According to Bagley (1999) and Tulu et al., (2018), abortion levels of up to 2% in cattle are generally regarded as normal, but figures upwards of 3-5% warrant investigation and intervention, in order to curtail further losses. Some studies have determined average abortion rates ranging from 4.55% in sheep to 5.61%-11.86% cattle (Abdelhadi et al. 2015; Dechicha et al. 2020). There is therefore no question that abortion figures of up to 77.6% at animal level, which are reported in this study, were very high and therefore constituted an outbreak that waranted further investigation.
A definitive diagnosis of the cause of abortion often presents a challenge because of the many infectious and non-infectious causes which are often responsible (Kumar et al. 2015; Tulu et al. 2018). Abortion also often follows an initial infection that may have occurred several weeks or months prior. Therefore, the causal agent is often undetectable when the abortion occurs (Tulu et al. 2018). The high cost of laboratory work to aid in diagnosing bovine abortion also compounds the problem (Tulu et al. 2018). Some investigations on causes of abortions have found that a significant proportion are due to unknown causes with some authors determining figures ranging from 47–67% (Macías-Rioseco et al. 2020; Wolf-Jäckel et al. 2020) in cattle, and 42% in sheep and 56% in goats (van den Brom et al. 2012). An abortion investigation in dairy cattle in Uruguay determined the aetiology in only 53% (54/102) of cases. About 51 of these cases were caused by infectious agents, with the following breakdown: Neospora caninum (29%), Coxiella burnetii (6%), Campylobacter fetus subsp. venerealis (2%) and the rest were Bovine Parainfluenza-3 virus, Bovine Viral Diarrhea virus (BVDV), Salmonella enterica serovar Newport, Leptospira interrogans, including opportunistic bacteria Escherichia coli, Streptococcus spp., Staphylococcus spp., Mannheimia spp., Trueperella pyogenes, and Providencia stuartii, (Macías-Rioseco et al. 2020). One study demonstrated that there was serological evidence of mixed infections in cattle among abortifacient agents, raising the potential for their synergism (Okumu et al. 2019). These studies point the notion of multiplicity of causes of livestock abortions as was found in the current study.
One study showed that in up to 38% of bovine abortions, inflammatory changes were found in the placenta and fetal organs without specific aetiology being identified (Wolf-Jäckel et al. 2020). The figure was between 4% and 10% in goats and 11–12% in sheep (Van Den Brom et al. 2012; Van Engelen et al. 2014). The reasons for the low success rate of a definitive diagnosis of the causes of abortion lies in the fact that, quite often, diagnostic procedures may be performed well after the inciting cause has disappeared from the body of the host (Kumar et al. 2015). In addition, pathognomonic gross lesions are uncommon, and in many cases they could have been obscured by autolysis (Kumar et al. 2015). Furthermore, toxic and genetic factors are not discernible in specimens available for examination, and many causes of abortion are unknown, or at present, there are no proper diagnostic procedures for their identification (Kumar et al. 2015).
In one study conducted in Algeria, 37.3% (32/59) of the farms surveyed had experienced abortions during the previous five years, although animal-level abortion rates were not indicated (Ghalmi et al. 2009). Our results indicate that 68.1% of surveyed farms experienced abortions. This could be due to the outbreak of abortions experienced at the time of the survey.
Twenty-three percent (12/52) of the caprine sera tested at one of the affected farms were positive for B. melitensis. This pathogen is a known cause of abortions in both sheep and goats (Das et al. 2008; Ndazigaruye et al. 2018). B. melitensis is an important cause, not only of abortions but also of orchitis and epididymitis in sheep and goats; it can also be an important cause of abortion in cattle in mixed farming practices. In a study investigating the causes of abortion in cows that were mixed and fed together with sheep and/or goats, PCR of 34 (28%) specimens out of 120 abortion samples including fetuses and milk tested positive for B. melitensis biovar 3 (Deng et al. 2020). This was the first study to demonstrate B. melitensis as the main etiological agent for cows mixed with sheep and/or goats. In the areas covered in the current study, mixed farming practices involving cattle and small ruminants (sheep & goats) is the norm rather than the exception. This might explain the abortions observed in both cattle and small ruminants in some farms.
The diagnosis of Brucella spp., as a cause of abortion in Africa has been through bacteriologic, serologic or molecular techniques and Brucella abortus biovar 3 is more associated with abortions in cattle (Ntirandekura et al. 2018). In a study of causes of abortion and risk factors associated with abortion, B. abortus accounted for 76,8% and 84,1% of abortions in cattle and sheep, respectively, while Mycoplasma accounted for 15,5% and 17,6% in cattle and sheep, respectively (Deng et al. 2020). Abortion-related risk factors included mixed farming, contact with other flocks, sheep brucellosis positivity and Mycoplasma positivity on serology (Deng et al. 2020; Ndazigaruye et al. 2018).
