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United States Department of Agriculture

Agricultural Research Service

Related Topics

Research Project: Molecular Genetics, Genomics, and Phylogenetics of Foodborne Zoonotic Parasites Affecting Food Safety and Public Health

Location: Animal Parasitic Diseases

2013 Annual Report


1a.Objectives (from AD-416):
1) Determine what genetic and genomic features distinguish Trichinella spiralis from Trichinella murrelli. 2) Utilize genomics to determine if microsatellite loci can be used to trace zoonotic outbreaks of Trichinella spiralis. 3) Determine the genetic features that account for the epidemic spread of certain strains of Toxoplasma gondii.


1b.Approach (from AD-416):
Investigations will be conducted to clarify how infections in wildlife influence the safety of pastured pork. Accordingly, first identify heritable differences between two related species of Trichinella, only one of which (T. spiralis) severely compromises pork safety by evading swine immunity. The other, Trichinella murrelli, predominates in North American wildlife but fails to thrive in swine. By comparing the genomes of these two parasites, the intent is to establish a basis for exploring what makes pigs so especially vulnerable to T. spiralis.

Secondly, develop the means to trace chains of transmission of Trichinella spp. Using markers which have already established the long-term dispersal history of T. spiralis (to the Americas in the pigs and rats brought by European colonists), researchers will attempt to discriminate instances of persistent on-farm transmission from sporadic introductions of T. spiralis to swine herds.

Finally, the genetics of T. gondii reproduction will be characterized. Both sexual and asexual reproduction can occur in T. gondii, and available data provide conflicting evidence as to the relative importance of each reproductive mode. These incongruous data leave in doubt whether this parasite evolves as an assemblage of distinct lineages, or whether it more closely resembles a coherent, interbreeding species. Additional data are needed to better resolve how T. gondii propagates and evolves. These results will help determine whether particular strains pose elevated food safety risk will help anticipate this parasite's evolutionary response to preventative interventions.


3.Progress Report:
Progress was made on all three of the project’s goals, which support NP108 Food Safety Component 1: Food-borne Contaminants. To determine features that distinguish Trichinella spiralis from Trichinella murrelli, a series of experiments established differences in protein expression by these two parasites at various stages of their development. Proteins expressed exclusively, or especially abundantly, by one of these two species were sequenced and characterized. Immunological assays were designed in order to establish which of these might offer the most potential to diagnose animals exposed to one or the other of these parasites, only one of which persists in swine to an extent that gravely threatens pork safety. To enhance efforts to trace outbreaks of trichinellosis, isolates known to share a transmission history were compared using variable genetic markers. Greater resolution will be sought using additional markers. Finally, the process of strain formation and dissemination in the agent of toxoplasmosis was explored as part of a consortium that has sequenced the genomes of tens of strains. When completed, we will have our clearest view yet of the extent of diversity in this ubiquitous foodborne parasite and the methods it employs to adapt and disseminate.


4.Accomplishments
1. Proteins show important variation among species of Trichinella. Of late, increasing concern has been raised about the possibility that trichinellosis (once nearly completely eliminated from the American pork supply) may resurge as a foodborne parasitic threat, given the growing practice of raising swine outdoors where they may more frequently encounter infections. If that were to occur, the health of Americans could be compromised and very significant costs to production and testing might ensue. ARS researchers at the Beltsville Agricultural Research Center were therefore interested in comparing a parasite capable of thriving and persisting in swine with a closely related parasite that is instead adapted to wildlife hosts. In spite of the fact that the parasites are far more similar than they are different, proteins were discovered that specifically characterize one or the other of these parasites. Ongoing work will seek to understand the biological significance of those differences, and will seek to exploit those differences as a basis to judge whether or not a given pig harbors a parasite that endangers human health. These findings will interest veterinarians, public health officials, producers, and consumers of pork and may influence policies governing risk reduction in free-range and organic animal husbandry.

