Climate Change and Fish-Parasite Relationships – A Review

1. Introduction

The threats of climate change to human society and natural ecosystems were highlighted in the fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) in 2007. A scientific consensus has been reached, among the scientific community, that finally admit that human activity causes the accumulation of greenhouse gases in the atmosphere, leading to increasing temperatures and causing environmental changes[1,2].

The theme on Climate Change has recently been addressed more frequently, due to the direct and indirect environmental impacts that affect the planet and causes several ecological changes, which affect all life forms[3,4,5]. Among the actions that contribute to climatic changes we can mention: the increase in the use of fossil fuels, deforestation, industrial activity and biomass burning, which results in an increase in the concentration of CO2 and volatile organic compounds such as butane and propane among others and will destroy the ozone layer that protect the planet[5,6].Activities such as livestock, agriculture, hydroelectric construction and highways are directly related to the occurrence of climate change, due to their environmental impacts [3,6,7,8,9]. The main results of climatic change include temperature rise, and variations in other climate factors such as precipitation and humidity[6,10,11].

The effects mentioned above affect all groups of living beings, their ecological relationships with the environment and also the relationships between trophic levels, leading to biodiversity loss in many taxonomic groups[3]. The resulting environmental changes, caused by both natural phenomena and human intervention, have a marked influence on the emergence and proliferation of zoonotic parasitic diseases[6,12]. Many infections, vector organisms, non-human reservoir species, and rate of pathogen replication are sensitive to climatic conditions[13,14]. For example, the blue crab (Collinectes sapidus) and its parasite, the dinoflagellate Hematodinium perezi, the later has its optimal growth temperature between 23 and 31 ⁰C, while this temperature range causes a decline in the host's immune response. This effect is also known among the host Chionoecetes opilio which decreases its immune response around 4 ⁰C and the parasite Hematodinium sp. has the reproduction increased around of 3 ⁰C, being stable until 10 ⁰C[15] .

The relationship between climate change and the incidence/prevalence of infectious/parasitic diseases has been addressed by different authors, considering certain regions of the planet[7,8,16,17,18,19,20,21] or using more general approaches, without considering the spatial distribution or any specific geographic region[9,10,11,12,22].

Parasitic species are associated with their hosts, and these with their ecological niche, this ecological niche suffers from climate change, directly affecting the environmental behavior of host species, and consequently the adaptations of parasitic species associated with their hosts. It is well known that seasonal changes in the environment can cause modifications in the parasite-host relationship. The presence or abundance of parasites is directly influenced by both the environment within the host, their micro-environment and/or the condition of the ecosystem, their macro-environment[23].

The repercussions of climate change are not limited to temperature effects on hosts and their parasites, but also include: alterations in water levels and flow regimes, eutrophication, stratification, changes in acidification and salinity, reduced ice cover, changes in ocean currents, increased ultra-violet (UV) light penetration, runoff and weather extremes[24].

One of the factors pointed out as temperature, directly influenced by species distribution patterns, favors the growth of host rates reservoirs of parasites and vectors, associated with the prevalence of parasites[17] besides affecting the life cycles of parasites[11] and consequently, something that can be linked to the emergence or reemergence of some parasitic diseases[21].

The interaction between climate change and the occurrence of diseases raises questions about the difficulties involved especially those that, highlights the fact of the cause-effect relationship between climate change and the incidence/prevalence of diseases, caused by parasites, where biotic or abiotic factors can interfere in host/parasite interaction[25]. Host-parasite systems with community-level interactions make them complex systems; in this complexity, it is difficult to assume climatic factors as the only cause, without taking into account the other related factors [25]. It is important to contextualize the difficulties raised[25], in which climate change attributed to global warming, can interfere in the dynamics of disease transmission, including parasite and host populations[11,21,22]. The aim this revision was understand as climate changes can interfere with parasite-hosts interactions.

2. Materials and Methods

This article is a bibliographic review. A survey was made on the “Web of Science” database a platform available for researching scientific references on the proposed topic. A survey of articles using the key words: climate change, parasites and fishes. Accessed at day June 2025.

