Emerging infectious diseases and parasites on fish are associated with pathogens that have recently increased in incidence, impact or geographic or host range. They may pose a greater threat to biodiversity through biomass loss and extinctions of host species than pathogens responsible for endemic diseases.
This is because the dynamics of the host-parasite interactions may differ as the pathogen has not coevolved with the host or the ecosystem in which they emerged.
An outbreak of an emerging disease may occur when the parasitic fauna of a species is introduced from its natural range into a new region at the same time as its host, providing the opportunity for host switching.
Given the lack of co-evolution between these new host species and the introduced pathogen, transmission rates and infection impacts may be high.
Indeed, it has been suggested that the introduction of pathogens into new host responses to parasite infection are important to understand as they form the basis of the population response.
In coevolved host: parasite relationships, infections tend to negatively impact host fitness, modulate the dynamics of host populations and have indirect consequences for non-host Infection costs are compensated by hosts through, for example, developing immune systems as an infection barrier and tolerance through alteration of life-history traits, particularly in the pre- reproductive life-span.
Populations through changes in the strength of inter specific competitive relationships.
This then impacts reproductive effort and body size as individuals allocate more resources to reproduction than growth and survival to ensure reproduction before resource depletion, castration or death.
How hosts respond to introduced pathogens where there has been no co-evolution is less clear; whilst expectations are of catastrophic outcomes through strong negative effects on host survivorship, these may not always be apparent, areas through human activities is one of the most important factors driving disease emergence in natural populations.
In freshwater ecosystems, the opportunity for fish parasites to be moved between regions by anthropogenic activities is high given that the rate of introduction of non-native fishes has doubled in the last 30 years, mainly due to the globalization of aquaculture.
Introduced diseases resulting through fish movements in aquaculture have already resulted in significant impacts in some fish, for example the infection of European eel Anguilla anguilla with Anguillicoloides crassus has been implicated as a major factor in their global decline.
Research on the host consequences of infection by introduced parasites has tended to focus on those that impact immediately on host population dynamics through high mortality rates, with sub-lethal impacts, such as alterations in host behavior and growth rates rarely considered despite their potentially significant consequences for wild populations.
Parasitic species can be found everywhere, and on every living organism. Their presence in their host is generally at equilibrium in aquatic organisms and the most common lifestyle on the planet (Marcogliese 2005).
Consequently, it is difficult to find any environment or organism that can be labeled as ‘pristine’ or parasite- free. When researchers describe control sites as being pristine, pathogen or disease-free, they are merely describing the lack of viruses, bacteria and xenobiotics, and are not generally referring to parasites.
There are times when changes in the environment (natural or anthropogenic) can change the state of balance of the parasite between host and nature, thus resulting in disease.
These changes can be environmental such as temperature, climate, or anthropogenic such as pollution and urbanization.
When the dynamic equilibrium between host and parasite is lost, some changes can occur within the host.
These changes can cause mechanical damage (fusion of gill lamellae, tissue replacement), physiological damage (cell proliferation, immune-modulation, altered growth, detrimental behavioral responses,) and/or reproductive damage.
The roles, functions, and life-styles of parasites help to characterize an ecosystem. Knowledge of parasites and parasitic communities, allows scientists to recognize the role of the fish host in the food web or ecosystem.
Mechanical Damage of Parasites on Fish
Fusion of gilllamellae: Many species of parasites invade the gills of fish. They can range from microscopic tubulinea or monogenea, to acroscopic annelida and arthropoda, and all can be viewed on the gill arches or nestled between the gill filaments.
Grossly visible reactions to these parasites on the fish may be noncritical and include a mild discoloration of the gill filaments or one or two white spots. In more critical cases, the fish may display heavy eroding, massive discolorations (often paler), numerous white spots, and increased mucus secretion (Toksen 2007).
Tissue Replacement: Parasite loads in individual fish can often rise to such high numbers that they occupy the majority of the total area of a specific organ.
Physiological Damage of Fish Parasites
Cell Proliferation: Proliferation of a single type of cell can cause detrimental effects in the fish host. This same proliferation of cell types is found in human diseases such as cancer.
Immuno modulation: All parasites have evolved ways to evade the host’s immune response and host immune systems have evolved numerous ways to counter these evasive strategies (Sitja-Bobadilla 2008).
A trade-off is established that is essential to the survival of the parasite and provokes a state of illness in the host, which is not necessarily lethal (Sitja-Bobadilla 2008).
However, when a parasite efficiently evades the host immune system, it may damage the host, but actually reduce damage to the parasite (Sitja-Bobadilla 2008). Some parasites have evolved strategies that use the host immune system to aid their attachment to the fish host.
Detrimental Behavioral Responses: Although parasites generally do not cause negative impacts to their host, occasionally parasites can develop in such a way as to alter their host’s behavior.
This usually occurs with parasites that have complex lifecycles, as it may be more difficult for them to go from one host to the next.
For example, the behavior of arthropods, the intermediate hosts of acanthocephalan parasites, may show various changes when infested, including changes in activity, photoreaction, escape behavior, substrate color choice, and vertical distribution.
Altered Growth: Altered growth is perhaps the most difficult mechanism to validate effects due to parasitism.
In many studies, researchers have determined that altered growth (delayed growth, stunting) only occurs in extreme laboratory conditions, and would not be observed in the wild.
This may be because parasite infested “stunted” fish may not survive in the wild, and they be taken more readily by predation.
For the most part, parasites depend on host-derived energy for growth and development, and so they are potentially affected by the host’s ability to acquire nutrients under competitive foraging scenarios.
Research by Barber (2005) found that the fastest growing fish developed the largest parasites; therefore, faster growing hosts apparently provide ideal environments for growing parasites.
Reproductive Damage of Parasites on Fish
Parasites often influence their hosts through the diversion of resources either directly by using up energy and nutrients or indirectly by increasing the activity of the immune system.
In summary, parasites on fish affect fish health through mechanical, physical and reproductive damage.
These changes can reduce growth, fecundity and survival, change behavior and sexual characteristics, and result in many other maladaptive alterations of the infected host.
These changes could have significant consequences at not only the individual level, but population, community and ecosystem levels as well.
The use of parasites to discriminate among fish populations inhabiting sites of different environmental quality is conceptually possible.
It is difficult to find any environment or organism that can be labeled as ‘pristine’ or parasite-free.
Describing a control sites as being pristine, pathogen or disease-free, they are merely describing the lack of viruses, bacteria and xenobiotics, and are not generally referring to parasites.
The roles, functions, and life-styles of parasites help to characterize an ecosystem. Knowledge of parasites and parasitic communities, allows scientist to recognize the role of the fish host in the food web or ecosystem.
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