By Zachary Horn 

One Health recognises that human health, animal health and environmental health are interconnected to such a degree that successful management of one of these areas is dependent upon the others.(13) One Health therefore demand cooperation and collaboration between practitioners and experts across all three fields (i.e. human, animal/agricultural, and environmental health). 

One Health covers nearly all facets of public health and disaster management, including antimicrobial resistance, food safety and security, environmental contamination, ecological disasters, and occupational health.(13) 

Some of the most prominent health events occuring at the intersection of One Health are from zoonotic and vector-borne diseases. Surveillance data suggests that approximately 60% of emerging infectious diseases are zoonotic diseases, and approximately one billion cases of illness result from zoonotic infections annually.(14,15) 

Zoonotic pathogen emergence results from interactions between microbial diversity, environmental and ecological characteristics and stability, and the frequency of contact between animal reservoirs and humans.(16) For zoonotic pathogens to pose a pandemic threat, they must ‘spill over’ from a wild reservoir to humans (either directly or by an intermediary host), be transmissible between humans, and achieve chains of transmission that transcend geopolitical boundaries.(16) 

The humanitarian crises within the DRC highlight two examples of zoonotic disease driven One Health emergencies: Ebola and Malaria. 

Ebola

The DRC has been the site of several outbreaks of Ebola. Ebola is a deadly viral haemorrhagic fever caused by a filovirus. There are five species of Ebola: Bundibugyo ebolavirus, Reston ebolavirus, Sudan ebolavirus, Tai Forest ebolavirus, and Zaire ebolavirus.(15) While the average case fatality rate (CFR) of ebola is 50%, early Zaire ebolavirus outbreaks had a CFR approaching 90%.(17) 

Several challenges faced in responses to Ebola outbreaks have arisen from animal and ecological health factors.(15) One challenge is that the true natural reservoir of Ebola is yet to be definitively known but asymptomatic carrier states identified in fruit bats make them a key suspect.(15) Primates can carry and develop symptomatic, often fatal, disease from Ebola virus and act as an intermediary between humans and the natural reservoir.(15)

There is also evidence that dogs can contract Ebola, likely resulting in asymptomatic illness, fuelling ong concerns that dogs may also pose a significant risk of acting as another intermediary species between infected animals and humans.(18) 

Deforestation, armed conflict, attacks by rebel militias, and highly mobile populations are major drivers of increased interactions between humans and animals, but also complex changes in the microbial diversity within animal populations.(15,16) Additionally, hunting and butchering wildlife, or otherwise coming into contact with deceased animals, carry significant risks for cross-species transmission and spillover events.(16) Zaire ebolavirus has been identified as the cause of over 5,000 Gorilla deaths and there is evidence that mass fatalities among non-human primates may be a predictor of an emerging human threat.(19,20) 

Once Ebola has entered a human population, secondary transmission between individuals occurs via contact with the blood and body fluids of symptomatic patients.(21) The nature of the illness places healthcare workers and families at high risk for transmission, further increased by cultural practices surrounding the care of the ill and deceased. While recent developments in targeted vaccination in response to outbreaks are changing the way in which Ebola outbreaks unfold, Ebola continues to pose a profound threat to human health globally. 

While there are several complexities and challenges, Ebola crises in the DRC highlight the importance of a One Health approach. Environmental changes (including deforestation) and lacking infrastructure are increasing contact between humans and animals potentially carrying the devastating virus. Poverty, large populations, and food shortages are driving hunting and butchering behaviours that carry high risk for transmission of the virus from natural reservoirs and intermediary species to humans. 

Animal health surveillance may identify mass fatalities that warn nearby communities of a threat of an Ebola outbreak and inform safe food sourcing practices. Additionally, effective animal management, such as desexing and managing dogs in at-risk villages, may also make a significant contribution to reducing the overall burden of Ebola. 

Therefore, effective collaboration between human, animal, and environmental health sectors may rapidly detect the virus and prevent the emergence of Ebola outbreaks among human populations, reducing the overall  health burden and risk of further worsening existing humanitarian crises in the region. 

Malaria

In 2019, malaria was estimated to result in 409,000 deaths and 94% of the 229 million cases globally occurred within the WHO African region.(22) Malaria results from infection by one of five Plasmodium parasites (with Plasmodium falciparum and Plasmodium vivax posting the greatest risk) spread by bites from infected Anopheles mosquitoes.(23)

Malaria develops as an acute febrile illness, usually after 10-15 days from the bite of the infected mosquito.(23) Importantly, malaria due to Plasmodium falciparum can be fatal without treatment within 24 hours of symptoms onset.(23) 

The frequency and efficiency of transmission depends on parasite factors, vector factors, human factors, and environmental factors.(22) This highlights another important intersection between One Health domains, particularly in relation to both risk of exposure, and thus disease, and prevention. 