More than 629,403 papers with the main topic "Climate change" have been found on the search platform for scientific articles "Web of Science" for the most varied areas of Science (Figure 1).

However, only 3,064 articles were found relating climate change and parasites on the same platform. The first publication date back to the 1920s, with one article published in 1926, intituled “Effect of temperature and moisture on nematode root knot “, where the author, Godfrey, observes different levels of humidity and temperature and the development of the nematoda Heterodera radicicola plant parasite and the root development of plants. The author concludes that climatic variations are determinant for the development of the parasite, which in turn interferes with the root growth of plants[26]. Only, from 1999 onwards there was an annual continuity of publications involving climate change and parasites. In 2024, there were 275 articles published and 12,573 citations made.

Many articles have been published annually since the 2000s. However, most of the published articles were literature reviews and/or indirectly had climatic factors as a reference in the parasite-host interaction. And even less, 446 articles have related information between “climate change”, “parasite” and “fish” in freshwater or marine environment, the majority of the works were revisions on the subject proposed. The first paper was published in 2001, intitled “The status of the grey seal, Halicopterus grypus, in the northwest Atlantic”, that mention fishes in indirect way[27]. There were 45 articles published in 2024 and 2,086 citations (Figure 2).

In fact, only in 18 papers the authors are discussing the climate changes as the main causes of interference in the interaction fish/parasite relationship, in our survey (Table 1).

This table presents some studies that conclude that climatic factors directly interfered in the parasite-host relationship, with the factors being water temperature, air temperature, rainfall, salinity, dissolved oxygen, CO2 concentration. It was noted that in the central part of the European continent, the increase in river water temperature is related to the higher incidence of Proliferative Kidney Disease in salmonid species[29,30,31,32]. Showing that the high temperature in the waters of the rivers favors the proliferation of Myxozoa and increases the susceptibility of the host species to disease in lentic environment. The increase in temperature favored the proliferation of the myoliquefaction-causing Myxozoa in swordfish off the coast of Australia in a marine environment[33]. Another important factor is related to the biological cycle of parasites. Monoxene parasites tend to increase adaptability to climate change in the aquatic environment, reflecting in greater parasite intensity in their hosts[34,35]. For heteroxene parasites, they are subject to the number of intermediate hosts to close the biological cycle. Species that require more than one invertebrate intermediate host tend to decrease the population in the environment, caused by the dispersion, decrease or disappearance of the first or second intermediate invertebrate hosts[36]. The temperature of water, air and rainfall, when favorable to the first intermediate invertebrate hosts, favor the population of trematode parasites, reflecting in the greater intensity and prevalence in the environment. However, this condition is not a rule, there is also the decrease or disappearance of parasitic species, because of climate change, reflected in the increase in water temperature, air and rainfall, with the decrease or loss of the ecological niche associated with parasitic species.

3. Effect of Climate Change on Host Fish

In ectothermic animals, such as fish, the impact of climate change, is more severe, their body temperature and metabolism are closely regulated by their environmental water that make them especially vulnerable to temperature fluctuations inducing organic responses at all levels of their biology[46,47].

Aquatic organisms, and fish in particular, are affected by a variety of stressors caused by anthropogenic influences that lead to changes in environmental parameters. These in turn elicit stress responses of the organism in the sense that the affected organisms show reactions outside their normal range[48]. The impacts of multiple stressors in freshwater biota at spatial scales and ecosystems, point out that anthropic actions can interfere in water quality dynamics, these include chemical pollutants, nutrients, flow velocity, pH, dissolved oxygen, disturbances in light and temperature regimes, and many other physical-chemical variables that can be significantly altered[49].

Metabolic disorders and oxidative stress can be caused by increasing temperature, affecting the synthesis of release and actions of stress hormones[50]. Acute or prolonged exposure to high temperatures affects the functions of stress axes and responses to other stressors, compromising the long-term coping capacity of each organism[51].

