Table 2.

Effect of point mutations on viral virulence and tropism.

Strain	Isolate	Site	Variation	Changes in Virus	Reference
Purdue	PTV no.R29	Spike	665nt T to C	Loss of gut tropism; Reduced virulence	[67,99]
Purdue	PTV-ts-dmar	Spike	655nt G to T	Loss of gut tropism; Reduced virulence	[67,99]
Purdue, Miller	Miller M60, Purdue P115, attenuated H, AHHF, SHXB	Spike	1753nt T to G	Reduced virulence	[7,68]
Purdue	Purdue P115	Spike	aa145 P to L; aa147 C to R; aa155 C to F	Loss of sialic acid binding site; Reduced virulence; Loss of hemagglutinating activity	[105]
Purdue	PUR46-MAD	ORF 1a	637nt G to A	Reduced virulence	[106]

The evaluation of the virulence of the virus is based on the virulence of the virus as described by the author in the first report: if the individual challenge experimental piglets die or the pathological sections of the small intestine show severe lesions, this strain is a virulent strain.