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Reduced immune function predicts disease susceptibility in frogs infected with a deadly fungal pathogen.

Savage AE, Terrell KA, Gratwicke B, Mattheus NM, Augustine L, Fleischer RC - Conserv Physiol (2016)

Bottom Line: However, neither group differed from Bd-negative control frogs.The low BKA values in dying frogs compared with infected individuals without disease signs suggests that complement activity might signify different immunogenetic backgrounds or gene-by-environment interactions between the host, Bd and abiotic factors.We conclude that protein complement activity might be a useful predictor of Bd susceptibility and might help to explain differential disease outcomes in natural amphibian populations.

View Article: PubMed Central - PubMed

Affiliation: Center for Conservation and Evolutionary Genetics.

ABSTRACT
The relationship between amphibian immune function and disease susceptibility is of primary concern given current worldwide declines linked to the pathogenic fungus Batrachochytrium dendrobatidis (Bd). We experimentally infected lowland leopard frogs (Lithobates yavapaiensis) with Bd to test the hypothesis that infection causes physiological stress and stimulates humoral and cell-mediated immune function in the blood. We measured body mass, the ratio of circulating neutrophils to lymphocytes (a known indicator of physiological stress) and plasma bacterial killing ability (BKA; a measure of innate immune function). In early exposure (1-15 days post-infection), stress was elevated in Bd-positive vs. Bd-negative frogs, whereas other metrics were similar between the groups. At later stages (29-55 days post-infection), stress was increased in Bd-positive frogs with signs of chytridiomycosis compared with both Bd-positive frogs without disease signs and uninfected control frogs, which were similar to each other. Infection decreased growth during the same period, demonstrating that sustained resistance to Bd is energetically costly. Importantly, BKA was lower in Bd-positive frogs with disease than in those without signs of chytridiomycosis. However, neither group differed from Bd-negative control frogs. The low BKA values in dying frogs compared with infected individuals without disease signs suggests that complement activity might signify different immunogenetic backgrounds or gene-by-environment interactions between the host, Bd and abiotic factors. We conclude that protein complement activity might be a useful predictor of Bd susceptibility and might help to explain differential disease outcomes in natural amphibian populations.

No MeSH data available.


Related in: MedlinePlus

Physiological metrics among frogs exposed to Batrachochytrium dendrobatidis (Bd) and uninfected control frogs euthanized at early time points [1–15 days post-infection (DPI)] compared with late time points (29–55 DPI). Sample sizes vary among these metrics owing to lack of growth data for week 1 and limited blood volumes for many frogs. Top panel: change in mass in control frogs (n = 26), Bd-exposed frogs euthanized 1–15 DPI (n = 13) and Bd-infected frogs surviving 29–55 DPI with (n = 10) or without (n = 19) signs of chytridiomycosis. Middle panel: bacterial killing ability (BKA) of blood plasma from control frogs (n = 26), Bd-exposed frogs euthanized early (1–15 DPI, n = 18) and Bd-infected frogs surviving 29–55 DPI with (n = 10) or without (n = 17) signs of chytridiomycosis. Bottom panel: ratios of circulating neutrophils to lymphocytes (N/L) in control frogs (n = 12), Bd-exposed frogs euthanized early (1–15 DPI, n = 8) and Bd-infected frogs surviving 29–55 DPI with (n = 6) or without (n = 12) signs of chytridiomycosis. Groups with different letters represent statistically significant differences (P < 0.05). All values are shown ±SEM.
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COW011F3: Physiological metrics among frogs exposed to Batrachochytrium dendrobatidis (Bd) and uninfected control frogs euthanized at early time points [1–15 days post-infection (DPI)] compared with late time points (29–55 DPI). Sample sizes vary among these metrics owing to lack of growth data for week 1 and limited blood volumes for many frogs. Top panel: change in mass in control frogs (n = 26), Bd-exposed frogs euthanized 1–15 DPI (n = 13) and Bd-infected frogs surviving 29–55 DPI with (n = 10) or without (n = 19) signs of chytridiomycosis. Middle panel: bacterial killing ability (BKA) of blood plasma from control frogs (n = 26), Bd-exposed frogs euthanized early (1–15 DPI, n = 18) and Bd-infected frogs surviving 29–55 DPI with (n = 10) or without (n = 17) signs of chytridiomycosis. Bottom panel: ratios of circulating neutrophils to lymphocytes (N/L) in control frogs (n = 12), Bd-exposed frogs euthanized early (1–15 DPI, n = 8) and Bd-infected frogs surviving 29–55 DPI with (n = 6) or without (n = 12) signs of chytridiomycosis. Groups with different letters represent statistically significant differences (P < 0.05). All values are shown ±SEM.