The impacts of climate change on the functional diversity of freshwater fish, influenced the change in the distribution of fish species, with some species being able to lose up to one hundred percent of the areas to which they are climatically adapted in the Paraná-Paraguay River basin[31]. To this end, these impacts make it difficult for the hosts to interact with their respective ecological niches, reducing the community, either by death or abandonment of the place[51].

The declining biodiversity could also bring food problems. Climate change could lead to disruptions in ocean currents, which will stop bringing nutrients to Antarctica's shores, hampering the growth of krill, which is the base of all the oceans' food chain. This could lead to a decrease in marine biota and a low supply of food to communities traditionally consuming fish and/or seafood[52].

A study about current knowledge in climate change-related eco-immunology in teleosts fishes, focusing on temperature, hypoxia, salinity, and acidification showed that acute and chronic changes in temperature and dissolved oxygen compromise the immunity of fish, increasing susceptibility to disease[53]. The review highlights the need for further studies on acidification and multiple stressor interactions. In addition, it explores the acclimatization potential of the fish's immune response, although comprehensive conclusions cannot yet be drawn due to the limited number of epigenetic studies.

The formation of reactive oxygen species (ROS) responsible for oxidative stress is a common aspect generated by stress. The term oxidative stress is used for the state of imbalance between the generation of oxidants (free radicals and ROS) and the availability of endogenous antioxidants to eliminate ROS[54]. Many studies show that ROS play a central role in the pathogenesis and progression of many infectious diseases and inflammatory disorders[55,56].

Inflammation is a host defense mechanism, generating immune response to physical stimuli, external chemicals or pathogen presence that plays a crucial role in the removal of the pathogens in the hosts, involving an increased generation of ROS by target cells and the immune system[57]. As part of the inflammatory response, ROS are produced that facilitate the elimination of invasive tissue pathogens, however when produced for prolonged periods promote oxidative stress and problems related to chronic inflammation[56]. Immune system cells such as mast cells and leukocytes are recruited during inflammation, they migrate to the site of infection, leading to a “respiratory burst” due to increased oxygen uptake and therefore greater release and accumulation of ROS at the site of infection that stimulate pathways that lead to inflammation activation[55].

The immune system of vertebrates comprises cells, tissues and organs that serve to protect a host organism from infections by bacteria, viruses or parasites. Innate immunity is the first line of defense against invading pathogens. This type of immunity involves physical barriers such as skin and mucus, cellular components and soluble components, including cytokines and complement[58,59]. Adaptive immune response is slower but more efficient, and includes humoral components, B lymphocytes, and cell-mediated components such as T lymphocytes. Adaptive immunity is very specific in its targets and has a memory component that allows a faster response to a reinfection[60].

Fish have both innate and adaptive immunity. In the innate immune response, pathogens are detected by recognition receptors, which induce specific responses to various molecular patterns associated with pathogens (PAMPs) in the infected animal[61]. Water temperature can affect the immune system of fish. Acute and chronic changes in temperature also have different impacts on animals, with short-term episodes being offset by processes such as heat-shock protein response (HSPs), while chronic temperature variations are less likely to be resolved by such responses and can still impact the physiology of the organism[62,63].

In a study on the projection of climate change in Colossoma macropomum (Characiforme) in the Amazon, a greater leukocyte immune response was found in the proportions of lymphocytes and granulocytes. The test determined a change in climate for the year 2100, where the increase in temperature closely influenced the immune response of the host in relation to the degree of monogeneans parasites in the gills[35].

There is a limitation between the correlation of evidence that climate change is related to fish diseases[35]. Host-parasite relationships are affected by several factors, especially by environmental changes. For fish, changes in water temperature can lead to an increase in the intensity and virulence of the pathogen, decrease in immune resistance and increased frequency of disease outbreaks in the host. However, little attention has been paid to clarifying how dependence on fish temperature can directly affect parasites[64].

4. Climate Change and Fish Production

Aquaculture is defined as the cultivation of aquatic organisms under controlled conditions, with the objective of producing a marketable product in the most efficient and economical way possible. Aquaculture is among the most important sectors for food production, providing high quality animal protein and income generation and employment[35,64,65]. Aquaculture plays an important role in food production, in addition to a large international presence with an impact on the global economy[66].

