Globally Emerging Zoonotic Parasitic Diseases Essay
Globally Emerging Zoonotic Parasitic Diseases Essay
In recent years we have seen an increased prevalence in zoonotic parasitic infections. This is likely due to a number of factors, some of which are related to a shift in the way humans interact with animals. There are also however, other factors, not so clearly reflective of human-animal interactions. Globalisation has brought increased international trade of livestock resulting in international spread of disease, as well as increased tourism and adventure travel, resulting in exposure to new groups of zoonotic pathogens.
Land use is changing; the clearing of new areas for the cultivation of food and other land uses has resulted in human settlement in areas were animal populations and parasites were previously isolated from humans.Globally Emerging Zoonotic Parasitic Diseases Essay. In addition, displacement and stress of wildlife through development as well as global warming can lead to epidemics of wildlife disease with ensuring overflow infections into domestic animals and humans.Globally Emerging Zoonotic Parasitic Diseases Essay. Industrialisation has brought a change in the way food is manufactured. Farming animals in such close proximity creates a breeding ground for disease. Overpopulation, immigration and displacement due to conflict and natural disasters, has seen the migration of populations into large urban centres. This puts stress on the current infrastructure, resulting in inadequate water supplies and sanitation infrastructure to cope with the population load. Public health measures breakdown in times of war and poverty, consequently we see a re-emergence of infections that may have previously been controlled.
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Microbial adaptation and vector adaptation has borne antimicrobial and insecticide resistance, which in turn has seen the recurrence of previously controlled infections. The emerging infectious diseases are also attributed to population change; the ageing population, poverty, malnutrition and immunosuppression from other diseases such as HIV.
This literature review identifies five key emerging or re-emerging zoonotic parasitic diseases that have particularly important global impact, these are; Plasmodium knowlesi, Echinococcus granulosum, Cryptosporidium, Schistsomiasis, and Leishmaniasis. Globally Emerging Zoonotic Parasitic Diseases Essay.
Echinococcus granulosus
One of the parasites with significant global impact is Echinococcus granulosus. This is a cestode parasite for which dogs and other carnivores are the definitive host. Man is a dead-end host in which the disease Cystic Echinococcosis (CE) occurs, this is commonly known as hydatid disease. Disease is often transmitted from sheep or other cattle to dogs and then humans. Globally Emerging Zoonotic Parasitic Diseases Essay.
In humans cysts are caused by larval stages of the parasite. Hydatid cysts cause abdominal pain, hepatomegaly, cholestasis, biliary cirrhosis, portal hypertension, ascites, fever, anaemia and weight loss. Cysts may rupture into the peritoneal cavity, causing anaphylaxis or secondary CE. Abscess formation is possible due to bacterial infection of cysts. The mortality rates for untreated patients can be very high. Surgery for cyst removal is very complex with a very high risks of anaphylaxis and subsequent death in the event of intra-operative cyst rupture. Trend shows an increase in persons affected. YLDs global. There has been a steady increase in global YLD from 54, 957 in 2000, 63, 278 in 2010 and 76, 044 in 2016. Globally Emerging Zoonotic Parasitic Diseases Essay. Although control programmes against Echinococcus granulosus have been established in some countries, the parasite still has a wide geographical distribution affecting many countries of all continents, and in some areas it is a re-emerging problem. For example, alarming increases of the number of human cases have been reported from Bulgaria, Tibet, Kazakhstan and China, there is also a high incidence of disease in displaced populations, such as the Syrian refugee population.
Particularly CE is seen amongst low-income livestock raising communities, and those living in poverty in which households and animals (both domestic and stray) live in close proximity.
Control of CE has been infamously difficult, the cause for this is multifactorial. Instability of healthcare infrastructure as seen in developing countries and countries of conflict has resulted in the termination of pre-existing control programs for CE.
The presence of both stray dogs, and domestic dogs that are allowed to roam free will increase the spread of disease. Globally Emerging Zoonotic Parasitic Diseases Essay. Currently there is a lack of regulation of slaughter houses and dogs are able to eat offal that has been discarded off, thus continuing the life cycle of disease. Lack of regulation also means slaughter houses can continue to sell meat, despite it being affected by cysts. Other barriers include the cost of and accessibility of de-worming medications or vaccinations, and the difficulty in identifying affected dogs as dogs with Echinococcus granulosus are asymptomatic. Globally Emerging Zoonotic Parasitic Diseases Essay.
Schistosomiasis
Schistosomiasis, is a disease caused by digenetic blood trematodes. The three main species infecting humans are Schistosoma haematobium, S. japonicum, and S. mansoni. Various animals, such as dogs, cats, rodents, pigs, horses and goats serve as a reservoirs for S. japonicum, and dogs for S. mekongi.
