INTRODUCTION:

All aquatic organisms are influenced by climate change. Even small temperature changes alter fish metabolism and physiology in terms of growth, fecundity, feeding behavior, distribution, migration and abundance. The impacts of climate change revealed marked alterations in the distribution of aquatic organisms, their seasonal performance and physiological responses to habitat. Moreover, threats are also in future through significant climate warming in the next years. An increase of 20C to 30C in temperature is expected to high rise of extinction in 20% to 30% plant and animal species, while major changes to the structure and function of marine and other aquatic ecosystems are anticipated.

Climate change also showed profound effect on parasite distribution and disease in aquatic organisms. The ecological changes in the host or pathogen or both are directly linked to disease outbreak. The most apparent effects are resulted from the extension in the geographical range of pathogens. The increased temperature may cause thermal stress in aquatic animals, leading to reduced growth and defense mechanisms with altered behaviors. The distribution and abundance of pathogens must be affected by dominance over climate through introduction of invasive species, habitat alteration, agricultural practices and human activities. The climate change also includes alterations in water levels and flow regimes, changes in acidification, UV radiation, eutrophication, stratification, run-off and weather parameters. All these factors will have consequence for entire ecosystem and their food webs.

The host-parasite systems are affected by climate at community and population level. There prediction about particular parasite or pathogen is difficult due to species specific relationship and non-linear thresholds in disease dynamics and climatic processes. However, some general predictions do emerge. Longer growing seasons and high temperatures should affect parasite life cycle and host biology with frequent outbreaks of disease resulting from increased transmission rates. Parasites and pathogens with complex life cycles, or those in poikilothermic hosts, may be abnormally affected by global warming. Parasites may respond to increasing temperatures more strongly than their hosts.

Disease in Marine Organisms:

The disease in marine organisms are strongly related with climate change and mass mortalities have been documented in sea grasses, oysters, starfish, corals, abalone and sea urchins (Harwell, 1999). The population affected is key component in marine habitats with disease dispersal throughout on community level in the local ecosystem.

The most dramatic disease to emerge and spread in recent years across large spatial scales is associated with coral reefs. There, disease agents are often opportunistic facultative pathogens that infect corals suffering from physiological stress resulted by exposure of high temperatures. The altered temperature may increase virulence factors in pathogens (Lesser et al., 2007) and also their growth, development and transmission (Harvell et al., 2002). Aspergillosis is a fungal disease with optimal virulence of causative agent (Aspergillus sydowii) at 300C, whereas corals increasing their susceptibility to attack of pathogens. The staghorn corals (Acropora muricata) are sensitive to high temperature and attacked by black band disease (BBD).

The dermal infection of protozoa (Parkinsus marinus) causes significant loss in oyster population. Similarly, protozoa (Haplosporidium nelson) appear limited by low temperature, and spreads MSX disease in oysters. Black abalone (Haliotis cracherodii) is another mollusk that has experienced mass mortalities with periods of warmer sea temperature and ENSO events. The protozoa (Paramoeba invadens) causing massive mortalities of the green sea urchin (Strongylocentrotus droebachiensis) are associated with warmer sea surface temperature, and, also higher tropical storm and hurricane activity. The purple sea urchin has experienced numerous epidemics in past decades, caused by unknown pathogens. Recently, outbreaks of bitter crab disease in snow crabs (Chionoecetes opilio) of Newfoundland associated with warmer temperatures with parasitic dinoflagellates (Hematodinium spp.).

The impacts of these outbreaks are severe. They directly affect the fisheries of sea urchins, abalone, crabs and especially oysters. The foundation species experiences stress, and their subsequent loss significantly alters ecosystems and affects established local economics (Fischlin et al., 2007). The sea urchin acting as keystone consumers actually control the benthic landscape by preventing the establishment of kelp-dominated communities.

Disease in Freshwater Systems:

The general effects of global warming on parasites include rapid growth and maturation, early age of spring maturation, increased parasite mortality, parasitism and disease with earlier prolonged transmission with the possibility of continuous transmission. Climate changes effects are ore profound in poles and near equator due to existing seasonal temperature change for easy occurrence and transmission of parasites. The ectoparasitic crustacean (Argulus coregoni) that infects salmonids undergoes two annual life cycle instead of single generation in warmer climate. There is also other parasites proliferates in warmer water, demanding more extensive control measures or treatment. Gastropods transmit trematodes such as Diplostomum spp., a parasite which causes cataracts and blindness in fish, resulting in further problems in aquaculture (Chappel et al., 1995).

Studies on the effects of thermal effluents illustrate the impact of increasing temperature on host-parasite system (Macroglese, 2001). Parasite populations may remain constant, increase or decrease, as may the populations of their intermediate and definitive hosts (Pojmanska et al., 1987). Even similar parasites that infect the same snails and fish may display different seasonal transmission pattern under the same thermal enhanced conditions (Macroglese, 2001). The extension of transmission into winter months, it may cause during the hottest summer periods, as was observed for two trematodes (Ornithodiplostomum and Tylolelphys) of tropical fishes.

