Abstract
A seminatural, factorial‐design experiment was used to quantify dynamics of the pathogen Mycoplasma agassizii and upper respiratory tract disease in the Mojave desert tortoise (Gopherus agassizii) over 2 years. Groups of initially healthy animals were separated into serologically positive (seropositive), seronegative, and artificially infected groups and paired into 23 pens. We found no evidence of long‐term immune protection to M. agassizii or of immunological memory. Initially seronegative, healthy tortoises experienced an equal amount of disease when paired with other seronegative groups as when paired with seropositive and artificially infected groups—suggesting that recrudescence is as significant as transmission in introducing disease in individuals in this host–pathogen system. Artificially infected groups of tortoises showed reduced levels of morbidity when paired with initially seronegative animals—suggesting either a dilution effect or a strong effect of pathogen load in this system. Physiological dynamics within the host appear to be instrumental in producing morbidity, recrudescence, and infectiousness, and thus of population‐level dynamics. We suggest new avenues for studying diseases in long‐lived ectothermic vertebrates and a shift in modeling such diseases.
Highlights
Recrudescent diseases have predominantly been studied in humans—predominantly on a molecular level in certain viral diseases, in immunosuppressed individuals, or in cases where drug resistance can evolve (Okoro et al, 2012; Rentier et al, 1996; Slater, Griffin, Ghani, & Okell, 2016)
While mild upper respiratory tract disease (URTD) rates range from 0% to 15% on average within populations, recent evidence by quantitative PCR suggests that 20–85% of individuals within these populations may harbor subclinical infections—with very low loads of M. agassizii (Braun et al, 2014; Chava Weitzman, unpublished data; Sandmeier et al, 2013)
Recrudescence is expected to change the dynamics of host–pathogen systems in long-lived hosts, exhibiting chronic disease and low transmission rates (e.g., Wang et al, 2013)
Summary
Recrudescent diseases have predominantly been studied in humans—predominantly on a molecular level in certain viral diseases, in immunosuppressed individuals, or in cases where drug resistance can evolve (e.g., the varicella-zoster virus which can result in chicken pox and shingles, infections in HIV-positive patients, malaria) (Okoro et al, 2012; Rentier et al, 1996; Slater, Griffin, Ghani, & Okell, 2016). While mild URTD rates range from 0% to 15% on average within populations, recent evidence by quantitative PCR suggests that 20–85% of individuals within these populations may harbor subclinical infections—with very low loads of M. agassizii (Braun et al, 2014; Chava Weitzman, unpublished data; Sandmeier et al, 2013). Because disease progresses slowly in these animals, the long time period of this study was crucial to capture disease dynamics within both individuals and groups of tortoises that has not been evident in earlier studies (Brown et al, 1994; Schumacher et al, 1993). (4) At the end of the 2-year duration of the experiment, the number of animals in each group that seroconvert or show severe signs of URTD (visible exudate) should be associated with some combination of measured variables (e.g., initial antibody and infection status, exposure to other groups of animals, and loss in mass). Because serology and URTD are known to be decoupled within individuals (Braun et al, 2014; Sandmeier et al, 2013, 2017), our focus was on group-level measures of disease
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