Gastrointestinal nematodes (GIN) pose an existential threat to small ruminants such as sheep and goats (Hoste et al., 2010). The two species have evolved different strategies to minimise the threat (Hoste et al., 2008). In sheep, the immune response is the major mechanism of defence (McRae et al., 2015). The nematode Teladorsagia circumcincta is the dominant nematode in cool temperate areas of the world and possibly there is more known about the mechanisms of protective immunity to this nematode than any other (Stear et al., 1995). There are three major signs of immunity (Smith et al., 1984). The first is reduced establishment and survival which is strongly associated with a local type 1 hypersensitivity response (Miller, 1984; Murphy et al., 2010; Stear et al., 1995). The second sign of resistance is an increased number of inhibited larvae which has been weakly associated with a local IgA response (Smith et al., 1985; Strain et al., 2002) although there is some doubt as to whether the IgA response is quick enough to influence larval inhibition (Halliday et al., 2010). Further, larval inhibition can also be influenced by density-dependent Research paper Boer goats appear to lack a functional IgA and eosinophil response against natural nematode infection Basripuzi (Basripuzi is the surname of the first author.

This study was conducted to investigate potential interactions between the abomasal blood feeder Haemonchus contortus and the intestinal mucosal browser Trichostrongylus colubriformis among deliberately infected Boer goats. Faecal and blood samples were collected twice a week for eight weeks from 25 parasite-naïve goats. Correlation analysis and multiple linear regression models were conducted to explore the association between phenotypic variables and variables taken at necropsy. Positive associations were identified between total FEC and log T. colubriformis number (r = 0.62, p < 0.05) as well as between IgA and peripheral eosinophil counts (r = 0.65, p < 0.05). A negative correlation was observed between T. colubriformis and log H. contortus number (r=-0.56, p < 0.05). Multiple linear regression models show that H. contortus and T. colubriformis interacted with each other. T. colubriformis appeared to contribute more significantly to the variation of FEC than H. contortus. Coinfection induced an IgA response which was only effective against T. colubriformis but not protective against H. contortus infection. This could be seen via significant associations of IgA with both nematode species but with the effect of IgA differing for H. contortus and T. colubriformis. In this study, H. contortus infection was not detrimental to the goats with no observed impact on PCV. This could be because the growth of T. colubriformis as represented by its length was associated with reduced number and composite burden of H. contortus during co-infection, or possibly due to low infection dosage. Improved understanding of the impact of H. contortus and T. colubriformis and their interaction from natural co-infection studies is beneficial for a better understanding of the goat-parasite interaction and its potential impacts on the health and productivity of animals.