H5 Avian Influenza Reaches Stranded Cetaceans on Brazil’s Coast
Brazil’s shoreline surveillance has now documented highly pathogenic avian influenza RNA in stranded cetaceans, adding marine mammals to the long list of wildlife species touched by the global H5N1 panzootic. Between November 2023 and August 2025, investigators screened carcasses recovered along the coastlines of Rio Grande do Sul, Rio de Janeiro, and Ceará, and found influenza A viral RNA in 18 of 45 animals representing 13 species. Two samples carried the H5N1 strain itself.
The signal matters because the infected animals were not random marine curiosities. Franciscana dolphins and Lahille’s bottlenose dolphins were among the positive cases, both species already under serious conservation pressure. In a setting where mortality from strandings and environmental stress already threatens fragile populations, a viral pathogen circulating in the same waters creates a second biological burden that can be hard to see until carcasses start washing ashore.
Low Viral Loads, But a Clear Wildlife Signal
The molecular findings were technically delicate. Positive specimens showed cycle threshold values ranging from 28 to 37, corresponding to roughly 1.3 × 10^5 to 8.0 × 10^2 RNA copies/µL, values that approached the assay’s limit of detection. The H5N1-positive samples sat near that same threshold. In other words, the viral signal was real, but faint.
That pattern fits the state of the carcasses more than it weakens the result. Sampling focused on decomposition stages 2 through 4, when autolysis can erode RNA integrity and blunt the ability to recover infectious material. Advanced decay also makes histopathology harder to interpret, and the six IAV-positive animals that underwent tissue examination did not show lesions attributed to influenza infection. In marine mammal pathology, postmortem change can erase the very tissue injury investigators need to prove disease, even when viral nucleic acid remains detectable.
Still, the biology was not limited to the body surface. Viral RNA appeared in multiple internal tissues, including cerebral, oral, anal, intestinal, tracheal, and ocular samples, with blowhole swabs most often testing positive. Internal detection weakens the argument for simple environmental contamination from infected seabirds or shoreline exposure alone. The most plausible reading is true infection in at least a subset of the stranded animals.
Why Cetaceans Enter the H5N1 Story Now
The appearance of influenza A in dolphins and whales along Brazil’s coastline comes after major H5N1 events in South American pinnipeds that caused high mortality in 2023. That earlier wave established a route of ecological concern: once the virus moves efficiently through coastal wildlife communities, the marine environment can become a shared transmission space rather than a passive backdrop. Cetaceans do not mingle with seabirds in the same way pinnipeds do, but they occupy overlapping waters, feed in shared ecosystems, and may encounter contaminated prey, water, or carcasses.
Two species stand out from a conservation standpoint. The Franciscana dolphin is classified as vulnerable, and Lahille’s bottlenose dolphin as endangered. When a pathogen is detected in species already constrained by habitat loss, fisheries interactions, and low population resilience, each additional mortality pressure becomes more consequential than a simple case count suggests. A small number of infected individuals can matter when the species itself has little room for demographic loss.
The study also identified H5N1 in Franciscana and Clymene dolphins, which extends concern beyond a single taxonomic niche. Cetaceans are not usually considered core hosts for avian influenza, but mammalian spillover is no longer an outlier event. The broader panzootic has repeatedly shown that H5 viruses can adapt to new mammalian settings when ecological exposure is sustained.
Surveillance Gaps Remain Along the Wildlife-Marine Interface
The work depended on systematic beach surveys in Rio Grande do Sul and notification-based responses across all three coastal regions, a field strategy that is labor-intensive but essential when carcasses are the primary source of data. Standardized collection of date, location, species, sex, and decomposition stage made the findings usable for both epidemiology and wildlife conservation. Without that structure, the same carcasses would have yielded little more than scattered anecdote.
Yet the study also shows how much remains hidden. Most carcasses were too decomposed for detailed histologic interpretation, and no infectious virus isolation is described here. Viral RNA detection in dead animals cannot define transmission chains, tissue tropism in living cetaceans, or clinical disease severity with precision. It can, however, mark a geographic and host expansion of concern.
The practical implication is straightforward: marine mammal surveillance cannot stop at pinnipeds or obvious die-offs. Cetaceans along the Brazilian coast now sit inside the surveillance frame for H5 ecology, especially in areas where stranded animals can be collected before decomposition destroys the diagnostic record. Monitoring needs to continue because the coast is not a closed system, and avian influenza is moving through it with enough flexibility to cross species boundaries in ways that were once considered rare.
For conservation medicine and wildlife virology, the central question is no longer whether H5N1 can appear in cetaceans. It has. The harder task is determining how often that happens, which species are most exposed, and whether marine circulation of the virus is becoming persistent rather than episodic.