An emerging viral disease can be defined as a new occurrence of a disease because of: (a) the evolution or change of an existing virus or its spread to a new geographic area, species or ecological niche; (b) its rapidly increasing incidence, in terms of numbers of infected individuals or geographic range; or (c) a previously unrecognized disease or virus ^{[1][2][3][4][5]}[1,2,3,4,5]. A viral disease of the past (i.e., one previously considered to be controlled) that re-appears with an increased prevalence in an area with susceptible host populations, expands its host range or appears in a new clinical form, is usually termed as re-emergent viral disease ^[4][6][4,6]. Many recent human emerging viral diseases have an animal origin, some with a significant impact on animal or public health, such as SARS-CoV-2, and two viruses that infect passerines: influenza A virus (AIV) and West Nile virus (WNV). The advent of modern, more powerful sequencing and bioinformatics technologies has increased the discovery of novel viruses in animals, with or without causing disease, that may be pathogenic, emergent, or zoonotic. Many of these novel and potentially emerging viruses are found in avian species that are present in virtually every ecosystem. Surprisingly, although passerines are the most abundant and diverse avian species worldwide, little is known about the novel and emerging viruses that they host and their possible role in the emergence of new viral diseases in animals and humans.

Several factors are related to the development of emerging viruses and diseases as they enable infectious agents to evolve into new ecological niches, to reach and adapt to new hosts, and to spread more easily among the new hosts: urbanization and destruction of natural habitats, allowing humans and animals to live in close proximity; international travels and trade; climate change and changing ecosystems; changes in populations of reservoir hosts or intermediate insect vectors ^[4][5][4,5]. All these inter-dependent factors imply that the study of new or emerging viruses must be approached from a multisectoral and multidisciplinary perspective, framed in the One Health approach supported by the WHO, WOAH and FAO ^[4][5][4,5] (Figure 1). The international trade of passerines, mainly ornamental breeds, has been also associated with the introduction of novel viruses in some countries, which pose a risk to native or endemic species if they can jump the species barriers, such as described with avipoxvirus in New Zealand [7]. There is also reasonable concern that more vulnerable individual species (of all taxa, including Passeriformes) may be at risk of extinction from viral pathogens. It is suggested that island endemic species are particularly vulnerable to pathogens, especially introduced pathogens to which they have no prior contact and no innate immunity [7]. Other authors have pointed out that all species with a small geographic range, low population size and low genetic diversity may be highly vulnerable to extinction, not just those island endemic species [8]. However, the researchers are not aware of any evidence that any species has ever become extinct due to a viral pathogen.

One of the most important factors that determines viral emergence is related to the adaptation of a given virus to new host species and/or the concomitant appearance of changes in the environment that offer new opportunities for the virus to thrive [9]. Some emerging viruses have a broad range of hosts. For example, RNA viruses such as West Nile virus (WNV) and Avian Influenza virus (AIV) can infect hundreds of different bird species including passerines. Others, such as herpesviruses or papillomavirus, are usually considered to be specific for one or a few related species ^[9][10][9,10]. Many species may act both as natural viral reservoirs and as amplifying hosts in bird-vector-bird cycles, for example, some togaviruses and flaviviruses. Occasionally these viruses are transmitted to incidental (dead-end) hosts including humans, equids, and other mammals. Birds can also act as a gene source of emerging viruses in cross-species transmission, for example, new influenza viruses may evolve through the reassortment of different gene segments [11].

The potential adaptation of an emerging virus to a new host depends on viral transmission routes (shedding of virus and infection in individuals) and the possibility of reaching this new host is facilitated by close and prolonged contact between individuals (such as breeding facilities). Viral transmission in birds may be horizontal (between individuals) or vertical (from females to offspring, congenitally or through embryonated eggs such as avian leukosis virus). Horizontal transmission is the most frequent and can occur by direct contact between animals (aerosols, fluids, feces, wounds or by predation or scavenging), by indirect contact through fomites or contaminated material (such as water, food, or troughs), and by vectors. Common vectors are blood-feeding arthropods such as mosquitos, midges, and ticks, in which the virus is propagated, and viruses transmitted mainly by this route are collectively called arboviruses (from Arthropod-Borne viruses). Vector-borne transmission is an indirect route of increasing importance in emerging viral diseases, some of which are important zoonoses, such as flaviviruses and togaviruses. It should be noted that viruses can be transmitted by more than one route.

