Our study reports an overall animal-level seroprevalence of 5.7%, and to the best of our knowledge, this is the first study on N. caninum seroprevalence in cattle reported from Namibia. This observed prevalence is comparable to that reported within the region in South Africa, where 5.17% seroprevalence was reported in beef cattle (Chisi et al., 2013). However, unlike in the latter study, where all the dairy farms sampled were positive (Chisi et al., 2013), the two small-scale dairy herds sampled in the current study were both negative. These results could be because the dairy industry in Namibia is still in its infancy, with an estimated herd of only 3000 cattle (Bieldt, 2005). In addition, the largest dairy farm in the country was not sampled because it fell outside of the study area.
A similarly low seroprevalence rate of 3.8% was also detected in one study in Iran, where it was attributed to a combination of warm and dry climate followed by cold and dry conditions (Noori et al., 2019). These climatic conditions were deemed unfavourable for the survival of N. caninum oocysts in the environment (Noori et al., 2019). On the other hand, humid conditions coupled with mild to warm environmental temperatures are ideal for sporulation and survival of oocysts (Dubey et al., 2007) and have also been associated with higher incidences of N. caninum abortions (Wouda et al., 1999). Northwest China has a generally hot and dry climate with very little rainfall in the summer months compared to the other parts of the country, and the area has been found to have the lowest N. caninum seroprevalence rate of 9.4% in the country (Wei et al., 2022). These scenarios perfectly mirror the semi-arid conditions of the Khomas region of Namibia, where the current study was conducted.
A study on cattle in northern Tanzania (Arusha region) determined a seroprevalence rate of 21.5% (Semango et al., 2019), which is much higher than the current study. Given the semi-arid nature of Namibia’s climate, it could affect the survival and sporulation of the N. caninum oocysts in the environment (Dubey et al., 2007; Noori et al., 2019) and therefore reduce transmission. On the other hand, northern Tanzania is more humid and has higher average annual rainfall (Kimaro et al., 2018) compared to Namibia, which provides a more conducive environment for the sporulation of N. caninum oocysts. The same argument could also explain the relatively low prevalence rate in Namibia compared to other countries like the northern part of Algeria (Ghalmi et al., 2012); Argentina (Moore et al., 2003), China (Qian et al., 2017), Colombia (Llano et al., 2018), North and Central America, Asia, Europe and India (Hebbar et al., 2022).
The low animal-level N. caninum seroprevalence rate in the current study could also be attributed to the fact that 95% (698/736) of the samples tested were from beef cattle compared to only 5% (38/736) from dairy cattle. Beef cattle are less susceptible to N. caninum than dairy cattle, and therefore they tend to have lower seroprevalence rates (Quintanilla-Gozalo et al., 1999; Haddad et al., 2005; Fort et al., 2015; Ribeiro et al., 2019; Gharekhani et al., 2020).
Communal, dairy & resettlement establishments were all negative, possibly because the number of herds sampled from this category was very small; a larger sample size might have given a different result. Additionally, feliforms are less likely to be found in this category of establishments due to higher human population densities. However, the dog population is also expected to be increased with the increased human population, especially in communal setups (Butler and Bingham, 2000).
The current study found no significant association between the number of dogs at the farming establishments and N. caninum seropositivity. Similar findings were also made in studies in Tanzania (Mathew, 2017; Semango et al., 2019). However, since dogs are the definitive hosts of N. caninum (McAllister et al., 1998; Goodswen et al., 2013), these findings are somewhat surprising. Other studies have indeed confirmed that the presence of farm dogs increases the risk of N. caninum infection in cattle (Dubey et al., 2007; Robbe et al., 2016; Fávero et al., 2017; Abdeltif et al., 2022) and goats (Rodrigues et al., 2020), most likely through faecal contamination of pastures and open water sources with oocysts. The lack of significant association with the number of dogs found in the present study might have been caused by the fact that most of the farm dogs were confined to the homesteads and therefore did not have carte blanche access to livestock pastures. This, in turn, meant that the risk of pasture contamination, regardless of the number of dogs at the farming establishment, was markedly reduced.
However, N. caninum positive status was significantly associated (p < 0.05) with the presence of Feliformia (brown hyenas, leopards & cheetahs) but not black-backed jackals, despite most farmers reporting many sightings of jackals on their properties. Domestic dogs (Canis familiaris) and black-backed jackals (Canis mesomelas) look physically similar; therefore, the possibility of mistaking these two species in places where the presence of Feliformia was reported cannot be entirely disregarded. In a recent Namibian study, brown hyenas (Hyaena brunnea) and black-backed jackals (Canis mesomelas) were found to be seropositive to N. caninum (Seltmann et al., 2020); however, the significance of these findings in light of the results of the current study need to be further investigated. Furthermore, workers in Tanzania have also suggested the possible involvement of wildlife in the epidemiology of N. caninum after finding no association been dog ownership and cattle seropositivity (Semango et al., 2019).
In one study, self-rearing of replacement heifers was associated with an increased risk of bovine neosporosis (Otranto et al., 2003). However, the current study found no such association, despite all the establishments indicating that they self-reared replacement heifers. This could be explained by the low N. caninum seroprevalence rate in the Khomas region, which reduces the risk of vertical parasite transmission.
This study found no significant association between abortion history and N. caninum seropositivity. One study in Northeast Algeria made similar findings, and the authors concluded that those cows were resistant to N. caninum abortions (Abdeltif et al., 2022). However, further investigations would be needed, given the extreme biological importance of the latter findings. The same scenario could also be at play in the Khomas region of Namibia, especially considering that significant variations in seropositivity have been found between countries, within countries, regions in the same country, and breeds (Dubey et al., 2007). Another study in Southeastern Iran found no association between N. caninum seropositivity and a history of abortions (Noori et al., 2019). However, other studies have demonstrated a significant association between N. caninum seropositivity and a history of abortions (Moore et al., 2009; Ghalmi et al., 2012; Llano et al., 2018). As earlier noted, this discrepancy could be due to variations between countries or regions.
The findings in this study were generally in agreement with some studies in other parts of the world. However, the seroprevalence level of N. caninum in the Khomas region is much lower than in most other parts of the world. Furthermore, the role of Feliformia in the epidemiology of bovine neosporosis needs to be further investigated.