Visual appraisal of sheep is a notoriously inaccurate way of assessing worm burdens. Most of the economic loss from worms is from subclinical (i.e. not visible) worm infections. By the time a sheep looks 'wormy', the worm infection is advanced and production losses are significant.

Different worms produce different effects. Sheep worms can be broadly categorised as those that suck blood and those that don't.

1. Blood suckers
   Barbers pole worms (Haemonchus) live in the 4^th stomach, liver fluke live in the liver. Both suck blood, sheep become anaemic, lose condition and can die.
2. Ill thrift and scouring
   Brown stomach worm (Ostertagia) and black scour worm (Trichostrongylus spp) often live in the same sheep. Between them they cause the sheep to eat less and use food less efficiently. As a result, diseased sheep are underweight and their productivity reduces. The damage caused by these worms to the lining of the gut can also result in scouring or diarrhoea.

Infection and disease are not the same
Just because a sheep has worms (most have them anyway) it doesn't mean it has disease. Several things affect which worms a sheep gets, how many it gets and if they will cause disease.

For example: