Piscine transmissible amyloidotic encephalopathy

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Piscine transmissible amyloidotic encephalopathy (PTAE), more commonly known as mad fish disease, is a neurodegenerative disorder that affects the central nervous system of fish. It has to date been described only in farmed tuna developed by Lassgard Bioteknik, though a 2009 study has previously established PTAE infections in gilt-head sea bream.

The transmission of PTAE to humans, primarily via ingestion of infected fish, can result in Cariappa-Muren disease (CMD). Although PTAE entered the food chain in 2034, it went undiscovered until the first cases of CMD were reported in 2039. Since PTAE-affected tuna are considered to be the primary infection vector for the CMD pandemic, it precipitated a dramatic overhaul of food safety regulations and the collapse of the aquaculture industry.

ClassificationPermalink

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Piscine transmissible amyloidotic encephalopathy (PTAE) is classified as a transmissible spongiform encephalopathy (TSE) because its causative agent is considered to be a misfolded protein known as a prion. In the brain, these prions cause the native isoform of major prion proteins (designated as PrP^c) to misfold into the same infectious state (designated as PrP^Sc), which goes on to convert further PrP^c in an exponential cascade. This results in dense protein aggregates in the form of amyloid fibres that disrupt the normal tissue structure. ^[1]

Though there is as yet no consensus on the exact etiology of PTAE, a majority of researchers believes that it is most likely a variant of bovine spongiform encephalopathy (BSE), with an altered pathogenesis due to the cross-species transmission in fish. This finding is largely based on a 2009 research study, which had demonstrated that orally exposing gilt-head sea bream to homogenates with BSE could result in the fish developing a neurodegenerative amyloidosis. The study has since been replicated and found to be broadly consistent with PTAE histopathology. ^[2]

Opponents of this finding cite the 2009 study mentioning a lack of BSE-specific spongiosis as evidence for the hypothesis that PTAE resembles a novel fish amyloidosis more than a classical TSE transmitted to fish, an alternate explanation that the study’s authors also allowed for. In May 2044, researcher Lars Berenger and their colleagues published an article proposing that PTAE may not be a prion disease, but a parasitic encephalitis caused by Miamiensis avidus. ^[3] This article has been widely criticised for disregarding the established link between PTAE and Cariappa-Muren disease (CMD) in order to reach its conclusion.

Signs and symptomsPermalink

Fish infected with PTAE will not show any immediate symptoms due to the disease’s long incubation period. The clinical period is further delayed due to the significant potential for neurogenesis in fish. ^[4] It is thought that the conversion of PrP^c into PrP^Sc is countered, though not outpaced, by the fish brain’s capacity for regenerating a variety of tissues and complete neuronal diversity.

Once the clinical period begins, erratic swimming and behavioural abnormalities will become apparent and grow progressively worse. Death usually follows the onset of hypokinesia, which prevents fish from maintaining the swimming behaviour required to keep pumping water through their gills. Caged fish, as with aquaculture farming sites, have also been known to kill themselves by swimming into walls and nets as a result of ataxia. ^[5] Given the challenging nature of accurately determining anomalous behaviour in individual fish, neuropathological and immunohistochemical examinations of post-mortem brain tissue remain the most reliable diagnosis method of PTAE.

HistoryPermalink

DiscoveryPermalink

PTAE was first described in January 2039 by Sunil Cariappa and Connie Muren. It had been discovered by Cariappa in November 2038 when he linked an increased mortality rate in domestic cats to feeding diets that included Lassgard tuna. When this line of farmed tuna was found to be contaminated with a novel prion agent, the focus shifted to its transmission potential to humans, which led to the discovery of CMD. ^[6]

As a result, research into the origin and pathology of PTAE did not begin in earnest until the World Health Organisation (WHO) was able to conclusively establish a transmission link between PTAE and CMD after internal documentation on Lassgard Bioteknik’s closely guarded production process was leaked to the public in October 2039.

OriginPermalink

On April 19th 2040, the WHO published its official findings on PTAE. Based on the collaborative work of sixteen laboratories from eleven countries and an official investigation with full access to Lassgard Bioteknik facilities, the WHO announced that the most likely origin of PTAE was the company’s practice of using in-house developed processed animal protein (PAP) pellets as a feed stock for its farmed tuna. ^[7]

These pellets, which are less expensive than fish meal and allowed Lassgard Bioteknik’s to fatten up its tuna stock, were rendered from processed waste material of bovine origin, indicating that PTAE is technically a variant of BSE with additional passages in interspecies infection. When prions are passed between species, they take on the characteristics of the recipient species, which had led to the initial description of PTAE instead of a modified strain of BSE.

Immunoassays of Lassgard Bioteknik’s proprietary PAP materials have detected the presence of prion determinants, though molecular data has shown that these determinants do not wholly correspond with bovine recombinant prions. Based on this anomaly, a minority of researchers maintains that PTAE bears more properties of a novel fish amyloidosis than a classical TSE.

EpidemiologyPermalink

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Various spatial epidemiology studies have determined that PTAE’s infectivity rate was closely linked to Lassgard Bioteknik’s handling of its tuna as a product:

• Lassgard Bioteknik maintained high product prices, which led to restaurants and retail outlets processing entire Lassgard tuna, including the heads, in order to cut costs. As a result, processed Lassgard tuna were regularly contaminated with central nervous system tissues, which are more likely to contain a high titre of prions and therefore represent a greater transmission risk to humans. ^[8]
• The same risk also applied to the line of canned tuna which Lassgard Bioteknik began marketing and selling in early 2038. These cans generally contained processed fish meat rendered from the less prized parts of Lassgard tuna, such as the heads. Additionally, prions are not destroyed even if the fish or material containing them is canned, cooked, or refrigerated.
• When Lassgard Bioteknik was unable to remedy the PTAE epizootic that was causing its tuna stock to die off, the company began processing the tuna after they became symptomatic but before they would start exhibiting suicidal behaviour or dying. This ensured that shipped Lassgard tuna were in the clinical stage of PTAE, which generally shows the highest PrP^Sc count before onset of death.

See alsoPermalink

• Cariappa-Muren disease

ReferencesPermalink