2. Backyard pigs are more dangerous than wild boar as a potential source of human trichinellosis. Americans rarely experience trichinellosis, and most cases can be attributed to consuming poorly cooked wild game. Conventionally raised pork no longer conveys appreciable risk, owing to progress in sanitation and control over the swine diet. In Romania, human outbreaks routinely persist, owing to either the hunting of wild boar or the slaughter of domesticated pigs raised by families for their own consumption. Working with a team of Romanian veterinarians, ARS researchers in Beltsville, MD established that pigs raised in for familial consumption actually are twice as likely to be infected as are wild boar. More encouragingly, the team confirmed that swine raised in Romania using modern confinement procedures were entirely free of infection. These data underscore that risk varies not so much as a function of geography, but rather as a function of the type of animal husbandry practiced. Risk would appear especially elevated where swine are raised near people, but without proper attention to rodent control or to the quality of swine feed. These data will interest veterinarians, epidemiologists, and public health officials concerned to reduce human exposure to illness. They may also bear on policies governing agricultural trade with Europe.

3. Detected early infections with Trichinella in swine may reduce public health risks. Parasitic infection with species of Trichinella is only rarely reported among American consumers of conventionally raised pork, but remains a concern for consumers of wild game and has been raised as an emerging concern for organic and free-range pork. Although most species of Trichinella excrete substances to which both pigs and people raise strong immune responses, the earliest stages of infection can go undetected (because immature parasites fail to make their presence known to the immune system) and certain parasites never develop the structures most irritating to our immune systems. ARS researchers at the Beltsville Agricultural Research Center, working in collaboration with colleagues from China and France, identified antigens expressed at particular stages of parasite development in order to overcome such limitations. This accomplishment provides a means to reduce the risk of “false-negative” tests for this parasite, which could further help ensure the safety of pork to American consumers.

4. Parasitic infections provide clues to fighting cancer. Although eliminating animal-borne parasites from the food supply unquestionably benefits food safety, experiments in laboratory animals has demonstrated properties of such parasites that could benefit human health. For example, mice prone to certain cancers are protected by infection with parasites of Trichinella. Other mice prone to inflammatory bowel disease can have their symptoms reduced by exposure to parasitic infection. Working with colleagues from China, ARS researchers at the Beltsville Agricultural Research Center explored the basis of such tumor and inflammatory suppression. By identifying how parasites reprogram cells in ways that prevent their cancerous growth, and modify the signals that otherwise induce inflammation, these findings may promote new avenues for fighting cancer and autoimmune disease. These results will be of interest to clinicians, veterinarians, parasitologists, immunologists, and cancer biologists.

5. Newly discovered variants of Toxoplasma are especially dangerous. Infection with Toxoplasma gondii has variable clinical outcomes, ranging from asymptomatic lifelong infection to serious neurological disease, blindness, and congenital defects. Preventing the most serious outcomes depends, in part, on reducing the exposure of women to infection while they are pregnant. But additional variation in risk evidently derives from the genotype of the parasite. Working with an international team, ARS scientists at the Beltsville Agricultural Research Center identified new parasite variants that provoke especially marked veterinary disease. Clinicians and epidemiologists will need to be on guard for the possibility that these genotypes may convey elevated risk to human health.

6. Ancient diversity among parasites found in fish. Parasites belonging to the Coccidia include members that threaten veterinary and human health. Although the best-studied of these organisms infect mammals and birds, the diversity of this parasitic group is vast and their ultimate origins heretofore unknown. In a groundbreaking study, ARS researchers at the Beltsville Agricultural Research Center working with Hungarian colleagues established that the coccidia infecting fish harbor especially ancient evolutionary diversity, and may well be the ultimate source of infections that plague people and our livestock. This information will be of interest to evolutionary biologists as a step forward in our understanding of the history of life, but may well have practical application as it points to the possibility of foodborne infection with uncharacterized parasites via the consumption of uncooked fish.

