Disease investigation in a New Zealand high-health pig herd reveals a divergent porcine adenovirus

Viruses are invisible invaders that constantly test the boundaries of life, often hiding in plain sight within the bodies of animals we rely on for food. Among these, adenoviruses are a widespread family of pathogens known to infect mammals, birds, and reptiles. In humans and many other animals, these viruses are notorious for causing a variety of illnesses, from common colds and eye infections to more severe conditions affecting the liver or gut. In pigs, however, these viruses have long been considered minor troublemakers, typically causing only mild digestive issues or no symptoms at all. Because they are so common and usually so harmless, scientists have paid them relatively little attention, leaving a significant gap in our understanding of their true diversity and potential to cause serious harm. This uncertainty becomes particularly critical when dealing with high-health herds, which are groups of pigs raised in strictly controlled environments to exclude specific diseases. These herds are essentially immunologically unexposed, meaning the animals have never encountered many common pathogens before, making them uniquely vulnerable if a new or unusual virus is introduced.

In April 2025, a 320-sow herd in New Zealand, raised from piglets born via hysterectomy to ensure they were free of common bacterial diseases, faced a sudden and severe health crisis. The outbreak began with piglets just five to six days old, who stopped eating, became lethargic, and developed a distinct, widespread redness across their skin. Unlike typical piglet illnesses that involve diarrhea, these animals showed no digestive signs, leading veterinarians to suspect a systemic infection attacking the blood and internal organs. At the same time, the farm saw a sharp, alarming spike in the number of mummified fetuses—dead piglets that had dried out in the womb—far exceeding the farm's usual baseline. The situation was puzzling because the herd was designed to be free of known threats, yet the disease was spreading rapidly and killing neonates.

To solve this mystery, researchers collected tissue samples from a sick piglet and from several of the mummified fetuses. They first looked under a microscope to see what damage the virus had caused. Inside the cells of the piglet's liver, kidneys, spleen, and intestines, they found something striking: the nuclei of the cells were filled with dark, dense clumps. These clumps were viral factories, where the virus was replicating so aggressively that it was destroying the cell from the inside out. The virus was particularly active in the cells lining the blood vessels, suggesting it had spread throughout the entire body via the bloodstream. To identify the culprit, the team used a powerful technique called metatranscriptomics, which involves reading all the genetic material in a sample to see what is present. This approach allowed them to reconstruct the complete genetic blueprint of the virus directly from the infected tissues without needing to grow it in a lab first.

The genetic analysis revealed a surprise. The virus was an adenovirus, but it was unlike any porcine adenovirus previously described. The researchers named it Porcine adenovirus 6. While known pig adenoviruses usually cause mild, self-limiting diarrhea, this new strain was highly abundant in the tissues of the sick piglet and was also found in the mummified fetuses. Its genetic structure was remarkably different from its known cousins; it shared only about half of its genetic code with the closest known pig adenoviruses. Instead, its closest genetic relatives were adenoviruses found in bats and even a polar bear, suggesting this virus might have jumped from another mammal into the pig herd. The virus possessed the standard genetic layout of an adenovirus, but its sequence was so divergent that it represented a completely new lineage within the group.

The investigation also ruled out other common suspects. The team tested for a wide range of viruses known to cause reproductive failure and death in pigs, such as porcine circovirus and various herpesviruses, but found no evidence that they were driving the outbreak. The only other virus detected was a porcine kobuvirus, but it was present in very low amounts and is typically associated with mild gut issues, making it an unlikely cause of the severe, systemic disease seen in the piglets. The evidence pointed squarely at the novel adenovirus as the primary cause. The virus was found in high quantities in the blood vessels and organs of the dying piglet and was present in the fetuses, linking it directly to the reproductive losses and the skin redness observed in the live animals.

The severity of the outbreak appears to be a direct result of the herd's unique history. Because these pigs were raised in a high-health, disease-free environment, they had never been exposed to this virus before and possessed no natural immunity to fight it. In a conventional herd, where pigs are constantly exposed to various pathogens, the animals would likely have developed some level of protection, and the virus might have caused only mild symptoms or gone unnoticed. In this case, the lack of prior exposure allowed the virus to spread unchecked, causing widespread tissue damage and death. The outbreak was transient, resolving within six weeks as the surviving herd likely developed immunity, a pattern consistent with other disease events in this specific herd where the severity of illness diminished as the animals became exposed to the pathogens.

This discovery highlights a blind spot in our understanding of viral diversity in domestic animals. While scientists have cataloged many viruses, the genetic landscape of adenoviruses in pigs remains poorly mapped. The existence of such a divergent virus, capable of causing severe systemic disease when introduced to an unexposed population, suggests that there are likely other unrecognised viral lineages circulating in the world. The findings serve as a reminder that high-health herds, while protected from known threats, may be particularly vulnerable to emerging or cross-species infections that have not yet been encountered. By identifying this novel virus and linking it to a severe disease outbreak, the researchers have expanded the known family tree of porcine adenoviruses and underscored the importance of monitoring not just for known diseases, but for the unexpected pathogens that can arise when the balance of immunity is disrupted.