TRICHINELLOSIS (TRICHINOSIS) TIMELINE
Detailed timeline of U.S. Department of Agriculture’s involvement of over a century of trichinellosis research
Italy, Austria-Hungary, Spain, Germany, Romania, Greece, and Denmark prohibited the importation of American pork alleging the presence of trichinae.
U.S. Department of State issued inquiries to the pork industry about meat safety.
Microscopic survey of pork products for trichinae was made in Atlanta, Boston, Chicago, Montreal, and Washington, D.C.
Bureau of Animal Industry’s Meat Inspection Division began microscopic examination for the detection of trichinae in pork to be exported to countries requiring such inspection.
Charles W. Stiles found no evidence in Germany that cases of trichinosis originated from American pork, but he determined that microscopic inspection was inadequate.
Microscopic inspection of pork was discontinued.
B. H. Ransom discovered that trichinae in pork could be destroyed by refrigeration of 5° F for 20 days.
The destruction of trichinae by refrigeration of infected pork was reported by U.S. Department of Agriculture (USDA) as a control measure for trichinosis.
Ransom and Benjamin Schwartz established definitely that a temperature of 137 °F was ample for the destruction of trichinae.
Schwartz demonstrated that the vitality of trichinae could be destroyed by massive doses of x-rays.
Schwartz authored a USDA leaflet titled “Trichinosis, A Disease Caused by Eating Raw Pork”.
Schwartz devised a method for preparing trichinae antigen free from muscle tissue and tested it on pigs experimentally infected with trichinae.
Schwartz initiated an investigation to determine the current frequency of Trichinella spiralis infection in swine in the United States.
The Beltsville Parasitology Laboratory began a new project to investigate the effectiveness of the current meat inspection procedures designed to detect and inactivate larvae of Trichinella spiralis in pork products.
A new antigen containing the metabolic products of live trichina larvae was prepared and tested on hogs by the intracutaneous method.
Investigations of trichinae in hogs confirmed that more garbage-fed hogs were infected and harbored heavier infections than grain-fed hogs.
L. A. Spindler, in cooperation with O. G. Hankins of the Animal Husbandry Division's Meats Laboratory, showed that dehydration of pork containing viable trichinae to a 3 percent moisture level at temperatures of 102 to 120 °F would kill all the parasites.
Spindler reported that rapid dehydration of trichinous pork lowered the thermal death point of the parasites.
States passed laws against feeding raw garbage to swine, in an effort to control the viral disease vesicular exanthema. As a result of these measures, trichinae infection decreased dramatically. http://www.aphis.usda.gov/vs/trichinae/docs/fact_sheet.htm
John S. Andrews with D. E. Zinter and N. E. Schultz tested a pooled-sample digestion technique developed in 1967 by W. J. Zimmermann of Iowa State University designed to facilitate the examination for trichinae of all hogs slaughtered in modern abattoirs with high-speed slaughtering capability. This procedure is now the world standard for recovering larvae from muscle of all animals, and the mandatory method for meat inspection in countries where inspection is required (e.g., European Union).
K. D. Murrell and Gerhard A. Schad, University of Pennsylvania, in an epidemiological study, provided evidence that swine trichinosis was more prevalent in the swine of the northeastern United States than previously thought, especially in production systems that feed food waste.
J. Ralph Lichtenfels and Murrell, using the scanning electron microscope, examined the three ”species” of Trichinella spiralis that were reported to be distinguishable on morphological grounds. Their results demonstrated this approach was not reliable.
Murrell and Schad investigated the role of syvatic animals as reservoir hosts of Trichinella spiralis, and found a relatively high prevalence in wildlife in the U.S. Middle Atlantic region, especially in black bears in Pennsylvania. Importantly, isolates of Trichinella were found to include both Trichinella spiralis, and another genotype that, unlike Trichinella spiralis, had very low infectivity in pigs.
Anthony Kotula, Beltsville Agricultural Research Center (BARC) Meat Science Laboratory, Murrell and H. R. Gamble, Animal Parasitology Institute, developed new thermal death curves for Trichinella spiralis trichinae in pork which became the standard for USDA recommendations on cooking and freezing for the de-vitalization of pork for home consumption and for commercial pork processing.
Gamble and Murrell develop a serological method (ELISA) for detecting anti-Trichinella antibody in pigs using a highly specific secretory/excretory product (ES) from the muscle larval stage. This overcame the non-specificity and low sensitivity problems plaguing all existing serological tests; a U.S. patent was issued for this antigen. It remains the international gold standard for immunodiagnosis and was the basis for several commercial tests that are widely used. Gamble later developed a molecular procedure to produce recombinant diagnostic antigens, and a monoclonal antibody for specific diagnostic tests.
Murrell and H. P. Marti of the Animal Parasitology Institute, discovered that the most important immune protective mechanism in pigs against Trichinella spiralis is antibody-mediated and attacks the newborn larvae migrating in the blood stream. Murrell, Marti and Gamble were eventually awarded a U.S. patent for a vaccine based on the antigens of the newborn larvae.
