Depressed performance then may be a consequence of internal parasitism. Adult worms graze the lining or mucosa and cause irritation and fluid loss, interfering with the digestive function of the stomach. Feed conversion for weight gain, body maintenance, reproductive fitness and milk production is then compromised. Larvae or immature worms invade the gastric glands, which are pits in the lining of the stomach, disrupting normal structure and function of these glands. This compounds the effects of parasitism.
Adults lay microscopic eggs, which are passed in the feces and hatch in dung pats.
Microscopic larvae are then dispersed, with some trickling deep into the soil and others carried onto surrounding grass. Cattle become infected while grazing. Upon passage of grass to the stomach, infective larvae penetrate into gastric glands. Larvae usually emerge from the glands in relatively small numbers after one to two weeks, then mature so that glandular disruption is not so dramatic and therefore is of less clinical consequence. These larvae then rapidly increase in size, emerge in large numbers and cause massive destruction to the gastric glands, bringing stomach function to a virtual halt.