The current pandemic caused by COV-SARS-2 is the result of the transmission of a virus from animals to humans. Today, we know the root cause was a virus found in bats. We also know that before reaching us it landed and mutated in one other species. This intermediate species remains unidentified.
The ability that some viruses have to jump amongst species is concerning because it makes us potentially vulnerable. This is why, the contact that we have with wild and domestic animals has become a source of interest for many, as we can see by the amount of articles published about this subject (here, here and here a few examples)
Figure 1: A wet market in Asia: lack of health controls, poor hygiene and placing different animals in close contact are the main causes of epidemics.
Some are pointing fingers to intensive animal production and using this pandemic to move forward their anti-farming agenda by stating that the root cause of this and other epidemics is to be found in industrial farming. According to their rationale, intensive farming is a dangerous activity, full of risks for our health, so it must be much more controlled or directly banned.
This is why we must look at the facts and see whether industrial farming has really been connected to some of the epidemics that mankind has suffered in the last decades. Besides, we will also see what is the inherent risk –if any- that farming has in the potential jump of harmful microorganisms from animals to us.
To start, we need to define intensive farming. We could say that it is a form of animal husbandry in which production benefits from economies of scale due to the high output of those operations. This means that cutting-edge technologies are used (in genetics, nutrition and veterinary care) in order to maximize the output. Those operations are highly mechanized in order to reduce labor force.
Industrial farming was born in the US and the UK in the 50’s. The synthesis and manufacturing of vitamins (especially vitamin D) allowed the administration of these compounds all year round and avoided seasonal variances.
Moreover, the development of vaccines and veterinary treatments made it possible to produce a high number of animals indoors in a way that they did not develop diseases. It also allowed to reduce costs and, as a consequence, milk, eggs and meat became much more affordable.
Today, this production system is the most common, although we should not forget that, in many regions, especially in developing countries, family farms and backyard production are very common and an important source of animal protein for a big part of the population.
Figure 2: An intensive poultry farm.
Now, we will see what impact –if any- intensive farming has had on the origin and the dissemination of epidemics. We need to understand, first, what are the risk factors that make it easier for a pathogen to jump from an animal species to humans. A recent paper from California and Melbourne Universities explains this point (the work is focused on viral transmission, but its implications are valid for all microorganisms). According to this publication, there are three conditions that a virus must accomplish in order to have better chances to jump and get to us:
- The animal must be abundant and should have a relative frequent contact with humans.
- That animal order must have many different species.
- That species must carry many potentially dangerous viruses.
Stating the obvious, we could conclude that if an animal carries many pathogens, but the species itself is close to extinction, then, there will not be many chances that this particular species will contact with humans and that the viruses it may carry make the jump.
The paper points also out that, if this order of animals presents many species, and these species carry many potentially dangerous viruses, then, the likelihood of this virus getting into humans is considerably higher.
We have that rodents, quiropters (bats) and primates present many different species and many of them do carry potentially harmful germs. Then, we have to be especially careful with these animals.
Figure 3: Risk of a virus jumping amongst species: On the left, species according to number of virus they carry and number of species in every order of those animals. On the right, the number of viruses they carry vs. their global population.
Domestic animals do not carry a high number of potentially dangerous viruses. However, there are many domestic animals. Their sheer number together with the fact that they live close to us, could potentially increase the risk of getting diseases from them.
So, let’s see how risky industrial farming is. We see below all the epidemics with more than a thousand human fatalities since 1900.
Figure 4: Epidemics with more than 1000 fatalities since 1900 (click to enlarge)
Industrial farming started in a few countries in the 50’s. Then and today, there are still many countries in which this type of production is not the most common, with important numbers of animals being still raised in house backyards.
We can see, in the slide above that, since 1950, there have been five epidemics in which domestic animals have been involved. Below are the impact and implications of each one of them:
- 1957-1958 Asian flu: It caused 1-4 million deaths. The virus was the result of a combination of human and avian strains. Back then, industrial farming was not present in Asia. Conversely, the presence of birds in living rooms and apartments was common.
