In Malaysia, the molecular detection of parasites of veterinary importance in livestock such as Giardia, Cryptosporidium[17, 18], Neospora caninum have been reported. However, there is a conspicuous lack of molecular data focusing on strongylid parasites, the most pathogenic group of GIP to livestock in Malaysia. In the present study, the most common strongylid parasite, H. contortus infection was found in 22.4% (93 of 416) of studied animals, comprising 92 goats (58.6% of 157) and one deer (1.4% of 70). A number of drug resistance studies in Malaysia have indicated that H. contortus remains the most widespread strongylid species (73–97%) in small ruminants (i.e., goats and sheep) [20–23]. These studies have indirectly acknowledged the preponderance of H. contortus infection in Malaysia and the current study further confirms this notion. Likewise, the predominance of this parasite species in goats has been reported worldwide. In comparison with previous studies, the prevalence of strongylids noted in this study was much lower than Kenya (90%) , Zimbabwe (88-97%)  and Brazil (96.9%) .
In contrast, H. contortus was less common among the studied domesticated deer. Similar findings were also noted in the red deer in Stelvio National Park, one of the main protected areas of north-eastern Italy (1.3%)  and roe deer in the northwest of Iberian Peninsula, Spain (1.4%) . The results indicated this species might not be a major threat to the wellness of deer . Nonetheless, this finding must not be generalized and a more comprehensive study in the country should be conducted.
Trichostrongylus infection was also observed in the present study. Although Trichostrongylus is less significant to livestock compared to H. contortus, its impact on livestock cannot be underestimated [5, 29]. In Malaysia, a series of drug resistance studies reported that Trichostrongylus (5–26%) was the second predominant strongylid parasite species among livestock after H. contortus (73–97%) [20–23]. In recent years, there is an increasing trend of Trichostrongylus infection in small ruminants (personal communication, Veterinary Research Institute, Malaysia). Not surprisingly, more than half of the strongylid infected goats and deer in the present study were positive for Trichostrongylus. Among the Trichostrongylus species, T. colubriformis was the most common species in goats. The infection of small ruminants with this species appears to be common with a wide spectrum of prevalence rates, as high as 78–100% in Nigeria [30, 31], more than 90% in France  and as low as 9.8% in Iran . Apart from small ruminants, T. colubriformis infection has also been reported in other livestock including cattle [5, 29, 34]. However, none of the Trichostrongylus species was detected among cattle samples in the present study.
As for T. axei, its predomination in temperate zones around the world have been pointed out , such as Nigeria (69.2%) , Australia (overall more than 90%)  and Zimbabwe (88–97%) . However, in Malaysia (a tropical country), there is only one study reporting that T. axei was the most common strongylid parasite as observed from post-mortem examination of small ruminants . This is in contrast with the findings of the present study, where a very low frequency of T. axei was demonstrated. It is important to point out that the current status of T. axei in Malaysia remains unknown and therefore pinpoints the need for additional concerted research efforts in future.
Based on the results, none of the cattle and swine samples were positive for H. contortus and Trichostrongylus spp. Generally, Haemonchus placei, Cooperia pectinata, Cooperia punctate and Ostertagia spp. were the dominant strongylid parasites in cattle, notably in Kenya and Netherlands [38–40]. Attempts to amplify these species using the primer sets of Gasser et al.  were made but no positive samples found in the present study (unpublished data). Interestingly, the canine specific hookworm species (Ancylostoma caninum) was detected in one of the cattle sample in the present study as confirmed by DNA sequencing (data not shown). There is a high possibility of the cattle being a mechanical transporter. However, the actual factor(s) that contribute to this rare case need to be further investigated.
With regards to swine, absence of H. contortus, T. axei and T. colubriformis was observed among current studied swine samples. Although the occasional existence of T. axei has been described [5, 29], its prevalence remained low (<5%)  and less significant to swine. As compared to T. axei, T. colubriformis is more common in swine where the natural incidental infections of T. colubriformis have been reported in Hungary, Australia, Russia and United Kingdom . However, little attention has been paid to gastrointestinal parasitic infections in swine in Malaysia. It is crucial that a comprehensive coverage of the current status of parasitic infections in swine populations in Malaysia is conducted in future.
Even though there have been a number of publications stating co-infections of strongylid in animals [29, 30], limited scientific reports demonstrated the real situation within host especially Malaysian livestock. The present study demonstrated that co-infections with H. contortus and T. colubriformis were predominant in goats. Fakae and Chiejina  have reported the co-occurrence of these strongylids (i.e., H. contortus and T. colubriformis) in goats ranging from 90% to 100%, which was significantly higher than the present study. With regards to co-existence of H. contortus and T. axei in goats, only a low number of strongylid positive samples were observed. Given the limited information currently available regarding the Malaysian T. axei, this result is crucial in filling the gap of knowledge of parasitic infection among ruminants in Malaysia.
Co-infection between H. contortus or T. colubriformis with other gastrointestinal parasites in small ruminants has been reported. Among these two strongylid species, T. colubriformis commonly co-occurred with other GIP to produce a more severe impact compared to single infection to the host. For example, Ostertagia circumcincta co-infected with T. colubriformis has been shown to significantly reduce wool growth in lambs (up to 66%)  while Eimeria spp. and T. colubriformis infections resulted in enteritis . As for H. contortus, severe impacts (i.e., inappetence, severe scouring, and reduction in live body weight and death) have also been demonstrated during co-infection with Eimeria spp. .
With regards to co-infection of H. contortus or T. colubriformis, a series of studies reported the significant association between milk production and co-occurrence of these two strongylid species in dairy goats, which caused the reduction in milk yields (13.0-25.1%) in goats with highest milk production at the initial stage of the study . In addition, Chartier and Hoste  found that repeated exposure to the mixture of H. contortus and T. colubriformis caused goats with high milk production to suffer more severe pathophysiological disturbances (increase in pepsinogen concentration and decrease in inorganic phosphate concentrations) and severe depression in milk yields (-8 – -35%). Given that the severe pathophysiology and productivity attributable to the co-infection of gastrointestinal parasites, the current results suggest the strongylids co-infected individuals might also suffer with similar impact of this phenomenon. Therefore, there is now an urgent need to investigate the co-infection status of strongylid parasites among Malaysian livestock for better treatment management.