If there is a pig equivalent to COVID-19, it might be Porcine Reproductive and Respiratory Syndrome (PRRS). Occurring worldwide and affecting pigs of all ages, PRRS has a major impact on the pork industry each year. Given its prevalence and severity, the disease is a focus of projects like "Disease-induced hypothyroidism following PRRSV infection”.
“PRRSV is a virus that impacts both the reproductive and respiratory side of things,” said Dr. Glenn Hamonic, Postdoctoral Fellow, Western College of Veterinary Medicine, University of Saskatchewan. “It infects both feeders and piglets, and can also be transferred by pregnant gilts or sows to the fetus, resulting in significant levels of fetal death.”
In North America alone, PRRS costs the pork industry hundreds of millions of dollars annually. To make matters worse, the virus mutates quite readily, producing thousands of strains and making vaccination difficult. This has sparked considerable research on alternative ways of managing the disease, such as looking for genetic markers associated with better PRRS outcomes.
“Since 2012, Dr. John Harding’s research team at the University of Saskatchewan has been focusing on the reproductive side of PRRS, seeking genetic markers that are associated with an increase in fetal viability following infection. We recently found one such marker near the DIO2 gene, which is responsible for the conversion of the thyroid hormone T4 to its more active form, T3. That is how we wound up thinking about the thyroid hormone in relation to PRRS.”
Researchers then began looking at banked serum samples and found that almost every fetus infected by the virus was extremely hypothyroid (lacking in thyroid hormone). They noticed the same situation in PRRS-infected piglets, and it caught their attention because thyroid hormone is critical to growth and development in piglets and fetuses.
“With the fetuses, we spent a lot of time with the question of why they are dying during PRRS infection. Hypothyroidism could be a contributing factor, so a big part of our research is figuring out why fetuses and pigs are becoming hypothyroid.”
Ordinarily, the hypothalamus and pituitary detect that an animal is hypothyroid and work to correct the problem, but this is not happening when PRRS strikes.
“When you talk about managing a large number of fetuses in a litter, interruptions in growth can be a major problem. How much of that is due to hypothyroidism, and how much due to infection, is not yet clear, as nobody has characterized this before. We have identified the points of disruption, but how is the virus inhibiting the fetus’s ability to produce thyroid hormone and to sense when levels are low?”
If they can determine why this disruption is occurring, scientists may be able to limit the impact on piglet growth and fetal mortality.
“The reality with the PRRS virus is that unless they come out with a foolproof vaccine that covers the thousands of strains in circulation, there will be great demand for alternative measures that can work across multiple strains. Given that piglets infected with two different strains of PRRS showed a big drop in thyroid hormone, and that those levels returned to normal after recovery, it seems there is a significant link between hypothyroidism and PRRS.”
The brilliance of resilience
That link could be highly relevant to the Genome-Alberta led project on disease resilience in pigs. If fetuses and piglets are more resilient in the face of PRRS infection, the impact on the thyroid hormone levels may not be as severe, resulting in a better disease result and fewer problems for producers.
“What really excites me about this work is that I spent years in disease research and never thought to look at hormones. This new focus sparks some key questions for science going forward: How does the impact of the thyroid hormone translate to average daily gain across infection? If we can find the link between hypothyroidism and PRRS, will this improve fetal and piglet outcomes directly, or will it decrease their viral load, which in turn will improve their outcomes?”
There are still answers to be found in this area, but if science is asking the right questions, that’s a pretty good start.