The herd-level prevalence of Fasciola hepatica infection in the goat population of Poland

Fasciola hepatica, also known as the common liver fluke (in contrast to the giant liver flukes – Fasciola gigantica and Fascioloides magna), is a globally distributed trematode parasite. It poses a risk to human and animal health, as well as contributes to losses in livestock production in Europe and globally (8, 27). A wide range of mammals may serve as definitive hosts of F. hepatica, however the disease plays a major role in domestic and wild ruminants (27). F. hepatica is a digenetic trematode of the Trematoda class and Digenea subclass with an indirect life cycle, in which the intermediate hosts are water snails of the Lymnaeoidea superfamily. The risk of disease depends on climatic factors such as optimal temperature (above 10°C) and high soil moisture which enable the intermediate hosts and free-living larvae of F. hepatica to thrive. Infection of the definitive host occurs via the oral route by ingestion of infective metacercariae – encysted larvae attached to plants growing near water pools. Fasciolosis may follow an acute, subacute (both mainly involving traumatic hepatitis), or chronic course (due to hyperplastic cholangitis) depending on the rate of parasite ingestion. Contrary to sheep and cattle, goats are unlikely to suffer from an acute form of the disease, as their browsing habits preclude massive ingestion of metacercariae in a short period of time. However, chronic infection by a low number of trematodes may lead to production losses because of reduced growth rates, fertility, milk yield, and carcass weight and inferior carcass composition (45, 51).

Traditionally, the diagnosis of fasciolosis was based on direct detection of fluke eggs in fresh faeces using the faecal sedimentation method (coproscopy). The method is considered to have very high diagnostic specificity (Sp) but low diagnostic sensitivity (Se) due to the 8–12-week prepatent period of fasciolosis, the paucity of eggs present frequently in a faecal sample and the intermittency of egg shedding (26). Several molecular diagnostic methods detecting Fasciola DNA have recently been developed; however, their reproducibility is still limited (hence their use is restricted to reference laboratories) and they have not overcome the main problem of the long prepatent period because Fasciola eggs are the source of DNA to be detected (6). Immunoenzymatic assays (sandwich ELISAs) detecting in feces metabolites released by late immature and adult flukes into the bile (coproantigen) currently ensure higher Se (4, 24, 28, 29, 49). Also, ELISAs detecting specific antibodies against the common liver fluke in the serum or milk overcome the problem of false negative results during the prepatent period, because antibodies against F. hepatica are detectable in serum as early as 3–4 weeks post infection and remain detectable for a very long time (1, 28, 31, 38, 44, 48). Therefore, serological ELISAs appear to be the most optimal diagnostic tools for large-scale surveys.

The prevalence of F. hepatica infection in Polish cattle has been thoroughly studied over the previous 20 years (17, 18, 21, 22, 23, 32, 47), and it is known to vary between geographical regions and seasons within a range from 7% to 80%. Data regarding fasciolosis in small ruminants are limited, not only in Poland but generally in Europe (5), although they suffice to suggest that the prevalence differs considerably between geographical regions (42). Very few local studies conducted on F. hepatica infection in sheep and goats in Poland have shown that prevalence is below 10% (15, 40); however, data from large-scale epidemiological surveys are lacking. Therefore, the aim of the study was to determine the herd-level prevalence of F. hepatica infection in the goat population of Poland by serological and coproscopic examinations.

The serological survey enrolled 33 randomly selected herds counting from 9 to 183 adult goats with the median (IQR) of 35 (23–61) goats. Twelve herds (36.4%) were small, 17 (51.5%) were medium, and 4 (12.1%) large. In 32 of 33 herds, between 1 and 34 male goats (median of 3) were kept. The herds were located in 9 of the 16 provinces of Poland (Table 1). Between 9 and 168 adult goats with the median (IQR) of 33 (19–49) were tested per herd, which amounted to 1,464 adult goats (134 males and 1,330 females) in total.

Seropositive goats were detected in 11 of 33 herds, which corresponded to a herd-level seroprevalence of 33.3% (CI 95%: 19.7%–50.4%). At the animal level, a positive result was found in only 17 of 1,464 tested goats (1 male and 16 females) (1.2%, CI 95%: 0.7%–1.9%). Two positive MM3-SERO ELISA results were indicative of a medium infestation and 15 positive results corresponded to a weak infestation. In 8 herds only a single goat was seropositive, while two, three and four goats were seropositive in the 3 remaining herds. The within-herd seroprevalence ranged from 0.8% to 11.1% with the median (IQR) of 2.3% (1.5%–4.6%) (Table 2). There were 14 inconclusive serological results, 12 of them in herds in which positive results were also found.

The serological status of the herd was not significantly associated with its geographical location (with the province in which the farm was located) (P = 0.291) or with any other investigated risk factor (Table 3). The proportion of seropositive males (0.7%; 1/134) was not significantly different from the proportion of seropositive females (1.2%; 16/1330; P = 0.617).

Faecal samples of 2,770 goats from 214 herds were tested, including 17 of 33 serologically tested herds (51.5%; 8 of 11 seropositive and 9 of 22 seronegative herds). Between 1 and 83 faecal samples (median (IQR) of 10 (5–15)) per herd were tested. The herds were located in all provinces of Poland (Table 1). The herd-level prevalence of F. hepatica infection according to coproscopy turned out to be 0.5% (CI 95%: 0.1%–2.6%), which was a significantly lower figure compared to the seroprevalence (P < 0.001) (Fig. 1).

