Preparation for turnout
As surely as spring follows winter, the return of the national dairy herd to fresh
pasture over the coming weeks will be followed on some farms by the sounds of
coughing and explosive diarrhoea.
But while the turning of the seasons is unavoidable, there is no reason why turnout of
young cattle from their winter quarters should lead to outbreaks of disease due to
lung- and gut worm infestations, according to a leading cattle vet.
John Cook of the Carlisle-based Paragon Veterinary Group says farmers and their
veterinary advisors have both effective anthelmintic products and the knowledge of
worm biology needed to achieve good parasite control - so for the majority of
circumstances there is no reason why any cattle living in UK conditions should suffer
an incident of clinical disease. The only reason some farms still experience lung- and
gut worm cases is because of a lack of attention to the right combination of treatment
and management strategies needed for that particular herd.
Planning how to deal with worm parasites throughout the grazing season needs to
begin at least six weeks before turnout. Vaccination against lung worm requires two
doses and time to allow the immune system of the treated calf to respond. The time
when cattle can be put out to pasture obviously depends on local weather conditions
and will usually be later in northern latitudes so preparations should therefore begin at
some time from the middle of February onwards, he points out.
However, the lungworm Dictyocaulus viviparus is very much a special case among
the various worm species that cause disease in cattle under British conditions. It is
more mercurial than those worms found in the gastrointestinal tract and farmers can’t
guarantee that their stock will experience the continual low level exposure needed to
develop natural immunity. So it is worth giving nature a helping hand and fortunately
there is relatively inexpensive and highly effective vaccine product that will give
lifelong immunity.
So, for the more predictable gut worms there is no need for a vaccine - even if one
were available. Preventing disease depends more on predicting the level of risk and
managing the herd accordingly. Certainly there is no need for expensive technological
fixes as the cheap, old established ‘white drench’ type products gives all the
anthelmintic power that is needed in most situations, Mr Cook says.
As in the sheep industry, anthelmintic resistance has emerged in the species of worms
found in cattle and certainly does pose a serious threat. But the risks of these
problems developing on a particular farm are largely determined by how frequently
anthelmintics are used. The hazards may be minimised by a combination of two
strategies. These are light use of appropriate anthelmintics to suppress worm numbers
at key times in the grazing season and evasion, avoiding exposure of naïve young
stock by moving them on to clean pasture before significant numbers of immature
worms can build up on pasture.
A ‘mix and match’ approach combining elements of both strategies is the best way
forward for most farmers, Mr Cook says. This will take into account both the effects
of variation in environmental conditions and the various pros and cons of the two
basic approaches. With repeat treatment to suppress the worm population, for
example, there is the risk that the cattle will not be exposed to enough worms to
ensure immunity is subsequent grazing seasons. Meanwhile, there are also potential
risks in trying to evade worms - the dose and move strategy involving treating young
stock with anthelmintic before moving them to clean pasture may create resistance
problems. Small numbers of worms are likely to survive treatment and then become
the only parasite strains found on the new pasture.
Other factors will also have to enter the equation, most notably the environmental
conditions that will determine the numbers of immature parasites surviving on the
pasture. After a cold winter, the initial number of worms is likely to be low and so the
build up of infectivity on grass will be delayed. Farmers also need to take account of
the effects of cold stress on the worms which can create a state of arrested
development in any surviving members of the main gutworm species Ostertagia
ostertagi. These may then become activated several months later causing type II
ostertagiasis in older cattle in their second spring rather than the classic type I disease
in the summer of the first grazing season.
Yet another issue that should be taken into account is the intended use of the
particular animal. With those cattle being kept for fattening and eventual sale for
slaughter, the treatment strategy is likely to be different from that needed for
replacement heifers. In the former group, suppression of the worm burden during the
height of the grazing season can be achieved with a drench or a sustained release
anthelmintic bolus system. But for valuable breeding stock likely to spend several
seasons on the pasture, it is worth investing in the more sophisticated - and therefore
expensive - pulse release boluses. These can provide the right balance between
treatment and intermittent exposure to worms that will ensure lifelong immunity, Mr
Cook says.