Reproduction in Domestic Animals

18 SM Rhindingestion’ for these may be very different from that ofadults, e.g. mammalian foetuses are nourished viamaternal blood and developing birds by egg yolk andwhile young mammals consume milk, adults eat animalor plant material. Similarly, larval stages of insects andother invertebrates may have very different diets toadults. In addition to differences in food source,differences in metabolism with age mean that it isunwise to extrapolate from effects of exposure in adultsto those in immature animals, because the relationshipbetween the amounts of pollutants ‘ingested’ at thedifferent stages and the respective tissue concentrationsmay differ with differences in metabolism. Furthermore,indirect measures of exposure of immature animals topollutants, for example, through measurements inmammalian milk or umbilical cord blood may be ofless value than tissue concentrations in the foetus ⁄ immatureanimal for the prediction of physiological effectssince the former does not take account of effects ofmetabolism and excretion in the offspring.While concentrations of pollutants in human (Dekoningand Karmaus 2000; Darnerud et al. 2001) andsheep milk (Rhind et al. 2007b), and human umbilicalcord blood, amniotic fluid and meconium (Dallaireet al. 2003; Barr et al. 2007) have been measured, thereare relatively few reports of foetal tissue concentrations(Bosse et al. 1996; Dekoning and Karmaus 2000) in anyspecies and so direct assessment of risk to the foetus isnot possible. Concentrations of PTMs (Rhind 2005) andseveral PCBs and PBDEs (Rhind et al. 2007a) have beenshown to be lower in fetal sheep than in the correspondingmaternal tissue but individual concentrationsdiffer greatly with congener (PCB < 20 ng ⁄ kg to3.8 lg ⁄ kg; PBDE < 20 ng ⁄ kg to 30 lg ⁄ kg liver drymatter), as does the relationship between adult andfoetal levels. Schecter et al. (2007) provided the firstreport of PBDE levels in human foetuses; mean totalPBDE concentrations were 23 ng ⁄ kg lipid.Interestingly, sheep maternal and foetal concentrationsof neither PTMs (Rhind et al. 2005a) nor PCBsand PBDEs (SM Rhind, unpublished observations) weresignificantly correlated. However, related studies havealso shown that very small increases in maternalexposure to pastures fertilized with sewage sludge whichcontains multiple pollutants, result in changes in structureand function of the foetal testis (Paul et al. 2005)and ovary (Fowler et al. 2008). Collectively, theseobservations show that the relationships between pollutantexposure and effect are highly complex, that foetalexposure cannot be extrapolated from adult profiles,and that small increases in exposure to multiple pollutantscan adversely affect foetal reproductive development.Immediately after birth, the main source of nutrient inmammals is milk. The transfer of PTMs to the offspringthrough milk is probably relatively unimportant (Wilkinsonet al. 2003) but exposure to lipophilic organicpollutants through milk may be significant, particularlyin species such as marine mammals which have a highmilk fat content (Duinker and Hillebrand 1979). On theother hand, studies comparing milk concentrations atearly and late stages of lactation in sheep (periods of fatmobilization and deposition respectively) indicate thatmilk concentrations of phthalate and alkyl phenol arenot elevated by increased maternal fat mobilizationduring early lactation (Rhind et al. 2007b) suggestingthat maternal fat may not be a significant contributor toneonatal pollutant exposure in this species.Food-related exposureWhile food is generally regarded as the major source ofexposure in vertebrates, in some circumstances foodrelatedroutes may also be important. There is a risk forall species (vertebrate and invertebrate) of exposure tocontaminated water or air as a result of the processes offood acquisition and ingestion. For example, dairy cowsdrinking from canals in The Netherlands which arecontaminated with sewage, and therefore a cocktail ofpollutants, have been shown to have reduced reproductiveefficiency and lower milk production (Meijer et al.1999). Most of the pollutants that are likely to bepresent in food are relatively insoluble in water and soare not normally ingested with drinking water. However,if bound to suspended solids in the water, suchpollutants could be ingested. In addition, these pollutantsmay interact with steroids from the contraceptivepill and alkyl phenols which are typically present in theaqueous component of sewage (Institute for Environmentand Health 1999; Bowman et al. 2003).Similarly, surface application of sewage sludge topastures has been shown to be associated with compromisedfoetal testis and ovary development (Paul et al.2005; Fowler et al. 2008). Since systemic uptake ofpollutants by plants is very limited (Wild and Jones1992), it is likely that the associated exposure topollutants results from ingestion of soil along with theherbage, particularly when the herbage mass is small (upto 30% of dry matter intake of sheep may be soil;Thornton and Abrahams 1983), as a result of surfacecontamination of the herbage with soil particles.In specific circumstances, air-borne pollutants linkedto the food source could, at least theoretically, contributeto the body burden of pollutants, e.g. domesticanimals may inhale additional pollutants when grazingpastures fertilized with sewage sludge, owing to theretention of volatilized chemicals within the sward.Concentrations of many pollutant classes in normal airare measurable (David and Sandra 2001; U.S. CentralPollution Control Board Newsletter 2001; Lee et al.2004) and there is evidence that urban air-bornepollution can cause DNA damage in humans (Whyattet al. 1998). Thus, the potential for additional exposures,resulting from the process of acquiring food,should be taken into account.Age-related effectsThe degree of pollutant accumulation through fooddepends on animal age not only because of the differenceswith age in diet type but also because olderanimals may be exposed to pollutants for longer and, asshown in some species of fish, may accumulate more(Darnerud et al. 2001). Furthermore, it has been shownthat when exposure is prolonged, low, and apparentlyharmless, pollutant concentrations can exert biologicalÓ 2008 Macaulay Land Use Research Institute