Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, previously 2019-nCoV) is suspected of having originated in 2019 in China from a coronavirus infected bat of the genus Rhinolophus. Following the initial emergence, possibly facilitated by a mammalian bridge host, SARS-CoV-2 is currently transmitted across the globe via efficient human-to-human transmission. Results obtained from experimental studies indicate that animal species such as cats, ferrets, raccoon dogs, cynomolgus macaques, rhesus macaques, white-tailed deer, rabbits, Egyptian fruit bats, and Syrian hamsters are susceptible to SARS-CoV-2 infection, and that cat-to-cat and ferret-to-ferret transmission can take place via contact and air. However, natural infections of SARS-CoV-2 have been reported only in pet dogs and cats, tigers, lions, snow leopards, pumas, and gorillas at zoos, and farmed mink and ferrets. Even though human-to-animal spillover has been reported at several instances, SARS-CoV-2 transmission from animals-to-humans has only been reported from mink-to-humans in mink farms. Following the rapid transmission of SARS-CoV-2 within the mink population, a new mink-associated SARS-CoV-2 variant emerged that was identified in both humans and mink. The increasing reports of SARS-CoV-2 in carnivores indicate the higher susceptibility of animal species belonging to this order. The sporadic reports of SARS-CoV-2 infection in domestic and wild animal species require further investigation to determine if SARS-CoV-2 or related Betacoronaviruses can get established in kept, feral or wild animal populations, which may eventually act as viral reservoirs. This review analyzes the current evidence of SARS-CoV-2 natural infection in domestic and wild animal species and their possible implications on public health.
In today's society, dog companionship is widely prevalent worldwide. In the United States, 63 million households have a pet dog, a majority of which consider their dog a member of their family (6). In addition to living in our homes, dogs have also become increasingly widespread in applications to assist individuals with disabilities as assistance dogs. During and following World War I, formal training of dogs as assistance animals began particularly for individuals with visual impairments in Germany and the United States (7). Following World War II, formal training for other roles, such as mobility and hearing assistance, started to increase in prevalence. Over the decades, the roles of assistance dogs have expanded to assist numerous disabilities and conditions including medical conditions such as epilepsy and diabetes and mental health disorders such as posttraumatic stress disorder (PTSD). At the same time, society has also seen increasing applications of dogs incorporated into working roles including detection, hunting, herding, and protection (8, 9).
A biopsychosocial perspective of how biological, psychological, and social influences may impact one another (solid lined arrows) and influence human health and well-being (represented here by the large thick circular shape).
This dynamic biopsychosocial approach to understanding health and well-being is appealing to the field of human-animal interaction (HAI) because of the dynamic nature of the relationship between humans and animals. For example, a person may acquire many dogs over his/her lifetime, perhaps from childhood to old age, and each of those dogs may sequentially develop from puppyhood to old age in that time. Behaviorally, the way the human and the dog interact is likely to be different across the lifespans of both species. From a biopsychosocial model perspective, the dynamic nature of the human-canine relationship may differentially interact with each of the three influencers (biological, psychological, and social) of human health and well-being over the trajectories of both beings. Notably, these influencers are not fixed, but rather have an interactional effect with each other over time.
Another psychological outcome related to dog interaction that receives considerable research attention is anxiety. Studies have found that short-term, unstructured interactions with a therapy dog can significantly reduce self-reported anxiety and distress levels [e.g., (18)]. For example, children with their pet dog or a therapy dog present during a stressful task exhibit lower perceived stress and more positive affect compared to when alone (19), when a parent was present (20), or when a stuffed dog was present (21). In addition to psychological mechanisms, there are social and biological mechanisms at play as well. In these short-term stressful contexts, a dog may serve as both a comforting, nonjudgmental presence as well as a positive tactile and sensory distraction. Dog interaction might also reduce anxiety and distress by influencing emotion regulation while coping with a stressor (22). During animal-assisted therapy, having a dog present during psychotherapy such as cognitive behavioral therapy can aid in decreasing self-reported anxious arousal and distress for patients who have experienced trauma, making the therapeutic treatment process more effective (23).
Dogs may also provide a source of motivation; for example, people with dogs are more likely to comply with the rigors of their daily life (31). The relationship with a pet dog may provide motivation to do things that may be less desirable. For example, for older adults who own pets, it is not uncommon for them to be more involved in daily life activities because of the need to take care of their animals (32). Likewise, children also complete less desired activities due to their relationship with the dog [for a discussion of this topic see (33)].
Similar research has also highlighted the value of dogs for children with disorders of executive functioning and self-regulation, especially autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD). For some children with ASD, dogs may provide a calming and positive presence (37) and may both reduce anxiety (38) and improve problematic behaviors (39). Parents report that both pet dogs and service dogs can provide certain benefits for children with ASD, including benefits to children's moods, sleep, and behavior (40, 41). Therapy dogs have also been found to be impactful in supporting children with ADHD in their emotional regulation (42) and aspects of character development (43). Nevertheless, the outcome of dog interactions may not be positive for all individuals with ASD and ADHD; despite evidence of psychological benefits of dog interaction for some children, others may be fearful or become over-stimulated by dogs (44).
However, one of the most popular physiological measures in HAI research is the stress hormone cortisol (57). Studies have found that short-term interactions with a dog can decrease both subjective stress and circulating cortisol concentrations [e.g., (58)]. Cohabitating with a dog has also been found to impact circulating cortisol after waking among children with ASD (39) and military veterans with PTSD (59). Experimental studies have also examined how having a dog present may modulate the stress response and cortisol secretion among individuals undergoing a stressful situation. Among adults, studies have found that having a dog present during a socially stressful paradigm can attenuate cortisol compared to when alone or with a human friend (60). A recent randomized controlled trial similarly found that interacting with a therapy dog, for 20 min, two times per week, over a 4-week period resulted in reduced cortisol (basal and diurnal measurement) among typically developing and special needs school children compared to the same duration and length of delivery for a yoga relaxation or a classroom as usual control group (61). However, it is of note that many methodologically rigorous studies have not found significant effects of interacting with a dog on physiological parameters, including salivary cortisol (21, 62, 63). A recent review of salivary bioscience research in human-animal interaction concluded that significant variation exists with regards to sampling paradigms, storage and assaying methods, and analytic strategies, contributing to variation in findings across the field (57).
As research quantifying the physiological outcomes from dog interaction continues to increase, so does research attempting to understand the underlying mechanisms of action leading to stress reduction. One theoretical rationale for dogs' stress-reducing benefits consists of the dog's ability to provide non-judgmental social support (60), improve positive affect (64), and provide a calming presence (22). Dogs may also contribute to a feeling of perceived safety and provide a tactile and grounding comfort (65). For these reasons, dogs are often incorporated into treatment and recovery for individuals who have experienced a traumatic event (66). Another mechanism contributing to these stress reducing benefits may be tactile stimulation and distraction derived from petting or stroking a dog. For example, Beetz et al. (67) found that the more time a child spent stroking the dog before a stressful task, the larger the magnitude of cortisol decrease. In fact, calming tactile interactions such as stroking, touching, and petting may be a key mechanism explaining animal-specific benefits to stress physiology, as touch is more socially appropriate in interactions with animals than as with other people (22). While there are many hypothesized mechanisms underlying positive psychophysiological change following human-dog interaction, more research is needed to determine how individual differences in humans, animals, and the human-animal relationship affects outcomes (21, 57, 62, 63).
The mechanisms that underly positive human-dog interactions are likely to be interrelated and broadly, yet differentially, impactful across the three influencers of health (biological,