7. Food-borne bacteria have similar genomes despite very different origins. Listeria monocytogenes is an important foodborne infection, and it is important to identify the major routes of human exposure as well as the efficacy of measures taken to render food products free of contamination. Here, ARS scientists in Beltsville, MD worked with a team from Denmark to characterize and compare the genomes of two isolates derived from separate fish processing plants. In spite of their distinct origins, and in spite of a several year interval between isolations, these two isolates proved to be almost identical to each other, more so than to any other isolate presently recorded. Particular attributes of this strain of Listeria.


Review Publications
Turner, M., Lenhart, S., Rosenthal, B.M., Sullivan, A., Zhao, X. 2013. Modeling effective transmission strategies and control of the world’s most successful parasite. Theoretical Population Biology. 86:50-61.

Zhao, Y., Liu, M.Y., Wang, X.L., Liu, X.L., Yang, Y., Zou, H.B., Sun, S.M., Yu, L., Rosenthal, B.M., Shi, H.N., Boireau, P., Wu, X.P. 2013. Modulation of inflammatory bowel disease in a mouse model following infection with Trichinella spiralis. Veterinary Parasitology. 194:211-216.

Rosenthal, B.M., Ajioka, J.W. 2012. Population genetics, diversity and spread of virulence in Toxoplasma gondii. In: David, L.S., Howlett, B.J., Heitman, J., editors. Evolution of Virulence in Eukaryotic Microbes. Boston, MA: Wiley-Blackwell. p. 231-245.

Holch, A., Webb, K., Lukjancenko, O., Ussery, D., Rosenthal, B.M., Gram, L. 2013. Genome sequencing identifies two nearly unchanged strains of persistent Listeria monocytogenes isolated in two different fish processing plants sampled six years apart. International Journal of Food Microbiology. 79:2944-2951.

Wang, X.L., Liu, M.Y., Sun, S.M., Liu, Y., Wang, X.R., Chu, L.X., Rosenthal, B.M., Shi, H.N., Boireau, P., Wang, F., Zhao, Y., Wu, X.P. 2013. An anti-tumor protein produced by Trichinella spiralis and identified by screening a T7 phage display library, induces apoptosis in human hepatoma H7402 cells. Veterinary Parasitology. 194:186-188.

Wu, X.P., Liu, X.L., Wang, W.L., Blaga, R., Fu, B.Q., Liu, P., Bai, X., Wang, Z.J., Rosenthal, B.M., Shi, H.N., Sandrine, L., Vallee, I., Boireau, P., Wang, F., Zhou, X.N., Zhao, Y., Liu, M.Y. 2013. Unique antigenic gene expression at different developmental stages of Trichinella pseudospiralis. Veterinary Parasitology. 194:198-201.

Pastiu, A.I., Gyorke, A., Blaga, R., Mircean, V., Rosenthal, B.M., Cozma, V. 2013. In Romania, exposure to Toxoplasma gondii occurs twice as often in swine raised for familial consumption as in hunted wild boar, but occurs rarely if ever among fattening pigs raised in confinement. Parasitology Research. 112(6):2403-2407.

Liu, P., Wu, X.P., Bai, X., Wang, X.L., Yu, L., Rosenthal, B.M., Blaga, R., Lacour, S., Vallee, I., Boireau, P., Gherman, C., Oltean, M., Wang, F., Zhao, Y., Liu, M.Y. 2013. Screening of early antigen genes of adult-stage Trichinella spiralis using pig serum from different stages of early infection. Veterinary Parasitology. 194:222-225.

Molnar, K., Ostoros, G., Dunams, D.B., Rosenthal, B.M. 2012. Eimeria that infect fish are diverse and are related to, but distinct from, those that infect terrestrial vertebrates. Infection, Genetics and Evolution. 12(8):1810-1815.

Last Modified: 7/25/2014
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