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The main clinical sign of N. caninum infection in cattle is abortion, although stillborn calves and the birth of calves with congenital neurological defects have also been reported (Buxton et al. 1997a; Uggla et al. 1998). A brief period of pyrexia has been reported from challenge studies (Innes et al. 2001a; Macaldowie et al. 2004).

2.3.1 Abortion

Abortion occurs most commonly at 5-6 months of gestation, but Neospora-associated abortion has been reported to occur from 3 months gestation to full term (Dubey 2003a). Earlier loss of pregnancy may occur but is less likely to be recognised or fetal material be available to confirm the diagnosis (Innes et al. 2005). Fetal death before 5 months gestation is likely to lead to mummification and retention in the uterus, whilst fetuses that die in the earliest stages of gestation may be reabsorbed. The evidence that N. caninum

infection causes early embryonic loss or infertility is contradictory, with Latham (2003) reporting an increased number of services per conception in infected cattle, while López- Gatius et al. (2005b) found no effect on conception rate. Moreover, Santolaria et al. (2009) found that cows that aborted due to N. caninum infection rapidly returned to normal fertility.

Cattle infected with N. caninum have a 3 to 7-fold greater risk of abortion than non- infected cattle (Thurmond and Hietala 1997a; Wouda et al. 1998a). Likewise, there is evidence that the risk of abortion in congenitally-infected heifers is much higher in their first pregnancy than in their non-infected herd-mates (Weston et al. 2005). Together, such results suggest that the immunocompetence of the host when it first encounters the parasite affects the immune response. This notion is supported by studies in which experimental challenge of cattle with tachyzoites prior to pregnancy prevented abortion and/or vertical transmission to their calves (Williams et al. 2000; Innes et al. 2001a).

The pathogenesis of Neospora-associated abortion is not completely understood as although characteristic histopathologic lesions can be identified in aborted fetuses, similar lesions are found in some congenitally-infected but clinically-normal calves (Gondim et al. 2004c). The severity and outcome of fetal lesions probably depend primarily upon fetal age and immunocompetence (Innes 2007a). There is also evidence that abortion may result from the maternal immune response to N. caninum within the placenta (Dubey et al. 2006) or from a maternal systemic change from a predominantly T helper (Th) 2-type regulatory response to a predominantly Th1-type pro-inflammatory response, which will assist in the control of parasitaemia but is detrimental to the maintenance of pregnancy (Raghupathy 1997).

Experimental infections in cattle have shown that infection in the first trimester of pregnancy may have more severe consequences for the fetus than infection later in gestation (Barr et al. 1994b; Williams et al. 2000; Maley et al. 2003; Macaldowie et al. 2004). Fortunately, transmission to the fetus during early pregnancy is not inevitable, as several studies have reported only 50-83% vertical transmission when a tachyzoite challenge was given on Day 70 of gestation compared with 100% vertical transmission when challenged on Day 140 or 210 of gestation (Williams et al. 2000; Innes et al. 2001a; Macaldowie et al. 2004).

Challenge trials of pregnant sheep have shown that N. caninum infection can cause abortion and vertical transmission (Buxton et al. 1998) but it is not thought to be an important abortifacient in sheep or goats under natural conditions (Owen and Trees 1999; Hässig et al. 2003; Eleni et al. 2004).

2.3.2 Stillbirths and congenital abnormalities

Infection of cows with N. caninum commonly results in an increased incidence of stillborn calves (Waldner et al. 1998; Stenlund et al. 1999) and more rarely, in a range of congenital neurological abnormalities (Dubey et al. 2006). The latter include low birthweight and recumbency (O'Toole and Jeffrey 1987), flexural deformities of the limbs (Dubey and de Lahunta 1993), ataxia, encephalomyelitis and proprioceptive deficits (Barr et al. 1993), exophthalmia or asymmetric appearance of the eyes (O'Toole and Jeffrey 1987), scoliosis (Bryan et al. 1994), hydrocephalus (Dubey et al. 1998) and narrowing of the spinal cord (Dubey et al. 1990b).

A recent study (Brickell et al. 2010) has reported an increased risk of perinatal mortality (defined as stillborn calves or dying within 24 hours of parturition) amongst heifers infected with N. caninum in their first (p < 0.01) and second (p < 0.1) parity. It was not, however, clear from the study whether any of these exhibited clinical signs consistent with congenital neosporosis.

2.3.3 Other production effects

No other health consequences have been reported in cattle infected with N. caninum. There is, however, a consistently increased risk of culling that has been reported in several studies from different countries and production systems (Thurmond and Hietala 1996; Tiwari et al. 2005; Bartels et al. 2006a). In most studies, the increased risk of culling can be attributed to abortion. Interestingly, a study of one Californian dairy herd showed that the risk of culling was increased in seropositive cattle independent of abortion (Thurmond and Hietala 1996), whereas another study carried out among dairy herds in Ontario, Canada, found that seropositivity was not associated with culling (Cramer et al. 2002).

Whether infection with N. caninum (once the effects of abortion are excluded) affects milk production is less certain, as some studies have reported that yield is decreased by 3 - 4% (Thurmond and Hietala 1997b; Hernandez et al. 2001; Romero et al. 2005). In another study this effect only occurred in the first 100 days of lactation (Bartels et al. 2006a). Two other studies found no effect of seropositivity on milk production (Hobson et al. 2002; van Leeuwen et al. 2002) and one New Zealand study reported increased milk production in seropositive cows (Pfeiffer et al. 2000). It might be that lesions similar to

those in aborted fetuses occur in congenitally infected animals and may compromise organ function and thereby limit a cow's ability to produce large quantities of milk, or even that the energy cost of the immune response might affect milk production; but it is difficult to imagine how infection with N. caninum could have a positive effect on milk production.

Studies on N. caninum infection in beef cattle have found that seropositive calves have lower weaning weights and an increased risk of culling (Kasari et al. 1999), as well as lower growth rates after weaning resulting in a lower carcass weight and profit (Barling et al. 2000b).

In addition to the aforementioned losses there may be rebreeding costs, expenses associated with establishing a diagnosis and replacement costs if aborting cows are culled. Not all of these costs will be relevant to all cattle farming enterprises but the net result will be a decrease in farm profitability.