Zearalenone is produced by a number of the Fusarium species. The association of Fusarium spp. and zearalenone with hyperoestrogenism and infertility has been well recognised as a danger in concentrate feeding of pigs, poultry, cattle and horses. In NZ there has been considerable research into the presence of zearalenone in pastures ingested by sheep and the subsequent effects on fertility.
The culmination of this work has been the positive identification of zearalenone intoxication as a major cause of reduced lambing percentages in North Island flocks.
Zearalenone is a fungal toxin produced by a wide range of fungi in both mouldy grains and pastures. It is one of the many toxins produced by several of the fungal species Fusarium. Three of the zearalenone producing species are common in New Zealand pasture and it is unusual to find a pasture sample that does not contain at least one of the zearalenone producing Fusarium. Fusarium are generally saprophytic and like Pithomyces chartarum grow preferentially on dead plant material.
Fusarium growth is greatest during late summer and autumn and, under warm dry conditions, they form numerous large, characteristically shaped spores. Spore production is not related to toxin production so that spore counting cannot be used as a way of monitoring pasture toxicity.
There does not seem to be a particular weather pattern associated with Fusarium growth and high pasture zearalenone and the widespread distribution of toxic pastures suggests that the conditions required for fungal growth and toxin production are not restrictive. The three zearalenone producing Fusarium also have different optimal growth conditions. Fungi of the Neotyphodium genus, common in NZ pastures (in ryegrass), may also produce zearalenone.
Observations suggest that the minimum toxic dose for short term exposure (10 - 15 days) is approximately 1mg per day and is less if exposure is longer (more than 20 days). Many flocks in NZ may have zearalenone intakes which chronically exceed these levels. Such pastures may have zearalenone levels of 1mg per kg dry matter or higher over several months of the year.
Therefore at a ewe maintenance level of 1kg dry matter per day the risk of toxic zearalenone intake from these pastures is real.
In general pasture levels of zearalenone begin to rise in late summer and early autumn and fall to low levels in winter.
Zearalenone is converted to the oestrogenic α-zearalenol, and has a structural similarity to oestradiol 17β, which is the naturally occurring oestrogen in sheep.
Zearalenone and zearalenol are very similar to the hormonal growth promoter zeranol. Oestrogenic EC50 (in pM) were measured as: oestradiol 17β - 2.8, α-zearalenol - 50, zeranol - 106, zearalenone - 1639, β-zearalenol - 20,000.
Metabolism of α-zearalenol to zeranol takes place in cattle, sheep and horses, but probably not pigs. this can lead to zeranol residues above the MRL.
Acute intoxication in female animals is manifested by hyper oestrogenism. Male animals are also susceptible to zearalenone. In cows symptoms seen include, enlarged mammary glands and prolapse of the vagina and rectum. Similar changes are seen in sows and piglets. Initially, affected sows show enlargement or swelling of the vulva similar to that seen in oestrus. If this continues to an extreme degree, eversion of the vagina will occur. In gilts there is precocious mammary development and while the reproductive performance of pregnant sows may not be affected, abortion has been known to result from feeding on fusarium affected corn.
In male animals there is impaired libido and spermatogenesis, testicular atrophy and preputial enlargement.
Provided the clinical changes are not too severe nor prolonged, removal of the animals from the affected feed usually allows recovery in 7 10 days. In the case of male animals, recovery from testicular atrophy may take several months.
The major effect of zearalenone occurs before mating. Ewes ingesting zearalenone show changes in oestrous behaviour (decreased cycle lengths, increased oestrus duration) and reductions in ovulation and fertilisation rates that result in an increase in the number of barren ewes and few ewes with twins.
The effect of zearalenone on the reproductive performance of ewes
| Zearalenone dose | Fusarium culture | Control | |
| Oestrus duration (h) | 40 | 26 | 16 |
| Proportion ovulation (%) | 55 | 53 | 88 |
| Ovulation rate (%) | 1.6 | 1.5 | 1.9 |
| Fertilisation rate (%) | 29 | 43 | 100 |
| Fertile ewes | 9 | 16 | 58 |
The effect of zearalenone on reproductive performance depends on both the magnitude of the daily intake and the length of time the animals are exposed to the toxin. The higher the daily dose and the longer the exposure, the greater the effect and the longer it lasts after zearalenone exposure ceases.
Research data suggests that ovulation rates (and thus lambing percentages) will fall by about 5% for every mg zearalenone ingested per day for short term exposure and by about twice this rate if exposure to the toxin is prolonged.
Dosing with 6mg zearalenone daily for 30 days did not have an adverse effect on spermatogenesis or fertility. It seems unlikely that zearalenone pastures grazed by rams will have an effect on their reproductive performance.Not fatal.
Pasture zearalenone concentrations at any one site are extremely variable and samples collected from within small areas can vary widely; up to 100 fold range between highest and lowest zearalenone concentrations. Because of these variations pasture sampling does not fairly represent the material actually eaten by grazing sheep and is not considered a good indicator of the potential risk of zearalenone induced infertility.
Ingested zearalenone is converted to the more oestrogenic derivative zearalenol before being conjugated to glucuronic acid to aid excretion via the urine. In ruminants it is possible, even probable, that ingested zearalenone is converted in the rumen before being absorbed.
Zearalenone metabolites appear in urine soon after zearalenone ingestion and reach a peak 3 - 6 hours later. Residues can be detected for several days after sheep are experimentally dosed with zearalenone.
An ELISA procedure for measuring urinary “zearalenone” (i.e. the total zearalenone and zearalenol released by hydrolysis of the glucuronide conjugate) has been developed. This technique is reliable and has been used to survey the effects of zearalenone on flock fertility over many areas of New Zealand. Data from this survey is shown in Table 61 after Towers (1992).
Mean urine zearalenone: creatinine levels in survey flocks and estimated zearalenone intakes and risk categories
| Z:Cr* Ratio | Percentage of Flocks | Zearalenone Intake** | Risk Category | Expected Result | |
| 1991 % | 1992 % | mg per day | |||
| 0 - 0.4 | 32 | 56 | 0 1 | Low | Unlikely to affect ewe fertility*** |
| 0.4 - 0.8 | 36 | 35 | 1 - 3 | Moderate | Prolonged intakes depresses fertility |
| >0.8 | 32 | 9 | >3 | High | Fertility depressed |
| n=53 | n=226 | ||||
* ng zearalenone per mg creatinine, ** estimated from urinary Z:Cr ratio, *** short term effect only – prolonged exposure to zearalenone levels approaching 1 mg/day also expected to depress fertility
Survey results clearly show zearalenone intoxication is a major cause of poor reproductive performance in ewe flocks in North Island NZ, flocks in the more northerly regions being at greater risk. Zearalenone intoxication may be a problem in some South Island areas as well.
Other causes of infertility.
None.
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