Aquaculture can contribute to compensating for the negative impacts that climate change can have on fisheries[67]. However, mobile organisms, such as fish, may be tolerant of some environmental changes or may escape these changes by moving from one place to another, but aquaculture fish raised in cages or ponds in fixed places cannot move to avoid these changes [67,68].

Climate changes can be considered stressful and promote the emergence of diseases in the growing environment of aquaculture[39,62], since temperature is an environmental factor that affects productivity in fish farming, it can affect the growth, reproduction and behavior of fish in nature and aquaculture, changing individual and population dynamics[1]. The increase in infectious diseases in aquaculture is associated with large economic losses. These diseases are controlled by the treatment of fish with chemicals and medications such as antibiotics. This seemingly stable state, in which the effective transmission of a disease is controlled by drugs, is highly delicate and can easily lose balance if environmental conditions change. One of these parameters is water temperature, which promotes the occurrence of diseases in nature and also in fish farms, since they obtain water from natural sources and the water temperature in the facilities accompanies environmental change [31,32,69]. Some studies have addressed the relationship between temperature modulation and fish susceptibility to pathogens[70,71].

Research on the impacts of climate change on aquaculture and fisheries improves understanding of environmental effects and their consequences for aquatic ecosystems. Highlighting the socioeconomic implications, providing essential data for public policies. The promotion of sustainable adaptation strategies and international cooperation to mitigate these impacts. In addition to investment in research and innovation to address climate challenges and ensure global food security[72].

The impact of climate change on aquaculture can vary in relation to both types (e.g., water temperature, saline water intrusion, ocean acidification) and extent of climate change, depending on climate zones (temperate, arid, tropical or Mediterranean), geographical areas (inland, sea or coastal), types of aquaculture production systems and aquatic species that are grown. In the Northern Hemisphere, rising temperatures can result in mild weather and positive effects on animal growth, but it can also introduce new diseases. In the Southern Hemisphere, the increase in temperature can result in heat stress, floods, droughts and extreme weather conditions[73]. In addition, ocean productivity decreases due to climate change and raw materials for fishmeal and fish oil may be affected[73,74]. In this circumstance, climate change indirectly puts more pressure on improving characteristics related to food efficiency, especially in carnivorous species.

The protozoan Ichthyophthirius multifiliis is one of the main parasites that causes damage to fish farms. Due to lesions proven by these agents in intense infections combined with the enormous reproductive capacity of the protozoa, these can cause high mortality rates even in populations of native species[75]. This fact confirms the potential for dissemination of parasitological agents. In addition, these can act as vectors, bringing new pathogens from different regions and further intensifying the parasite load when it comes to introduced in farmed fish. The intensity of infection was higher in polluted and eutrophicated lakes, demonstrating the importance of environmental characteristics in the proliferation of these agents. Thus, the food supply on farmed fish contributes significantly to the eutrophication process of the environment. Associating this issue with rising temperatures, these environments are always more susceptible to new pathogens and will never be completely free of parasites[76].

The interactions between hosts-parasites can be influence in multiples variables in function of the temperature. The host and parasite thermal preferences curves, the prevalence, abundance, intensity and modes of transmission, the distribution and availability of the hosts are factors can be positively or negatively affected by temperature[24,28].

Whenever the increase in temperature does not exceed the tolerance curve of the parasite or the host, it favours the intensity of parasitism and its transmission. In a generalized scenario where the host suffers from oxygen stress, the metabolism of the parasites increases[24].

As an example, fish species with great commercial importance may suffer more from the emergence of diseases and parasitic infections due to changes in water temperature in the marine environment. The myxosporid Kudoa musculoliquefaciens responsible for muscle myoliquefaction in swordfish (Xiphias gladius), increased its infection power with a variable from 2,500 to 290,000 spores/g of muscle directly in the marine environment at warmer temperatures between 26 ⁰C-28 ⁰C[33]. While the ectoparasite Lepeophtheirus salmonis, which can cause a decrease in the production of Atlantic salmon (Salmo salar) by spoliative action on the host, had its potential for proliferation and infection at temperatures of up to 22 ⁰C[40].

