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on May 7, 2008

Roundworms damage the health defences

Ascarid worm parasite infestation is a long-known problem that has returned to the headlines because of its impact on the pig's ability to resist other disease challenges.

We all know our pigs can be infested by roundworms. This may not seem a worry as long as no complaints are voiced by the slaughterhouse. But it is worthwhile to sound the alarm from time to time and review our anti-parasite procedures, because it is absolutely certain that uncontrolled endoparasitic infestations cost money.

All indications from extensive experience are that the cost in European terms can be as high as €5 per pig slaughtered. The economic damage will hurt especially in these times of high feed cost and low market prices. Even in more normal situations a problem with worms is able to decrease the herd's labour income by 10-15%. But when low prices mean less earned per pig while feed costs are higher, the impact may be nearer to 50%. As a research publication in 2007 confirmed, an uncontrolled roundworm infestation will lead to a further decline in the labour income of the pig unit by decreasing the daily growth and increasing the feed conversion rate.

It is always a good idea to refer first to what we know about the life cycle of the parasite before formulating our options for worm control. In the case of the roundworm (Ascaris suis), the cycle is special because the pig serves both as the host and as an intermediate host so there is no need for the parasite to leave the body.

Between the ingestion of the eggs and the development to an adult worm, it takes a minimum of 35 days. After the pig ingests embryo-stage eggs, the larvae will soon be set free to penetrate the gut wall. They can then make use of the blood vessels of the digestive system to reach the liver. Once inside the liver their presence is evident as the so-called milk spots formed by migrating larvae. The next step in the larval migration is from the liver to the lungs, where they also cause damage.

Finally, the still-evolving larvae will be coughed up and swallowed down again by the pig. During this second passage in the digestive system, they will make themselves comfortable in the small intestine and become adult worms. As an adult they can produce up to 200 000 eggs per day. These are excreted with the faeces and so the cycle is complete.

Roundworms do more than simply steal nutrients from the host animal. The tissue damage they create during their migration through the organs is a further challenge to the pig's health, more in the lungs than the liver. Milk spots in the liver will have vanished almost completely after 6 weeks because of this organ's high capacity for regenerating itself. However, the lungs do not heal themselves in the same way. Do not forget also that the lungs are in contact with the outside air and all it contains, from dust to gases such as ammonia as well as bacteria and viruses. Damaged lungs are more open to further assaults on their correct functioning.

The correlation between lung and liver damage due to ascarid migration has been demonstrated many times. In a recent Dutch study, finishing units with chronic respiratory problems were compared to other sites that did not have this health challenge while being similar in other main respects. As might be expected, lung damage was more frequent and more severe on units with respiratory disease. But the study found additionally that there was a statistically significant increase in the frequency of milk-spot livers on these problem farms. The finding illustrated the direct link between the migration of roundworm larvae and the respiratory disease complex calculated to be costing these units an average of €19.29 per slaughter pig produced.

Ideally, the pig's own defence system prevents intrusion by possible pathogens. Where that is not possible, these defences try to neutralise the intruder and dispose of it. For example this means, at times of peace and harmony for the health of the pigs on a unit, that the respiratory tract of each pig will trap and eject any intruder. Dust and germs are caught in the mucus produced by the mucosal cells and are brought up again by the hair-like cilia lining the upper respiratory tract.

What happens when the threat increases? At one level the defences remain intact because mucus production can be increased and the animal starts coughing to speed up the expulsion. There are also the white blood cells and antibodies in the bloodstream to counter attacks, alongside the mechanisms present in the lungs and digestive system.

As with all defensive operations, however, success in practice depends on having enough supplies of weaponry and a correct co-ordination of actions. This is where the roundworm hits. The parasite diverts essential nutrients away from the host and so disturbs the supply chain. It also disrupts co-ordination, by attracting the attention of the immune system at the expense of functions that should be directed to control bacterial and viral attacks.

More scientific data appeared last year, highlighting how migrating ascarid larvae have a negative effect on the immune response of the pig. The researchers had created 2 groups of young pigs and made sure one group was kept clean of Ascaris suis infestations while the other was trickle-infected with ascarid eggs twice each week during the course of the trial.

Three weeks into the trial period, a vaccine against Mycoplasma hyopneumoniae was given to all the pigs. Checks another 3 weeks later found mycoplasmal antibodies in the blood of every pig in the non-roundworm group, but in only 33% of the animals that had received ascarid eggs. After a further week, the pigs of both groups were deliberately infected with M. hyo. At slaughter, only 78% of the ascarid-infested group showed antibodies to the mycoplasma, compared with 100% of the control group.

Chronic respiratory problems in the nursery and grow-finish departments represent one of the biggest health challenges facing the modern pig industry. It has become common to talk about the porcine respiratory disease complex or PRDC and to recognise that a specific diagnosis of any episode is extremely difficult because of the number of microbes that may be involved. Worst still, the excellent drugs and vaccines available to us for dealing with this complex cannot provide a solution in many cases because a high infection pressure has met pigs with a lowered disease resistance.

This illustrates the principle of needing to balance immunity and infection pressure. Specifically regarding ascarid intestinal parasites, it explains why a strategic deworming programme is important in helping to set the right conditions for a good control of a respiratory problem. Strategic deworming aims to stop infection of the piglets by the sow. The most secure method is to treat all sows every 3 months we find that this frequency is necessary especially in group gestation housing systems, where sows can become re-infected much easier than in individual stalls. Special attention should be given to new gilts, who present the greatest danger of re-infecting the sow herd.

In the earliest stages of the pig's life we try to prevent infection going from sow to piglet, in order to allow the piglet's immune system to react to vaccinations such as for mycoplasma and PRRS. However, the wormer should also be applied in the finishing period as this is where we expect the largest economic damage to occur. Our objective needs to be to prevent the development of new adult worms so no new eggs can be shed into the pigs' environment.

Remember it takes an egg a minimum of 35 days to develop into an adult worm. With that in mind, deworming is recommended to consist of regular treatments given at a maximum interval of 5 weeks from the start of the finishing round. The product chosen must be active against all stages of the roundworm's life cycle. Otherwise, new adult worms will be formed in the gut of the finishing pig before 5 weeks and all previous control efforts will have been for nothing.

An as-yet unpublished study in the Netherlands has given data clearly demonstrating the improvement in liver and lung defects achieved both at farm level and on the slaughter line, by implementing a programme of strategic deworming that featured the molecule flubendazole.

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Ascarid roundworms and their larvae can cause damage in the intestines, liver and lungs that opens the pig to more health troubles from bacteria and viruses.
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