By Dr. David R. Smith, professor, MSU College of Veterinary Medicine
The term One Health encompasses the concept that the health of humans, animals, and the environment are inter-related. The current coronavirus pandemic illustrates why One Health is important and what we can learn from the coronaviruses of animals. [https://www.cdc.gov/onehealth/index.html]
Coronaviruses are enveloped RNA viruses. These characteristics mean they are relatively easily to destroy with disinfectants, require relatively close contact to transmit, and that their genetic makeup changes over time. It is believed that the pandemic coronavirus (SARS-CoV-2) mutated from a coronavirus that originally infected animals, probably bats. This is not the first coronavirus to do so, nor will it be the last. The coronavirus diseases of animals have common characteristics that help us understand what is happening with the current pandemic. [https://www.bmj.com/content/368/bmj.m634]
Coronavirus infections are common to many species of animal, including man. However, the various strains of coronavirus are specific to a species of animal. For example, there are coronaviruses of dogs, cats, cattle, ferrets, mink, mice, pigs, poultry and rabbits, but they are specific to each host animal. For example, dogs cannot be infected with the cat coronavirus. Occasionally, however, the genetic makeup of the virus changes enough to move from one host species to another, for example, as recently happened with the pandemic strain of virus.
The coronaviruses infect and reproduce in the cells of the respiratory or digestive tract and this means that infection with the virus may cause respiratory system disease, digestive system disease or a combination of both. Sometimes there are differences in the signs of disease from a particular coronavirus infection. For example, the coronaviruses of cattle cause diarrhea in very young calves, respiratory disease in older calves, and outbreaks of diarrhea in herds of adult cattle. It is not uncommon that the severity of coronavirus diseases is associated with age. For example, some coronaviruses of pigs may cause illness in pigs of all ages, but the disease is particularly severe, often fatal, in very young baby pigs.
Because coronaviruses infect the cells of the respiratory tract and the digestive system, transmission of the virus from one host to another might occur through airborne or fecal exposure to the virus. The coronaviruses are not hardy outside of the animal host, but there are times when environmental contamination aid transmission. Winter dysentery in cattle is more likely to occur in herds settings where animal feed may be contaminated with manure or where barns are not well ventilated [https://pubmed.ncbi.nlm.nih.gov/9706203/]. In 2013, porcine epidemic diarrhea virus was introduced into the U.S. pig population, most likely from contaminated feed that originated from China. That epidemic killed over one million pigs in this country. [https://www.merckvetmanual.com/digestive-system/intestinal-diseases-in-…].
Herd immunity is a population-level phenomenon that occurs when individuals without immunity are protected from infection because many others in the group are immune. Respiratory disease in calves, and winter dysentery in adult cattle, occur as outbreaks when many cattle in the herd no longer have sufficient immunity and herd immunity is lost. [https://pubmed.ncbi.nlm.nih.gov/31590904/]
Vaccines have been developed to stimulate immunity against a few coronavirus diseases of domestic animals and poultry, but the effectiveness of these vaccines is limited. Vaccination against a coronavirus of cats has been associated with an increased occurrence of a severe fatal coronavirus disease called feline infectious peritonitis.
Veterinarians rely on biosecurity and biocontainment strategies to prevent or control coronavirus diseases in domestic animals. These actions include protecting vulnerable groups from exposure to coronavirus and preventing animals from close contact with potentially infected animals [https://pubmed.ncbi.nlm.nih.gov/12064166/][https://pubmed.ncbi.nlm.nih…]. We see these strategies being used in the current coronavirus pandemic in the form of travel restrictions and social distancing.
Coronavirus diseases are widespread among animals and they serve as important zoonotic pathogens of people. Zoonotic diseases are spread from animals to people. Prior to the current coronavirus pandemic there have been other coronaviruses of animals that have changed their genetic makeup to become capable of infecting humans. One of the coronaviruses that causes the common cold, called OC43, mutated from cattle in the late 19th century. Nobody knows for sure, but some researchers have speculated that when this coronavirus first infected people, it may have been responsible for a pandemic in the early 1890’s that killed a million people around the world. More recently, in November of 2002, a coronavirus of civet cats emerged in China as a human pathogen causing the disease of severe acute respiratory syndrome, or SARS. That coronavirus spread to people in 26 countries and caused 774 deaths before disappearing in July 2003. In 2012, a coronavirus of dromedary camels emerged in people as a disease called Middle East respiratory syndrome, or MERS. This disease has spread to 27 countries, has caused over 800 deaths, mostly in Saudi Arabia, and continues to occur. [https://www.cdc.gov/coronavirus/types.html]