Parasitic Risk
When cattle graze in the pasture, they are likely to be infested by many parasites, whose diversity is extremely important. Generally, these parasites require one (autoecious) or several (heteroecious) intermediate hosts to infest the final host.
Once installed, these parasites have a direct impact on livestock, either on quantitative (delayed growth, reduction in milk production) or qualitative level (quality of wool, statute of carcass, quality of milk, etc). It has therefore essential to treat these infections.
Primary control that was implemented was the inoculation of the cattle with chemical molecules, known as anthelminthics (AHs), which attack parasites and neutralise them. This treatment is called chemotherapy. It is also used as a preventive treatment on plots in order to limit the installation of parasites.
Two observations were made concerning the direct use of these molecules over time:
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The appearance of parasitic strains resistant to these anthelminthic molecules, which render those molecules ineffective, requiring the use of other molecules. These resistant strains were observed on sheep and goat farms (Jackson & Coop, 20001) ; Kaplan, 20042)), but not yet on cattle farms. It is important to notice the appearance of ‘multi-resistant’ strains which are not sensitive to any chemical molecules, making any chemotherapy attempt totally useless. There are not many ways left to fight these multi-resistant parasites.
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Environmental impact of these molecules. Once introduced in the organism of the livestock, these molecules are sooner or later released (by dejections, death of the animal, etc.), putting them in direct contact with environment. Unsuited for other fauna, these molecules are quickly becoming toxic, specially for Microfauna (rodents, insectivores, etc…) (Van Wyk et al., 19973))
Parasitic Contamination and Recycling
It is important to conceive that the mere presence of parasites in the direct environment of livestock is not enough to present a real risk. The biological cycle of the parasite, the host immunity and climatic conditions are all factors that influence the capacity of parasitic contamination.
When the combination of these factors is favourable to parasites, a slightly contaminated plot can reach very significant contamination within few weeks. For Ostertagia ostertagi, the length of larvae evolution (and thus the time necessary for contamination of the plot) depends on the spring temperature. To evolve from an egg to a larva of third stage, it takes one week at 22°C, more than four weeks below 10°C (Chauvin, 20094)).
Certain contaminations require several parasitic cycles. The first generation of parasites have to give birth to a generation of larvae. These larvae have to grow to an adult state thus forming the second generation of parasites. They will give birth to the third generation of parasites (via a generation of larvae), and so on. Thus the third or fourth generation of adults (more or less) will contaminate the plot occupied by the first generation to a significant degree.
Preventive Measures and Alternative Treatments
If we want anthelminthic chemical molecules to retain their efficiency in order to preserve the environment from their residues, it is important to use other preventive or treatment methods.
It has been shown that in some cases, a mere rotation of pastures reduced the rate of ruminant contamination. Regular contact with parasites enables animals to develop acquired immunity, meaning a development of natural defences limiting a multiplication or installation of parasites. However, this acquired defence disappears if the parasite disappears.
In the case of such immunities, parasites are concentrated in a small minority of the herd (10 to 15%) in order to exceed the contamination threshold (from which effects are observed on animal performance (qualitatively or quantitatively), as observed on dairy cows (Agneessens et al. 20005)).
Possibilities of natural anthelminthic molecules were also explored. Replacing chemical molecules, such molecules are present naturally in some legumes. The presence of these forage plants in pastures would thus significantly decrease the use of chemotherapy (preventive or active) (Hoste et al., 2006 6)e.g.