By Lisa Whitfield

Worms are a significant risk for stock kept at pasture. The risk is greatest for young stock, who need to develop immunity to worms in order to be able to clear themselves of parasites. The full development of immunity to important species of worms takes 18 to 20 months in cattle. In the meantime there is lingering susceptibility to the development of production-limiting disease if good management practices are not put in place and followed.

We are lucky to have only a small number of worms which we need to manage in cattle. Common worms which you need to be aware of are: Cooperia oncophora, Trichostrogyle spp., Ostertagia ostertagi, and Dictyocaulus viviparus (lungworm).

Cooperia oncophora

Cooperia inhabits the small intestine. Numbers peak on pasture in autumn, and it is most likely to affect youngstock at this stage, during their first year. Cooperia doesn’t like cold weather so doesn’t persist well between autumn and spring when there is a good winter in between. Cattle typically develop immunity to this worm by one year old.

Gastrointestinal disease from Cooperia infestation is frequently on farms which practice exclusive reliance on single-action macrocyclic lactone pour-on drenches for their calves. There is widespread resistance of Cooperia to macrocyclic lactone drenches.

Trichostrongylus species

Three Trichostrongylus species can affect cattle in New Zealand – T. axei, T, vitrinus and T. colubriformis. These worms can inhabit both the abomasum and the small intestine of their hosts. Immunity to Trichostrongylus does not develop until young stock are about 18 months old. Larvae are tolerant to the cold and numbers peak on pasture in spring, so immunity to these worms follows this peak of exposure.

Ostertagia ostertagi

This is considered the most significant worm of cattle in NZ. Ostertagia inhabits the abomasum and has the potential to cause significant damage to the lining of this organ. Larval numbers peak in autumn so disease is often seen at this time. Larvae also become dormant in the lining of the abomasum over winter, so disease can also occur the following spring and summer if good immunity has not been developed.

There is low efficacy of levamisole and benzimidazole drenches against Ostertagia. On most farms, macrocyclic lactones are effective in controlling Ostertagia, however, resistance to macrocyclic lactone drenches has been found in New Zealand and this should be of concern to all cattle farmers.

Dictyocaulus viviparus (Lungworm)

Lungworm lives in the airways of cattle where it causes inflammation and irritation, and in severe cases leads to the development of secondary pneumonia. It is quite common for this worm to cause disease in young cattle in New Zealand. Immunity develops within a few months of exposure. Where there is frequent drenching of youngstock such that exposure is insufficient to cause a good immune response, it is possible to see lungworm disease in mature cattle.

Rearing Practices which underpin good worm management

Healthy, well fed, and stress-free animals have a head start on the process of immune system development against worms. The provision of good quality nutrition in good quantity is fundamental to the good health of young stock.

Healthy calves regardless of breed grow from 0.6kg to over 1.2kg per day and failure to achieve this indicates a management problem which should be identified and actively addressed. In order to be able to quantify that they are achieving this weight gain, you need to regularly weigh your youngstock.

Guesstimating or eyeballing will not provide the level of detail required to know whether young stock are meeting their growth targets on a short-term basis. Individual animals which fall below this target are good candidates for a targeted drench programme. They should also be examined for other common diseases of young stock.

Grazing management practices which see areas of pasture grazed exclusively by young stock with no cross-grazing by adult cattle or other species of livestock year on year, and under the right climatic conditions, can see the build up of worms in the environment and very high infection pressure on successive groups of calves grazing there.

Regularly shifting young stock rather than having them graze pastures to low levels reduces exposure to the areas of pasture which carry the highest larval loads – that is the few centimetres of pasture closest to the ground. Grazing strategy should balance access to good quality pasture with not grazing too close to the ground.

Physiological stressors such as weaning, underfeeding, poor weather conditions, lack of shelter, and concurrent diseases, will all suppress immune responses and increase the risk of worm burdens getting out of hand.

To ensure worm management of cattle is sustainable, we need to modify our rearing practices to give our stock their best chance at dealing with worms themselves, rather than relying on drenches alone.

  • Lisa Whitfield, is a Manawatu production animal veterinarian.