Di Blasio and coworkers isolated several of what they referred to as neglected opportunistic infections as causes of abortion, namely, Escherichia coli, Acinetobacter lwoffii, Staphylococcus spp, Streptococcus spp, Streptococcus uberis, Streptococcus suis, Trueperella pyogenes, Mannheimia haemolytica, bacillus cereus and Norcardia spp which they indicated as seemingly emerging and co-placing the major infectious players in bovine and caprine reproductive failure (Di Blasio et al. 2019).
In the current study, Trueperella pyogenes was isolated only once from a cow abortion case (Table 9). T. pyogenes is a normal inhabitant of domestic animals' respiratory, urogenital and gastrointestinal tracts, and it, therefore, causes opportunistic infections (Di Blasio et al. 2019; Macías-Rioseco et al. 2020; Ponnusamy et al. 2017). However, it is recognised as a critical etiological agent in bovine endometritis and metritis (Goldstone et al. 2014) and has also been implicated in sporadic abortions (Ponnusamy et al. 2017; Wolf-Jäckel et al. 2020). In some studies T. pyogenes was found to account for 3% (5/162) to 10% (27/280) of cases of bovine abortions which were studied (Syrjälä et al. 2007; Wolf-Jäckel et al. 2020). It is, therefore, reasonable to assume that T. pyogenes was the cause of abortion in this particular case.
A total of 63.3% (19/30) of bacterial culture results yielded various species of Enterococcus as the abortive agents, predominantly E. mundtii 26.7% (8/30), E. casseliflavus and E. gallinarum each at 10% (3/30). In an earlier study on livestock abortion outbreak in Namibia from 1991 to 1992, E. casseliflavus was isolated in 33% (30/91) of the culture samples (Basson, unpublished data). In another study of the causes of abortions in small ruminants E. casseliflavus was identified as an abortive agent, among other infectious causes (Schnydrig et al. 2017). In yet another study, Enterococcus species were also isolated in mastitic cow milk as well as from the genital tracts of aborted women (Hamzah and Kadim 2018). It is therefore possible that Enterococcus spp. could have been responsible for at least some of the abortions encountered in the current study.
Streptococcus suis was only isolated from one bovine fetus in the current study and is generally also regarded as an opportunistic infection in ruminants (Macías-Rioseco et al. 2020). There is, however, a report where it was implicated as the cause in a bovine abortion case (Higgins et al. 1990). It is therefore plausible that S. suis was a possible cause of abortion in this case.
Streptococcus spp., Enterococcus spp., Brucella spp. and E. coli were the most significant bacteria isolated from cases of epididymo-orchitis in bulls, rams and bucks. However, Brucella spp. and E. coli were only isolated from bucks. Streptococcus spp. can cause orchitis and overt testicular abscessation in cattle (Mahmoud et al. 2020) as well as epididymitis in rams (Searson 1976). The genus Brucella has a great tropism for the genitourinary system of cattle, goats, sheep and pigs, which is caused by the high concentration of erythritol in the testicular and placentary tissues of these animals (Colmenero et al. 2007). In particular, Brucella ovis has a predilection for the genital tract of sheep where it causes epididymitis and, rarely, abortions (Ridler and West 2011). Brucella abortus, Mycobacterium tuberculosis, Truperella pyogenes, Histophilus somni, Bovine Herpes Virus 1 (IBR/IPV) and Mycoplasma spp. have been implicated as the most common causes of orchitis in bulls (Bell 2006). Some authors contend that epididymo-orchitis is rare (approximately 1%) in breeding bucks and that B. melitensis and Corynebacterium pseudotuberculosis are the commonest causes (Stewart and Shipley 2014). However, the findings of this study seem to contradict that notion, as evidenced by the high number of orchitis cases in bulls, rams and bucks at Farm A, ranging from 33–40% during this outbreak.
It is therefore reasonable to assume that the Brucella spp. that were isolated in the current study were at least partially responsible for some of the cases of epididymo-orchitis that were observed in bucks in this study. Enterococcus spp., along with other bacteria like Pseudomonas spp. and Klebsiella spp., have also been implicated in causing epididymo-orchitis in man (Ryan et al. 2018), but no study has implicated this bacteria as a cause in domestic animals. However, the current study has demonstrated that Enterococcus spp. should be considered as a cause of epididymo-orchitis in bulls, rams and bucks, as well as abortions.
Non-infectious causes of abortion in cattle include chemical, physical, genetic, hormonal and iatrogenic factors (Ghalmi et al. 2009; Souza et al. 2018). One study provided nutritional causes such as starvation or energy deficiency which can result in embryonic or fetal death; imbalance of protein which can result in the lowering of uterine pH thereby reducing embryonic survival; iodine deficiency which can result in fetal goitre and increased neonatal death as well as abortion; manganese deficiency which results in lowered fertility; phytotoxicity associated with accumulation of nitrates in plants during dry spells which can result in abortions and congenital causes such as bovine leukocyte deficiency (Zekarias et al. 2019). In addition, Tulu and others mentioned heat stress, season, serum progesterone levels after conception, twin pregnancy, vaccination status against abortifacient diseases, communal grazing hygiene and animal management as non-infectious causes of abortion (Tulu et al. 2018).
