Sandscript #30 – A long-running story: the enduring search for health security common to humans, livestock and wildlife
Read here the second article of Sandscript 30th issue
- North-East Chad, somewhere towards Abéché in a region that is now gazetted as the Ouadi Rimé – Ouadi Achim Faunal Reserve (OROAFR). Farcha Veterinary Research Laboratory (now IRED, Livestock Development Research Institute) is on a field mission to study the epidemiology of Rift Valley fever (RVF) (Maurice & Baille, 1967), a zoonotic arbovirus infection (a viral disease transmitted by arthropod vectors) that affects both humans and domestic and wild ruminants.
The mission entails taking blood samples from livestock and wildlife to look for antibodies against RVF, indicating past infection with the virus. For the wildlife, the mission has to kill (it was the only method known at the time) 24 individuals in good health belonging to five different antelope species: 12 dorcas gazelles, 6 dama gazelles, 2 red-fronted gazelles, 3 scimitar-horned oryx and even a tiang as this antelope was in the habit of migrating that far north during the rainy season. Back in N’Djamena, the laboratory carries out serological analyses for RVF and discovers that, in addition to 20% of all the sheep sampled being seropositive, 46% of the antelopes sampled in the Abéché region (11 including 2 of the 3 oryx) were seropositive (Maurice & Baille, 1967). RVF will then be confirmed on several occasions in Chad, both in humans, including some casualties (e.g., Durand et al., 2003), and domestic ruminants (e.g., Ringot et al., 2003; Fayiz Abakar et al., 2014).
This mission was altogether banal, almost routine. However, it proves a valuable learning experience for us today.
We thus observe that in 1967 the Farcha laboratory had already adopted an integrated approach to research by exploring the role of both wild animals and domestic animals in the epidemiology of diseases.
This was 37 years before the Wildlife Conservation Society’s founding symposium in New York in 2004, which launched the “One World One Health” approach in spectacular fashion, that is nowadays on the tip of everyone’s tongue. Moreover, in 1998, six years before the famous symposium, the Farcha laboratory – along with its partners – undertook a long interdisciplinary research series on the health of nomadic pastoralists and their livestock in the Sahel, bringing together a number of eminent specialists in human and animal health (Montavon et al., 2013).
In reality, the story of “One World One Health” in the Farcha laboratory had begun several decades before “One Health’s” baptism in New York. Our predecessors were already exploring pathologies shared between humans and animals and influenced by the environment. Often more generalist than we are today, many of them were much more than health specialists; they were “naturalist-scientists” in the true sense of the term, with broad skills in many sciences such as zoology, botany, climatology and agrostology (the scientific study of grasses and pastoral rangeland).
They quite naturally incorporated these different sciences into their work. Now quite obsolete, the term “naturalist-scientist” has largely disappeared. With progress in science, scientists have become ultra-specialized, slightly losing sight of the cross functional nature of disciplines, which have become more in-depth, verticalized as it were. This is probably why health specialists felt the need to turn to a more horizontal approach such as “One Health” to recover the multidisciplinary method of our naturalist-scientist elders. Incidentally, certain sciences have perhaps even disappeared at the same time, such as agrostology, the science of grassland, which used to be a major discipline in the Sahel as demonstrated by the many publications from the last century.
To return to our 1967 mission, some twenty years later in the early nineties, not a single scimitar-horned oryx was to be seen in the region or anywhere in Africa for that matter. The species was declared extinct in the wild in 2000 (IUCN SSC Antelope Specialist Group, 2016). It took a further twenty or so years before a scimitar-horned oryx reintroduction program began in Chad in 2016 under the aegis of the Chadian Ministry of Environment, Fisheries and Sustainable Development, Sahara Conservation Fund (now SaharaConservation) and the Environment Agency – Abu Dhabi, United Arab Emirates (UAE).
Between March 2016 and January 2017, the program reintroduced three batches of 25 oryx each i.e., 75 oryx, from the UAE into OROAFR. On 11 February 2018, a fourth batch of 75 oryx was reintroduced and placed in quarantine enclosures to acclimatize before they were released into the reserve on 6 August of the same year i.e., almost six months after their arrival. Eighteen days after their release, two of these oryx wearing GPS collars died on the same day at a distance of 98 km from each other, beginning an episode of mass mortality as 40 oryx from the fourth batch (53%) died in the space of 40 days. Only four oryx from the first three batches were affected. These batches had not been subjected to the constraints of the fourth batch: (i) on the one hand, containment in quarantine under heavy rainfall with forced exposure to an arthropods outbreak, (ii) on the other hand, the abrupt transition from their diet, which was extremely rich during quarantine, to natural Sahelian grassland, which was exceptionally abundant during this intense rainy season, but consequently was all the poorer in terms of nutrition. It is also possible that the oryx in the previous batches had been moderately exposed to the RVF virus during prior normal rainy seasons, thus allowing them to develop the immunity of a “natural vaccination”.