Chytridiomycosis caused by the chytrid fungus is responsible for global mass mortalities, population declines and extinctions of amphibian species (Kringer et al., 2007). This pathogen is more prevalent during cooler months, ceasing to grow at 290C and dying at 300C to 320C (Kolarova, 2007). Climate conditions are linked to chytrid outbreaks and the extinction of at least two frog species in the mountain of Costa Rica (Pounds et al., 2006). These conditions in tropical montane environments may be shifting towards optimum for chytrid epizootics. The mid-altitudinal range promotes chytrid transmission in areas where it is previously checked by warm daytime and cool night-time temperature (Pounds et al., 2006). It is suggested that global warming will reduce the impact of chytridomycosis in low-altitude frog populations in the tropics and sub-tropics. However, in mountains and temperate lowlands, chytrid infection may be expected to spread, contributing to further amphibian declines and possible extinctions (Pounds et al., 2006).

Disease transmission through Ecosystem instability:

Any negative effects of climate change on biodiversity acting as buffer may affect disease transmission. The dilution effect exhibited by incompetent disease reservoirs decreases the impact of highly competent reservoirs and reduces disease risk (Schmidt et al., 2001). The terrestrial model of Lyme disease and risk proposed that a richness of higher species reduces the risk of transmitting Lyme disease to humans. This model should apply to any disease or parasite that require horizontal transmission, non-host specific, differential reservoir hosts to transmit the pathogen and when the most competent vector or intermediate host is dominant. Thus, anthropogenic changes that affect biodiversity will increase disease risk (LoGludice et al., 2003). Epidemiological models suggest that high species richness may buffer disease prevalence when transmission is dependent on a vector or intermediate host. This will have significant approach with climate change and pathogen extends their distributional range from areas of high diversity into areas of low diversity (Dobson, 2004).

The food web structure also influences the occurrence of disease and parasitism. Any perturbations that reduce predator abundance may reduce prey mortality rates, resulting in epizootics of density-dependent pathogens in prey populations. This may occur through increase in the prevalence of disease in host populations where the pathogen already exists or spread infection to new host populations (Holt et al., 2006).

Parasites and disease of foundation species or keystone parasites, may affect their populations through ecosystems. Thus, parasite and disease outbreaks in these organisms may have powerful and long-lasting effects on entire ecosystems (Bruno et al., 2007).

Mass mortalities of mud-snail and amphipods have been observed (Holt et al., 2006). These organisms serve as first and second intermediate hosts in the life cycles of microphallid trematodes, shorebirds and waterfowl being the definitive hosts. The mud-snail population was declined and the amphipods disappeared altogether. The declines coincided with abnormally high temperatures. Moreover, the amphipod constituted a major food source for migrating shore-birds (Mouritsen et al., 2005). High-temperatures increased the development rate and release of free-living infective stages of the microphallid trematodes, thus causing extensive mortality among the gastropod hosts and eliminating the amphipod population (Mouritsen et al., 2002). It is experimentally demonstrated that infection levels and parasite-induced mortality in amphipodfs also increase under warmer conditions. The development and transmission rates of the parasites, under conditions of high temperatures and extended growing seasons, are more sensitive than their invertebrate intermediate hosts. In this case, mortality is additive and with higher infection levels can lead to epizootics and cause fluctuations in the amphipod population (Mouritsen et al., 1997). Also, an echinostome trematode (Curtularia australis) spends their life on cokle (Austrovenus stutchburyi) as intermediate host. This parasitic infection in cokle resulted in alteration of the surface topography and reduced bioturbation of the salmonids. Basically, cercarial production is more responsive to temperature than other physiological processes (Poulin, 2006).

Higher temperature may also increase the development and transmission of P. invadens, the parasite of green sea urchins. More frequent outbreaks of this parasite will restrict sea urchin populations and ensure the maintenance of kelp forests and their associated community of organisms. Another parasite of sea urchin (Echinomermella masti) showed same response.

Thus, parasites can regulate host populations, influencing the composition, structure and function of biological communities (Marcogliese, 2005).

Stressors and Climatic change for Disease in Aquatic Organisms:

Climate change increases habitat loss through fragmentation, pollution, introduction of alien species, hypoxia and altered hydrology. Furthermore, these anthropogenic perturbations may confound the interception of the effects through climate change. Any kind of stress increases the susceptibility of fish and other organisms to disease (Pickering, 1989).

The combination of direct and indirect effects may result in the non-linear responses to climate change by organisms, communities and population. High temperatures in surface waters and greater hypoxia in bottom waters will confine fish to narrower bands of tolerable conditions. The reduction in dissolved Oxygen and high temperature could lead to increased pathogenicity of gill parasites, respiratory problems and even death (Pojmanska et al., 1980). Combined effects of parasites are greater in polluted waters, compared to reference sites (Thilakaratne et al., 2007). Thermal stress is amplified in molluscs and fishes when infected with parasites and positively correlated with total number of parasites in various body organs. Thus parasites may limit the ability of animals to maintain homeostasis at higher environmental temperatures (Lutterschmidt et al., 2007). The effects of the parasites are context-dependent as cited in the case of mud-snail and grey treefrog (Hyla versicolor). Eutrophication is predicted to increase in aquatic ecosystems, and expected to affect parasite distribution and abundance with host-parasite interactions. The severity of aspergillosis in sea fans and deformities in frogs by the trematode may be cited as example.

Thus, it may be notable that the negative effects of disease will not only increase with global warming, but be compounded by other stressors.

CONCLUSION:

Several water-borne diseases in both human and aquatic organisms are linked to climatic events with outbreak of causative agents for transmission. These outbreaks in foundation or keystone species must affecting entire ecosystem. There is much evidence to suggest that parasite and disease transmission will increase with global warming.

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Received on 21.09.2020 Modified on 12.10.2020

Research J. Science and Tech. 2020; 12(4):323-326.

DOI: 10.5958/2349-2988.2020.00048.0