Detection of Herpesvirus Infection (RaHV-1) in amphibians from commercial, experimental, and recreational breeding in São Paulo State, Brazil

Frog farming has become a promising activity in Brazil, which ranks as one of the largest producers of captive-bred frogs. Although Brazil is among the world'sleading producers of frog meat, regulation of frog farming remains highly deficient, and the industry faces cultural, economic, governmental, and health-related obstacles, hindering its expansion in the country. Additionally, this sector encounters challenges related to inadequate management, environmental stress, and outbreaks of infectious diseases. Among the diseases that can affect amphibians, herpesvirus infections, belonging to the Alloherpesviridae family, have drawn attention due to their ability to cause skin lesions, tumors, and adenocarcinomas, negatively impacting animal development and producer profitability.

Herpesviruses, in general, are known for establishing long-term latent infections in their hosts. These viruses have large genomes with double-stranded DNA and are classified into three main subfamilies based on target cell types and replication cycles: 1) Alphaherpesvirinae: typically cause acute infections and establish latency in neurons. 2) Betaherpesvirinae: characterized by longer lytic cycles and latency in immune system cells. 3) Gammaherpesvirinae: tend to associate with lymphatic system cells and may be linked to cancers in their hosts. However, herpesviruses that infect amphibians, such as RaHV-1, belong to the Alloherpesviridae family, distinct from the three subfamilies of herpesviruses that affect mammals, birds, and other warm-blooded vertebrates. Amphibians are ecologically and economically significant (VALENCIA-AGUIAR et al., 2013). Due to their thin skin, which facilitates gas exchange between their bodies and the environment, these animals are highly susceptible to ecosystem changes (JORGENSEN, 1994). Most species undergo a life cycle with two phases, a predominantly aquatic larval phase and a terrestrial adult phase. This characteristic makes amphibians excellent bioindicators of environmental quality (SEWELL and GRIFFITHS, 2009). However, this sensitivity to environmental changes, such as deforestation, pollution, and global warming, has led to significant population declines (GRANT et al., 2020; DIETRICH et al., 2020).

This class of vertebrates is divided into three main orders: Anura (including frogs, toads, and tree frogs), Caudata (salamanders and newts), and Gymnophiona (caecilians or blind snakes) (FROST et al., 2021). Some species of the Anura order are traded globally for human consumption. Among them, the North American bullfrog (Aquarana catesbeiana) stands out due to its easy adaptation to various management systems worldwide (OLIVEIRA, 2015). Frog farming represents a significant economic potential for producing countries since, in addition to meat, various by-products such as flour, pâté, hamburgers, and preserves can be marketed (OLIVEIRA, 2015). However, international trade in frogs has contributed to the spread of emerging pathogens, as the species appears resistant to high infection levels (JENKINSON et al., 2016; SCHLOEGEL et al., 2009; BRUNNER et al., 2019).

Genetic studies attribute the co-evolution of this viral group with their hosts, where latent infections occur without causing severe diseases with high mortality, thereby favoring the spread of herpesviruses (FRANCO & ROECHE, 2007; CATROXO et al., 2003). Latency can be defined as viral persistence in which the virus remains within cells in a non-pathogenic form, with intermittent periods of reactivation and shedding (ADAMEK et al., 1996). Any latent carrier animal is a potential source of infection, as the virus can resume multiplication and be shed through secretions under low resistance conditions, promoting disease spread. During latency, the viral episomal genome becomes circular, and gene expression is limited. Reactivation is associated with various stressors, such as transport, adverse weather conditions, overcrowding, and concurrent infections (QUINN et al., 2005).

Herpesviruses spread primarily through three routes: cell-to-cell, bloodstream, and nervous system. Cell-to-cell transmission, common in these viruses, occurs through intercellular bridges that prevent antibody action. The virus can also reach the nervous system, being transported to specific ganglia neurons, such as the trigeminal or sacral, depending on the infection site. Blood circulation (in lymphocytes) also facilitates its dissemination, allowing the virus to reach reproductive organs, though viremia is brief. In animals with low immunity, infection can range from asymptomatic to mild but may worsen with high viral doses. As a result, some animals may be asymptomatic carriers of viruses and, when exposed to a drop in immunity, may trigger a viral process, even if the viral presence cannot be linked to the animal'scause of death, and it is unknown whether the viral particle quantity found contributes to worsening the condition (MEYER, 2001; LOPEZ & SILVA, 2019).

Studies on the presence of herpesvirus in bullfrogs (Lithobates catesbeianus) are limited in Brazil, motivating this research to detect the occurrence of this pathogen in commercial and experimental facilities in São Paulo State, Brazil.