Finally, the very last mortality in the oryx precisely coincided with the final rain of the 2018 wet season. It is important to note that in 2018 there had been twice as much rain as normal, triggering a cascade of phenomena, beginning with a population boom in arthropods: acarids on one side, particularly Hyalomma sp. ticks that passed on babesiosis to the oryx, and insects on the other, with a massive infestation of Haematobia sp. biting flies that exhausted the oryx with their incessant harassment, blood predation and the propagation of cutaneous streptothricosis. Autopsies also revealed several cases of haemorrhagic septicaemia.
A female oryx in particular intrigued us with its sudden death just after having aborted, despite being in very good general health. The clinical suspicion of RVF was reinforced by post-mortem. Samples were taken following the precautions required for this zoonosis. Laboratory examinations at CIRAD in Montpellier, WOAH’s (World Organisation for Animal Health) international reference laboratory for RVF, made it possible not only to isolate but also to sequence the RVF virus. In addition, the blood samples from five of the nine oryx analyzed for RVF (55%) tested positive by ELISA and/or PCR tests.
We know that the environment plays a major role in RVF’s epidemiology. Climate-related events such as very heavy rainfall (El Niño phenomenon) can lead to an outbreak of mosquitoes that are vectors for the virus.
During inter-epizootic so-called ‘silences’ (between mortality episodes), the virus is thought to insidiously circulate in domestic animals, particularly small ruminants, even in the absence of wild animals (Lefèvre, 1997). The virus also subsists by vertical transmission (from the mother to her eggs) in mosquitoes of the Aedes genus, which preferentially feed on domestic animals. Moreover, during the same period and in the same region, the outbreak of insects caused by the exceptional rains not only triggered the RVF episode in the oryx but also a serious epizootic of another arbovirus disease, African horse sickness (transmitted by a biting midge of the Culicoides genus), which led to the mass mortality of donkeys and horses around Abéché. And later on, still in the same region, yet another arbovirus infection occurred, this time a Chikungunya epidemic affecting humans, the virus responsible also being transmitted by mosquitoes of the Aedes genus.
In the course of this story, we could not fail to notice that the oryx that were seropositive for RVF in 1967 did not present any signs of disease and were therefore probably healthy carriers, as were the seropositive sheep sampled at the time. Thus, the oryx might have co-evolved with the virus, selecting, if not natural resistance to the disease, at least reduced sensitivity, which incidentally disappeared with the extinction of the species in the wild.
While the oryx involved in the mass mortality in Chad in 2018 belong to the same species as the now extinct population studied in 1967, the individuals concerned have a completely different history and nature. These animals are from several generations of stock born and raised in captivity in the UAE in an environment free from RVF and under strict sanitary controls. These oryx seem to behave differently to their ancestors in Chad, demonstrating “naive” status with regard to RVF, as is said of living beings faced with unprecedented situations.
Therefore, naive oryx from the UAE were infected by the RVF virus in OROAFR, where we know that the virus has been circulating for over fifty years. The oryx could not have arrived in Chad carrying the virus as RVF’s incubation period only lasts from 12 hours to 6 days (WOAH, 2022), whereas they were hit by the disease almost six months after their arrival in Chad.
This mortality episode in oryx is an exceptional case. It is rare to detect both: (i) the RVF virus in wild animals and (ii) a strong clinical expression in these same wild animals. It is also rare to observe so clearly the concurrent triggering by a climate-related event of: (i) a coinfestation by external and internal parasites and (ii) a coinfection by bacterial and viral pathogens.
Today, the long-running story of “One Health” in Chad continues to be written on many topics, including the question of biodiversity conservation in the Sahel.
Co-Chair, Antelope Specialist Group SSC, (Species Survival Commission)
International Union for Conservation of Nature
Richard Ngandolo Bongo Nare
Deputy Director General,
Livestock development Research Institute (IRED), Chad