That horse did show neurologic symptoms, although animals with that strain of EHV - 1 more often have a disease that causes respiratory infection or abortion in pregnant mares. (equimanagement.com)
Cytochrome P450 (CYP) enzymes well known for their involvement in the metabolism of tobacco carcinogens are also involved in estrogen metabolism, and many are regulated by estrogens (e.g. CYP 1A1, CYP 1B1, CYP 17) 5354, CYP 1B1, in particular, is upregulated by equilin and equilinin (hormones from pregnant mares found in Premarin ®) which converts these estrogens to 4 - OH (pro-carcinogenic) metabolites. (lmreview.com)
Mares should be immunized four weeks in advance of foaling with vaccines specifically approved for pregnant mares. (lenavetclinic.com)