Developing Immune Profiles of Endangered Australian Sea Lion (Neophoca cinerea) Pups Within the Context of Endemic Hookworm (Uncinaria sanguinis) Infection

Abstract

As a top predator, the endangered Australian sea lion (Neophoca cinerea) is a sentinel of ecosystem change, where population trends can reflect broader shifts in the marine environment. The population of this endemic pinniped was historically diminished by commercial sealing, and recovery has been slowed by fishery interactions, disease and, potentially, pollutants. Hookworm infects 100% of neonatal pups and has been identified as a contributor to population decline. Here, a multivariable approach using traditional serological and novel molecular tools such as qPCR and ddPCR was used to examine immune phenotypes of developing Australian sea lion pups infected with the endemic hookworm (Uncinaria sanguinis) from two South Australian colonies. Results show changing immunophenotypes throughout the patent period of infection represented by pro-inflammatory cytokines (IL-6), IgG and acute-phase proteins. Although cytokines may prove useful as markers of resistance, in this study, IL-6 is determined to be an early biomarker of inflammation in Australian sea lion pups, excluding the alternative hypothesis. Additionally, immunological differences between animals from high- and low-intensity hookworm seasons, as well as ivermectin-treated animals, indicate hookworm infection modulation of the host immune response, as evidenced by a lower IL-6 mRNA expression in the non-treated groups. This study of the Australian sea lion is an example of an ecoimmunological approach to disease investigation, which can be applied to evaluate the impact of environmental and anthropogenic factors on susceptibility to infectious diseases in free-ranging species

Mitigating disease risk in an endangered pinniped: early hookworm elimination optimizes the growth and health of Australian sea lion pups

“…Significant between group differences in lymphocyte count coincided with mid-to late patent hookworm infection, indicating antigenic stimulation and a host immune response. This has been confirmed in an immunological study of hookworm infected Australian sea lions, showing a positive association of increasing pup age with B-lymphocyte count and immunoglobulin G (IgG; antibody) concentration across the period of hookworm patency ( 51 ). While there is the potential for early hookworm elimination to negatively impact lymphocyte ontogenesis, comparable lymphocyte counts in treated and control pups of the oldest two age cohorts by the study conclusion suggest any impact is likely to be short term.…”

“…In the present study, a longer follow-up period identified significantly higher TPP concentrations ( p < 0.001) in untreated pups of all three age cohorts at second recapture (combined median time [range] post recruitment 63 d [41–147 d]; ages 72 d [51–128 d], 82 d [51–142], and 88 d [55–168 d] for the <14 d, <70 cm, and ≥70 cm cohorts, respectively – all approximating the age of natural hookworm elimination). The above mentioned study of the immune response to hookworm infection in the Australian sea lion provides some explanation for the apparent discrepancies and suggests an inflammatory contribution to increased TPP concentration in untreated pups ( 51 ). In that study, the start of the patent period was marked by a peak in IL-6 (a pro-inflammatory cytokine) gene expression indicating early stage inflammation, followed initially by an increase in positive acute phase proteins (proteins increasing early in the inflammatory process), and then by the increase in B-lymphocyte count and IgG production.…”

“…Observations were consistent with an antihelminth inflammatory response, peaking at the time of natural hookworm elimination. Superimposed on this inflammatory contribution to TPP concentration was an age-related increase in albumin concentration, reaching a maximum in the postpatent period ( 51 ). This followed an initial decline in the very early stages of the inflammatory response (confirming albumin as a negative acute phase protein in the Australian sea lion).…”

A novel multi-variate immunological approach, reveals immune variation associated with environmental conditions, and co-infection in the koala (Phascolarctos cinereus)

“…It has the potential to identify protective responses 6 , and predict subsequent effects on population fitness 7 and disease dynamics 6 , 8 . Immunological responses can be modulated by diverse internal factors such as sex, age, stress, pathogen burdens 7 , 9 – 13 , and external factors such as season, anthropogenic pressures and environmental spatial and temporal features 9 , 14 – 17 . Thus, they promise to provide an indicator of real-time impacts of potential disease drivers.…”

“…As a result, most wildlife immunological studies perform separate analyses of each variable. To overcome these challenges, an integrative multi-variate approach assessing the various arms of the immune response in conjunction with host, pathogen and environmental data 6 , would enable more confident identification of biologically relevant immunological variations and mechanisms of complex systems 6 , 18 , 19 and profiles characteristic of different infection and health states 14 . While classical and cross-species techniques of measuring immunological responses such as bacterial killing and complement assays are frequently used and demonstrate value to assess immune function, they represent only one or limited components of the immune system 20 .…”