The length of worms is determined by the hosts
when a population is exposed to the same infectious agent, not all individuals show susceptibility to the infection. To find out the reason, M J Stear and colleagues of the Glasgow University Veterinary School, uk , investigated the basis of resistance to nematode infection in sheep. Nematode worms are major parasites of plants and animals that cause lethal infections ( Nature , Vol 389, No 6646).
The researchers monitored sheeps from the same flock of a commercial farm. They studied them for the load of nematode worms, which the sheeps carried during their life span and also at the time of death. Most of the parasites belonged to a single specie called Ostertogia circumcincta . At least 25 O circumcincta were measured from each sheep and data were analysed to find whether sheeps that were genetically related to each other at a level higher than the average degree of relatedness in the flock, also resembled each other in respect to some property of the parasites that they carried.
The researchers also estimated heritability -- the extent to which genetic factors in a host might be used as predictors of the severity of parasite infection. It was found that heritability of parasites was close to zero. There was no evidence that the parasite load in an individual was influenced by genetic variability from one lamb to another. But heritability of worm length was found extremely high -- when sheep carried worms that were unusually long or short, their relatives too tended to carry worms that were unusually long or short.
It shows that worm length in female worms is strongly correlated with the number of eggs in that female's uterus, and therefore, with the number of young that she gives birth to. If a worm is long, the fecundity would be high. So genetic factors in a host that regulate the length to which a worm can grow, indirectly serve to control the increase in the worm population.
Though the researchers have not been able to know about the factors, they speculate that the quantity and specificity of a type of immunoglobulin known as IgA may be one factor. If true, this would be useful means of distinguishing between resistant and susceptible lambs.
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