This disease was first associated in 1983 with a rod-shaped virus named lettuce big-vein associated virus (LBVaV), which is transmitted by the obligately parasitic soil-inhabiting fungus, Olpidium brassicae.
[1] However, in 2000, a second virus, Mirafiori lettuce big-vein virus (MLBVV), was found in lettuce showing big-vein symptoms.
Furthermore, since the lettuce infected with LBVaV alone doesn't develop the symptoms while the infected with NLBVV one does, the latter virus is considered to be a main agent of the big-vein disease.
[2] Affected plants have veins that become large and clear, causing the rest of the leaf to become ruffled.
Severely infected plants may fail to form a lettuce head.