Different Anopheles genuses demonstrate different affinity for various aquatic habitats, ranging from water pooled in household items through to rainy seasons in dense tropical rainforests.(22) Climate change is a pertinent environmental health threat as it is a significant driver of shifting geographic distribution of vectors, and thus the geographical spread of the vector-borne diseases they carry.(24) 

Environmental factors, including environmental controls, are therefore important determinants of malaria exposure as well as potential avenues for preventing malaria spread. Important environmental interventions typically include ensuring that containers do not collect rainwater and ensuring that permanent bodies of water do not remain stagnant), but also need to consider the larger impact that climate change has on environmental health. 

Another commonly used environmental control is the use of insecticides to eliminate mosquitoes; however, the effectiveness of this intervention is threatened by insecticide resistance with 73 countries reporting resistance to at least one common insecticides and 28 countries reporting resistance to all common insecticides.(22) 

The risk of severe malaria outbreaks is increased when mosquito species have a long life-span and demonstrate preferential affinity for human-biting, as opposed to animal-biting.(22) However, this risk of exposure is not determined by vector factors alone; the risk is either exacerbated or mitigated by the way in which human behaviours facilitate or prevent exposure. 

Population growth, increased population mobility, and trade practices increase the risk of exposure between vectors and humans, and thus increase the risk of transmission of vector-borne diseases to humans.(24) Important malaria preventative interventions include the use of mosquito nets to directly prevent exposure and chemoprophylaxis to prevent disease regardless of exposure.(22) 

In addition to directly increasing the risk of exposure by increasing contact between vectors and humans, these factors also result in drastic changes in biodiversity which is further associated with increased risk for vector-borne diseases as an additional environmental factor.(24) 

In terms of pertinent human factors, humans develop partial immunity to malaria with progressive exposure to a specific parasite species.(22) This means that immunity is geographically specific according to the distribution of the specific parasite species.(22) However, immunity dwindles after prolonged periods without exposure, and thus humans can still contract malaria from a parasite of a different species or if an individual returns to the region they grew up in after a period long enough to allow immunity to dwindle.(22) 

In summary, effective environmental controls reduce the risk of malaria by preventing mosquito incursions or reducing their presence in human dwellings, while climate change and the environmental consequences of industrial behaviours increase the risk of exposure. Human behaviours, such as mosquito avoidance strategies and chemoprophylaxis, reduce the risk of exposure or disease, while behaviours such as logging/deforestation, inadequate mosquito avoidance (including due to lack of resources), and migration increase the risk of exposure or disease. The fact that such a fine balance between all of these factors is required to prevent severe malarial illness perfectly highlights that a One health approach has the potential to achieve better health outcomes than management of a single domain alone.

References

13.CDC. One Health basics. CDC [Internet]. 2018. Available from:  https://www.cdc.gov/onehealth/basics/index.html
14.Jones KE, Patel N, Levy M, Storeygard A, Balk D, Gittleman J, et al. Global trends in emerging infectious diseases. Nature. 2008;451:990-994.
15.Sikakulya FK, Mulisya O, Munyambalu DK, Bunduki GK. Ebola in the Eastern Democratic Republic of Congo: One Health approach to infectious disease control. One Health. 2019;9:100117
16.Wolfe ND, Daszak P, Kilpatrick AN, Burke DS. Bushmeat hunting, deforestation, and prediction of zoonotic disease emergence. Emerg Infect Dis. 2005;11(12):1822-1827. 
17.World Health Organisation. Ebola virus disease. World Health Organisation [Internet]. 2021 Feb 23. Available from: https://www.who.int/news-room/fact-sheets/detail/ebola-virus-disease
18.Allela L, Bourry O, Poullot R, Delicat A, Yaba P, Kumulungui B, et al. Ebola virus antibody prevalence in dogs and human risk. Emerg Infect Dis. 2005;11(3):385-390.
19.Bermejo M, Rodriguez-Teijeiro JD, Illera G, Barroso A, Vila C, Walsh PD. Ebola outbreak killed 5000 gorillas. Science. 2006;314(5805):1564. 
20.Kelly TR, Machalaba C, Karesh WB, Crook PZ, Gilardi K, Nziza J, et al. Implementing One Health approaches to confront emerging and re-emerging zoonotic disease threats: Lessons from PREDICT. One Health Outlook. 2020;2(1). 
21.Crazen JM, Kanapathipillai R, Campion EW, Rubin EJ, Hammer SM, Morrissey S, et al. Ebola and quarantine. New Engl J Med. 2014;371(21):2029-2030.
22.World Health Organisation. World malaria report: 2020. World Health Organisation [Internet]. 2020. Available from: https://www.who.int/publications/i/item/9789240015791
23.World Health Organisation. Malaria. World Health Organisation [Internet]. 2021 Apr 1. Available from: https://www.who.int/news-room/fact-sheets/detail/malaria
24.Faburay B. The case for a ‘one health’ approach to combating vector-borne diseases. Infect Ecol Epidemiology. 2015;5:28132.