IBR stands for Infectious Bovine Rhinotracheitis — a disease caused by Bovine Herpesvirus type 1 (BoHV-1, sometimes written BHV-1 or BoAHV-1).

Like all herpesviruses, BoHV-1 has one defining trick: once a cow is infected, she's infected for life.

Her immune system can suppress the virus, but never fully clear it. The virus retreats into nerve cells and stays dormant — sometimes for years — until something tips the balance. That something is almost always stress.

When an infected cow is stressed by calving, transport, mixing with new mobs, feed changes, or extreme weather, the virus can reactivate. She begins shedding it again, often without showing any signs herself, and exposes every animal she's near.

This is why IBR behaves the way it does in New Zealand: quiet for long stretches, then occasionally explosive — particularly when naïve animals meet a recently reactivated carrier.

BoHV-1 can cause two distinct patterns of disease:

The respiratory form — affecting the upper airways, with nasal discharge, conjunctivitis, coughing, fever, and milk drop. This is what gave IBR its original name in the United States: "red nose."

The reproductive form — affecting the genital tract, with reduced conception rates and potential early embryonic loss. This form is harder to spot because it shows up as a number on an empty rate report, not a sick cow in the yard.

Both forms come from the same virus. Which one shows up depends on the strain, the animal, and the route of infection.