Increases and decreases in marine disease reports in an era of global change

Abstract

Outbreaks of marine infectious diseases have caused widespread mass mortalities, but the lack of baseline data has precluded evaluating whether disease is increasing or decreasing in the ocean. We use an established literature proxy method from Ward and Lafferty (Ward and Lafferty 2004 PLoS Biology2, e120 (doi:10.1371/journal.pbio.0020120)) to analyse a 44-year global record of normalized disease reports from 1970 to 2013. Major marine hosts are combined into nine taxonomic groups, from seagrasses to marine mammals, to assess disease swings, defined as positive or negative multi-decadal shifts in disease reports across related hosts. Normalized disease reports increased significantly between 1970 and 2013 in corals and urchins, indicating positive disease swings in these environmentally sensitive ectotherms. Coral disease reports in the Caribbean correlated with increasing temperature anomalies, supporting the hypothesis that warming oceans drive infectious coral diseases. Meanwhile, disease risk may also decrease in a changing ocean. Disease reports decreased significantly in fishes and elasmobranchs, which have experienced steep human-induced population declines and diminishing population density that, while concerning, may reduce disease. The increases and decreases in disease reports across the 44-year record transcend short-term fluctuations and regional variation. Our results show that long-term changes in disease reports coincide with recent decades of widespread environmental change in the ocean.

Keywords: infectious disease; literature proxy; marine; temperature anomalies.

Figure 1.

Comparison of trends over time for reports of disease in focal marine taxa from 1970 to 2013, with the right panel zoomed to the more recent 2001–2013 time period and divided into groups with (a) increases, (b) decreases and (c) neither increases nor decreases. Correlations with time were evaluated using Spearman's ρ, with horizontal lines indicating ±1 standard deviation about the mean of the dataset (1970–2013). The percentage on the vertical axes refers to the per cent of total reports per taxon per year that reported disease. Red (blue) outlines denote significant positive (negative) correlations (ρ) with time (significance level 0.05); dashed lines indicated non-significant correlations (significance level between 0.05 and 0.1). Significance (p) values were adjusted for multiple testing using Holm's Bonferroni method (n = 10 for 2001–2013, n = 9 for 1970–2013). (Online version in colour.)

Figure 2.

Comparisons between rabies records and the rabies literature proxy for 1970–2013 and 2001–2013 based on correlations using Spearman's ρ. Rabies records is the number of reports per year, while rabies % is the per cent of total reports per taxon per year that report an incident of disease. Red outlines denote positive correlations with time and blue outlines denote negative correlations with time. (Online version in colour.)

Figure 3.

The proportion of Caribbean reef pixels exposed to bleaching-level heat stress (DHM ≥ 1°C-months, 1970–84; DHW ≥ 4°C-weeks, 1985–2013) and normalized disease reports for corals in the Caribbean both increase over time. (Online version in colour.)