10 April 1998

NOT ONLY THE CALF AT RISK

MUCH has been written over the years about how to protect the calf in its first grazing season from the problem of worms, writes independent vet Dr Tony Andrews.

Much less, has been said about the second grazing season. At present this appears to be becoming more important, as suggested by the increase in clinical outbreaks of husk predominantly in dairy heifers and cows.

Disease results when there is an imbalance between immunity and exposure. The difficulty arises when animals with inadequate immunity are exposed to high levels of pasture challenge. Thus, if animals are entering the main herd without adequate immunity to the lungworm parasite, they may breakdown with husk at some time during their adult life.

Has husk increased?

The answer does seem to be definitely yes. Using VIDA data, the number of husk diagnoses annually by the Veterinary Investigation Centres in the 70s and 80s was about 100 or usually less.

A rise was seen in 93 with a 95% increase and disease in adults was a noticeable feature. There then followed a reduction in 94 and 95 only for there then to be a rise.

Thus the VI Centres showed a rise of 136% from 1995 to 1996 and 62% from 96 to 97. Support for an increase in disease is also suggested from the data of the National Animal Disease Information Service. There was an overall rise of 67% in husk outbreaks between 95 and 96. A similar increase occurred between 96 and 97 with a rise of 44% in youngstock and over a doubling for adult disease.

But while these rises are alarming, they must not be taken out of context; most herds of cattle still do not suffer from clinical outbreaks of husk. Indeed, the increase is probably part of the epidemiology pattern of the parasite which has been influenced by weather conditions and changes, altered agricultural management practices and economic trends. These last two have in turn led to different worming and vaccination practices, a marked reduction in lungworm vaccination – compared with the 80s – and in some farmers minds the consideration that vaccination and worming are synonymous and do the same job.

&#42 Immunity to worms

This is a complex situation; while exposure and immunity to stomach worms are predictable, those to husk are unpredictable. This is where the difficulty arises. But in both cases immunity does depend on exposure to the worms. Also overexposure to the worms in susceptible cattle will result in disease.

While most farms have infective husk larvae on them, not all do. Even when pastures are known to be infected, the time after entering the pasture until when they will be exposed to the larvae is variable and unpredictable.

The number of parasites to cause disease is very low, probably a few hundred. As the number needed to cause illness is low, it is easy to spread infection indirectly to a farm on boots, vehicle tyres and via natural vectors such as wildlife and on fungal spores.

How long infections can survive on pasture is also not known, but it is probably over a year. The story is even more complicated as some apparently healthy animals will contain small numbers of parasites in their lungs for long periods and not show any signs of disease. These larvae will help to seed the pasture for other animals to become infected or boost their immunity.

Immunity is, however, rapidly acquired after exposure to infection often in under two months or after vaccination.

It appears that protection from husk does not exist indefinitely and needs regular exposure to the lungworm infection on pasture to boost immunity. Thus if cattle are not exposed to infection at pasture for a while, probably a grazing season or less, that immunity may be partially or completely lost in some animals and signs of clinical disease develop when they again meet infective lungworm larvae.

This makes it much more difficult to ensure that cattle remain immune and helps explain husk outbreaks in older and adult cattle.

Avoiding lungworm is all about immunity and this only follows exposure to the parasite. In practice this can be through contact with natural infection on pasture and then the animals becoming infected and immune. The timing of this and the amount of infection encountered will be very variable and unpredictable.

This could well mean that animals develop disease and become very ill or die, or they may be exposed and develop immunity without any apparent signs of disease or they may not be exposed at all and so not develop immunity.

Thus a more predictable way to gain immunity is to allow them exposure to a controlled amount of lungworm larvae which have been castrated by irradiation so that they do not reach the mature adult stage. This is the principle of vaccination which has now been effectively used on farms for more than 30 years.

So how do we reduce the chances of parasitic disease in the second grazing season or later? There is no easy or complete answer to this. However there are some guidelines which may be useful to follow.

The first and most important thing is to discuss the situation with your vet. Between you, you should be able to determine likely previous exposure to husk and hence the probable immune status and how you should proceed with grazing and worming this season.

If the farm has never had husk on it, is not vaccinating, is not buying in cattle and is isolated from other cattle premises then there is probably no need to do anything different to the normal worming routine.

On farms where husk has occurred, even if a long time in the past, life is more difficult.

Whether or not vaccination was undertaken and whatever worming programme used, unless the cattle were exposed to the lungworm infection in the first grazing season and become immune, they may be susceptible to picking up sufficient infection to go down with disease.

