The current pandemic of coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is generally considered a zoonotic one (i.e., of animal origin). Little is known, however, about whether it has any animal reservoirs that may sustain infection in humans via direct or indirect contact.
A new study in the journal GeroScience deals with this aspect of the disease, especially with respect to preventing higher death rates in older adults.
Background
The review was conducted in the period ending in April 2021, after which a massive surge in cases has occurred in many parts of the world. The origin of the virus remains shrouded in mystery. However, most researchers agree that it spread to humans from an unknown animal reservoir.
The human SARS-CoV-2 and the strain of a very similar virus isolated from Chinese horseshoe bats showed 96% concordance in their genomes, which strongly suggests that the latter are the primary hosts of the former pathogen. However, the bat virus fails to attach strongly to the human angiotensin-converting enzyme 2 (hACE2) receptor.
Another similar virus isolated from Malayan pangolins shows a significantly greater affinity of binding for these receptors, however. With cases expanding, and newer variants with immune escape capabilities and higher transmissibility emerging in rapid succession, the world does not seem to be at the end of this taxing pandemic period.
The severity of the disease in some patients is among the most problematic aspects of the infection. Though only about 15% of infected patients show severe symptoms, with 5% going on to develop the critical disease, the 1% mortality rate is shockingly high, resulting in over four million deaths so far, when seen against the background of the hundreds of millions of infections occurring worldwide.
Apart from the respiratory distress and multi-organ dysfunction that characterize critical COVID-19, many patients develop long COVID-19, a syndrome that includes persistent fatigue, breathlessness or neurological symptoms lasting for weeks after the virus has been cleared from the body. In some patients, strokes, inflammation of the heart muscle, or lung fibrosis are some of the more severe manifestations.
COVID-19 mortality is especially high among older adults, especially after the age of 65 years. Men are also more vulnerable than women. The especially hard impact of the pandemic on Europe may be partially explained by the larger proportion of older people in many of these countries, especially within nursing homes hit by outbreaks.
What did the study show?
The consensus at present is that this virus arose as a result of recombination between the bat variant and another coronavirus that infects an as-yet-unknown species. However, SARS-CoV-2 has shown itself capable of infecting cats, dogs and other species, though not poultry or pigs.
Some studies have shown that such infections occur in less than a tenth of the total cats or dogs kept as pets, accounting for 13% and 5% of dogs and cats known to be in families with a known COVID-19-positive individual. None of these pets had any respiratory symptoms.
This would suggest that the infection spread from humans to pets and not vice versa. Such a finding is reassuring since, in the USA alone, there are over 135 million pet cats and dogs alone, of which many hundreds of thousands are bound to have been exposed or infected.
People who are suspected or confirmed to be infected with SARS-CoV-2 should therefore minimize close direct contact with animals including companion animals; farm, zoo, or other captive animals; stray animals; and wildlife in order to limit any potential human-animal zoonotic transmission,” the researchers recommend.
This is to spare the animals themselves, as these pets do not shed the virus in quantities that can cause infection in others, animals or humans. However, animals at shelters may be infected, or their fur may potentially carry the virus.
As more people seek to adopt pets, the virus may be able to spread to new human hosts by direct contact with the fomites (like animal fur) or with infected animals themselves. To minimize this, pets should be kept away from sick individuals, and if the owner or caretaker is hospitalized, the pet should be kept at home and cared for rather than taken to a shelter.
This is to prevent potentially infected pets from being kept together at the same shelter and spreading the infection to new households. For similar reasons, older adults should not visit such shelters at present, say the authors. These recommendations agree with those of the US Centers for Disease Control and Prevention (CDC).
There is no obvious reason why older adults should not take their dogs or cats for a walk in the park, however, as even infected animals show low viral shedding. Pets should not have close contact with other animals.
Pet owners should keep from touching them without immediately cleansing their hands. They should probably also not have too much contact with them to avoid getting the virus from them or infecting their pets.
Another problem with pet adoption is that dishonest dealers are procuring pets by shady means without guaranteeing that these animals are healthy and uninfected. These pets may then carry pathogens into the pet shops or shelters, infecting others.
What about other animals?
Farm animals are mostly immune to this infection, including pigs, chickens, ducks and rabbits. However, farmed mink and ferrets are readily infected and develop symptomatic disease, with high viral loads and respiratory symptoms. This is therefore a two-way channel of transmission.
The inter-species jump is associated with the occurrence of adaptive mutations, which may confer immune evasion capabilities. So far, however, this has not been shown to cause a widespread outbreak.
Another area of concern is the use of COVID-19-susceptible laboratory animals such as dogs in experiments involving older scientists, exposing the latter to infection with the virus. Some studies are done on privately owned pets, and the owners of such animals must be trained to reduce the risks of getting the virus, as well as scientists who carry out procedures and observations on laboratory animals.
Mice of the genus Peromyscus have been used on a very large scale in research and can be readily infected by SARS-CoV-2 but without any symptoms. They are superior in many ways to humanized Mus musculus for SARS-CoV-2 infection modeling.
Peromyscus are found in the wild and in the laboratory and thus should be studied to understand the potential for reverse zoonosis. White-tailed deer and non-human primates are also susceptible to the virus and are important subjects for study.
Zoo animals and SARS-CoV-2
Great apes, tigers, lions, and minks have all been reported to be infected, presumably from humans, and some of these zoo animals had to be euthanized. Interestingly, an earlier SARS-like virus, the Middle East respiratory syndrome (MERS)-coronavirus, was identified first in dromedary camels before human infections were detected.
Conversely, SARS-CoV-2 has not been found in a single dromedary despite the fact that it has occurred in many countries with millions of camels. Nonetheless, zoos may be dangerous places for older adults, the frail or the immunocompromised, without proper precautions to avoid zoonotic infections.
What are the conclusions?
The risk posed by SARS-CoV-2 for elderly adults must be a top priority in all preventive strategies, as this is the highest risk group. Warnings against close contact with pets or animals at shelters, in zoos, in the wild, or at farms must be issued, though the risk of all of these as routes of transmission in the general population is low.
Animals may theoretically play a role by either establishing a reservoir for new strains of SARS-CoV-2 and infected companion animals are also potentially able to spread new strains of SARS-CoV-2 to other people and pets in the household,” write the researchers.
This mandates that hand hygiene and face shielding be practiced by those who come in contact with animals.
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