Given the requirement for the control of environmental parameters within aquaculture, recirculation systems provide total control of water temperature. Nevertheless, their economic feasibility is still uncertain. Conversely, cage aquaculture is integrating monitoring technologies to gather valuable data regarding the impact of climate change on fish and pathogens. Smart aquaculture, employing sensors, the internet of things, and data analytics, is increasingly becoming a reality for numerous commercial fish farms[77].

5. Climate Change and Parasites

The increase in temperature affects system transmission, with direct increase in the metabolism of the parasite, increasing the feeding or replication of the parasite in the host, increasing the damage and resulting in the production of a greater number of stages of transmission, as well as a faster spread of the disease in a single outbreak[15,69]. The increase in temperature can extend the duration of transmission windows, resulting in a general spread of the disease in the host population. Both mechanisms, alone or in combination, would result in a higher prevalence of the disease. However, some diseases could also show opposite effects with the increase in temperature if their ideal temperature for growth and transmission were lower[69,78].

The fact that climate change mainly affects fish communities, whether in cold or hot waters with the determining factors: ocean acidification, increased temperature, salinity, disposal of pollutants, among others, has shown significant changes in the disposition of host species. These factors are also influencing the reproduction, fertility and feeding of fish species. Parasites can be acquired by their hosts in two ways: phylogenetic and ecological. By the phylogenetic route, the parasites are inherited from species or groups of ancestral species, and by the ecological route, the parasites are acquired from the environment or from other host species[79].

In a study about climatic variations in the Neolithic period in Europe and their impacts on lake communities and their intestinal parasites in humans, 14 taxa were found, of which 4 taxa were attributed to fishes (Diphyllobothrium, Opistorchis, Echinostoma and Dioctophyme). It was found that in the periods where there was greater rainfall influence or melting associated with the rise in the water level of the lakes, mainly favored species of the genus Diphyllobothrium associated with increased fishing activity and the consumption of infected raw fish. While in the drier periods, with the decrease in the water level of the lakes, it favoured species of the genus Trichuris, closely related to agropastoral activities[42]. These changes reflect dietary and livelihood adaptations to climatic conditions.

Parasites with monoxenous development/life cycle, there may be dispersion or disappearance of the definitive host from a certain area where the parasite is present, which may disadvantage the entire parasite community in that region or ecological niche, with an increase in parasitic intensity in non-habitual hosts or definitive disappearance of the parasitic species. Parasites with heteroxenous development/life cycle, climate change could be even more severe. These parasitic species need one or more intermediate hosts to be able to complete their biological cycle. If these hosts are more susceptible to environmental variations, they may decrease or disappear from the environment, reflecting negatively on the parasitic biological cycle. These parasites necessarily need the intermediate host to evolve and be able to parasitize the definitive host, such as digenetic parasites[80,81].

Wood and collaborators analyzed 699 fish species from North America from 1880 to 2020. A total intensity of 17,702 parasites was found and divided into 85 taxa (n= 17,259 parasite specimens). Of all parasite species found, 52% require three or more obligate intermediate hosts, in these species there was a 10.9% decline in parasite abundance for each decade. Also, for these parasitic species, a 38% decline in parasite abundance was detected for every 1⁰C increase in sea temperature. For parasite species requiring one or two obligate intermediate hosts, no decline in parasite abundance was observed. The authors concluded that other factors may also contribute to the decline in the abundance of these parasites, despite the strong effect of temperature[45].

Parasites may respond to increasing temperatures more strongly than their hosts. Trematodes digenetic are among the most common parasites of vertebrates. They require a molluscan intermediate host in which infective cercariae are asexually produced and released into the water in search of the next host in the life cycle. In a review of temperature effects on cercarial shedding, it was shown that temperature differentially affected cercarial production, with increases in cercarial emergence from the first intermediate host, after an increase of few degrees in temperature.[82].

