Introduction
Echinococcosis is a zoonotic infection caused by Echinococcus granulosus, which is a small tapeworm and in the natural cycle, dogs and other canids are typical definitive hosts and ungulates, including sheep, goats, pigs and horses, are intermediate hosts in which hydatid cysts occur. The latter stage can also develop in humans (Eckert & Deplazes, Reference Eckert and Deplazes2004). Although E. granulosus penetrates deep between the villi of the small intestine of the definitive host, there are no pathogenic effects even in heavy infections (Eckert et al., Reference Eckert, Deplazes, Craig, Gemmell, Gottstein, Heath, Jenkins, Kamiya, Lightowlers, Eckert, Gemmell, Meslin and Pawlowski2001), suggesting that infected definitive hosts are asymptomatic carriers of the parasite. Furthermore, infections with E. granulosus cysts in the intermediate host are typically asymptomatic, except for a small number of cases with chronic and heavy infections. There are no reliable methods for the routine diagnosis of infections in living animals, but in rare cases cysts have been identified by ultrasonography alone or in conjunction with serum antibody detection (Eckert et al., Reference Eckert, Deplazes, Craig, Gemmell, Gottstein, Heath, Jenkins, Kamiya, Lightowlers, Eckert, Gemmell, Meslin and Pawlowski2001). A new ELISA with a high specificity and a sensitivity of 50% to 60% might be useful for detecting E. granulosus cysts in sheep on a flock basis but cannot be used for a reliable diagnosis of infected individuals (Kittelberger et al., Reference Kittelberger, Reichel, Jenner, Heath, Lightowlers, Moro, Ibrahem, Craig and O'Keefe2002). The most reliable diagnostic method is cyst detection during meat inspection or at post-mortem examination. Cystic echinococcosis in farm animals causes considerable economic problems due to loss of edible livers. Significant losses of meat and milk production and fleece values from infected sheep may also occur. These losses are of special significance in countries with low economic outputs where sheep production is of particular importance (Torgerson et al., Reference Torgerson, Dowling and Abo-Shehada2001). Iran is one of the endemic areas of echinococcosis and as Northwest Iran is one of the biggest territories for rearing domesticated animals, the present aim is to determine the prevalence of hydatid cysts in slaughtered animals in this territory. The site, intensity and fertility of cysts and the viability of their protoscoleces will also be investigated.
Materials and methods
Examination of slaughtered animals
This cross-sectional study was carried out on 5381 animals (928 cattle, 243 buffaloes, 3765 sheep and 445 goats) in one of the biggest territories for rearing domesticated farm animals in the Ardabil province, in Northwest Iran, from April 2004 to March 2005. This area exports more than 373,000 sheep and goats and 55,000 cattle to other provinces especially the capital city of Iran, Tehran, annually. During the study, the industrial slaughterhouse was visited periodically to examine the liver and lungs of slaughtered animals for the presence of cystic echinococcosis. The identity, sex, infected organ and number of cysts were noted and the intensity of infection categorized into light (1–10 cysts) and heavy (>10 cysts) infections. The number of animals condemned was also noted.
Examination of cysts and viability of protoscoleces
Individual cysts were grossly examined for any evidence of degeneration and calcification. Some cysts in sheep and cattle were randomly selected for fertility studies and to reduce intracystic pressure, the cyst wall was penetrated with a needle and opened up with a scalpel and scissors. The contents were transferred into a sterile container and examined microscopically (40 × ) for the presence of protoscoleces. Cysts which contained no protoscoleces as well as heavily supurative or calcified were considered unfertile. The viability of protoscoleces was assessed by the motility of flame cells together with staining with a 0.1% aqueous eosin solution (Smyth & Barrett, Reference Smyth and Barrett1980). Living protoscoleces did not take up the stain, unlike the dead ones.
Statistical analysis
Differences between the prevalences and intensities of infection relative to season, host sex and the mean viability of protoscoleces were assessed using Chi-square and student t-tests.
Results
The highest prevalence values were found in sheep (74.4%) and the lowest in buffaloes (11.9%). The prevalence of infection was higher in females compared with male animals, but this difference was only significant in sheep and cattle (table 1; P < 0.001).
Table 1 The prevalence (%) of hydatid cysts in male and female animals slaughtered in the Ardabil province, Northwest Iran, 2004–2005.
Cysts in cattle (28.5%), buffaloes (8.9%) and sheep (44.3%) were found mostly in lungs, but in goats, both liver and lungs were infected. The majority of cattle, buffaloes, sheep and goats harboured 1–10 cysts in the lungs and liver, although heavy infections (>10 cysts) invariably occurred in the lungs. The highest offal condemnation was seen in the lungs of sheep (13.4%) and the lowest in the liver of buffaloes (0.4%) (table 2). The decrease in the value of discarded carcasses was estimated of 51,900 US dollars based on market prices in 2004.
