The Silent Pandemics

Factory farms are breeding grounds for novel pathogens with high destructive capability. [Photo credit]

COVID-19 is not the only pandemic in the world today. At present there are dozens of active panzootics — animal pandemics — running rampant in farms around the world. Nowhere is this more prevalent than in farmed aquatic animal populations, who are not provided with the biosecurity measures deemed essential for preventing the emergence of novel diseases in other farming systems.

The global animal farming industry is fighting emergent diseases on many fronts. One-quarter of all the world’s pigs were culled in 2019 order to prevent the spread of African Swine Fever in Asia. The panzootic is ongoing, with the World Organisation for Animal Health releasing biweekly reports on the spread of the virus, which had spread to 193 new farms in Europe and 34 new farms in Asia in the two week period commencing 2020–10–02. [1]

Bird flu is another constant feature of 21st Century headlines. In February this year, H5N8 was identified on a chicken farm in the Central Sudair region of Saudi Arabia. Dropped from the faeces of wild birds overhead, the disease spread rapidly in the ultra-dense environment of the factory farm, causing the Saudi Arabian Agricultural Ministry to kill all 385,000 chickens, even the healthy ones, to prevent it from spreading to other farms.[2] In January, 13,000 turkeys were slaughtered in Poland following an H5N8 outbreak. In March, 32,000 ducks were culled in a farm in South Hungary. This month, an H5N8 outbreak was identified in the Chelyabinsk region of Russia, and a cull will almost certainly follow.

These pigs and chickens, who usually live in family groups of no more than a dozen, are farmed in sheds with tens of thousands of others. All the animals are selectively bred to reduce diversity, and this potent cocktail of density and genetic uniformity allows diseases to spread and mutate rapidly.

Each significant human pandemic in the past forty years has been of animal origin.

SARS, MERS, HIV, H1N1, and COVID-19 all developed in a wild animal population, then entered into human circulation via the food chain. It is widely acknowledged that maintaining a biosecure firewall between farmed animals and their wild counterparts is essential to prevent the transmission of these diseases, which easily result in catastrophic stock loss, and risk producing a new pathogen into the global farm population.

But for aquatic animals, even these well-established conventions are ignored. Aquatic animals are routinely packed in densities far greater than terrestrial animals, and are routinely exposed to wild animals. Sea water freely flows into aquaculture facilities, bringing wild pathogens in on the tide. Aquaculture facilities rapidly produce novel panzootics as a result.

Six new diseases have been identified in farmed shrimp since 1990. [Photo credit]

The most pronounced example of poor biosecurity leading to rapid production of novel diseases is shrimp farming, whose economic model depends on the constant introduction of wild animals into the farmed environment. Six novel shrimp diseases have emerged since 1990. Each has spread around the world, becoming endemic industry-limiting obstacles for shrimp farmers.

Yellowhead virus first emerged in a shrimp farm in the inner gulf of Thailand in 1990 and proved capable of causing 100% mortality in a population within 3–5 days of the first animal showing symptoms. The Thai shrimp farming industry collapsed within the year, with 90% of all farms in the gulf closing as they could not control the virus. [3] Mid Mortality Crop Syndrome is a viral infection which emerged in 1993, and regularly causes 80% stock death in prawn farms. [4] The White Spot Syndrome virus emerged in 1992 and is now endemic in shrimp populations on every continent. There is no effective cure, and current practice is to cull the whole pond following a single confirmed case of WSS. It is estimated to have wiped 15 billion dollars off the value of the shrimp industry since it emerged.

Shrimp aquaculture has undergone a meteoric rise over the past twenty years, with five times as many prawns farmed globally today than at the turn of the millennium.

Established broodstocks, which screen their animals for disease, cannot breed fast enough to keep up with the increasing demand for farmed shrimp, meaning juveniles are caught in the wild and brought into the farm to be fattened up. Between 25–35% of all farmed shrimp were born in the wild, and carry any type of disease with them into the farm.

To match demand, up to 35% of all farmed shrimp are caught at sea, and fattened up on-farm, bringing their diseases with them. [Photo credit]

Additionally, live animals caught at sea are regularly introduced to the pond as food. Squid, clams, and brine shrimps are all fed live to the farmed shrimp. An ALI estimate puts the number of Antarctic krill caught to be used as live feed at 113 billion annually. [5] Krill are themselves small Decapoda, who may themselves be either infected with a disease that can transfer to prawns, or may be carrying infected biomatter in their intestinal tracts.