- 1958-1959 Hong-Kong flu: Its death toll ranged from 0.5-2 million people. Here, humans strains mixed with avian and swine virus Again, industrial farming cannot be blamed for the outbreak of the epidemic, as no big farms were present in the region at that time.
- 2009-2010 Global flu: It caused between 0.1 and 0.6 million casualties. It started in Mexico and the virus seems to have been a combination of strains from human and swine origin. As of today, there is not as a full agreement whether the outbreak was caused by imported hogs which were not properly quarantined, or whether a vaccine strain was not properly inactivated and then was used in some pig herds.
- 2015 Indian flu: With 1,200 deaths, it is not proven that this epidemic had its origin in animals, although it is possible due to the close contact that many people in this region have with ducks and other birds. High density of population in crowded cities seems to have had an influence too.
- 2017 Japanese encephalitis: It caused more than 1,300 deaths. This virus is transmitted by mosquitoes. Many animals carry the virus from horses to birds. Among domestic animals, pigs also play a role. There is a vaccine, which has been administered to farmed hogs for decades. Based on this information it is not very likely that industrial farming played a role in this outbreak. However, the role of backyard pigs cannot be discarded.
What we see here is that, since industrial farming started 70 years ago, there has only been one epidemic episode, in 2009, in which big farms may have been somehow involved, although experts do not agree on this particular point.
We are now to look at what diseases have been controlled or eradicated thanks to intensive farming. As big farms become the norm in industrialized countries, the total number of farms is trending down. In these conditions, in-farm veterinary control becomes possible and it is much more efficient than when every house had a few heads of livestock.
If we take avian influenza as an example, we have seen that this virus can jump from birds to humans and even originate pandemics. In the US, the EU and in most poultry producing countries, this disease is closely monitored with frequent analytics at avian farms. Very strict bio-protection measures are taken in all production units in order to avoid the arrival of this pathogen. Here a few examples:
- Limiting visitors on the farm and minimizing foot traffic.
- Avoiding contact with wild and domestic fowl.
- Avoiding the sharing of farm equipment.
- Having a clean and functioning footbath at each entrance to the broiler house.
- Ensuring that all visitors or personnel have disinfected or new footwear before entering a house or facility.
- Making sure feed and water sources are covered and free of contaminants, limiting the attraction of wild fowl and pests.
- Having official signage clearly stating the farm is a biosecure zone and any unauthorized entry is strictly prohibited.
- Employing effective pest and wild bird management practices.
- Adequately training farmers, farm and company personnel in biosecurity and disease prevention.
All these measures are enforced thanks to the presence of many, well-trained veterinarians. Conversely, control becomes impossible and birds become a likely source of epidemics in countries where they live within houses, frequently in close contact with other species, and when contact with wild birds is likely to happen.
Figure 5: Avian influenza virus
Biosecurity measures, enforced in all intensive farms, make it extremely unlikely that an outbreak disseminates. This is a very practical and clear example of effective disease control that industrial farming allows us to do.
Producers today make huge efforts to eradicate diseases in animals. Thanks to them and to veterinary services, there are many diseases that have been eradicated from US herds, such as: brucellosis or tuberculosis. These diseases are however still present in the wildlife.
Figure 6: Disinfection of a truck before entering a farm.
There is one more reason why intensive farming protects us all against epidemics: The products that it delivers are heavily scrutinized and must get over many health controls. Besides, industrial farming bring prices of bioavailable, rich protein down.
Conversely, when access to animal products is limited is when people resort to hunting in order to get the protein they need. That is where close contact with wild species can cause the spreading of new viruses. This is how AIDS virus passed from monkeys to people, due to the close contact of humans with the fluids of primates after butchering them. It is also likely that hunting bats has caused today’s coronavirus pandemic.
Figure 7: Empty swine farm after the outbreak of African swine fever in China.
