Coronaviruses include transmissible gastroenteritis (TGE) virus, porcine epidemic diarrhea (PED) virus and porcine delta coronavirus (PDCoV). These viruses cause diarrhea and vomiting with subsequent dehydration and death, especially in young pigs. Fecal oral and fomite contamination are the main routes of virus transmission within and among herds. The consumption of feed and feed ingredients contaminated with PDCoV from feces can be a risk factor for transmission of swine enteric coronaviruses. Therefore, it is essential to understand the inactivation kinetics in feed ingredients and fecal matter to control transmission.
Previous approaches to minimize risk of coronavirus transmission
Although live and killed TGE and PED virus vaccines are available, they do not provide complete protection from infection. Deliberate feeding of naturally infected feces and virus-containing organs to sows has also been used to promote lactogenic immunity of piglets, which has demonstrated potential antibodies to offspring. These methods of disease control are not effective, and prevention of infection is the most cost effective method of control. The most common practices to prevent virus infection is to decrease virus transmission among herds with biosecurity protocols. In spite of multiple biosecurity protocols effective in preventing diseases such as porcine reproductive and respiratory syndrome (PRRS), swine coronavirus can be transmitted among herds rapidly, posing the question of greater environmental survival and unknown routes of transmission that are not included in current biosecurity protocols.
New solution to the problem
At the University of Minnesota, we investigated the survival of PDCov in feed and feed ingredients as well as the environment (fresh feces and slurry) as new sources of coronavirus transmission among herds. Results from our study demonstrated that storage of virus-spiked feed ingredients for 21 days did not result in complete inactivation of PDCoV (see table). Of the various feed ingredients evaluated, a longer time was required to inactivate the virus in feed ingredients containing low moisture and high ash content such as premix and meat and bone meal. These two ingredients appear to pose the greatest risk of transmitting PDCoV, especially premix that was only able to achieve a 1-log reduction in 21 days. The minimum infectious dose of PDCoV is unknown, but current data suggest that premix and meat and bone meal pose the greatest risk to infect pigs.
Of the various feed ingredients evaluated, a longer time was required to inactivate the virus in feed ingredients containing low moisture and high ash content such as premix and meat and bone meal.
The greatest virus inactivation was observed in low oil DDGS, medium oil DDGS, meat meal and milled corn (> 3 log) compared with meat and bone meal, complete feed and high oil DDGS (< 3 log). Feed ingredients were evaluated at 25C for extended periods of time, which decreased virus survival of feed ingredients. Different feed ingredients had different final virus concentrations. These results indicated that virus inactivation in feed and feed ingredients is complex and there may be multiple interactions that affect the integrity of the glycoprotein in the virus, which has the greatest role in protecting the virion from damage, and thereby preventing the virus from being inactivated.
Storing feed ingredients for up to 21 days at 25C reduces virus survival but it does not provide complete virus inactivation.
In addition, our results showed that heat treatment appears to be one of the most effective ways to reduce PDCoV survival in fresh feces and slurry. Organic matter such as feces and slurry pose a risk for extended periods of time except for at high temperatures of PDCoV in feces (60C) and slurry (25C), as they achieved the greatest log reduction in 28 days. In slurry, there appears to be a correlation between temperature and relative humidity that decreases the delta value. However, this pattern was not observed in fresh feces. These studies were conducted using a single dose of virus (3.5 x 105). The survival of virus in samples contaminated with larger amounts of virus is unknown.
Storing feed ingredients for up to 21 days at 25C reduces virus survival but it does not provide complete virus inactivation. There were differences in virus survival among feed ingredients, and the least inactivation was observed in the premix and meat and bone meal. In feces and slurry, virus inactivation was greater at higher temperatures, and relative humidity may influence inactivation kinetics at which materials were stored. Virus in fresh feces and slurry do not survive as well at 60C and 25C. Therefore, exposing samples to greater temperatures and storage periods may increase the inactivation kinetics of PDCoV and reduce potential of transmission.