Ideally, you need to know if the animals were exposed to infection the previous season. If they were, then there should be little worry in the second grazing season. But if exposure was minimal or non existent then they will be susceptible and like to develop clinical signs if grazing heavily infected pastures.

There is an ELISA blood test that gives an answer as to whether or not the cattle were exposed recently to infection. The test is expensive but if this is done at the end of the first grazing season on a sample of the cattle it should indicate whether or not they have picked up lungworm.

If the cattle were exposed to infection then they should be immune and so not cause difficulties in the next season.

Hopefully, these animals will again be exposed to infection in the second season and so further boost their immunity. Again if there is any doubt about this an ELISA test at the end of the season will confirm whether or not this is so.

Management history

In the case of those which do not show any signs of being exposed to infection by the ELISA or where not tested and the management history might suggest likely poor exposure to lungworm, decisions must be made.

If the animals were previously vaccinated then a booster before turnout in the second grazing season should be considered.

Generally, one dose should be sufficient. In those which have not previously been vaccinated then it is best to institute a full vaccination programme with the two doses of vaccine with a month between them and then not allowing them out until a fortnight after the second dose. When the animals go out, they should receive a booster to their immunity if it is a pasture grazed by cattle the previous year other than themselves. This should then mean that it is not necessary to worm for husk.

If the ELISA test is not undertaken but it is possible lungworm exposure was not very likely, then a blanket form of advice would be to vaccinate animals on infected farms before turnout in the second grazing season.

In herds not previously done then the routine two dose vaccine programme should be used. In those which have vaccinated and there is some doubt as to whether or not they have been exposed to husk, at least a single dose would be advisable.

If the farm is husk-infected then these second season vaccinated animals should still have an adequate boost to their immunity to prevent them developing disease when they enter the main herd. This again is where you should discuss with your vet as some farmers send away their heifers to other areas which might just not be infected and so not produce the required immunity boost. If there is any worry about this the second season grazing animals should again be tested for the presence of antibodies at the end of the grazing season. &#42

Almost all clinical cases of lungworm now involve heifers and cows rather than calves as was the case two decades ago.

Huskcontrol programme for the second grazing season

Farms with or without husk infection should always have a practical control programme for husk control or prevention worked out for the life of the animals on the farm with their vets.

Farms can be divided into those where husk infection occurs – most farms, and those which are uninfected – very few. An uninfected farm is one which has never had animals with husk signs present on it, all dung samples always negative for husk larvae, ELISA samples from all age cattle groups negative whenever sampled over several years If in doubt it is always best to consider the farm to be infected.

&#8226 Infected farm (most farms)

(a) First grazing season: Non-vaccinated EXPOSED TO INFECTION (Disease signs, larvae present in the dung or ELISA test samples positive)

Second grazing season probably safe from husk so allow to graze. A wormer should not be needed for husk, but an appropriate worming programme may be needed to control PGE.

(b) First grazing season: Vaccinated EXPOSED TO INFECTION (ELISA test samples positive) On most vaccinated farms the animals will be exposed and immune.

Second grazing season probably safe from husk to allow to graze. A wormer should not be needed for husk, but an appropriate worming programme may be needed to control PGE.

(c) First grazing season: Vaccinated NOT EXPOSED TO RECENT INFECTION (ELISA test samples negative or mainly negative)

Second grazing season: Consult your veterinary surgeon as these animals may not be immune. If immunity thought incomplete, revaccinate. It is probably safe to use a single dose of vaccine. A wormer should not be needed for husk, but an appropriate worming programme may be needed to control PGE.

(d) First grazing season: Non vaccinated NOT EXPOSED TO RECENT INFECTION (ELISA test samples negative or mainly negative)

Second grazing season: Vaccinate using usual two dose vaccine programme. A wormer should not be needed for husk, but an appropriate worming programme may be needed to control PGE.

&#8226 Uninfected farm – No stock bought in, not vaccinating, isolated from other cattle farms, always using same land for grazing. Continue worming as before.

If you must buy in, purchase animals which have not grazed.

If have to buy in grazed stock, house at once, and worm on at least two occasions at an interval of about a month or longer before they go outside. Use a suitable wormer which is able to deal with larvae included those inhibited or hypobiotic as well as adult worms.

This applies to adult cattle as well as heifers and youngstock. These measures should remove any residual infection – but be on your guard.