Endoscopy as a novel method for assessing endoparasite burdens in free-ranging European shags (Phalacrocorax aristotelis)



  1. Parasites are considered to be a key driver of evolutionary processes in wild animal populations. However, assessing host-endoparasite burdens non-destructively is problematic. Collection of faecal samples can be difficult, and faecal egg counts may not always be a reliable indicator of infection intensity.
  2. Here we report on endoscopy as a method for assessing natural burdens of nematode parasites Contracaecum rudolphii Hartwich, 1964, in a wild seabird, the European shag (Phalacrocorax aristotelis, L.). We aimed to measure natural individual parasite burdens and repeatability of burdens over time, and to verify that treatment with ivermectin removed parasites.
  3. Endoscopy was rapid, averaging 6 min (n = 157), with no obvious adverse effects on behaviour or breeding success compared to non-endoscoped birds.
  4. Nematode burdens in the oesophagus and proventriculus of conscious shags were counted and classified as absent, low, medium or high using a flexible gastroscope with a camera attachment that recorded video footage.
  5. Re-assessment of worm burdens was highly accurate, with 94% of randomly selected videos (n = 50) giving identical categorical scores, and 70% of worm counts (n = 40) giving the same total or differing by only one worm.
  6. All birds were parasitised by C. rudolphii. Natural burdens were significantly higher in males and in late breeders.
  7. Individuals had highly repeatable categorical parasite scores over time with 65% of control birds sampled more than once (n = 17; mean interval between assessments= 10·8 days) showing no change in scores. However, although the rank ordering of birds based on categorical scores remained constant, more finely resolved quantification indicated a slight seasonal decline in worm counts within individuals.
  8. Treatment with ivermectin (4 mg kg−1 of bird weight) resulted in complete removal of parasites. There was some evidence of temporal declines in worm counts with lower doses of ivermectin, including a dose (0·7 mg kg−1) previously shown to impact chick survival and growth.
  9. Endoscopy has considerable potential for investigating individual variation and temporal changes in endoparasite burdens and drug efficacy. Applicability and limitations of this method for other host-parasite systems are discussed.