Furthermore, genetic heterogeneity and clonal diversity, which leads to variability in parasite virulence, are also common among L. (Viannia) spp. parasites [ 10, 19, 20].
In this study, we have demonstrated the great variability in the virulence genes present in isolated Salmonella strains in both dead and live birds, and among birds of the same species.
Although experiments are complicated by enormous variability in strain virulence, it would be informative to see if resistance has increased compared with their findings, and those of Zinn et al. (1977).
Although pO157 has been reported in most O157 H7 strains (13 ), our study demonstrates a high variability in the putative virulence factor content of pO157 as well as a highly variable content of pO157-associated virulence factors among the O157 isolates screened.
More than 2,500 serotypes of Salmonella have been identified and significant variability has been found in virulence, infectious dose, and host.
Despite the variability in host tropism and virulence, orthopox viruses exhibit a high degree of similarity in morphology, life cycle, and structure of the assembled virus.
In addition to the introduction and loss of specific genes in the global M1T1 strain, a high degree of variability in the expression of virulence genes among isolates belonging to this clonal strain was reported (3, 36 ).
Our results based on the higher resolution provided by massive sequencing techniques, however, point to an unexpectedly high level of intraspecies variability in relevant gene families involved in virulence, such as the ALS family of adhesins.
