Mathiason also attributed the higher-than-expected CWD-positive rate in fawns to highly sensitive amplification assays. “That’s permitting us to be able to detect smaller and smaller quantities [of CWD prions] that may not have been noticed by conventional tests like IHC, ELISA, or Western blot,” she said.
Samuel said that CWD-positive fawns may die of infection soon after they’re born, although CWD clinical signs typically don’t develop until 1.5 to 2 years after infection.
“Maybe there’s a tendency for CWD-positive mothers to not do as well at raising fawns, but that wouldn’t be enough to explain it” Samuel said. “Another possibility is that these apparent infections are occurring in different tissues than we’re used to sampling and through a very different route than the usual oral route, so maybe these don’t turn into full-blown infections.”
Sonja Christensen, PhD, an assistant professor in Michigan State University’s Department of Fisheries and Wildlife, who likewise was not a study author, pointed out that results in the lab often look different from what is seen in free-ranging cervid populations. “There are a lot of uncertainties about what happened from when that fawn was born to when it showed up on a necropsy table or in the back of a hunter’s truck,” she said.
In addition, wildlife biologists are limited to hunter-harvested animals, and many hunters don’t select fawns. “So certainly their data set probably reflected that,” Christensen said. “And if anything, this provides some grounds for looking more at fawns in hunter harvests in the future for surveillance specifically for CWD.”
‘Something missing in the safety net’
James St. John / Flickr cc
Christensen said the study findings may have substantial implications for deer populations: “We now know that those fawns can be infected with CWD at birth and may be infectious to other animals much, much earlier—even if they look healthy. That has huge implications for wild-deer population trajectories, because now we have to be thinking of another risk factor on the landscape that’s infecting animals within their own population.”
She also noted that fawns often travel outside of their family unit when trying to establish their own home range. “With those excursions, if you have a positive fawn, more contacts might occur with other uninfected or susceptible deer outside of their regular family group,” she said. “So that’s another risk factor on the landscape.”
The study, she added, shows that even a very young and seemingly very healthy fawn isn’t necessarily CWD-free: “So really trying to be judicious with moving animals around and reducing the spread of potential contacts among deer that could be infectious when you don’t really know their CWD status is really important.”
Samuel said vertical transmission is unlikely to be significant and probably won’t change management strategies, unless it can be shown that vertical CWD transmission is killing a large number of fawns.
Female yearlings, which could have potentially been exposed to CWD for 1.5 years, have a 25% infection rate, compared with 35% for adult does. In contrast, CWD infections rise from 25% in yearling males to 50% over the following 1 to 2 years.
“There’s obviously a lot more infection going on for males compared to females,” he said. “While the paper is interesting and well done, I simply don’t see how vertical transmission can be considered an important route of infection based on data from Wisconsin deer.”
Mathiason agreed that factors other than vertical transmission likely have more bearing on management strategies. “I just think as we see populations with increasing prevalence rates, it’s a real issue,” she said. “And if we think about placental materials being left in the environment, it’s yet another contributor to infectivity within the environment as well.”
Tom Hauge, MS, a wildlife biologist and retired director of wildlife management for the Wisconsin Department of Natural Resources, said the study results might encourage deer farmers to test fawns before moving them from one facility to the next.
“The industry, despite the USDA [US Department of Agriculture] having herd certification programs and things like that, has really struggled with it,” said Hauge, who wasn’t involved in the study. “An awful lot of herds that were certified CWD-free have somehow turned out to be positive. And so that points out that there’s something missing in the safety net.”