| Parasitic Gastro-Enteritis of Cattle I |
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By Stephen Champion, BSc (Hons) BVM&S MRCVS, WHAT IS PGE? Parasitic gastro-enteritis refers to the infection of the abomasum (the true stomach) and the intestines with nematode worms. Once infection builds up, cattle become progressively ill-thriven, which has serious economic and welfare implications. The loss of productivity from reduction in weight gain is tangible in affected cattle, and is significant! The most significant parasite is the abomasal worm, called Ostertagia ostertagi. Infections with this worm are most commonly seen in young calves during the summer and early autumn, and is called Type I disease. The worm larvae are ingested by the cattle when grazing, from the grass around dung pats, enter the abomasum and become adults in about three weeks. During this time they cause severe damage to the lining of the abomasum, leading to digestive disturbances and the inability of the calf to digest and utilise its grass. This leads to poor weight gain and scour. Meanwhile the recently developed adult worms produce millions of eggs, which are passed out onto the pasture to complete the lifecycle. Organic producers required to monitor faecal egg counts will find high numbers in the dung of cattle with type I disease. There is another facet to the lifecycle of this worm. Some of the larvae acquired in the Autumn arrest their growth in the abomasum, and remain as larvae throughout the winter. Then, in the early spring, they recommence the development, causing massive damage within the stomachs of your carefully nurtured yearling cattle. This is called Type II disease and can cause massive early season losses in productivity and even deaths. Faecal egg counts will be negative, as adult worms are not involved in type II disease. In addition, infective larvae are capable of surviving over the winter on the pasture. Calves turned out in the spring ingest these overwintered larvae, which develop into adults. These contaminate the pasture with parasite eggs passed in the dung, which develop into a new generation of larvae as the season progresses. As larvae build up on the pasture, disease in cattle starts to occur, from mid-summer onwards. This is a fundamental point to understand, as it forms the basis of several worm-control methods. Spring-born beef calves are generally not grazing sufficiently in early spring to recycle the overwintered larvae. Type I disease can however be seen in autumn-born calves grazing from April. The rate of development of larvae to the infective stage, and their longevity on the pasture, is dependent on temperature and rainfall. In temperate climates this leads to a fairly predictable cyclical appearance of these species on the herbage, which can be useful to know. Infections with some species of the intestinal worms are known to add to the deleterious effects of Ostertagia over the summer, although do not usually produce such serious effects in their own right. Tapeworms, which are not nematode worms, are generally of little importance in cattle, although a certain part of their lifecycle can infect humans and cause disease. This rarely happens, however, in the UK, but is of great importance abroad. TYPES OF WORMERS USED IN CATTLE IN THE UK Anthelmintics (wormers) are widely used in both the treatment and prevention of PGE in cattle. On the basis of their chemical structure, and mode of action, they are divided into several main groups. It is important to understand that the main groups exist, so that rotation of anthelmintic groups can be planned from year to year, thus minimising the likelihood of parasites becoming resistant to these products. The names of the groups have been simplified for the purposes of this article, such that they may be easily remembered when perusing the pharmacy shelves. Group 1 anthelmintics are the white drenches or benzimidazoles, and first appeared on the market in the 1960s. Since then, they have become safer and more numerous, and function by disrupting the energy metabolism of the worms, killing both adult worms and their eggs, which is beneficial in some types of dosing strategies. They are usually given as a drench by mouth or as a bolus. Examples are fenbendazole (Panacur) and oxfendazole (Autoworm). Group 2 anthelmintics are the levamisole type, and cause nerve paralysis of the worms. They can be administered by drench, injection, pour-on or bolus, and are sometimes combined with other products to product activity against liver fluke in addition. Group 3 anthelmintics are the avermectin type, and include such products as ivermectin (Ivomec) and doramectin (Dectomax). They are often called the endectocides because they have the ability to kill external parasites too, such as certain mites and lice. For this reason, they are useful at or before housing. They work again by paralysing the worms and are available for administration by all the major routes. An extremely useful property of this group of compounds is their ability to be stored in the animal’s body fat, from where they are slowly released. This affords extended periods of protection and hence the long dosing intervals, especially important in extensively grazed cattle. However, great care should be taken when dosing dairy cattle with this group, as most products have very long milk withdrawal periods! METHODS OF ANTHELMINTIC DOSING Many anthelmintics, most notably the benzimidazoles, are given by oral drench, and are available as suspensions for this purpose. Care must be taken to shake these before use and to give the correct dose. This is achieved by estimating or preferably weighing animals carefully, since doses are based on the live-weight of the animal to be dosed. The drenching equipment must be correctly calibrated and in good working order and the dose given carefully over the back of the tongue, so that it enters the rumen. Broad-spectrum anthelmintics, such as levamisole and the endectocides, can be given by injection. It is important to use sterile and sharp needles, which are changed frequently, to minimise risk of abscess formation at the injection site. This can be a very effective method of worming cattle. Several preparations are available for in-feed inclusion. It is difficult to ensure that all animals receive the correct dose of wormer, and better methods are available. Pour-on preparations contain either levamisole or the endectocides, and are applied along the flattest part of the back from withers to tail-head. This is another effective method of administration, since the active compound is absorbed through the skin or hair follicles. Care should be taken to avoid treatment when the animal is wet, or rain is expected within two hours of treatment. Farmers in Perthshire this summer need not apply! Bolus devices are a popular and labour-saving means of administering anthelmintics to cattle. Given at the start of the grazing season, they can protect right through the season. Great care should be taken when administering, and the correct dosing gun used. The gun should be inserted over the back of the tongue and the plunger depressed as the animal begins to swallow. Bolus devices fall into two catagories: the sustained-released devices and the pulse-release devices. Sustained-release devices, such as the Ivomec SR Bolus, release anthelmintic continuously over a period of time, and work by a number of different mechanisms. The Ivomec bolus, for example, consists of a semi-solid formulation of Ivermectin, enclosed in a semi-permeable membrane. It is held in place in the first or second stomach by an iron densifier at one end and releases active ingredient over 135 days. Pulse-release devices, such as the Autoworm range, consist of five to seven tablets of a benzimidazole mounted on a central metal core with a weighted end. The release of each tablet is cleverly determined by the corrosion rate of the central core and occurs at three-week intervals, giving an active life of the bolus of up to 147 days. The pulse-release devices allow regular periods of time during the grazing season when the cattle are not exposed to the anthelmintic. Cattle are thus exposed to worm larvae on the pasture for a limited period of time, which encourages a natural immunity to develop without the risk of a serious worm burden. In the next edition of Farming Scotland, Stephen will discuss how the strategic use of anthelmintics and clean grazing can be best employed to control gastrointestinal worms in cattle. |
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