Italian researchers have in a new study found that cetaceans which include aquatic mammals such as whales, dolphins, and porpoises could also become infected with the SARS-CoV-2 virus and could also end up as potential viral reservoirs.
The study findings were published in the peer reviewed journal: Pathogens.
Transmission of human pathogens to non-human animals, including wildlife, occurs more regularly than often thought [18,19]. SARS-CoV-2 appears to have a striking ability to infect a broad range of distantly related mammals. Due to its high transmissibility and prevalence, the virus may spread to susceptible, wild non-human mammal populations.
As we enter the third year of the pandemic, SARS-CoV-2 infection continues its global spread. Multiple factors are driving its transmission. As the virus continues to evolve, the emergence of variants poses new challenges to public health.
SARS-CoV-2 has been demonstrated in the wastewater and rivers of countries with high COVID-19 caseloads [20,21]. The coastal ocean is the ultimate sink for urban sewage. The risk of SARS-CoV-2 transmission in the receiving coastal water bodies should not be underestimated, though its stability in water is lower than that of other known non-
enveloped human enteric viruses with waterborne transmission [20,22]. Regarding the marine environment, SARS-CoV-2 will suffer an aggressive treatment because of UV radiation and heat; salinity and PH negatively affect the viral vitality and viability in the marine environment; by contrast plastic and organic material influence positively the viral persistence in the environment . Virus concentration reduces rapidly at high temperatures but can persist long in cold waters. At high titer it may survive more than 20 days at 4 ℃ and for 7 days at 22 ℃ .
Viral concentration decreases rapidly at high temperatures but its survival in cold waters remains a threat for marine mammals.
Viruses in raw wastewater are not readily removed by treatment and thus become environmental pollutants. Although the ocean provides for rapid dilution of sewage, its self-depuration capacity is finite, especially in coastal areas.
Marine water may become a conduit for zoonotic transmission of SARS-CoV-2 to marine wildlife at a virus concentration above the infection dose level. The range of infectious doses for direct seawater contact is unknown and the degree of exposure is difficult to estimate; nonetheless, a potential impact assessment of virus pollution in coastal marine waters is warranted.
In addition, as the Mediterranean is a “closed sea basin”, a “concentrating activity” towards chemical pollutants as well as towards infectious pathogens is possible.
In fact, microplastic pollution is one of the emerging threats across the globe and is becoming a topic of intense study for environmental researchers and the Mediterranean Sea has been recognized as a target hotspot of the world. 
As recently described in literature pathogens are capable of associating with microplastics in contaminated seawater, with more parasites adhering to microfiber surfaces as compared with microbeads. Given the global presence of microplastic, this could be a novel pathway by which anthropogenic pollutants may be mediating pathogen transmission in the marine environment, with important ramifications for wildlife and human health .
In the ongoing COVID-19 pandemic, direct and indirect exposure occurs when animals share the same space with infected humans. Positive molecular findings in animals cohabiting with COVID-19-positive humans (e.g., pets, farmed mink, big cats, and gorillas in zoos , have raised concern about the role animals may play in amplifying and spreading the virus and establishing reservoirs in the vicinity of humans. Animal infection studies published so far suggest that SARS-CoV-2 efficiently replicates in ferrets, cats , and rabbits  but poorly in dogs, pigs , and cattle . Genome analysis strongly suggests that zoonotic spillover of SARS-CoV-2 from farmed mink to humans occurred in the Netherlands and Poland [29,30]. Researchers at the Centro de Pesquisa e Conservação de Mamíferos Aquáticos do Instituto Chico Mendes de Biodiversidade (ICMBio/CMA) in Brazil found that Antillean manatees can be infected with SARS-CoV-2 and confirmed the first case of COVID-19 in a Sirenia species .
Current knowledge indicates that wildlife does not play a significant role in the transmission of SARS-CoV-2 to humans, but its spread in animal populations can affect their health status and facilitate the emergence of new virus variants. In addition to domestic animals, free-ranging, captive, or farmed wild animals (e.g., big cats, minks, ferrets, North American white-tailed deer, and great apes) have been found positive for SARS-CoV-2 infection . To date, farmed mink and pet hamsters have been shown capable of infecting humans with the SARS-CoV-2 virus. The introduction of SARS-CoV- 2 to wildlife could ensue in the establishment of animal reservoirs.
As in terrestrial species, emerging viruses in marine mammals may be associated with other diseases such as neoplasia, epizootics, and zoonotic disease, involving a complex pathogenesis with noninfectious cofactors such as anthropogenic toxins, biotoxins, immunologic suppression, and other environmental stressors.
