Monday, March 17, 2014

Modeling of the Heat Inactivation of the Coronavirus Porcine Epidemic Diarrhea Virus

by Dr. Ray Nims

A new variant of porcine epidemic diarrhea virus (PEDV, a member of the enveloped virus family Coronaviridae) has been circulating within the United States since 2013 and as of the end of January 2014 has been reported in 23 states. Due to the relatively high mortality rate caused in swine, this virus is causing severe economic losses. Mitigation of the risk of spread of this virus during husbandry and transport of livestock is of relatively great importance to the pork industry.

Thomas et al. (2013) examined the combination of temperature and time required to inactivate PEDV in swine feces on a metal surface. These conditions were investigated as possible options for livestock transport trailer decontamination when washing and disinfecting are not otherwise possible. Their results are shown in the table below. A 10-minute exposure to temperatures below 160 °F (~71 °C) failed to result in complete inactivation of the virus. As mentioned in the Thomas et, a 10-minute treatment at a temperature of 160 °F resulted in complete inactivation, as indicated by their pig bioassay, although this temperature was stated to be impractical for routine disinfection due to both cost and adverse impact on the equipment being sanitized.

While intermediate temperatures between 160 °F and 145 °F (~63 °C) were not examined empirically by Thomas et al., it is possible that a temperature lower than 160 °F might retain sufficient effectiveness to be useful for the purpose of sanitizing transport trailers. The relationship between time required for heat inactivation of viruses and temperature is essentially a power function (not a linear function). One may take advantage of this fact in modeling the time required for 1 log10 inactivation (the so-called decimal reduction value, or D) at various temperatures if one has empirical data for at least three temperatures. Since heat inactivation often displays first-order kinetics with respect to time, knowing the D value for a virus allows one to estimate the log10 inactivation after a fixed period of time (e.g., 10 minutes) at temperature. Heat inactivation data including D values at three temperatures were not found specifically for PEDV, but such data do exist for three other coronaviruses, including two from the same Genus (Alphacoronavirus) as PEDV, namely canine coronavirus and transmissible gastroenteritis virus, and one from the Torovirus Genus, namely Berne virus.

The figure below represents modeling of heat inactivation of coronaviruses in a liquid matrix caused by 10 minutes exposure to various temperatures. Since it is not known which of the two Alphacoronaviruses represent the best model virus for PEDV, modeling was also done based on the average D values for canine coronavirus and transmissible gastroenteritis virus (see line labeled Alphacoronavirus). Heat inactivation can very matrix-dependent, and the rate of inactivation of viruses in swine feces may not be represented accurately by these data for a culture medium matrix. However in the absence of specific data for PEDV in feces, the plots below may be used as an estimate of the temperature/time requirements for PEDV inactivation. If PEDV inactivation is similar to that reported for canine coronavirus, 10 minutes heating at 68 °C (~154 °F) may afford sufficient inactivation. If the data for porcine transmissible gastroenteritis virus are, however, more typical of that for PEDV, then 10 minutes heating at temperatures lower than 160 °F (71 °C) will probably fail to lead to complete inactivation. 

There is always the option, however, of increasing the exposure time from 10 minutes to, say, one hour. As shown below for the “average Alphacoronavirus” one hour heating at 63 °C (~145 °F) is estimated to result in >8 log10 inactivation.
 Perhaps these modeling estimates can guide the way for additional empirical studies to assay the effectiveness of this intermediate set of temperature/time conditions for mitigating PEDV risk during swine transport.