In the current study, the number of bovine liver samples which had low levels of copper, zinc, manganese, selenium or iron ranged from (17%) 3/18 for iron to 83% (15/18) for manganese. Trace element deficiencies have also been implicated in causing abortions (Kumar et al. 2015). Minerals such as copper, cobalt, selenium, manganese, iodine, zinc and iron can influence the reproductive performance of ruminants (Kumar et al. 2015; Zekarias et al. 2019) and have been reported to be a predisposing factor for the occurrence of retention of placentas and repeat breeding in dairy cows, abortion and weak calf syndrome (Balmurugan et al. 2017; Yasothai 2014). However, the progesterone metabolite allopregnanolone has also been implicated in the pathogenesis of weak calf syndrome (Riedel et al. 2018), although other authors suggest a genetic predisposition (Zepeda-batista et al. 2018). In another study, low serum concentration levels of zinc and high serum copper levels have been associated with increased risk of fetal loss (Graham et al. 1994). Copper deficiency has also been reported to be associated with early embryonic death and resorption of the embryo, increased chances of retained placenta and necrosis of the placenta (Yasothai 2014).
It would therefore appear from the mineral results that micronutrient imbalances were at least a contributing factor to the abortion/stillbirth and epididymo-orchitis outbreak. Copper and zinc deficiency appeared to be a common thread in all of the results, and both minerals are essential for metabolism and proper function of the immune system (Fraker and King 2004; Minatel 2000). This might explain the many species of bacteria which were isolated. It is the authors' contention that in-utero growth retardation as a result of maternal malnutrition or over-nutrition in conjunction with the immunosuppressive effects of the micronutrient imbalances were significant factors during this outbreak.
As regards the pathology in the bull, this could very well be related to the underlying copper deficiency noted. Copper deficiency has a significant negative impact on the male reproductive tract, and in addition, the immunosuppressive effects would predispose to the development of epididymitis.
Toxins from certain species of plants such as Ponderosa pine needles (Pinus ponderosa), locoweed (Oxytropis or Astragalus spp.), broomweed (Guttierrezia microcephala) as well as coumarins from rat poison, many grasses, moldy sweet clover, heat stress can also cause abortions (Kumar et al. 2015; Tibary 2021). However, none of these causes were proven or suspected to be responsible for any of the cases of abortion in the current study, although they cannot be entirely ruled out.
Swerczek and Dorton (2019) reported that Mare Reproductive Loss Syndrome (MRLS) has been known to affect horses in Kentucky for decades, causing, among other things, outbreaks of spontaneous abortions. The authors further noted that these fetal losses, with an unknown aetiology, but associated with MRLS occurred in mares and other livestock that were grazing in spring pastures affected by climatic and environmental factors like droughts, cold-stress, nitrogenous fertilisers, and herbicides; factors that cause nitrate to accumulate in these pastures. This excessive accumulation of nitrate was reported to result in the formation of toxic and pathogenic abiotic nanoparticles in the amniotic fluid and pathognomonic placental lesions consistent with the MRLS. The authors then hypothesised that this recently discovered mechanism of action for the pathogenesis of fetal losses might be a predisposing factor for a host of opportunistic diseases in livestock (Swerczek and Dorton 2019). The Kentucky scenario mirrors the circumstances of the outbreaks of abortions in Namibia in the periods 1991-1992 and 2016-2018.
The years 1991-1992 and 2015-2016 were marked by severe droughts in Namibia (Devereux and Næraa 1996; Holloway 2000; Kahiurika 2016; van Rensburg and Tortajada 2021). It is therefore possible, as reported by Swerczek and Dorton (2019), that these drought periods which preceded the major outbreaks of abortions in the country could have resulted from excessive accumulation of nitrate in the pastures, which when consumed by pregnant animals, could have caused the formation of toxic and pathogenic abiotic nanoparticles in the amniotic fluid thereby predisposing the pregnant animals to a host of opportunistic infections resulting in the outbreaks of abortions.
In summary, the study identified a multiplicity of possible etiologies of the reproductive problems that occurred in the central regions of Namibia from 2016 to 2018. Any number of combinations of the causes identified in this study, could cause the observed outbreaks. However, why these conditions occurred in these species, parts of the country and at these related time periods remains elusive. Perhaps, controlled focused studies of future abortions, orchitis and epididymitis in specific animal species may shed light on the temporal and spatial relationships of the reproductive problems and, hence, their etiologies.