Highlights from the past two NAHMS national swine studies

Eric Bush, DVM, MS; Nina Stanton, BS

Since 1990, the United States Department of Agriculture's National Animal Health Monitoring System (NAHMS) has conducted 12 national studies for eight different livestock groups. National studies provide scientifically sound, statistically valid national information used for education, research, policy development, and the overall improvement of animal industries. These studies provide a snapshot of industry health and management practices and address information gaps identified by a needs assessment process.1-3

NAHMS national studies include the states that have adequate numbers of animals so that at least 70% of the producers and animals within the continental United States are represented in the survey sample. Each study is based on producers from a probability-selected sample using National Agricultural Statistics Service (NASS) sampling frames. Respondent data are weighted to reflect selection probability and adjusted to account for nonrespondents. National estimates, therefore, reflect the population from which the sample was selected and include variance estimates calculated to reflect the study design.4,5

NAHMS staff are currently working with pork industry leaders to prepare for the third national swine study, Swine 2000, which will begin on-farm visits this summer. Practitioners serve as key advisors to most producers and play an important role in helping selected producers decide whether or not to participate in a NAHMS study. This paper seeks to equip practitioners and other consultants with knowledge and resources to help producers make an informed decision regarding participation in the NAHMS Swine 2000 study.

Producer evaluations of past studies

In 1990 and 1995, NAHMS collected data from hundreds of pork producers across the country. At the conclusion of each study, participating producers completed evaluations to assist NAHMS staff in developing future studies. Seventy-nine percent of producers that participated in the NAHMS 1990 National Swine Survey and 88% from the Swine '95 study indicated they would participate again. Over 85% and 94% of producers from the two studies, respectively, indicated they would recommend other producers participate.6

Producers' chief reason for participating was the recognized benefit to the entire United States pork industry, as indicated by 89% of respondents to the 1990 survey, and 86% of respondents to the 1995 study. Other reasons for participation mentioned less frequently by producers were study incentives (free laboratory testing of blood, feed, and feces for porcine reproductive and respiratory syndrome virus [PRRSV], mycotoxins, and Salmonella) and expectations of direct benefits from farm data collected during the study.

Benefits to pork industry from past studies

Information derived from NAHMS studies fills a niche for health, management, and biological data summarized at a national level. A top priority is that data be collected in a professional and confidential manner. Merged data from both laboratory testing and questionnaires were beneficial to the pork industry for epidemiological research by government and universities, policy development by industry organizations and government, and education.

Laboratory testing of biological samples collected during NAHMS studies provides objective prevalence data on swine pathogens, foodborne pathogens, and emerging diseases. This information allows stakeholders within the United States and from other countries to make health assessments of the United States national swine herd based on fact rather than perception. For instance:

  • During the Swine '95 study, NAHMS provided timely data on a baseline seroprevalence of Salmonella shedding in finisher hogs.7 New meat inspection regulations and interest in reducing Salmonella contamination through Hazard Analysis and Critical Control Points (HACCP) had stimulated the need for this information on farm.
  • NAHMS found no evidence of the Escherichia coli 0157:H7 bacteria, typically associated with other food products, among the 4229 swine fecal samples tested during the Swine '95 study. When Japan was experiencing an E. coli 0157:H7 outbreak, these results helped reassure the Japanese market that United States pork was safe, thereby minimizing the impact of the outbreak on demand for United States pork.
  • Blood samples collected from sows in the 1990 National Swine Survey were stored in a serum bank and later used to determine the national prevalence and the initial geographic distribution of the PRRSV soon after it was identified.8 PRRSV research continued during the NAHMS Swine '95 study (just prior to widespread adoption of the PRRSV vaccine), when blood samples collected from breeding and market hogs were tested to provide a broader estimate of how many animals and herds were affected by this virus.
  • Researchers from the University of Tennessee and Iowa State University tested serum samples banked during the NAHMS 1990 and 1995 studies to confirm that Toxoplasma gondii seroprevalence in United States pigs is on a decline.9 While fewer United States pigs are infected with Toxoplasma, it is a growing public health concern.
  • Swine '95 blood samples made it possible for ELANCO and NOBL Laboratories to estimate Lawsonia intracellularis exposure in finishing hogs and breeding animals. Many people in the industry believed that the seroprevalence of ileitis had increased in occurrence in the previous years, and this research showed that considerably more United States pigs were exposed to the organism than expected.10

Some of the most valuable benefits from the NAHMS studies are from analysis of management data in connection with laboratory test results. By knowing how tested animals were managed, NAHMS can identify management practices that pork producers can avoid or apply to reduce possible public health threats and improve product quality. In addition, management information provides facts that support development of national programs and policies affecting the pork industry. Examples from the NAHMS swine studies include:

  • Data analysts compared Swine '95 management and Salmonella test data to identify on-farm management practices that could reduce both the spread of Salmonella within a herd and transmissions through the food chain. They found pelleted feed and feed mixed off the farm presented a greater risk for shedding of Salmonella.11,12
  • Researchers from the University of Tennessee and the Iowa State University did similar risk factor analyses for Toxoplasma. They identified the extent to which access of cats to the swine facilities and hog access to the outdoors contribute to the spread of the parasite.13
  • The significance of findings from such risk factor analyses are often interpreted and applied in conjunction with industry working groups. The NPPC Salmonella and Toxoplasma Working Groups have incorporated NAHMS findings as they seek to identify herd-level good production practices (GPPs) to reduce foodborne transmission. The Swine '95 study provided estimates of seroprevalence and described the extent to which related management practices were used to help define the challenges future certification programs would face.
  • The prevalence of Trichinae infection, another parasite found in United States hogs, had not been evaluated since the 1970s. By the mid 1990s, the pork industry knew the prevalence of Trichinae had declined, but was unsure by how much. On behalf of the National Trichinae Research Project, NAHMS evaluated the Trichinae seroprevalence in sows (during the 1990 study) and finisher pigs (through the Swine '95 study).14 The Trichinae Working Group used the information on risk factors to evaluate the feasibility of establishing a Trichinae certification program and to judge how effective such a program was likely to be.
  • The Environmental Protection Agency (EPA) is using NAHMS Swine '95 information to revise their effluent guidelines for swine production. To date, these guidelines have been "one size fits all." With the NAHMS information, the EPA, in cooperation with the NPPC, is designing the guidelines to fit specific industry segments.

A third benefit of NAHMS data is the widespread use of national information to support education throughout the pork industry. Countless specific examples exist for the inclusion of NAHMS estimates by various departments within many universities where NAHMS publications are applied to research efforts and used in the classroom or for producer education. Other educational uses of NAHMS data are for company training and education booklets.

NAHMS Swine 2000 national study

What's next for NAHMS' swine projects? NAHMS is working with input from industry members to ensure continual success in addressing health issues affecting United States pork production. Their current focus is to prioritize animal health issues to be incorporated into a Swine 2000 study. Information on this upcoming study and its planned benefits will be released in the coming months. Current information is available under the swine section at the NAHMS web site at

References - refereed

1. Bush EJ, Gardner IA. Animal Health Surveillance in the United States via the National Animal Health Monitoring System (NAHMS). Epidemiol Sante Anim. 1995;27:113-126.

3. Wineland NE, Dargatz DA. The national animal health monitoring system. A source of on-farm information. Vet Clin North Am Food Anim Pract. 1998;14(1):127-139.

4. Losinger WC, Bush EJ, Hill GW, Smith MA, Garber LP, Rodriguez JM, Kane G. Design and implementation of the United States National Animal Health Monitoring System 1995 National Swine Study. Prev Vet Med. 1998 Feb 27;34(2-3):147-159.

5. Dargatz DA. Analysis of survey data. Prev Vet Med. 1996;28:225-237.

8. Bautista EM, Morrison RB, Goyal SM, Collins JE, Annelli JF. Seroprevalence of PRRS virus in the United States. Swine Health Prod. 1993;1(6):4-8.

9. Patton. S, Zimmerman JJ, Roberts T, Faulkner CT, Diderrich VR, Assadi-Rad A, Davies PR, Kliebenstein JB. Seroprevalence of Toxoplasma gondii in hogs in the NAHMS. J Eukaryotic Microbiology. October 1996;121S.

14. Gamble HR, Bush EJ. Seroprevalence of Trichinella infection in domestic swine based on the National Animal Health Monitoring System's 1990 and 1995 swine surveys. Vet Parasitol. 1999;80(4):303-310.

References - nonrefereed

2. Gardner IA. Epidemiological research and surveillance in swine production in the USA. 1st France-Japan Workshop on Epidemiology. Tokyo, Japan. September, 1995.

6. USDA:APHIS:VS. Producer evaluations of on-farm VS monitoring and surveillance activities. Fort Collins, Colorado: Centers for Epidemiology and Animal Health. 1994. NAHMS publication. N145.0694.

7. Fedorka-Cray PJ, Bush EJ, Thomas L. Results of the NAHMS Swine '95 Grower/Finisher Survey. Proc USAHA. Little Rock, Arkansas, 1996.

10. Bane DP, Norby B, Gardner IA, Roof MB, Knittel JP, Bush EJ. Prevalence and Management risk factors associated with Lawsonia intracellularis seropositivity in the U.S. swine herd. University of Minnesota. Supplement to Allen D. Leman Swine Conference Proceedings. St Paul, Minnesota. September 1997;19.

11. Bush EJ, Fedorka-Cray PJ. Management factors associated with shedding of Salmonella in the NAHMS Swine '95: Grower/Finisher study. Pork Quality and Safety Summit. 1998;66-71.

12. Bush EJ, Wagner BA, Fedorka-Cray PJ. Risk factors associated with shedding of Salmonella by US finishing hogs. Proc of the 3rd International Symp on the Epidemiology and Control of Salmonella in pork. Washington, DC. August 1999;106-8.

13. Kliebenstein JB, Patton S, Zimmerman JJ, Hu X, Hallam A, Roberts T, Bush EJ. Toxoplasma gondii in United States swine operations: An assessment of management factors. AEEMA Epidemiologie et Sante Animale VIII. Paris, France. 1997;05.26.1