However, the increase in temperature can benefit parasites with an heteroxenous cycle, up to the point of host death, reflecting negatively on the parasite's biological cycle[83]. Fluctuations as low as 0 – 1 ⁰C can reflect on the dynamics of parasite distribution, from exponential growth to parasite extinction[84].

The environmental conditions, as the temperature affect directly Free-living stages of parasites [85]. The effects of temperature on free-living stages have been analysed in different groups of parasites. The best studies were with trematodes [86].

The average increase across trematode species was a 200-fold increase with a 10°C rise in temperature, which is much higher than the two-to-three-fold elevation predicted by standard metabolic principles. Small increases in temperature should promote the proliferation and emergence of cercariae, with little or no reduction in transmission efficiency. Even short-term spikes in cercarial emergence are sufficient to cause mortality in the next host, with consequences that may cascade through the ecosystem [82]. Furthermore, there may be serious consequences for human populations at risk of trematode diseases, such as schistosomiasis, where the cercariae infect the man directly, if temperatures increase[24]. In contrast, Morley and Lewis (2013)[87], in another meta-analysis of low and midlatitude species parasites showed a declining at higher temperatures, while cercarial emergence was not affected between 20 and 25 C.

The impact of climate change on infection by the trematode parasite Clinostomum complanatum in Trichogaster fasciatus forage fish in the Aligarh region, India, over seven years has been observed. The results show that infection depends on the temperature of the air at the surface, highlighting the biological relevance of climate change in the ecology of this parasite. At temperatures above 40 0C, it favoured the stabilization of the infection in the intermediate host. The model also showed that the greater the evaporation of the waters, the smaller the area of the water depth, favouring the success of infection of the first intermediate host, positively interfering in the biological cycle of the parasite. This is the first study to implicate air temperature as an environmental variable that may contribute to parasitism in fish hosts[38].

Carlson et al. (2017), analyzed the variations in parasite habitat loss influenced by differences in general circulation models (GCMs). In the most optimistic scenario of Representative Greenhouse Gas Concentration (RCP 2.6), the average loss of native habitat of parasites was 20.2%, while in the most pessimistic scenario (RCP 8.5), the loss was 37.4%. No parasitic species has completely lost its suitable habitat in all climate scenarios, on average, species have lost 29.0% of their total habitat without dispersal. 86 species have lost more than 50% of their habitat, and 8 species have lost more than 80%. Even accounting for dispersal, 202 parasitic species still lost habitat by 2070, and 32 species lost more than half of their adequate global habitat. Despite these losses, parasitic species gained an average of 16.2% of suitable habitat, with 29 species doubling their area and 7 species tripling it, and none of these 7 species has zoonotic potential or records of human infection (Table 2)[81].

Monogeneans (Platyhelminths) are the most common and most diverse ectoparasites of fish, usually live on external surfaces, such as the skin, fins, head, gills and eyes, oral and branchial cavities[88]. Haptor is the main organ for fixing these parasites to their hosts and is the main morphological characteristic used to distinguish these from cestode and digenea parasites[89]. They have monoxenous life cycles and stages of rapid transmission, feed on mucus and epithelial cells, which results in damage to host tissue, such as deep wounds in the epidermis that can reach the dermis. The presence of these ectoparasites is correlated with increased susceptibility to secondary infections such as bacterial infections and disturbances in the host immune system[90]. They are widespread in aquaculture, including closed, semi-closed and open systems, and are responsible for economic losses. Therefore, the fight against infectious diseases, which is already considered one of the main current challenges of fish farming, can become an important issue in climate change scenarios[69,91].

In a senary of climate change until 2100, with the increase in temperature, a higher intensity of monogeneans Notozothecium janauachense, Anacanthorus spathulatus and Mymarothecium boegeri was observed in the host C. macropomum, with a 200% increase in mean intensity and a prevalence of 100% in the hosts. The authors concluded that the environment favours activity and increase in parasitic metabolism[92].

