Dr David Burden gives an overview of the occurrence of Lungworm, its lifecycle and those factors that can give rise to an increase in its prevelance.
The pig lungworm, Metastrongylus apri, is a slender white nematode up to 6cm long that lives in the bronchial airways of the lung. Eggs laid by worms in the lungs are coughed up into the mouth, swallowed and pass out of the pig in the faeces. If these eggs are eaten by earthworms, they hatch and burrow through the gut wall into the circulatory system where they continue to grow for about 10 days before reaching the third larval stage. The larvae then stop growing and become dormant for the remaining life of the earthworm (3-4 years). If the earthworm is eaten by a pig, these ‘hibernating’ larvae become active, penetrate the intestines and make their way to the lungs via the lymph and blood vessels. Once in the lungs they mature and complete the life cycle (See figure 1 below). The migrating larvae irritate the airways, rupture small blood vessels and cause inflammation. This damage makes the pig susceptible to secondary bacterial and viral infections of the sort that can lead to pneumonia. Infected pigs often have a persistent cough, poor body condition and slow growth.
Figure 1 - Life Cycle of Metastrongylus apri
A glance at the scientific literature reveals a paucity of research work on the pig lungworm. The general view in the UK during the 1960s and 1970s was that the worm was relatively rare and the incidence was declining, so why waste scarce research budgets on it. At that time, pig farming was becoming ever more intensified, with fewer and fewer pigs having any opportunity to forage for earthworms. It was a reasonable view then, that lungworm would be no further trouble. However, in the 1980s, there was a significant movement towards outdoor pig production. This has been sustained and now about 25% of UK sows are kept outdoors. Under these conditions, surely lungworm would make a comeback. So far, this does not seem to be the case. Surprisingly, cases of parasitic pneumonia in pigs recorded through the Veterinary Investigation Service have not increased dramatically over the last ten years. A few cases have been reported from small outdoor units and from farms keeping ‘wild boar’ but it does not appear to be a major problem in large outdoor herds. In the absence of robust epidemiological studies on this, we can only guess at why this should be.
At the moment, outdoor pigs mainly form part of the arable crop rotation programme on a mixed farm, where they occupy the land for a year or two before moving on to the next field. This rotation appears to help reduce the exposure of pigs to lungworm by taking the pigs away from infected earthworms before the parasite population can build up significantly. By the time the pigs return to the same field, the infected earthworms have run their normal lifespan and have died. The practice of nose ringing outdoor sows may also be helping by reducing their digging and foraging activity. So, it appears that for the time being, the lungworm is surviving mainly in units such as small organic farms or free range ‘wild boar’ units that do not or cannot rotate their habitats. The balance for the moment is slightly tipped in favour of the pig but parasites are great opportunists and a small change of habitat condition or pig management practice could easily and inadvertently tip it the other way to favour the worm.
Some veterinary medicine companies recognise lungworm as a potential problem and ensure that their new products and formulations are licenced for use against it. ADAS can help in this by carrying out the efficacy studies required for product registration in our specialist farm animal health CRO facilities.
ADAS is actively engaged in running anthelmintic efficacy studies in cattle, sheep and pigs to the GLP (Good Laboratory Practice) standard required for product registration throughout the EU. The expertise and on-site facilities together with the network of field officers available, allows ADAS to undertake laboratory and field based studies on a whole range of parasites found in UK livestock. We are, therefore, well placed to work on pig lungworm should the need arise.
For more information on any of the above please contact Simon Jewell on 01529 309278 or email simon.jewell@adas.co.uk.