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United States Department of Agriculture

Agricultural Research Service

Research Project: MOLECULAR SYSTEMATICS AND COMPARATIVE POPULATION GENETICS OF PARASITIC ORGANISMS THAT THREATEN FOOD SAFETY AND SECURITY
2008 Annual Report


1a.Objectives (from AD-416)
Objective 1: Employing single-gene and genomic approaches, improve diagnosis of protists and nematodes that parasitize major food animals and that facilitate establishment, internalization, and survival of bacterial pathogens in produce.

Subobjective A: better characterize the molecular epidemiology of parasitic coccidia and trichinella.

Subobjective B: better characterize those bacterophagous eukaryotic microbes that may convey and help establish in produce pathogenic bacteria.

Objective 2: Develop a molecular phylogeny of coccidia in fish in order to better define their potential risk to food safety and security, and in order to better understand the relationship between Eimeriidae (including the agents of avian coccidiosis) and the Sarcocystidae (including the agent of human toxoplasmosis).

Objective 3: Better define the historical and ongoing interactions among wildlife and livestock reservoirs of Toxoplasma gondii through comparative population genetic analysis.


1b.Approach (from AD-416)
Several genes will be sequenced from parasites obtained from a wide array of animals, both domesticated and wild. These will be compared to each other, and to sequences obtained from human beings, in order to define the diversity and epidemiology of these parasite species. Homologues will be characterized for genes whose global variation has already begun to be studied in T. gondii, including the Intergenic Spacer sequence between rRNA genes, beta tubulin introns, and others. Characterization of microsatellite alleles will be considered as a second approach which, although requiring a greater investment of time and resources, should provide greater population genetic resolution than is possible with current loci.


3.Progress Report
Significant progress was made towards understanding how Agricultural practices influence the genetic diversity and geographic distribution of animal parasites. Through a series of population-based studies of genetic diversity, evidence was identified that domesticating and transporting livestock has promoted the global dissemination of parasites, including certain parasites that cause human disease. This information may influence the assessment of risk to human health derived from pasture management of swine, and the exposure of cattle to parasites excreted by dogs and wild canids. Studies were also completed showing the regional prevalence in retail beef, of parasites excreted by dogs, and confirmed no instances of parasites capable of inducing human disease. Significant progress was made towards understanding patterns of dissemination and differentiation in Perkinsus marinus, the cause of Dermo disease in oysters, and strides were made in elucidating the diversity of coccidian parasites in fish.


4.Accomplishments
1. Dissemination of Toxoplasma gondii between North and South America has generally been limited, but successful parasite strains on each continent are identical at a particular chromosome. This major advance, published in the prestigious Proceedings of the National Academy of Sciences, revises our understanding of how strains of foodborne parasites diversify and disseminate. These findings provide a basis for focusing attention on particular portions of the genome in seeking the basis of its transmission success. Moreover, they highlight regional differences in how the parasite is transmitted: North American parasite populations are dominated by a handful of stable genetic clones (which may be passed down from generation to generation without substantial genetic change) whereas other populations appear to undergo more frequent recombination (which can only occur in cats). Thus, different management approaches may be appropriate for safeguarding food safety and public health. This work aligns with National Program 108, Food Safety (animal and plant products); Components 1a Pathogen, Toxins and Chemical Contaminants Preharvest, sections ii) Epidemiology and iii) Ecology, Host Pathogen and Chemical Contaminants Relationships.

2. Livestock domestication and dissemination has reduced the global diversity of Sarcocystis cruzi, a ubiquitous cattle parasite. Our laboratory seeks to understand the consequences of agricultural practices on the distribution and evolution of zoonotic pathogens. Here, we used a prevalent parasite that cycles between cattle and dogs, as a model to understand how livestock management can influence the dissemination and diversification of infectious agents. Compared to similar parasites found in wildlife hosts, those in domestic cattle were uniform in their small subunit ribosomal RNA gene sequence. Indeed, this gene did not differ even among parasites isolated from cattle in the United States, Argentina, or China. This finding suggests that animal husbandry may have inadvertently fostered the dissemination of a particular parasite lineage. Ongoing investigations will determine whether this phenomenon, established here in a parasite which sickens cattle but poses no known risk to human health, also occurs in pathogens which people can contract from livestock. This work aligns with National Program 108, Food Safety (animal and plant products); Components 1a Pathogen, Toxins and Chemical Contaminants Preharvest, sections ii) Epidemiology and iii) Ecology, Host Pathogen and Chemical Contaminants Relationships.

3. Established broader host distribution for parasites related to Toxoplasma gondii. One of the peculiar attributes of Toxoplasma gondii, a foodborne parasite that threatens the health of pregnant women, their fetuses, and persons with HIV-AIDS, is its broad host range. Although this parasite can complete its life cycle only in cats, it can infect virtually all warm-blooded vertebrates including many food animals. To better understand the basis of such a broad host range, and to determine whether this ability uniquely characterizes T. gondii, we conducted a series of genetic studies to establish the identity of parasites, in various hosts, that resembled a related parasites species: Sarcocystis neurona. Although this parasite was initially identified as a cause for neurological disease in horses, our studies this year affirmed its occurrence in marine mammals and found evidence for highly similar parasites in unanticipated places, including Geotropically birds. National Program 108, Food Safety (animal and plant products); Components 1a Pathogen, Toxins and Chemical Contaminants Preharvest, sections ii) Epidemiology and iii) Ecology, Host Pathogen and Chemical Contaminants Relationships.


6.Technology Transfer

Number of Non-Peer Reviewed Presentations and Proceedings3

Review Publications
Khan, A., Fux, B., Su, C., Dubey, J.P., Darde, M.L., Ajioka, J.W., Rosenthal, B.M., Sibley, L.D. 2007. Recent global swee[p of Toxoplasma gondii driven by a single monomorphic chromosome. Proceedings of the National Academy of Sciences. 104:14872-14877.

Rosenthal, B.M., Dunams, D.B., Pritt, B. 2008. Restricted genetic diversity in the ubiquitous cattle parasite, sarcocystis cruzi. Infection, Genetics and Evolution.(5):588-592.

Last Modified: 12/27/2014
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