Fig. 1.

The herd-level prevalence of Fasciola hepatica infection in the goat population of Poland according to serological testing with an MM3-SERO ELISA and a coproscopic test

In only one herd, located in central Poland, a single positive faecal sample was found. This goat was also seropositive and its serological result indicated medium infestation. As the goat was emaciated, it was euthanised on the owner’s request and subjected to a necropsy. The post-mortem examination revealed liver lesions typical of chronic fasciolosis. The organ was fibrotic, with an irregular nodular surface and an opaque capsule. Upon cutting the thickened biliary ducts filled with numerous mature flukes were found.

Our study shows that F. hepatica infection in goats in Poland occurs sporadically without any geographical pattern. Although seropositive goats were found in one third of tested herds, antibodies were detected only in single animals in each seropositive herd. Such a low number of seropositive results raises doubts as to the true status of these animals, given that coproscopy confirmed patent F. hepatica infection in only one seropositive animal. Only a test of a perfect Sp could ensure sufficiently high positive predictive value to draw reliable conclusions (45).

The MM3-SERO ELISA used in our study appears to be a very specific test according to studies so far carried out in sheep (31) and cattle (30). Such a high Sp is claimed to result from the use of a novel antigen captured by MM3 monoclonal antibodies and from controlling for potential unspecific cross-reactions by the use of additional wells on a plate which are coated solely by MM3 monoclonal antibodies. The very optimistic results presented in these studies should, however, be treated with great caution. They were all obtained by the same scientific team and have not been verified later by any other studies. Moreover, the MM3-SERO ELISA has never been validated on goats, although a large-scale cross-sectional study based on this ELISA was published for the goat population of Galicia, Spain (39). The MM3-SERO ELISA is based on anti-ruminant IgG1 conjugate; therefore the chance is low that it would work worse in goats than in sheep, but still an assumption that it is perfectly specific in goats should be made very cautiously.

Undoubtedly, our study shows that F. hepatica infection occurs in the Polish goat population, since the infection of one serologically positive goat was confirmed by the coproscopy and then by the post-mortem examination. A very low prevalence in coproscopy compared to seroprevalence has been observed in previous studies from other countries (13) and may be explained in two ways. First, it may be the consequence of the low Se of a single coproscopic examination. Coproscopy is known to detect F. hepatic infection from 8–9 weeks post infection at the earliest (2, 25), and only if a sufficient number of eggs is excreted in faeces. As a consequence, the Se of coproscopy is rarely estimated at more than 50% although few studies on the diagnostic accuracy of coproscopy in small ruminants have been published (25). The use of a coproantigen ELISA instead of coproscopy would have undoubtedly increased the Se, as it has been shown to detect even more positive goats than serological ELISAs (49). Secondly, it may result from past infections which have already been cleared by animals’ immune systems. As antibodies to F. hepatica are known to persist long beyond the period of active infection (31), the seroprevalence from our study may better correspond to the actual level of goat exposure to F. hepatica infection in Poland than do the estimates based on the coproscopy. The exposure to F. hepatica in goats is certainly very low compared to the situation observed in Polish cattle (19) and probably also in Polish sheep, although data on fasciolosis in sheep are rather scarce (15). The risk of fasciolosis is mostly determined by the presence and extensity of infected snails, in Europe mainly freshwater snails of the species Galba truncatula (10, 41, 43). According to previous studies, the prevalence of F. hepatica infection in Polish snails is quite high (20). Nevertheless, these gastropods can only survive in wet conditions, while goats tend to avoid wetlands and damp pastures. Moreover, goats prefer browsing to grazing, which is associated with frequent changes of place of feeding and precludes ingestion of large quantities of metacercariae even on highly contaminated pastures (45). According to our observations resulting from many years of veterinary service on Polish goat farms, goats rarely share pastures with reservoir hosts of F. hepatica such as sheep and cattle.

Moreover, the common use of benzimidazoles on goat farms in Poland may lead to unintentional and somehow accidental reduction of active F. hepatica infection in goats. Most antiparasitic treatments are implemented based on common opinions about the high risk which tapeworms or liver flukes pose for ruminants without any attempt to seek medical confirmation of a true infection prior to treatment (36). Our study indicates that such a practice is totally pointless in light of the extremely rare occurrence of F. hepatica infection in Poland. Furthermore, the unjustified use of anthelmintics stimulates the development of resistance of gastrointestinal nematodes to benzimidazoles, which is already a substantial problem of goat herds worldwide and also in Poland (33, 34, 35). This observation makes proper parasitological diagnostics in a goat herd crucial to maintain the effectiveness of anthelmintic drugs as well as animal health and welfare.

F. hepatica infection occurs sporadically in the Polish goat population and its prevalence is much lower than in cattle or sheep. Therefore, treatment or prevention against fasciolosis should only be considered if it has been reliably confirmed by a positive result of a highly accurate diagnostic test. This also applies to goats inhabiting geographical areas where F. hepatica infection appears to be widespread in cattle and sheep.