To the authors' knowledge, this is Namibia's first study on sheep and goats toxoplasmosis. This study determined animal-level prevalence rates of 3.68% and 0.29% in sheep and goats, respectively, which are very low compared to South Africa, India, Iran and Italy, where seroprevalence rates ranging from 27–64.46% were determined (Gazzinos et al., 2015; Sharif et al., 2015; Bachan et al., 2018; Tagwireyi et al., 2019). These low prevalence rates in Namibia might be due to the tropical semi-arid climate in the country, which is characterised by hot and dry summers interspaced with erratic rainfall and predominantly cold and dry weather in winter. These conditions create an unfavourable environment for the T. gondii oocysts to sporulate (Yan et al., 2016). Generally, T. gondii infections tend to be high in warm and humid environments but low in hot and dry climates (Meerburg and Kijlstra, 2009).
Animals reared under extensive conditions have been found to have lower seroprevalence rates than those reared under intensive or semi-intensive conditions (Tzanidakis et al., 2012; Tagwireyi et al., 2019). All small ruminants in the current study were grazed extensively, which could also have contributed to the low seroprevalence rates observed in the current study. Extensive grazing conditions in central Namibia are generally hot and dry for most of the year, which might interfere with the sporulation of T. gondii oocysts and, therefore, infection rates.
Interestingly, a recent study among pregnant women in Windhoek, Namibia, also found a similarly low seroprevalence rate of 2.61% (n = 344); the authors noted that this figure was low compared to other developing countries, and they attributed this to geographical and climatic factors (Colf et al., 2020). Furthermore, in an earlier study by Colf and co-workers, only 0.961% (n = 312) of blood donors in central Namibia were seropositive to T. gondii (Colf et al., 2014). Therefore, the low seroprevalence rates in the current study agree with these findings and demonstrate that T. gondii infections in sheep, goats and humans in the Khomas region are very low compared to the rest of the world, in which one-third of the globe’s human population is estimated to be infected (Tegegne et al., 2016).
The current study found that sheep had significantly higher animal-level and herd-level prevalences than goats. Similar trends have been reported in many parts of the world, including Northern Italy (Gazzinos et al., 2015), Africa, Caribbean Islands, Greece, Portugal, Spain, Poland (Stelzer et al., 2019) and Colombia (Martínez-Rodriguez et al., 2020). In one study in South Africa, Tagwirei and colleagues also found a higher seroprevalence in sheep than goats (Tagwireyi et al., 2019), although it was not statistically significant. However, studies in the USA and Egypt recorded a higher seroprevalence in goats than sheep (Mishra et al., 2016; Al-Kappany et al., 2018). Therefore, with few exceptions, the current study and reports from other parts of the world seem to indicate that sheep could be more susceptible to T. gondii than goats.
The significant association between seropositivity to T. gondii and a history of abortions found in the current study is not surprising. For example, in one study in Northern Iraq, sheep with a history of abortion within the previous twelve months were 13.4 times more likely to be seropositive (Al Hamada et al., 2019). One study in Central Ethiopia (Gebremedhin et al., 2013) and a review by Stelzer and colleagues also reported a similar association (Stelzer et al., 2019). However, in one recent report in Egypt by Abdelbaset and co-workers, no such association was found (Abdelbaset et al., 2020).
The total number of sheep at the farming establishment is another significant risk factor determined in the current study. Since sheep appear more susceptible to T. gondii infection than goats, as also established in this study, it is logical that the higher the number of sheep at an establishment, the greater the risk of toxoplasmosis. However, this disagrees with an Italian study where seropositivity decreased with increasing flock size (Cenci-Goga et al., 2013). It is worth noting, however, that this Italian research studied large flock sizes numbering 300–400 sheep, unlike the current study in which flock sizes were much smaller. Additionally, climatic conditions in Tuscany, Italy, are very different from the semi-arid Namibian conditions, which could affect the epidemiology of the disease.
Farm size was also positively associated with T. gondii seropositivity, implying that larger farming establishments had a significantly higher seropositivity rate. However, this type of relationship was unexpected. Logically, there should be an inverse relationship between farm size and seropositivity because the probability of small ruminants ingesting T. gondii sporulated oocysts in a smaller farming establishment would be higher. For instance, Corbellini and co-workers reported an inverse relationship between farm size and N. caninum seropositivity (Corbellini et al., 2006). On the other hand, larger farms in Namibia would be expected to have more wildlife carnivores which have been reported to be seropositive to T. gondii (Seltmann et al., 2020). Therefore, the chances of such carnivores contaminating the pastures would be higher on larger farms.
In conclusion, small ruminants in the Khomas region have been exposed to T. gondii, although the seroprevalence is low compared to other countries. Sheep appear more susceptible to T. gondii infections than goats; therefore, any control measures should be first targeted toward this species. However, given the very low prevalence rate found, introducing wholesale control measures is probably uneconomical at this stage. However, T. gondii should be included in the abortion screening tests at the Central Veterinary Laboratory. In this way, the country's toxoplasmosis status can be continuously monitored.