Surveillance and assessment for SARS-CoV-2 in marine environments should be activated to eliminate COVID-19; caution is needed to prevent SARS-CoV-2 transmission
to threatened species and aquatic populations in the vicinity of human activities. The Food and Agriculture Organization (FAO), the World Organization for Animal Health (OIE), and the World Health Organization (WHO) have called on countries to take necessary steps to reduce the risk of SARS-CoV-2 transmission between humans and wildlife, to reduce the risk of new variants, and to protect human and wildlife health.
To date, despite the presumed susceptibility of marine mammals to SARS-CoV-2 [1,6,7,32], infection of wild marine mammals had not yet been confirmed. Due to many cetacean species being classified as critically endangered and exposed to escalating anthropogenic stressors, we, as a National Reference Centre for Diagnostic Investigations on Stranded Marine Mammals, decided to increase the surveillance of cetaceans and screen cetaceans stranded on the Italian coast for SARS-CoV-2 infection during the COVID-19 pandemic.
Our study underlines the importance of monitoring marine mammal populations for SARS-CoV-2 infection. Through the collaboration of the network of Istituti Zooprofilattici Sperimentali, we are carrying out a monitoring activity on marine mammals stranded on the Italian coast which are known to be potentially susceptible to SARS-CoV-2. Such measures are intended to protect the health of animals and humans.
In addition, the number of animal cases of SARS-CoV-2 infection, albeit often occasional, continues to rise.
Within this context, white-tailed deer in the United State (the Eastern States and Iowa) have attracted attention after several human SARS-CoV-2 “variants of concern” and “variants of interest” were identified in the deer. According to the OIE Report SARS-COV-2 in Animals of 30.04.2022, 675 outbreaks in animals have been reported globally, affecting 23 species in 36 countries (https://www.woah.org/app/uploads/2022/05/sars-cov-2-situation-report-12.pdf; accessed on 30 April 2022).
With a virus so competent at spilling over into a range of phylogenetically unrelated species, the fear is that—though the pandemic is presently under control in human populations—the virus could remain in an animal population (especially in the wild), ready to spill back into humans and trigger a new epidemic cycle. In this regard, the suspected origin of SARS-CoV-2 from a primary animal reservoir (most likely a Rinolophus spp. bat), followed or not by an intermediate (and hitherto unknown) host species, should be kept in mind, because as in other agents responsible for “emerging infectious diseases”, the proven or suspect origin is the animal kingdom in at least 70% of them .
Another aim of the present study was to evaluate the expression and the effect of age, sex, and species on the expression of the viral host cell receptor ACE2 in the lung tissue of S. coeruleoalba and T. truncatus. Previous studies investigating ACE2 receptor expression in animal species hypothesized the susceptibility of some species to SARS-CoV-2 [7,32].
Univariate analysis of ACE2 receptor expression and origin of cetacean specimens (p = 0.054) suggested an effect on receptor expression. To assess the direction of this effect, we performed regression analysis but found no statistically significant association for ACE2 protein expression, although captive individuals appeared to express ACE2 more often than wild individuals; the small study sample size might explain the lack of statistical significance despite this apparent difference. Further investigation on more tissue samples is therefore warranted.
Neutrophil granulocytes and macrophages play a key role in inflammatory and immune response in terrestrial and aquatic mammals. We noted in the cetacean lung tissues a peculiar mode of compartmentalization of macrophages that suggested different functional specialization. The macrophage cytotype, the so-called pulmonary intravascular macrophage (PIM), probably involved in the uptake and subsequent phagocytosis of foreign elements (particulate matter of physical or biological origin) is carried by the blood. It plays a role complementary to the other macrophage cytotype residing in the lung, namely, the alveolar macrophage .
As revealed by IF, ACE2/CD68 co-localization was higher when an infectious agent was present and was probably on activated type 1 macrophages (M1). These findings indicate that in cetaceans alveolar macrophages and PIM, as frontline immune cells, may be directly targeted in the course of SARS-CoV-2 infection .
Statistical analysis showed a significant difference in ACE2 receptor expression between wild and captive individuals. Although this finding needs further analysis in a larger sample, one possible explanation is the level of chronic stress to which captive specimens are exposed, which could lead to higher cortisol production in these animals. Given the anti-inflammatory action of cortisol, this could have implications for the production of ACE2 (which has an antiphlogistic action). Moreover, the frequent use of drugs in captive animals including anti-inflammatory drugs could be a valuable explanation.
The differential susceptibility of domestic and wild animal species to SARS-CoV-2 infection is primarily driven by their homology with human ACE2 and the viral receptor’s region directly interacting with the viral spike (S) protein receptor-binding domain (RBD). The homology with other SARS-CoV-2 host cell receptors, such as neuropilin 1 (NP1)  could be an area of focus for further studies.