Viruses aren’t the only area of concern. While data on antibiotic use in aquaculture is limited, a 2003 study found that 74% of Thai shrimp farms routinely applied blanket antibiotic treatments to grow-out ponds. [6] Aquaculture farmers have no method for tracking animals in a tank, meaning they do not have individualised health plans for the animals. This means that drugs in aquaculture systems aren’t given to sick prawns, and are rather applied en masse to the whole population. This swiftly leads to antibiotic-resistant strains of bacteria, such as AHPND, which emerged in China in 2009. The bacteria which causes the disease, V. parahaemolyticus, has immunity to multiple antibiotics with human applications. [7] It can survive for three weeks in seawater, and there is no effective treatment for an infected population. This disease continues to cause 100% stock loss on farms around the world. [8]

Intensive farming practices virtually guarantee the emergence of antibiotic-resistant strains of bacteria over time. [Photo credit]

Commercial aquaculture is fighting the hydra of disease with all the wrong weapons.

Blanket medicinal treatments, the absence of biosecurity between the farmed and wild reservoirs, and the regular introduction of wild animals into the farm ecosystem means that, even if it managed to chop off a pathogenic, two more would inevitably grow in its place. Aquaculture enjoys a mortality rate far higher than would ever be permitted in terrestrial aquaculture. The 2017 Norwegian Aquaculture Analysis reported a mortality rate of 16%, and states in its executive summary that “53 million salmon [die annually] inside the cage”. [9] This means that more than twice as many salmon die ‘in the cage’ in Norway every year than there are dairy cows currently alive in the entirety of Europe. (n=22.6 million)

Commercial fish farming is becoming more and more marginal as these diseases limit the industry’s potential for growth. The high price and growing consumer demand for fish products allow these unsustainable practices to continue, but only by externalising the hidden costs of growing microbial resistance and rampant animal suffering.

The Coronavirus pandemic has taken place against a backdrop of numerous animal pandemics, each with significant human causal factors. Many of these happen underwater, out of sight for all but the most conscientious of consumers. The sheer scale of the neglectedness of aquatic animal welfare may make it hard for the public to visualize how the health and resilience of animals in aquatic farm systems is intertwined with the economic health, public resilience, and security of human society.

This journey must begin with a public recognition that fish, crustaceans, and other aquatic animals are individuals capable of suffering, and their health and wellbeing should be regulated with the same rigor and sense of importance as any of their terrestrial counterparts.

Originally published on the Aquatic Life Institute blog.



[3] Patmasiriwat, Direk, Onno Kuik, and Sunil Pednekar. The shrimp aquaculture sector in Thailand: a review of economic, environmental and trade Issues. CREED, 1998.

[4] Cullen, B.R., and Owens, L. (2004) Mid-crop mortality syndrome in Australian prawn farming: a case study. In: Papers from Styli 2003: thirty years of shrimp farming in New Caledonia (38) pp. 223–228. From: Styli 2003: thirty years of shrimp farming in New Caledonia, 2–6 June 2003, New Caledonia.

[5] Borthwick et. al., (2021) “Blue Loss, Estimating How Many Aquatic Animals are Hidden in the Food System” Aquatic Life Institute, March 2021. Available at:

[6] Holmström, Katrin, et al. “Antibiotic use in shrimp farming and implications for environmental impacts and human health.” International journal of food science & technology 38.3 (2003): 255–266.

[7] Tan, C. W., Rukayadi, Y., Hasan, H., Thung, T. Y., Lee, E., Rollon, W. D., … & Radu, S. (2020). Prevalence and antibiotic resistance patterns of Vibrio parahaemolyticus isolated from different types of seafood in Selangor, Malaysia. Saudi Journal of Biological Sciences, 27(6), 1602–1608.

[8] Santos, H. M., Tsai, C. Y., Maquiling, K. R. A., Tayo, L. L., Mariatulqabtiah, A. R., Lee, C. W., & Chuang, K. P. (2020). Diagnosis and potential treatments for acute hepatopancreatic necrosis disease (AHPND): a review. Aquaculture International, 28(1), 169–185.

[9] EYGM. “The Norwegian Aquaculture Analysis 2017” (2017)



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Mark Borthwick

Mark Borthwick

Traditional storyteller, animal ethicist, and effective altruist based in the Lake District, England. @MDBorthwick