All currently confirmed prion diseases are mammalian:

Transmissible spongiform encephalopathy

https://en.wikipedia.org/wiki/Transmissible_spongiform_encep...

There is possibility that ostriches can have some form spongiform encephalopathy:

Spongiform encephalopathy in a red-necked ostrich (Struthio camelus) (1991)

https://pubmed.ncbi.nlm.nih.gov/1887441/

Luckily, even most mammalian prions have some troubles jumping between mammal species.

For example, sheep scrapie can be transmitted to primates in laboratory, but to date considered non-transmissible to humans in nature (fortunately, as it is insidious and regulations on sheep and goats slaughter are laxer than on bovine):

Transmission of scrapie prions to primate after an extended silent incubation period

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4485159/

"However, one cynomolgus macaque exhibited obvious neurological signs more than 9 years (110 months) after intracerebral exposure to a high dose of a sheep classical scrapie isolate (25 mg of brain)."

BSE prions are much more infectious, not only for humans and ruminants, but for other species too, for example, cats and hamsters.

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Personally, I don't expect fish or insects to be sources of infection, only potential (if relatively unlikely) transmitters.

That said, apparently nearly everything can be transmitter for prions:

* Fish have prion proteins too (currently, no fish prion diseases are known):

Fish models in prion biology: underwater issues

https://pubmed.ncbi.nlm.nih.gov/20933080/

"To date, the occurrence of TSEs in lower vertebrates like fish and birds has received only limited attention, despite the fact that these animals possess bona fide PrPs."

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* Birds can transmit prions in their poop, spreading them at great distances (keep it in mind when clearing bird feces):

Crows don't digest prions, may transport them to other locations

https://www.sciencedaily.com/releases/2012/10/121017181250.h...

"Crows fed on prion-infected brains from mice can transmit these infectious agents in their feces and may play a role in the geographic spread of diseases caused by prions, such as chronic wasting disease or scrapie."

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* Insects and parasites can be prion infection vectors:

Could ectoparasites act as vectors for prion diseases?

https://pubmed.ncbi.nlm.nih.gov/12786866/

"Fly larvae and mites were exposed to brain-infected material and were readily able to transmit scrapie to hamsters. New lines of evidence have confirmed that adult flies are also able to express prion proteins."

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* Plants are acting as prion vectors:

Grass plants bind, retain, uptake and transport infectious prions

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4449294/

"Prions bind to plants and bound-PrPSc efficiently sustain prion replication [...] These results indicate that leaves and roots can efficiently bind PrPSc, which remains able to catalyze PrPC to PrPSc conversion, leading to prion replication. [...] After exposure, plants were extensively washed 5 times with water and animals fed with dried material orally. [...] All animals that ingested prion contaminated leaves and roots developed typical prion disease. Although the incubation times were significantly longer in animals ingesting prions attached to leaves and roots as compared with those fed directly with the brain material, the differences were not as high as one could have expected (Fig. 2A)."

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* Excrements can transmit prions, thus coprophagous animals, insects included, can too:

Transmission and Detection of Prions in Feces

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2803675/

"Oral exposure to prion-tainted blood, urine, saliva and feces have been suggested as modes of transmission for CWD and scrapie among herbivores susceptible to these diseases. Both CWD and scrapie infection is thought likely to enter the body through gut-associated lymphoid tissues, in Peyer's patches in the alimentary tract. Moreover, the presence of the infectious isoform of the prion protein, designated PrPSc, in Peyer's patches suggests alimentary shedding of CWD and scrapie prions into feces. [...]

When noninfected Syrian hamsters were cohabitated with Syrian hamsters orally infected with Sc237 prions, we observed 80–100% infection rates within 14 days after oral challenge."

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* Even dust can transmit prions:

Circulation of prions within dust on a scrapie affected farm

https://pubmed.ncbi.nlm.nih.gov/25889731/

"Using protein misfolding cyclic amplification we demonstrate that scrapie PrP(Sc) can be detected within circulating dusts that are present on a farm that is naturally contaminated with sheep scrapie."

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* Even ashes from not sufficiently hot cremation can transmit prions:

Infectivity studies of both ash and air emissions from simulated incineration of scrapie-contaminated tissues

https://pubmed.ncbi.nlm.nih.gov/15575075/

"We investigated the effectiveness of 15 min exposures to 600 and 1000 degrees C in continuous flow normal and starved-air incineration-like conditions to inactivate samples [...] yielded a total of two transmissions among 21 inoculated animals from the ash of a single specimen burned in normal air at 600 degrees C."

I wonder, how bad is prion contamination of the Ganges river in India?