In a recent study published in Emerging infectious diseases magazine, researchers discuss infection of the common frog Rana is temporary Tadpoles with Ranid Herpesvirus 3.
As a result of infectious diseases, global amphibian populations are rapidly declining. Declining amphibian numbers lead to a significant reduction in biodiversity and essential biomass, which may affect human health. Two new types of herpesviruses have been found in the common toad and the common toad, named ranid herpesvirus 3 (RaHV3) and povonoid herpesvirus 1, respectively.
send: Zn3 herpesvirus infection in the common frog Rana temporal tadpoles, Norway. Image credit: David Daniel Fotografie/Shutterstock
In this study, the researchers explored the possibility of RaHV3 infection in common wild toads before they undergo transformation.
In 2022, 14 samples of tadpoles were collected from five ponds in Norway. Sample batches contained three to 13 tadpoles each and were taken from nearby ponds or areas. Collection was performed after adult frogs were discovered with lesions similar to those observed in RaHV3 infection earlier in the year. Ponds were sampled two to three times between early June and mid-July 2022. The tadpoles were categorized according to Gosner’s developmental stages, which ran from stages 26 to 36.
Tadpoles were collected and humanely euthanized using tricaine methanesulfonate. Deoxyribonucleic acid (DNA) was extracted from half of each tadpole, and the other half fixed in 10% buffered formalin. The partial genome sequence of RaHV3 was amplified. DNA was extracted from multiple tadpoles obtained at each sampling date and aquarium location. If a positive signal for RaHV3 was found by polymerase chain reaction (PCR), all tadpoles from the same site were tested.
RaHV3 was detected in 2 out of 14 batches of tadpole DNA using qualitative and quantitative PCR. Positive samples were collected from two of the five sites tested. Among 13 tadpoles from Lillehammer Pond and 4 from Skytta Pond, six and one tadpole were found to be positive for RaHV3 DNA, respectively. Genome equivalents ranged from 2 x 101 and 2 x 107. A 100% match with the reference strain RaHV3_FO1_2015 was found after sequencing of the obtained amplicons through qualitative PCR.
No differences were detected in histological sections of tadpoles that showed positive and negative RaHV3 PCR results. However, the potential effect of RaHV3-related modifications could have been hidden due to the significant autolysis detected in the tissue sections under investigation.
The results of the study confirmed the hypothesis that frog infection occurs during pre-embryonic or larval stages. leopard frogs (pipe lithopats) were only susceptible to RaHV1 infection during early embryonic stages. It is unclear why pre-transformed frogs are more susceptible to herpesvirus infection. Precrystal frogs may be more susceptible to natural infections due to the absence of keratinized skin and the significant differences in their immune systems compared to life stages after homozygosity.
The research indicated the possibility of sporadic infection with RaHV3 in different groups of tadpoles, along with a higher lethality of the virus in infected tadpoles. The second hypothesis suggested that it would be difficult to identify RaHV3-positive tadpoles in natural environments because of their rapid death and exhumation. Alloherpesvirus, a type of herpesvirus, infects animals and tends to cause more severe illness and death in young individuals than in adults. RaHV3 has the potential to cause higher mortality rates in infected tadpoles, resulting in lower detection rates among the surviving tadpole population.
The study provides new insights into the pathogenesis of RaHV3 disease and its potential impact on both pre- and post-transformation life stages in amphibian hosts. Studying the effects of RaHV3 and other herpesviruses on frog populations is critical to amphibian conservation efforts, biodiversity conservation, and ultimately the well-being of people and the planet.