I would like to finish with a hypothesis: Recently, the virus of the African swine fever has caused havoc across Asia. This disease, harmless to people, has destroyed a big portion of the Asian pig population. I think it is likely that the deaths of millions of hogs have pushed some small farmers to find their meat in wild animals, which happen to carry coronavirus.
It is a possibility we cannot discard. In any case, it is clear that controlled, veterinary monitored animal production has virtually no connection to epidemics, quite the opposite, it is a safety net that will help us to avoid new epidemics in the future.
Bibliography:
- Kilbourne ED. Influenza pandemics of the 20th century. Emerg Infect Dis. 2006 Jan;12(1):9-14. doi: 10.3201/eid1201.051254. PMID: 16494710; PMCID: PMC3291411.
- The Mount Sinai Hospital / Mount Sinai School of Medicine. “2009 swine flu pandemic originated in Mexico, researchers discover.” ScienceDaily. ScienceDaily, 27 June 2016. <www.sciencedaily.com/releases/2016/06/160627160935.htm>.
- Jones, S., Nelson-Sathi, S., Wang, Y. et al.Evolutionary, genetic, structural characterization and its functional implications for the influenza A (H1N1) infection outbreak in India from 2009 to 2017. Sci Rep 9, 14690 (2019). https://doi.org/10.1038/s41598-019-51097-w
- Johnson CK, Hitchens PL, Pandit PS, Rushmore J, Evans TS, Young CCW, Doyle MM. 2020 Global shifts in mammalian population trends reveal key predictors of virus spillover risk. Proc. R. Soc. B 287: 20192736. http://dx.doi.org/10.1098/rspb.2019.2736
- Kilbourne ED. Influenza pandemics of the 20th century. Emerg Infect Dis. 2006 Jan;12(1):9-14. doi: 10.3201/eid1201.051254. PMID: 16494710; PMCID: PMC3291411.
- Saunders-Hastings PR, Krewski D. Reviewing the History of Pandemic Influenza: Understanding Patterns of Emergence and Transmission. Pathogens. 2016 Dec 6;5(4):66. doi: 10.3390/pathogens5040066. PMID: 27929449; PMCID: PMC5198166.
- Gibbs, A.J., Armstrong, J.S. & Downie, J.C. From where did the 2009 ‘swine-origin’ influenza A virus (H1N1) emerge?. Virol J6, 207 (2009). https://doi.org/10.1186/1743-422X-6-207
- Mena I, Nelson MI, Quezada-Monroy F, Dutta J, Cortes-Fernández R, Lara-Puente JH, Castro-Peralta F, Cunha LF, Trovão NS, Lozano-Dubernard B, Rambaut A, van Bakel H, García-Sastre A. Origins of the 2009 H1N1 influenza pandemic in swine in Mexico. Elife. 2016 Jun 28;5:e16777. doi: 10.7554/eLife.16777. PMID: 27350259; PMCID: PMC4957980.
- Sara Jones et al. Evolutionary, genetic, structural characterization and its functional implications for the infuenza A (H1N1) infection outbreak in India from 2009 to 2017. Nature scientific reports. (2019) 9:14690 | https://doi.org/10.1038/s41598-019-51097-w
- Karen L.Mansfield et al. Japanese encephalitis virus infection, diagnosis and control in domestic animals. Veterinary Microbiology. Volume 201, March 2017, Pages 85-92
- Kayed AS, Kandeil A, Gomaa MR, et al. Surveillance for avian influenza viruses in wild birds at live bird markets, Egypt, 2014‐2016. Influenza Other Respi Viruses. 2019;00:1–8. https://doi.org/10.1111/irv.12634
- Alders, R., Awuni, J.A., Bagnol, B. et al.Impact of Avian Influenza on Village Poultry Production Globally. EcoHealth 11, 63–72 (2014). https://doi.org/10.1007/s10393-013-0867-x
- Robertson et al. Disease Control, Prevention and On-Farm Biosecurity: The Role of Veterinary Epidemiology. Engineering. Volume 6, Issue 1, February 2020, Pages 20-25