Table 2 The prevalence (%) of hydatid cysts and offal condemnation rate in the liver and lungs of slaughtered animals in the Ardabil province, Northwest Iran, 2004–2005.
Data showed significant seasonal patterns for hydatidosis only in sheep (P < 0.001), and highest prevalence values were observed in sheep during autumn and winter (table 3). The fertility rate of cysts and the viability of protoscoleces from fertile cysts in the lungs or liver of sheep and cattle are shown in table 4, where cysts from the lungs and liver of sheep were more fertile than those of cattle (P < 0.05).
Table 3 Seasonal prevalence (%) of hydatid cysts in animals slaughtered in the Ardabil province, Northwest Iran, 2004–2005.
n number of animals examined.
* P < 0.001; df = 3; χ2 = 39.61.
Table 4 The fertility of cysts and the viability of protoscoleces from the liver and lungs of slaughtered animals in the Ardabil province, Northwest Iran, 2004–2005.
Discussion
Hydatid disease is an important medical and veterinary problem in Iran. Domestic intermediate hosts (cattle, sheep, goats and buffaloes) are major reservoirs for the disease in humans. The widespread distribution and nature of the life cycle of E. granulosus suggest that there will always be a risk of re-introducing the cestode as long as live animals are imported. One of the territories from which Tehran (the capital city of Iran) imports live ruminants is Northwest Iran where hydatidosis is widespread in domesticated farm animals. Diagnosis of hydatid in intermediate hosts is hindered by the absence of reliable tests for use in live animals. Although various imaging techniques have been used for the diagnosis of cystic hydatid disease in humans, there are problems associated with their application to ruminants. Most prevalence studies have relied on slaughter data (Macpherson, Reference Macpherson1981; Baldock et al., Reference Baldock, Arthur and Lawrence1985), as these are an economical way of collecting and analysing information on livestock disease, particularly subclinical conditions. Also, lesions of cystic echinococcosis usually remain for the life of the animal, and so, at post-mortem it is possible to tell whether or not an animal is infected (Njoroge et al., Reference Njoroge, Mbithi, Gathuma, Wachira, Magambo and Zeyhle2000).
The present study shows that 74.4% of sheep were infected with hydatid cysts. Dalimi et al. (Reference Dalimi, Motamedi, Hosseini, Mohammadian, Malaki, Ghamari and Ghaffari Far2002) reported that the mean prevalence of hydatidosis of sheep in various parts of Iran was 8.1% with a range of 1% to 27.5%. The prevalence in cattle was 38.3%, with a mean prevalence of 12% (1–28%) in cattle in different parts of Iran. Up to 20% of goats were infected with a mean prevalence of 6.5% (0.5–20%). Up to 11.9% of buffaloes were infected with a mean prevalence of 17.3% (1.5–57.8%).
Hydatidosis is a common disease in the Middle East, especially in countries neighbouring Iran. In Iraq, prevalence values of 4.5–44% in sheep, 3.1–26.7% in goats, and 4.3–13.9% in cattle have been reported (Molan, Reference Molan1993; Saeed et al., Reference Saeed, Kapel, Saida, Willingham and Nansen2000). In Turkey, 26.6% of sheep, 22.1% of goats and 13.5% of cattle were infected with this disease (Umur, Reference Umur2003). In Kuwait, 0.2–11.2% of sheep, 5% of goats, 32.5–40.2% of cattle (Hassounah & Behbehani, Reference Hassounah and Behbehani1976), and in Pakistan, 5.5–9.6% of cattle, 8.3% of sheep, 7.5% of goats and 12.3–49% of buffaloes were infected (Munir et al., Reference Munir, Anwar and Chaudhry1982; Khan & Haseeb, Reference Khan and Haseeb1984). Prevalence values in India were 8.9–69.0% of buffaloes, 9.7–68.9% of cattle, 2.3–93.3% of sheep, and 1.1–72.7% of goats (Mathur & Khanna, Reference Mathur and Khanna1977; Prasad & Mandal, Reference Prasad and Mandal1978, Reference Prasad and Mandal1979; Abraham et al., Reference Abraham, Pillai and Iyer1980; Kosalaraman & Ranganathan, Reference Kosalaraman and Ranganathan1980; Prabhakaran et al., Reference Prabhakaran, Soman, Iyer and Abraham1980; Islam, Reference Islam1981; Deka et al., Reference Deka, Srivastava and Chhabra1983, Reference Deka, Borkakoty and Lahkar1985; Rao, Reference Rao1985) and in Syria, 4.5% of sheep, 2.3% of goats and 5.2% of cattle were infected (Dajani, Reference Dajani1978). In the present study, sheep, apart from being the most frequently infected, are the most slaughtered animals for human consumption in abattoirs in this locality in Iran followed by cattle, goats and buffaloes. Furthermore, as 59.1% of sheep infections were fertile, they are potential sources of infection to dogs. Offal, especially the liver and lungs, is usually offered to domestic dogs or is dumped in rubbish bins, outside houses, where stray dogs can easily feed on it. On the other hand only 19.3% of cattle cysts were fertile, and most cysts are sterile and degenerate, so cattle are less likely sources of infection to dogs.