Infection occurs when parasite larvae, released by freshwater snails, penetrate the skin when in contact with infested water. Transmission occurs when people suffering from schistosomiasis contaminate freshwater sources with their excreta containing parasite eggs. Adult worms live in blood vessels where they release eggs. Some of the eggs become trapped in body tissues, causing immune reactions and progressive organ damage. Other serious clinical manifestations include chronic blood loss, cognitive impairment and diminished academic performance.
Additionally, schistosomiasis can cause female and male genital lesions resulting in pain, infertility, stigma, and increased risk of contracting HIV [REF M]. Ultimately, life-threatening health effects such as cancer of the bladder as well as renal and liver failure can ensue.
While the global burden of disease is underestimated, it is still believed that over 200 million people are infected worldwide and Schistosomiasis causes 3. 3 million DALYs. As with cryptosporidium and many other infectious diseases, lack of access to safe drinking water and adequate sanitation as well as increased contact with host animals results in increased risk of disease. Globally Emerging Zoonotic Parasitic Diseases Essay.
With the rise of global migration, international trade, and global environmental challenges such as climate change, it is not surprising that the interactions between humans and other animals are shifting. Salient infectious diseases, such as malaria and HIV (which have high burdens of disease), attract sophisticated public health frameworks and funding from global/regional organisations, such as the WHO. This unfortunately detracts attention from the many emerging zoonoses that fall under the radar as neglected tropical diseases (NTDs). This review considers the available literature and the attribution of burden of disease to the most insidious NTDs and recommends which five are deserving of policy prioritisation. In line with WHO analyses of NTDs, intestinal nematode infections, leishmaniasis, schistosomiasis, and lymphatic filariasis should be prioritised, as well as the burden of disease of cryptosporidiosis, which is largely underestimated. Both monitoring and treatment/prevention control methods for cryptosporidiosis are suggested and explored.
The Global Burden of Disease of Zoonotic Parasites: Top Five Contenders for Priority Consideration.
When considering global burden of disease (BOD) of infectious origin, it is usually certain ‘poster-child’ diseases that come to mind, such as tuberculosis, malaria, avian influenza, and HIV. This is reflected in global prevention strategies and allocation of resources [1]; however, it also means that there is a large body of infectious diseases called neglected tropical diseases (NTDs) that may fall below the global public health radar and either continue untreated or pose possible pandemic risk [2]. Specifically, zoonotic parasitic infections are of interest, considering the shifting interactions between humans and other animals as well as global trade and agriculture—with the rise of One Health as a public health discipline within the past decade [3,4]. This literature review will consider emerging parasitic zoonoses that fall into the category of NTDs and contribute the most to the global BOD. Upon outlining the relevant literature and presenting the top five zoonotic parasitic diseases in terms of global BOD, an integrated control program will be suggested for cryptosporidiosis, which is largely neglected in salient public health literature and public health policy.
On the topic of emerging parasitic zoonoses, a piquing observation was made by Thompson and Deplazes (2011): ‘In the context of emerging diseases, parasite zoonoses figure prominently, undoubtedly because of the shifting interactions between humans and other animals.’ [5]. Regarding the sheer number of emerging infectious diseases, the use of ‘prominently’ is definitely accounted for, with over 60% of around 400 identified infectious diseases since 1940 being zoonotic [4].Globally Emerging Zoonotic Parasitic Diseases Essay. However, it is important to view this in light of global BOD (which is often underestimated for these diseases) and the relative methods of transmission of these zoonoses. For example, certain zoonotic parasites may affect humans as incidental hosts (e.g., toxoplasmosis), where infection is dependent upon human behaviour/interaction with the definite hosts, while other zoonotic parasites readily infect humans and are more easily spread through environmental modes such as rain runoff, flooding, vectors, and aerosol spread [6].
For the purposes of this review, the global BOD framework proposed by Hotez et al. (2014) [7] will be used to explore the top five parasitic zoonoses (Table 1): (i) cryptosporidiosis; (ii) intestinal nematode infections (INI); (iii) leishmaniasis; (iv) schistosomiasis; and (v) lymphatic filariasis (LF). Despite listing BOD calculations in terms of disability adjusted life years (DALYs), which can further be broken down into components of years lived with disability (YLD) and years lost to premature mortality (YLL) [8], the analysis done by Hotez et al. (2014) through the World Health Organization (WHO) is not without its criticisms [7]. Briefly, criticisms of the analysis include existing gaps in data (especially in poorer parts of the world) and estimates of population health being prone to differing cultural frameworks—somewhat obscuring the extent of the BOD due to a given disease [9]. A full critique of that analysis is beyond the scope of this review; however, what remains important to remember is the economic limitations of instituting public health prevention strategies on a global scale and how resource management is necessary when dealing with a largely neglected subset of global infectious diseases [9]. Globally Emerging Zoonotic Parasitic Diseases Essay.
