Lungworms (Metastrongylus spp.) demonstrated in domestic pigs with respiratory disease: was there a clinical relevance?

BACKGROUND: An outdoor pig herd was affected by severe respiratory disease in one out of three pastures. At necropsy, Mycoplasma hyopneumoniae and Pasteurella multocida were detected in the lungs, as well as the lung worm Metastrongylus apri. The life cycle of Metastrongylus spp. includes earthworms as intermediate hosts, and since domestic pigs mainly are reared indoors, lungworm infections have not been diagnosed in domestic pigs in Sweden for decades, not even in pigs reared outdoors. Therefore, this disease outbreak was scrutinised from the view of validating the impact of Metastrongylus spp. RESULTS: At the time of the disease outbreak, neither eggs of Metastrongylus spp., Trichuris suis nor Ascaris suum were detected in faeces of pigs aged ten weeks. In contrast, five-months-old pigs at the pasture with respiratory disease shed up to 3800 eggs per gram (Epg) of Ascaris suum and up to 1100 Epg of Trichuris suis, whereas eggs of these parasites were not demonstrated in healthy pigs aged six months at another pasture. Low numbers of eggs from Metastrongylus spp. (< 150 Epg) were seen in faecal samples from both these age categories. At slaughter, seven weeks later, ten normal weighted pigs in the preceding healthy batch were compared with ten normal weighted and five small pigs from the affected batch. Healing Mycoplasma-like pneumonic lesions were seen in all groups. All small pigs shed eggs of Ascaris suum in the faeces, compared to around 50% of the larger pigs. Metastrongylus spp. were demonstrated in 13 of the 25 pigs (52%), representing all groups included. CONCLUSION: As Metastrongylus spp. were demonstrated regardless of health status, and also in another healthy outdoor herd, the impact of Metastrongylus spp. on the outbreak of respiratory disease was depreciated. Instead, a possible association with a high burden of Ascaris suum was suggested to have preceded the severe outbreak with respiratory disease in the affected herd. Further, it was concluded that Metastrongylus spp. will escape detection at routine meat inspections made at slaughterhouses, and as they appeared to generally not induce clinical signs of respiratory disease Metastrongylus spp. may be more common in outdoor production than previously believed.