How to set the phenotype in GWAS analysis?
Simply, First of all, you can group samples as survived animals and dead samples and you have a binary phenotype.

In a different approach, you can use survival time (time to death) as an ordinal variable phenotype. If you want you can utilize survived sample by assign a large death time value to them.

In the third approach, you can cluster the survival time (time to death) into the desired $n$ groups by the simple procedure like $k$-means. and you have coarse grain ordinal variable you can add survived sample group at the end of The Spectrum.