The high prevalence of hydatid cysts in domesticated farm animals, especially in sheep and cattle, is likely to be related to the presence of green pastures, an abundance of animals, a shortage of industrial animal husbandry, and the presence of stray dogs infected with E. granulosus. There is only one industrial animal husbandry, Keshtosanate Moghan in Northwest Iran and unpublished data showed that the prevalence in industrial animals is less than that in traditional animal husbandry, i.e. cattle 0.7%; sheep 4.1%, and goats 0%. The low prevalence of hydatidosis in industrial animal husbandry may be explained by using grass cultivated in surrounded farms, where dogs are prohibited.
In the present study, the lungs of sheep, cattle and buffaloes were found to be more commonly infected than livers, as previously reported by Khan et al. (Reference Khan, El-Buni and Ali2001) and Dalimi et al. (Reference Dalimi, Motamedi, Hosseini, Mohammadian, Malaki, Ghamari and Ghaffari Far2002). Gusbi et al. (Reference Gusbi, Awan and Beesley1987, Reference Gusbi, Awan and Beesley1990) and Ibrahim & Craig (Reference Ibrahim and Craig1998) also reported that the liver was the predominant infected site in sheep, goats and cattle. In the Middle East, the most common location of hydatid cysts in sheep is the liver followed by the lungs (Al-Yaman et al., Reference Al-Yaman, Assaf, Hailat and Abdel-Hafez1985; Abdel-Hafez & Al-Yaman, Reference Abdel-Hafez and Al-Yaman1989; Abo-Shehada, Reference Abo-Shehada1993; Kamhawi et al., Reference Kamhawi, Hijjawi, Abu-Ghazaleh and Abbas1995).
Present data on the distribution of hydatid cyst in different seasons, show that only sheep showed a significant variation (P < 0.001), with hydatidosis being prevalent in the autumn and winter. The sources of slaughtered animals might be the reason for this, as near the end of the autumn, and at the beginning of winter, a higher proportion of sheep harbouring hydatid cysts are slaughtered for migratory tribal people. (Hoghoughi, Reference Hoghoughi1971; Oryan et al., Reference Oryan, Moghaddar and Gaur1994). During the spring and summer, sheep are moved to the Sabalan mountain slopes in the Ardabil province and are returned in mid-autumn to Dashte Moghan. By this time, shepherds send old sheep to the abattoir, and these animals are usually thinner with a low efficiency and likely to be infected with cysts. Therefore, the accumulation of cysts is higher in the autumn and winter compared with other seasons. On the contrary, migration is not a part of rearing cattle, buffaloes and goats as these animals are usually kept in houses and stables.
Data on the prevalence and fertility of cysts in various domestic herbivores provide reliable indicators of the importance of each type of animal as a potential source of infection to dogs. Cysts, depending on the geographical situation, host, site, size and type of cyst may have different rates of fertility. In the present case, fertility rates of hepatic cysts of sheep and cattle were 68.5 and 19.4%, respectively, whereas the fertility rates of pulmonary cysts of sheep and cattle were 48.8% and 19.3%, respectively. In sheep, unlike cattle, the fertility of hepatic cysts was higher than that in lungs. Dalimi et al. (Reference Dalimi, Motamedi, Hosseini, Mohammadian, Malaki, Ghamari and Ghaffari Far2002) reported that the fertility of cysts in the liver of sheep in western Iran was higher than that in the lungs, but vice versa in the case of cattle. As in the present study, Gusbi et al. (Reference Gusbi, Awan and Beesley1987) and Ibrahim & Craig (Reference Ibrahim and Craig1998) also reported that the hepatic cysts of Libyan sheep were more fertile than those in the lungs.
The proportion of viable protoscoleces from fertile cysts from sheep and cattle were 61% and 32%, respectively, and this was confirmed by Dalimi et al. (Reference Dalimi, Motamedi, Hosseini, Mohammadian, Malaki, Ghamari and Ghaffari Far2002) in western Iran where viability in sheep (82%) was higher than that in cattle (75%).
In conclusion, as the prevalence and intensity of infection and fertility of hydatid cysts in sheep is higher than those in other animals, sheep clearly have an important role to play in the continuation of the E. granulosus life cycle in Northwest Iran. Therefore the safe disposal of infected offal, especially of sheep, will significantly reduce the transmission of cysts from slaughterhouses to potential hosts in this region.
Acknowledgements
This work was financially supported by Ardabil University of Medical Sciences. Grateful thanks are extended to Mr Ali Jafari for his assistance in preparing this paper.
