VN October 2021

Vetnuus | Oktober 2021 36 Regulars I Zoetis Livestock Column Gastrointestinal helminth infections are well-known to cause significant constraints in the productivity of animals. The extent of gastrointestinal helminth infection in pasture-based dairy herds are often underestimated, but under the right circumstances, grazing dairy cows and heifers can be heavily infected with worms. Excessive gastrointestinal worm infections can affect the health and welfare of the animal and have devastating economic consequences by affecting essential aspects such as growth, production and fertility. 1,2 An increasing amount of studies have demonstrated the impact it can have on milk yield. 4 Understanding the dynamics of host-parasite interaction is the first step in attempting control of gastrointestinal helminth infections. Pathogenesis Cattle nematodes will differ in their life cycles and pathogenicity, but let’s take Ostertagia ostertagi , commonly known as brown stomach worm, as an example. After ingesting the infective larvae of this parasite, it encysts itself into the abomasal wall, where it interferes with the normal functioning of the gastric glands. 1 After a moulting phase, adult parasites emerge, causing additional tissue damage. This tissue damage then leads to the characteristic foul-smelling diarrhoea accompanied by anorexia. 1 Emerged adults feed on the mucosal lining of the gut and start laying eggs. The entire developmental process, on average, takes approximately 21 days. 1 Risk of infection The risk of exposure to the infective larvae on pasture is dependent on multiple factors such as fecundity of the specific helminth species, climate, humidity, stocking density, the duration of the grazing season and time spent grazing each day. 1,2 Some helminth species can reproduce at a faster rate than others, for example, Cooperia spp. vs Ostertagia ostertagi . 1 High fecundity combined with optimal conditions of temperature and environmental moisture can lead to massive pasture contamination. 1,2 While the correct temperature is essential to the development of the helminth from the egg stage into the infective larvae stage, the level of moisture in the environment is just as crucial. 1 Both temperature and humidity are also vital for the migration of the infective larvae away from the dung pat – a survival mechanism employed by the helminth to increase its chances of ingestion by the next host. 1 Intensive grazing conditions with high stocking density will naturally lead to animals depositing more manure (and worm eggs along with it) per area of pasture. The infection pressure on the pasture will therefore be a factor of the frequency and physical amount of manure deposited. Interestingly, studies have shown that the frequency of defaecation is related to animal age – on average, a calf will defaecate 6.23 times per day compared to 8.1 times for a cow. 1 The amount of manure deposited per defaecation, on the other hand, is determined more by the weight of the animal. Therefore, if the weight, age and eggs per gram (EPG) of manure for an animal is available, one can make a rough calculation of the pasture contamination contributed by that animal over a period of time. 1 It has been estimated that a cow and calf kept together on pasture can deposit up to 51 million helminth eggs in roughly 3 tons of manure over five months. 1 Zoetis Animal HealthPages FOR ANIMALS. FOR HEALTH. FOR YOU. The control of gastrointestinal helminths in pastured dairy cows Dr Chantelle Erwee, Zoetis South Africa (Pty) Ltd, Technical Manager: Ruminants