinical disease has reportedly increased in Denmark and the Netherlands (DANMAP 2010; MARAN 2007).

The health impacts of organic production must be considered, as the organic accreditation process states that for animal products to be sold under this label they must not be treated with antibiotics or synthetic anthelmintic (worm-preventing) drugs (USDA–AMS 2011). Additionally, organic production may mean “natural” or pasture-based rearing. When coupled with animal welfare concerns, significant housing changes might impact animal health. For example, increased exposure to the soil and vermin may increase the prevalence of zoonotic diseases in livestock.

Healthy Animals Make Safer Food

The recent focus of the “One Health” concept highlights the premise that animal health is important to human health and well-being; this is particularly noted regarding zoonotic diseases, which transmit directly from animals to humans (One World, One Health 2008). Additionally, the impact of animal health on the incidence of human foodborne diseases needs to be considered. A long-standing premise of the U.S. food safety inspection system is that healthy livestock are essential for a safe food supply. This premise is the primary motivation for the U.S. Department of Agriculture’s (USDA) antemortem inspection and gross pathological postmortem inspection as initiated in the Meat Inspection Act of 1906.

In addition to overtly ill animals, there is a growing body of evidence showing that chronically, previously, and not visibly ill animals are more likely to be contaminated with foodborne pathogens after processing in the abattoir (slaughterhouse). These animals, however, may go unnoticed during antemortem (live animal) inspection, and thus questions arise concerning the potential impacts of these animals entering the food supply on public health risk from foodborne pathogens.

Evidence to Support the Direct Public Health Impact of Animal Health

Three general indications support the premise that healthy animals make safer food and, conversely, that unhealthy or marginally healthy (not visibly ill) animals increase foodborne risk: (1) USDA Food Safety and Inspection Service (FSIS) federal regulations, (2) indirect evidence, and (3) epidemiologic studies with risk modeling.

USDA Food Safety and Inspection Service Regulations

The USDA’s FSIS Public Health Veterinarians and other Inspection Program Personnel (IPP) are given the responsibility through the Federal Meat Inspection Act and the Poultry Products Inspection Act for ensuring the safety and security of the nation’s food supply. This is accomplished, in part, through the examination of live animals (antemortem inspection) for disease and through inspection of each carcass after harvest (postmortem inspection) to ensure that they are safe for human consumption. Antemortem inspection is a screening process to remove obviously diseased animals from the food supply and to identify animals that require a more extensive postmortem examination by an FSIS veterinarian. When conducting antemortem inspections, the IPP evaluate animals for signs of disease as well as any signs that the animals may have been recently treated, resulting in illegal drug residues.

Those animals that are considered healthy enough for harvest then receive postmortem inspection. The IPP again inspect the carcasses for signs of disease or contamination (as in the case of not visibly ill animals), or for indications that they may have been recently treated (e.g., injection sites). The IPP either can have contamination removed or, if they suspect a disease condition, can have the carcass and all the internal parts held for veterinary examination. In some cases, the entire carcass is condemned. The FSIS has delineated certain conditions that can be correlated definitively to public health in humans (e.g., infectious conditions and fecal contamination). In addition, the IPP are trained to identify certain zoonotic conditions—those conditions that are known to infect humans (USDA–FSIS 2009).

Indirect Evidence

Indirect evidence suggests that subclinically ill (not visibly ill) animals also contribute to public health risk (Andreasen, Musing, and Krogsgaard 2001; Russell 2003) (see Figure 1). Subclinical illness may therefore increase carcass contamination in a variety of ways. Animals stressed or immune compromised by long-term, low-grade illness are more likely to be infected with foodborne pathogens, especially Salmonella (Salak-Johnson and McGlone 2007; Noyes, Feeney, and Pijoan 1990). Additionally, animals with abscesses or other significant lesions will require extra trimming or further handling during the harvest process, and this handling may increase the likelihood of cross-contamination (Olsen et al. 2003; Rosenquist et al. 2006). Finally, certain illnesses or conditions may increase the chance of human error during the harvest process. For example, an adhesion may cause intestines to adhere to the body cavity; during evisceration, extra force may be required, leading to leakage or spilling of intestinal contents. Given the percentage of swine carrying Salmonella in the gastrointestinal tract at the time of harvest, if leakage occurs, there is approximately a 40% probability of Salmonella contamination (Hurd et al. 2002; Hurd et al. 2004; Rostagno et al. 2003). Carcass contamination is assumed to lead to product contamination and foodborne illness.

Other concerns include the presence of abscesses on the liver or around the heart sac in cattle, which can lead to abscess spillage when removing the viscera. Peritonitis, a common inflammation of the peritoneal cavity, may arise from a number of causes. Animals sent to slaughter that unknowingly healed improperly will appear healthy at antemortem inspection but are subsequently identified by the internal adhesions. These compromised animals are at increased risk for potential cross-contamination during harvest.

In pigs, the spleen, which has several important functions because of the large number of macrophages, including filtering the bacteria from the body, can be the site of multiple abscesses when recovering from a bacterial infection, which, if accidentally cut, could serve as a source of cross-contamination. Additionally, splenic torsion leading to adhesions of the spleen and liver to the intestines may cause difficulties during the evisceration process.

Lung lesions observed at harvest are a good measure of pig health on-farm. Morrison, Pijoan, and Leman (1986) investigated the relationship between growth rate and feed conversion in pigs with lung lesions. In 13 of 23 studies, they observed that a reduction in growth rate and feed conversion was associated with pigs that had lung lesions but observed no association in the remaining 10 studies (Morrison, Pijoan, and Leman 1986). A more recent study (Regula et al. 2000) found the presence of lung lesions at harvest to be associated with lower average daily gain, an indicator of animal health.

Further Reading

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December 2012

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