Apart from the inevitable testing of both cattle and sheep this has been a relatively quiet month for us on the farms. Because of this I thought I might pass on to you the contents of an article I came across the other day about gut parasites in cattle so that you can make more informed decisions as to the best way to worm stock for the time that they are out at grass. In cattle there are around twenty different gut parasites. Some are more abundant than others and they each occupy a specific part of the gastro intestinal tract.

In the Abomasum or true stomach the main ones are Ostertagia ostertagi, Cooperia Species and Trochostrongylus axei. Of these the first two are particularly common in younger cattle, it has been found that most calves have large numbers of Cooperia worms during the first part of the summer, but they readily develop an immunity to the worms and the eggs in their faeces decline towards the end of the summer and remain low after that.

Ostertagia on the other hand do not usually start to cause problems until the second half of the grazing season. Immunity to this parasite takes longer to develop and it is not until the host animal has been exposed to the worm to at least two years that a significant immunity develops. Immunity to ostertagia is not complete and adult animals will still carry a significant worm burden. Absolute numbers are difficult to estimate from faeces samples. Each parasite lays only a small number of eggs. When there are more worms present in the gut each worm lays even fewer eggs so the absolute numbers are difficult to calculate.

Trichostrongylus axei is generally though of as being responsible for parasitic abomasitis of cattle. This is a parasite that cattle share with sheep so it tends to occur most frequently on farms where there is mixed grazing.

The parasites that occupy the small intestine are Cooperia Species, Trichostrongyl species, Nematodirus and Strongyloides papillosus. You may be familiar with Nematodirus as a pathogen of young lambs in the spring. It can occasionally cause a similar problem in calves when they are first turned out, especially if they are on a field that has been grazed by lambs in previous years.

The large intestine is occupied by a species of worm called Oesophagostomum radiatum and by Trichuris species. Both of these are quite un common and are not usually of any great significance.

Gut parasites go through five larval stages before they mature inside the host animal and can lay eggs. This can occur in as little as fifteen days for Cooperia Species, most of the time the patent period is calculated as three weeks. Ostertagia has a cunning trick of delayed development of larvae ingested early in the winter. The fourth larval stage goes into a suspended state and does not continue until late in the winter and early spring. This is how massive numbers of eggs are deposited on the pasture early in the spring.

Studies show that even in warm and moist conditions the larvae only migrate a short distance from the dung pats. They are not able to feed and can only move five centimeters or so with the limited energy that they can muster. In wet conditions when there is heavy rain droplets can carry them for thirty centimeters or more and deposit then high on the grass stems where they are most likely to be taken in by a grazing animal. Most cattle will not graze close to cow pats unless they are forced to do so by heavy stocking rates.

When you think about it most suckler cows have a fairly good immunity to round worms and will pass only a limited number of eggs in their faeces. The problem is that, as you know, they produce a large quantity of faeces, up to thirty kilograms a day, and this can mean 30,000 warm eggs are added to the pasture every day by each of your cows. Calves that are born in the spring do not take in significant quantities of grass until they are six or so months of age. Usually they will have only a modest worm burden up to the middle part of the summer. When the weather turns wet in the autumn they start to be challenged by a more significant worm burden but many of them will have acquired a good immunity by then to limit the numbers of adult worms in their gastro intestinal system. Autumn born calves face more problems, they spend the winter away from the parasites and start to consume significant quantities of grass in the spring when the pasture contamination is at its peak and when the immune system of the host has not been primed to fight off the infestation.

The aim of a worming programme is to minimize the output of new worm eggs onto the pasture as well as to optimize the growth potential of the calves. For autumn born calves first turned out in the spring the first dose of wormer should be given after three to four weeks to reduce the output of eggs from the first crop of worms. The treatments need to be repeated at three week intervals if your wormer has no persistant activity, or up to eight week intervals for wormers that have a residual activity in the animal. If the summer is dry few larvae will be consumed and the interval between doses can be increased. Also if you are able to move the calves onto new pastures or silage aftermath in the middle of the summer they will get a break from the worm challenge. Dosing should start again in the autumn three weeks after there has been a spell of wet weather. It is important to use an efficient wormer when the calves come in for the winter to control ostertagia and also to prevent the build up of lice and mange that can be a significant nuisance in the winter months.

In theory adult cows should have a good working immunity to their gut parasites and only shed a moderate number of eggs onto the pasture each day. While this does hold true for suckler cows, that do not have a fluke burden it does not apply to high yielding dairy cows who have a stressful life and are challenged by the requirements for high yields and infections such as Bovine Viral Diarrhea and Salmonella dublin which will compromise their immune system. Most dairy herds will show a production response after an annual anthelminctic treatment. It must also have an effect of reducing the egg output onto tightly grazed pastures and bringing the down the daily challenge from worm larvae.

If you buy in adult cows it would be wise to treat them soon after they come as they are likely to be newly calved and stressed with the move to a new herd.

This is the time that they shed most eggs onto the pasture.

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