To better understand the challenges associated with the COVID-19 disease in humans, poultry health professionals can draw on many years of experience attempting to control avian coronavirus infectious bronchitis virus in poultry.
It is important to emphasize the COVID-19 virus is not associated with poultry or poultry products. Coronaviruses are divided into Alpha-, Beta-, Gamma- and Delta- coronavirus groups. Coronaviruses are responsible for a wide variety of existing and emerging diseases in humans and other mammals including food animals, as well as in birds including poultry.
Diseases associated with coronavirus infections cover a wide range including respiratory, enteric, neurological, renal and hepatic. The coronavirus IBV affects poultry, causing respiratory disease in chickens and is in the avian Gammacoronavirus group. Avian viruses in the Gammacoronavirus group do not infect or cause disease in humans.
The COVID-19 virus is in the Betacoronavirus group, along with SARS-CoV and MERS-CoV. It was previously shown that SARS-CoV does not infect or cause disease in poultry. Because the COVID-19 virus belongs to the same group as SARS-CoV and uses the same ACE-2 host cell receptor, it is highly unlikely that the COVID-19 virus will infect or cause disease in poultry, but it remains to be scientifically proven.
Based on knowledge at hand, and current lack of evidence of bird infections with the COVID- 19 virus, poultry and poultry products are not considered to be a source of COVID-19 infection for humans. The COVID-19 virus is primarily spread among people via respiratory droplets that contain the virus, with infections occurring via the nose, eyes and mouth.
Though highly infectious, it is an enveloped virus — one that is easily killed by soap and common disinfectants. Where do coronaviruses come from? Bats are widely accepted as the reservoir for mammalian (Alpha- and Beta-) coronaviruses. There are about 1240 different bat species harbouring as many or more different coronavirus types.
SARS-CoV and MERS-CoV came from a bat reservoir, infected an intermediate host then jumped to humans. It is likely that the COVID-19 virus originated from bats. Furthermore, preliminary data show some viruses isolated from bats to be close relatives. An intermediate host for the COVID-19 virus has not been identified yet.
The reservoir for avian coronaviruses, including IBV, is not clear. There are some closely related viruses in wild and domestic birds — pheasants, ducks, geese and pigeons, to name a few — but unequivocal evidence of a true reservoir is lacking. Why is it difficult to produce a vaccine against coronaviruses?
Protective immunity against a respiratory disease, such as infectious bronchitis virus in poultry or COVID-19 in humans, requires a strong local immune response.
In poultry, we achieve this by using live attenuated vaccines, but live coronavirus vaccines are difficult to produce because attenuation often renders them unable to produce a strong local immune response. Attenuation is accomplished by passage of the virus in a laboratory host system, for example embryonating chicken eggs or cell culture, but there is a fine line between attenuation and maintaining the virus’ ability to infect and induce an immune response.
Over attenuation renders the vaccine safe but not immunogenic, whereas under attenuation will create a vaccine capable of inducing a strong immune response but may cause a severe vaccine reaction. Then there is the problem of back passage of the vaccine in the host leading to a pathogenic virus.
Based on our knowledge of producing vaccines against IBV, production of a live-attenuated vaccine against COVID-19 with acceptable safety and efficacy may prove to be very difficult.
What about using killed vaccines against coronaviruses? There are killed or inactivated vaccines against IBV for poultry, however they are difficult to develop.
Chemicals such as formalin or beta-Propiolactone used to kill the virus can destroy the integrity of the spikes, thus yielding a vaccine that does not induce a protective immune response.
In addition, for killed vaccines to be effective, they must be given after a live-attenuated ‘priming’ vaccine, which as discussed above has significant safety issues. In humans, killed vaccines against respiratory viruses are used, for example against influenza virus, but this requires growing the virus to high titers, inactivating it and using safe adjuvants.
This has proved difficult to achieve for human coronaviruses. Why don’t we have recombinant vaccines against IBV in poultry? And could a recombinant vaccine be developed for the COVID-19 virus? The coronavirus surface-spike glycoproteins are embedded in a lipid envelope and have conformationally dependent epitopes that induce neutralizing antibodies in the host.
When the spike protein is removed from the virus envelope or when it is expressed in a laboratory system, those conformationally dependent epitopes are not faithfully reproduced. Thus, vectors such as fowl pox and herpesvirus of turkeys have not been suitable vaccine platforms for expressing coronavirus spikes.
Spike protein production by the virus in a natural infection is very specific and difficult to mimic, thus other recombinant vaccines, such as DNA vaccines, RNA vaccines and subunit vaccines, do not accurately reproduce spike. Additionally, these vaccines usually do not stimulate adequate local immunity and have to be given many times to provide any protection.
Then there are genetically altered coronavirus vaccines derived from infectious clones. Making changes to the genome of the pathogenic virus to create a safe, attenuated live coronavirus vaccine that is still capable of inducing an effective immune response is complex and often results in non-viable viruses or insufficient protection.
It can be a fast track to finding a vaccine candidate, but the safety of these live vaccines must be rigorously tested. Fortunately, it is not all bad news. The immune system of a bird is very different from that of a human. What doesn’t work in poultry may actually work well in humans. In addition, financially we can do a lot more in development and delivery of vaccines in humans than in poultry, where tight margins make it necessary for vaccines to improve the welfare of the flock while still yielding a satisfactory return on investment.
The goals and performance parameters for human vaccines are obviously much different than production agriculture. Currently, there are many different coronavirus vaccines and platforms being developed or optimized for human use. How do different serotypes or genetic types of coronavirus complicate vaccine development?
In poultry, there are many types – serotypes or genetic – of IBV that do not cross protect. Consequently, recovery from one type does not immunise the bird against another type. For this reason, we have had to develop a number of different IBV vaccines, such as Ark, Mass, Conn, DE, etc., to control the disease. Fortunately, there appears to be only one type of COVID-19 virus circulating in humans.
However, full genome sequencing has shown that the virus is changing. A number of mutations have been observed, but none appear to be maintained at this time, which suggests they are not important for transmission or virulence. From a vaccine standpoint, only one type of COVID-19 virus circulating in humans means that only one vaccine type should be needed to protect against this disease.
Are there treatments for coronaviruses? For humans, we have antiviral drugs for influenza, but there have not been any successful drugs developed specifically for coronaviruses. Drugs against the viral-encoded proteases have been tried, as well as drugs that interfere with entry and egress of the virus from the host cell.
These and many other potential antivirals are presently being tested by several companies. Two drugs currently in the news, chloroquine and hydroxychloroquine are being examined for their ability to ameliorate COVID-19 infections. These drugs have been used against malaria, lupus and rheumatoid arthritis for many years.
Preliminary data out of China indicate that the drugs stop the spread of the COVID-19 virus in cell culture and are somewhat effective in treating humans.
However, until controlled clinical trials are conducted, their effectiveness against COVID-19 remains in question. It is likely that this pandemic will not be over anytime soon.
In the meantime, follow government and World Health Organization recommendations to protect yourself and your family. Poultry flocks do not appear to be at risk.
Mark W. Jackwood, MS, Ph.D. Department of Population Health, Poultry Diagnostic and Research Centre, College of Veterinary Medicine, University of Georgia