Alternatively, viruses can enter the body via epithelial or the superficial mucosa of respiratory, gastrointestinal, and urogenital tracts. The most common routes of infection are ingestion of contaminated water or food (the fecal-oral route, for instance, picornaviruses) or inhalation of droplets expelled by an infected individual (respiratory route, for instance, herpesviruses and metapneumoviruses) or contaminated surfaces. Respiratory viruses can also be transmitted by contact with eye mucosa. In airborne transmission, viruses can spread over long distances through small respiratory aerosols that can remain suspended and travel in the air, such as influenza virus [12]. Viruses transmitted through ingestion usually are non-enveloped, resistant to low pH and to the acids in the digestive tract, and often produce diarrhea, causing large amounts of virus to be shed into the environment (where they can remain infectious for a long period of time) [9]. Diverse pathogens are probably transmitted between wild birds and domestic birds and poultry when feed and water are contaminated with feces in open aviaries and free-range farms. Describing the cloacal virome is essential for understanding the ecology of viruses circulating in the environment, identifying new virus-host relationships, and defining the risk of virus emergence [13]. However, data on the cloacal virome of wild or domestic passerines are still very scarce and, besides previous study in French Guiana and Spain [13], there are only a few other studies in China ^[14][15][14,15] and Australia/New Zealand ^[7][16][17][7,16,17]. Increased investigation of the virome and emerging viruses of passerines is vital for predicting future outbreaks and spillovers that can affect birds and other animals, humans and biodiversity ^[13][15][13,15].

Greater than two-thirds of viral taxa that infect humans are considered to be zoonotic: they are able to infect non-human vertebrates and may circulate in non-human reservoirs [18]. The alternative hosts for most zoonotic viruses are mammals (rodents, ungulates, other primates, carnivores, and bats). Birds are a much less important reservoir for zoonotic disease than mammals. Though less than 20% of zoonotic viruses share avian hosts [18], current data shows that birds are an important potential source for zoonotic viruses.

Passeriformes is the most diverse avian order. These are songbirds and perching birds with well-known species including sparrows, starlings, thrushes, magpies, crows, swallows, and finches. There are around 10,700 bird species, placed in 41 orders and 248 families, of which nearly 6400 species (59.6%) and 140 families (56.5%) are Passeriformes [19], making it by far the most speciose avian family. In addition, many passerine species are also extremely abundant. One recent estimate of the abundance of 92% of bird species determined that there are approximately 50 billion individual birds (albeit with high levels of uncertainty), of which 28 billion (56%) are Passeriformes [20]. This begs the question of whether the diversity of viruses circulating in Passeriformes is approximately equal to their share of avian diversity and abundance, and whether they could pose a significant risk to human and animal health and environmental balance.

Passerines include wild, urban, rural, and pet birds. Many passerine species are extremely abundant in the wild, but many populations of passerines are synanthropic species that live in close proximity to anthropogenic environments, which boosts the risk of the circulation of viral diseases between humans [21] poultry, and passerines (Figure 1). Pet passerines are usually bred for ornamental use, kept in captivity at home or in different types of aviaries (e.g., breeding facilities) that may have access to outdoors increasing the risk of contact with wild birds [22]. Migratory passerines can spread a wide variety of viruses over long distances, potentially being capable of infecting resident wild passerines (such as house sparrows, Passer domesticus), and these latter possibly contaminating pet birds living in open aviaries or poultry in farms [22]. For example, it has been suggested that small passerines could serve as bridge hosts for low-pathogenic avian influenza virus (LPAI) from infectious waterfowl to commercial turkey farms [23]. It is possible that the contact zone between agriculture and wildlife provides an interface where viruses could potentially circulate between wild and domestic birds [23] but probably vice versa as well. Passerines could play a larger role than previously thought in this interaction, a hypothesis that needs to be further studied.

The most important families of viruses affecting passerine birds are shown in Figure 2. Data on the bird species and families these viruses affect is summarized in Table 1, Tables S1 and S2 (can be downloaded at: https://www.mdpi.com/article/10.3390/microorganisms11092355/s1, Table S1: List of bird species; Table S2: List of avian families). Species recognized by the International Committee on Taxonomy of Viruses (ICTV) are italicized.