Murrell and R. J. Brake of the U.S. Department of Energy Sandia National Laboratory determined the gamma irradiation dosage for killing Trichinella spiralis muscle larvae. This was the data needed by both the USDA Food Safety and Inspection Service and the U.S. Food and Drug Administration to issue the first approvals for irradiation of red meat for controlling pathogens.
Schad, David A. Leiby of the University of Pennsylvania, and Murrell demonstrated in both natural and experimental on-farm conditions the important role of rats in transmitting Trichinella spiralis to farm pigs. John B. Dame, F. Stringfellow, and Murrell, using new molecular tools showed that Trichinella spiralis was interchanged between wildlife and farm pigs and rats surrounding infected farms, underscoring the role reservoir hosts play in the epidemiology of trichinellosis. In experimental studies carried out in Illinois, R. Hanbury, P. Dobey, and Murrell demonstrated that on-farm cannibalism is also a major mode of transmission, even when rats are controlled.
Dame and Murrell identified genetic markers which distinguished genotypes of Trichinella that varied in their infectivity to pigs. One isolate from bears was later determined to be a new species by the International Trichinella Reference Center in Rome, and was named Trichinella murrelli. This species, although poorly infective for pigs, is considered to be the most common species in wild animals in North America and responsible for most human cases derived from game meat.
Gamble and colleagues at Texas A&M University worked with USDA’s Food Safety and Inspection Service to validate the safety of commercial methods for the production of dry cured hams.
1988 to present
Gamble, and later Hill, worked with USDA’s Agricultural Marketing Service in establishing and maintaining training and quality assurance programs for carcass testing used to support U.S. exports of horsemeat (1988-2004) and pork (1996 to present).
Dante Zarlenga (API) and Gamble published the first sequencing data on diagnostic antigens for Trichinella spiralis. This work was subsequently taken up by many other researchers.
1990 to present
Researchers including Gamble and Hill supported national surveys for Trichinella (under the APHIS National Animal Health Monitoring Surveys) to document the decline and absence in Trichinella infection in conventionally raised pigs in the U.S. This information has been important in assuring the safety of U.S. pork when establishing trade agreements.
Murrell and Lichtenfels, with collaborators Edoardo Pozio and Giuseppe La Rosa of the International Trichinella Reference Center in Rome, made a substantial advancement in the control of trichinellosis by applying biochemical and biological methods to completely revise the systematics of the genus Trichinella. This clarified many puzzling observations on the biology of Trichinella and greatly enhanced understanding of the disease’s epidemiology, especially the identification of those few species (particularly Trichinella spiralis) which are the greatest risk for infection of pigs.
Zarlenga and colleagues introduced a multiplex PCR method that unequivocally differentiated the genotypes of Trichinella, particularly the encapsulated and unencapsultaed types. This method is the basic tool now in use world wide to distinguish Trichinella species.
Gamble, working with colleagues in the International Commission on Trichinellosis, developed the first in a series of guidelines on control of trichinellosis, establishing a consensus of international experts. Additional guidelines were subsequently developed and serve as references for international authorities such as the Codex Alimentarius and the World Organization for Animal Health.
Gamble and Christian Kapel of Denmark conducted a series of comparative studies in pigs on infectivity and antibody responses to Trichinella spiralis and several sylvatic species of Trichinella. The results showed that sylvatic species from North America have very low infectivity, and therefore, have a low risk for spillover to pigs with outdoor exposure.
Gamble and David G. Pyburn of USDA's Animal and Plant Health Inspection Service developed in collaboration with the U.S. pork producers a certification program to farms that achieve a designation as Trichinella-free.
Zarlenga, Benjamin Rosenthal, Eric P. Hoberg, Pozio, and La Rosa published a ground-breaking paper on the evolution and biogeography of the genus Trichinella.
Dolores E. Hill and colleagues determined the viability and infectivity of Trichinella spiralis in frozen horse muscle. Infected horse meat has been the infection source for a number of trichinellosis outbreaks, particularly in Europe; one outbreak in Paris was attributed to exported horse meat from the U.S. infected with Trichinella murrelli.
Rosenthal, Zarlenga, and colleagues published a study on the genetics of Trichinella spiralis that explains the role of humans in dispersing Trichinella spiralis throughout the world.
Hill and collaborators conducted an international ring trial to evaluate the specificity and sensitivity of the ELISA-ES antigen diagnostic test for pig infections. The results demonstrated that the test was very robust, accurate, and reproducible.
Hill and colleagues found that Trichinella spiralis does not survive in an independent sylvatic cycle, which has important implications for assessing the risk of Trichinella spiralis infecting pigs raised in extensive systems.
Zarlenga, in an international collaboration, published a draft genome of Trichinella spiralis, an important advance in understanding the biology of this parasite.