Several factors influence the responses of species to temperature rises, both intrinsic to each organism and extrinsic linked to the ecological niches in which they are inserted, which are directly subject to the environmental conditions existing in each geographic region and how these conditions are and will be affected or not by climate change[5,22].

Melo et al. (2024) analyzed 1,973 fish of the species Hoplias malabaricus in South America, found 120 parasites, two species of Monogeneans (ectoparasites): 17 Urocleidoides cuiabai and 44 U. eremitus; three species of Digenea (endoparasites): 15 Clinostomatopsis sorbens; 30 C. dimorphum and 14 Pseudosellacotyla lutzi. In a climate change scenario until 2050 using an atmosphere-ocean general circulation model, where the main variables analyzed were: temperature, precipitation, carbon and pH, where SSP (Shared Socioeconomic Path) of 4.5 for an optimistic scenario and SSP 8.5 for a pessimistic scenario was used. In an optimistic scenario, there was a loss of 93% of the niches suitable for all parasites, with a 100% loss for species C. dimorphum. In a pessimistic scenario, there was a loss of 97% of the suitable areas, leaving 3% of the niches for parasitic species. The increase in temperature can mainly influence the biological cycle of endoparasite species, as they require one or more intermediate hosts. While the host H. malabaricus showed adaptability to climate change[41].

Certain parasites may have a higher incidence with increasing temperatures, leading to a decrease in host species or even their extinction. In the literature there are reports of massive deaths among lions, frogs, birds, wild dogs, snails, mussels, storks, eagles, corals and various types of plants, all caused by pathogens. Fungi and viruses are especially sensitive to climate change and can quickly grow when the temperature rises, especially if this elevation is accompanied by increased humidity. Fungi and insects have their activity increased and can be responsible for the elimination of entire species of trees. Moreover, parasitosis in both cattle and wild animals are also greatly increased with higher temperatures[93].

The same impact of climate change on the diversity of animals and plants is predicted for parasitic diversity. Parasites, such as helminths, are known as modulators of the immune system, can have a synergistic effect on the effects of climate change on hosts. One suggests between host and parasites interaction is that the reactivity of the immune system of host vertebrates depends on exposure to parasites, which are important for the host to develop an adequate functional immune response. Parasitic-host interactions are the main force in the evolution of the immune system that is designed to maintain optimal basal immunity, minimizing the risk of developing potentially pathological reactions [94,95].

Modifications in environmental elements such as water bodies, soil moisture, vegetation cover, among others, interfere in parasites, vectors and their hosts, as well as in the interactions between them[96]. In the case of vectors of parasitic diseases, changes in temperature influence the life cycles of parasites. After all, a portion of them have part of the life cycle in the environment[10].

Climate change has brought about significant alterations in ecosystems, intricately affecting the interactions between parasites and their hosts. The response to these environmental changes is not uniform, varying considerably even among phylogenetically related species of both parasites and hosts [97]. This variability stems from physiological, ecological, and behavioural differences that influence each organism’s sensitivity to environmental shifts[98].

Moreover, many parasites possess complex life cycles involving multiple stages that may occur in different hosts or environments[86]. Each stage may respond differently to variations in temperature, humidity, and other climatic factors, directly impacting parasite survival, development, and transmission rates[86,98]. Studies have shown that these thermal responses are often non-linear, with infection rates increasing up to an optimal point and then declining under extreme temperatures[98].

Therefore, understanding the effects of climate change on host–parasite systems require an integrative approach that considers the ecological and physiological specificities of each species involved, as well as the interactions among the various components of the system across time and space[86,97].

Climate change may favor the emergence of infectious and parasitic diseases in fish in nature or in intensive, semi-intensive and open production systems. The increase in temperature can increase parasite metabolism, the adaptation of parasitic species to new hosts, the intensification of the most abundant parasitic species to search for new hosts, contributing to greater economic loss due to parasitic lesions in these hosts, greater parasitic contamination in fish production leading to a higher dietary risk for consumers. In addition to directly affecting the physiological and immunological dynamics of the hosts. However, future studies need to be carried out to be able to understand more deeply how climate changes can interfere with host-parasite adaptations and interactions.