Nematodirus Battus

Strongyl worms can have a complicated life cycle. They have adapted over the years to outwit the defence mechanisms of their host so that they can survive from one generation to the next. At their simplest they are ingested by a host animal, mature to the adult stage so that they can reproduce and lay eggs. The eggs pass through several larval stages either within gut of the host or on the pasture before they are mature enough to repeat the whole process in another host animal. Lungworms go through a more complicated journey as the larvae pass into the circulation of the host and eventually mature in the lungs. If successful they go on to shed their eggs in the deep airways of the lung where they gradually move upwards with the mucus to pass through the whole of the gastro intestinal tract before getting out onto the pasture once more.

Survival Strategy

Nematodirus has adapted a different survival strategy. It specifically targets lambs the eggs that are produced by the host animal do not mature straight away, they lay dormant on the pasture over the winter and all become active in the spring when the new season's lambs start to graze. Because of this the lambs are faced with an overwhelming infection that is difficult for them to cope with. In theory it is better to put lambs onto a different field each year, but that is not always possible. The worms can mature very quickly so to keep one step ahead of the parasite you might need to worm the lambs every three weeks to control the life cycle of the worm. Nematodirus affects lambs at the same time as they are trying to build up immunity to coccydiosis. The two diseases can occur at the same time and cause severe disease.

Contaminated Milk

Considering the amount of milk that is produced it not very often that mistakes are made and antibiotics and other residues find their way into the bulk tank. Indeed as far as I can tell the last survey that was carried out into this matter was more than ten years ago although more recent information is available from America. It is much more difficult to get samples tested now to determine if they are free of residues, as few of the milk companies seem to offer this service. The test kits that you can use on the farm do have their uses, but you need to be careful about how you interpret the results. Readings that trigger a positive result for the different drugs from the kits are not the same as the levels that are allowed by the milk companies.

The main reasons for a positive bulk test are accidentally allowing milk from treated cows to go into the tank, milking Dry Cows by mistake or not realising that recently purchased cows have been treated with antibiotics. Antibiotics are attracted to the fat in milk and this tends to stick to the jars. It is wrong to assume that it is safe to milk a treated cow into a recording jar so that it can be washed out afterwards. It is too easy for valves to leak and difficult to wash the jar thoroughly enough before the next cow is milked. The only safe way is to use a separate cluster and dump bucket. This can be difficult if you start to get a number of cows that are under treatment or recently calved and they all come in to be treated and milked at the same time.

Identify Cows under Treatment

More than half of the reported cases from America were from someone milking a cow under treatment, a cow that had been treated with dry cow tubes or cows that had not been correctly identified as under treatment. This is the most important thing to get right. You should have at least two ways of identifying cows in the herd that need to have their milk kept out of the bulk tank. This might be their number recorded on a board or in your computer system and coloured spray on their back legs or tape on both hocks. Recovering lame cows are a particular problem as there are few visual clues that they are under treatment and can easily be missed.

Special Cases.

Other grey areas are when you use the drugs in a different way to the recommendations by the manufacturer as can happen if you are milking three times a day. Also if you are injecting a cow because the mastitis is severe or are treating a cow for lameness and mastitis at the same time it is much more difficult to know when the milk will be clear to go in the tank. Some drugs are now licensed to be used together in combination but in general if you are injecting and using tubes at the same time you should use the standard milk withhold period of seven days.

Testing Kits

I must admit to having little experience with antibiotic screening tests that you can do on the farm. I think they should be used with caution, as they do not measure the same amount of antibiotics as the maximum residue levels allowed by the milk companies. Some of them are limited in the range of antibiotics that they detect. One of the better ones the Delvo SP measures Cloxacillin down to 20 parts per billion when 30 parts per billion is allowed so that is quite safe.

It will detect Tylosin at 100 parts per billion whereas the maximum residue level for this drug is 50 parts per billion. You could easily think that the sample was clear when in fact the milk should have been kept out for another day or so. When you come to rely on any test kit it is all too easy to run out of the bottles or chemicals that are needed to do the tests. So it would be better to use the kits just as a guide, especially if you have used a combination of drugs on one animal or if a cow has calved early, and have any doubtful sample checked elsewhere if at all possible.

Cryptosporidium

This parasite seems to be everywhere at the moment. Last month it turned up more times than everything else put together as the cause of scour in young calves diagnosed at our local Veterinary Investigation Laboratory. Just why it should have been so troublesome this year is not clear. It thrives best in a dirty environment, but there have been few straw problems this winter and there does not seem to have been a dramatic deterioration in the way that calves have been housed.

There is a drug now that helps to control the disease. It is given to young calves for seven days to stop the build up of the parasite in the calf so that if they do scour the symptoms will be milder and more treatable.

Cryptosporidium can be present at the same time as other diseases and it is worth controlling if you regularly loose young calves that are scouring when your usual treatments fail to work. 

BSE in Canada

It seems to have taken a long time for BSE to have been diagnosed from North America. The affected animal was six years old and came from a farm in Saskatchewan in Canada. It is unlikely that there can be any doubt about the diagnosis as it has generated so much publicity. The mother and some of the calves from the cow are still alive and they are to be investigated at post mortem to try and determine if any of them have early signs of the disease as well. It will be interesting to see the response from the meat industry in America.

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