Location: Animal Parasitic Diseases Laboratory2012 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 this project's objectives, which support NP108 Food Safety Component 1: Food-borne Contaminants. To commence assembly of the Trichinella murrelli genome, short sequence reads were successfully organized by scaffold assembly to homologous regions of the Trichinella spiralis genome as a proof of principle and to guide ongoing assembly and annotation efforts. To improve the ability to trace outbreaks of Trichinella spiralis, additional variable genetic markers (containing microsatellite repeats) were validated and shown to discriminate among field isolates. Analyzing these markers also demonstrated that in nature, animals can become infected more than once, but that they seldom ever do. This finding is relevant for considering the possible efficacy of vaccines intended to safeguard veterinary and public health. Finally, new insights were gained on the global diversity of Toxoplasma gondii, including a comprehensive description of parasite diversity using all extant systems for genetic characterization. This achievement provides researchers, the world over, a common set of reference strains, overcoming a history of distinct genotyping systems in different laboratories. The result should hasten communication and progress in the field of toxoplasmosis epidemiology. In addition, progress was made in understanding the identities, distributions, and risks associated with several other food-borne parasitic agents including those in the genera Sarcocystis and Besnoitia.
1. Genetics of Trichinella infections. Strong immune responses may strictly limit the opportunities for unrelated parasites, such as foodborne species of Trichinella, to mate. ARS researchers in Beltsville, Maryland examined the diversity of Trichinella larvae in naturally infected animals. Almost without exception, they found that animals had been successfully infected by only a single pair of parasites, as judged by the fact that larvae (their progeny) were full siblings. Therefore, immunity may typically preclude establishment of subsequent infections, a finding that further substantiates vaccination as a plausible strategy for future public health interventions.
2. Gene flow among dissimilar types of Trichinella. The spread of biological traits relevant to public health may be constrained by geographical, physical, and/or physiological barriers to reproduction among diverse forms of Trichinella. ARS biologists working in Beltsville, Maryland genotyped isolates of these food-borne parasites at loci (located in the nucleus) inherited from each parent, as well as at a locus (in the mitochondrion) inherited solely from the maternal line. Where each of two parasite lineages was known to infect wildlife, intermediate forms of the parasite (containing ancestry from both lines) were documented. In particular, one maternal lineage was found where two had been expected, substantiating the notion that introgression has resulted in the displacement, locally, of one by the other. These findings are among the first to explore the capacity of genes to flow from one lineage of Trichinella to another. Understanding whether traits can be shared among lineages is important, because a prevalent method to ensure the safety of meat (freezing) would be undermined if it certain traits (i.e. freeze resistance) were capable of being shared.
3. An unexpectedly widespread chromosomal variant in Toxoplasma gondii. Toxoplasma gondii, is an important and prevalent parasite that can be contracted either by eating contaminated meat or by ingesting contaminated produce or water. In spite of the fact that the parasite can undergo sexual recombination, producing myriad variants in certain locales, other strains are temporally stable and geographically widespread. The basis for persistence and dissemination of such strains was studied by ARS researchers in Beltsville, Maryland in collaboration with an international team of academic scientists. They found that a particular variant of one of the parasite’s chromosomes occurs in myriad, otherwise unrelated strain. This finding focuses attention on the genes encoded by this chromosome, as it may offer functional explanation for the basis of this parasite’s dispersal capacity, and new means to intervene in its transmission to food animals and to people.
Khan, A., Miller, N., Roos, D., Dubey, J.P., Ajzenburg, D., Darde, M., Ajioka, J.W., Rosenthal, B.M., Sibley, L. 2011. A monomorphic halotype of chromosome Ia is associated with widespread success in clonal and nonclonal populations of Toxoplasma gondii. mBio. 2:e00228-11.