Mentions: Blood samples were obtained in sufficient volume and quality (i.e. without clotting) to permit leucocyte counts in 38 individuals (n = 12 control, n = 8 early infection, n = 12 surviving and n = 6 dying). Across all samples (n = 38; Supplementary Appendix 1), lymphocytes were the predominant leucocyte in circulation (mean ± SEM, 88.0 ± 1.8%) followed by neutrophils (8.0 ± 1.5%; Fig. 2). Percentages of neutrophils were increased (t = −2.47, P = 0.018) and percentages of lymphocytes decreased (t = 2.85, P = 0.007) in infected vs. control animals. Given that changes in both cell types were consistent with a response to stress or infection, we focused subsequent analysis on N/L ratios, a single metric that is commonly used to quantify stress in vertebrates (Davis et al., 2008). These ratios differed (F = 6.38, P = 0.002) among uninfected, early infection, dying and surviving groups (Fig. 3). Specifically, post hoc comparisons revealed that the N/L ratio was increased in early infection Bd-positive frogs (q = 4.77, P = 0.010) and in dying frogs (q = 4.02, P = 0.036) compared with uninfected control animals. In contrast, surviving frogs demonstrated N/L ratios that were lower than other Bd-positive groups (q ≥ 3.91, P ≤ 0.043) and similar to uninfected control frogs. Other cell types were observed infrequently (eosinophils, 2.2 ± 0.6%; monocytes, 1.5 ± 0.4%; and basophils, 0.3 ± 0.1%), and proportions did not differ among all groups (Fig. 2).Figure 2:


Reduced immune function predicts disease susceptibility in frogs infected with a deadly fungal pathogen.

Savage AE, Terrell KA, Gratwicke B, Mattheus NM, Augustine L, Fleischer RC - Conserv Physiol (2016)

Physiological metrics among frogs exposed to Batrachochytrium dendrobatidis (Bd) and uninfected control frogs euthanized at early time points [1–15 days post-infection (DPI)] compared with late time points (29–55 DPI). Sample sizes vary among these metrics owing to lack of growth data for week 1 and limited blood volumes for many frogs. Top panel: change in mass in control frogs (n = 26), Bd-exposed frogs euthanized 1–15 DPI (n = 13) and Bd-infected frogs surviving 29–55 DPI with (n = 10) or without (n = 19) signs of chytridiomycosis. Middle panel: bacterial killing ability (BKA) of blood plasma from control frogs (n = 26), Bd-exposed frogs euthanized early (1–15 DPI, n = 18) and Bd-infected frogs surviving 29–55 DPI with (n = 10) or without (n = 17) signs of chytridiomycosis. Bottom panel: ratios of circulating neutrophils to lymphocytes (N/L) in control frogs (n = 12), Bd-exposed frogs euthanized early (1–15 DPI, n = 8) and Bd-infected frogs surviving 29–55 DPI with (n = 6) or without (n = 12) signs of chytridiomycosis. Groups with different letters represent statistically significant differences (P < 0.05). All values are shown ±SEM.
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Related In: Results  -  Collection

