Young stock (up to 18 months), cattle under nutritional stress or metabolic stress due to pregnancy or lactation, and bulls are generally more susceptible to internal parasites. High stocking rates, especially in wet and cool conditions that favour larval survival, will also subject cattle to higher worm levels.
Generally adult cows maintained in good body condition will have developed and are able to maintain a high level of immunity to worm infections. Hence throughout their life they will require minimal drenching.
Figure 1. A dam under high stress from newborn twins, as well as dry seasonal conditions, may develop scours due to worms. Image courtesy of Jenny Cotter
Genetic Predisposition: An exception to the above will be that a small number of adult female cattle (cows) within a herd will have a genetic susceptibility to parasites. Individuals are recognised by noting that the same cows repeatedly show signs of worms (evidence of scour, and a reduction in body condition score) when compared to the majority of the herd. These cows are best culled as soon as you are in a position to do so.
Bulls with high levels of circulating testosterone put more energy into growth and so have less capacity to use it in their immune system. Bulls, including unmarked bull calves, therefore run a risk of carrying higher worm burdens than other classes of cattle, or show more severe clinical signs of infections under moderate worm burdens when compared to females or castrates.
The development of a natural immunity to worms is the main basis of worm control, although its effectiveness varies among individuals due to genetic differences.
Born with little resistance to worms, calves only develop immunity once exposed for several months. With age, the majority of cattle develop a strong immunity to worms, which decreases the number of larval worms developing to adults, the number of eggs laid by female worms and the length of time worms survive inside the cattle.
The age of onset of immune protection varies widely among properties and environments, mostly due to differences in the duration and intensity of exposure to worm larvae. Compared to yearlings and younger animals, cattle older than about 2 years, carry lower worm burdens of stomach and intestine worms, and excrete fewer worm eggs for the size of the burden.
Calves at foot are calves running with a lactating cow on pasture until they wean (6-10 months of age). They are usually protected from developing a significant worm burden as a degree of worm protection is passed from the dam to the calf, via the milk, which continues their protection well beyond the period of colostrum absorption. As calves at foot are not entirely reliant on pasture for their nutrition, they also limit their intake of larvae until weaning. Cows with calves at foot should be provided with adequate worm free pasture and supplements to meet the needs of growth.
Artificially raised and dairy calves are very susceptible to significant worm burdens because they get reduced or no dam immunity from their mothers’ milk. Their inherited immunity ceases beyond the period of colostrum absorption. Calves that are solely reliant on pasture for their nutrition also have a longer exposure time for worm intake than do calves at foot.
Female cattle have shown a drop in immune competence against worms when they are unable to consume adequate amounts of feed to meet their requirements. This happens most frequently during late pregnancy and early lactation.
Cattle that in lower nutritional condition may show a lower immune competence to worms than those on good nutrition. Deficiencies in macro- or micro-minerals may also contribute to an animal’s failure to control worms. In particular, deficiencies in copper, selenium and cobalt can increase the susceptibility of cattle to worms.
Modern, efficient farming methods rely on increased stocking densities and the use of paddocks to control animal movements. Worms that survive in the gut release hundreds (and sometimes thousands) of eggs per day in the animal’s dung. These eggs develop into larvae that contaminate the pasture and infest the cattle in numbers beyond their immune system’s ability to cope.
High-risk areas include calf-rearing pens, permanent pastures on creek banks, irrigated pastures and any areas that have had a high stocking rate in the past.