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COW011F3: Physiological metrics among frogs exposed to Batrachochytrium dendrobatidis (Bd) and uninfected control frogs euthanized at early time points [1–15 days post-infection (DPI)] compared with late time points (29–55 DPI). Sample sizes vary among these metrics owing to lack of growth data for week 1 and limited blood volumes for many frogs. Top panel: change in mass in control frogs (n = 26), Bd-exposed frogs euthanized 1–15 DPI (n = 13) and Bd-infected frogs surviving 29–55 DPI with (n = 10) or without (n = 19) signs of chytridiomycosis. Middle panel: bacterial killing ability (BKA) of blood plasma from control frogs (n = 26), Bd-exposed frogs euthanized early (1–15 DPI, n = 18) and Bd-infected frogs surviving 29–55 DPI with (n = 10) or without (n = 17) signs of chytridiomycosis. Bottom panel: ratios of circulating neutrophils to lymphocytes (N/L) in control frogs (n = 12), Bd-exposed frogs euthanized early (1–15 DPI, n = 8) and Bd-infected frogs surviving 29–55 DPI with (n = 6) or without (n = 12) signs of chytridiomycosis. Groups with different letters represent statistically significant differences (P < 0.05). All values are shown ±SEM.
Mentions: Blood samples were obtained in sufficient volume and quality (i.e. without clotting) to permit leucocyte counts in 38 individuals (n = 12 control, n = 8 early infection, n = 12 surviving and n = 6 dying). Across all samples (n = 38; Supplementary Appendix 1), lymphocytes were the predominant leucocyte in circulation (mean ± SEM, 88.0 ± 1.8%) followed by neutrophils (8.0 ± 1.5%; Fig. 2). Percentages of neutrophils were increased (t = −2.47, P = 0.018) and percentages of lymphocytes decreased (t = 2.85, P = 0.007) in infected vs. control animals. Given that changes in both cell types were consistent with a response to stress or infection, we focused subsequent analysis on N/L ratios, a single metric that is commonly used to quantify stress in vertebrates (Davis et al., 2008). These ratios differed (F = 6.38, P = 0.002) among uninfected, early infection, dying and surviving groups (Fig. 3). Specifically, post hoc comparisons revealed that the N/L ratio was increased in early infection Bd-positive frogs (q = 4.77, P = 0.010) and in dying frogs (q = 4.02, P = 0.036) compared with uninfected control animals. In contrast, surviving frogs demonstrated N/L ratios that were lower than other Bd-positive groups (q ≥ 3.91, P ≤ 0.043) and similar to uninfected control frogs. Other cell types were observed infrequently (eosinophils, 2.2 ± 0.6%; monocytes, 1.5 ± 0.4%; and basophils, 0.3 ± 0.1%), and proportions did not differ among all groups (Fig. 2).Figure 2:

Bottom Line: However, neither group differed from Bd-negative control frogs.The low BKA values in dying frogs compared with infected individuals without disease signs suggests that complement activity might signify different immunogenetic backgrounds or gene-by-environment interactions between the host, Bd and abiotic factors.We conclude that protein complement activity might be a useful predictor of Bd susceptibility and might help to explain differential disease outcomes in natural amphibian populations.

View Article: PubMed Central - PubMed

Affiliation: Center for Conservation and Evolutionary Genetics.

ABSTRACT
The relationship between amphibian immune function and disease susceptibility is of primary concern given current worldwide declines linked to the pathogenic fungus Batrachochytrium dendrobatidis (Bd). We experimentally infected lowland leopard frogs (Lithobates yavapaiensis) with Bd to test the hypothesis that infection causes physiological stress and stimulates humoral and cell-mediated immune function in the blood. We measured body mass, the ratio of circulating neutrophils to lymphocytes (a known indicator of physiological stress) and plasma bacterial killing ability (BKA; a measure of innate immune function). In early exposure (1-15 days post-infection), stress was elevated in Bd-positive vs. Bd-negative frogs, whereas other metrics were similar between the groups. At later stages (29-55 days post-infection), stress was increased in Bd-positive frogs with signs of chytridiomycosis compared with both Bd-positive frogs without disease signs and uninfected control frogs, which were similar to each other. Infection decreased growth during the same period, demonstrating that sustained resistance to Bd is energetically costly. Importantly, BKA was lower in Bd-positive frogs with disease than in those without signs of chytridiomycosis. However, neither group differed from Bd-negative control frogs. The low BKA values in dying frogs compared with infected individuals without disease signs suggests that complement activity might signify different immunogenetic backgrounds or gene-by-environment interactions between the host, Bd and abiotic factors. We conclude that protein complement activity might be a useful predictor of Bd susceptibility and might help to explain differential disease outcomes in natural amphibian populations.

No MeSH data available.


Related in: MedlinePlus