Table 1
Summary of burden of disease (BOD) floor estimates of 5 most salient neglected tropical zoonotic parasitic diseases.
| Rank | Disease | BOD (million DALYs) |
|---|---|---|
| 1 | Cryptosporidiosis | 8.37 |
| 2 | Intestinal nematode infections | 5.16 |
| 3 | Leishmaniasis | 3.32 |
| 4 | Schistosomiasis | 3.31 |
| 5 | Lymphatic filariasis | 2.78 |
1. Cryptosporidiosis
Possibly the largest critique of the WHO’s analysis of NTDs is the choice to not include cryptosporidiosis in the NTD category despite it contributing significantly to the global BOD, with an estimate of 8.37 million DALYs according to 2010 data, which may be an underestimation in itself [7]. Recent evidence suggests that in the developed world, cryptosporidium diagnoses may reflect somewhere as low as 1% of true prevalence, with this being even lower in the developing world [10]. Considering the global distribution of protozoa of the Cryptosporidium genus and that immune-compromised patients and children are at risk of developing cryptosporidiosis, this suggests a greater BOD when paired with its diagnostic evasiveness [11]. In recent studies, estimates show that up to a quarter of children with diarrhoea are co-morbidly infected with cryptosporidium, which is associated with longer durations of diarrhoea, malnutrition, higher mortality rates, and immune-compromise [12].
According to the Centers for Disease Control and Prevention (CDC), transmission of C. parvum and C. hominis (the two primary species affecting humans) occurs mainly through drinking or eating contaminated water/food where thick-walled oocysts are ingested, allowing further parasite maturation in the intestines and reproduction into auto-infective thin-walled oocysts and thick-walled oocysts that pass through faeces into the environment [13]. Interestingly, this life cycle is not unique to humans with other Cryptosporidium spp. infecting essentially all mammals, including those in close domestic proximity to humans [13]. However, according to a recent study, no significant difference in BOD was found between households that used bottled water and those that used tap water, suggesting that the primary method of spread lies elsewhere [14]. Globally Emerging Zoonotic Parasitic Diseases Essay. When paired with higher levels of oocysts (which are quite resistant to chlorination attempts) in surface water at the end of rainy seasons, seasonal patterns in cryptosporidiosis suggest that rainwater-runoff in areas with poor infrastructure are perfect breeding grounds for Cryptosporidium spp. due to the hot and humid environment, so there is an increased risk of infection [15]. There is much we do not know about the extent of cryptosporidiosis globally; however, if the BOD is to be lowered and improved, accessible diagnostic methods must be made available globally (with animals also tested) [11]. Treatments targeting the reasons for infection, such as immunocompromised/AIDS or INIs, should also be prioritised and supplemented with antiparasitic therapy [11].
2. Intestinal Nematode Infections (INI)
DALY analyses of the global BOD of NTDs have systematically demonstrated the dominance and wide epidemiological spread of INIs with an estimated 5.19 million DALYs [7]. Of these, the majority are accounted for by hookworm diseases (3.23 million), followed by ascariasis (1.32 million), then by trichuriasis (0.64 million) [7]. Interestingly in the WHO analysis, strongyloidiasis was not included in the INI figure, which may result in a severe underestimation of the total BOD arising from nematodes. Regarding global prevalence, INIs are most prevalent in Asia with roughly 67% of cases; however, incidence rates controlled for country population are quite unvaried between major global regions [15]. The burden of INIs seems to vary greatly within major global regions, which makes sense considering the methods by which these diseases are spread—water runoff, rain, animals, and human migration amidst inadequate public sanitation and infrastructure [16].
The lifecycle of soil-transmitted helminths is important to consider, as the eggs of the helminth are passed from the definitive host into the environment at the infective stage [11,16,17]. The larvae of Strongyloides stercoralis, unlike other soil-transmitted helminths, are passed in faeces at the infective stage [11,16,17]. The pathological sequelae of Ascaris lumbricoides and Trichuris trichiura primarily involve symptomatic infection (and associated immunocompromise), wasting, and abdominopelvic problems, whereas the added complication of anaemia (from bleeding) is present in hookworm infections [17]. Globally Emerging Zoonotic Parasitic Diseases Essay. Maturation of eggs/larvae in the environment differ between helminthic species with some species requiring eggs to hatch into larvae (non-infective eggs) and others requiring the ingestion of eggs for maturation into adult worms; what remains constant is the need for soil/environmental infestation of eggs/larvae in order to complete the life cycle [17]. Eggs can survive from months to years in the environment, and with the addition of mammalian hosts the spread of a wider variety of zoonoses is made possible, with humans being incidental hosts of nematodes such as in the life cycle of Trichinella spp. [17].
3. Leishmaniasis
Leishmaniasis accounts for a significant proportion of global BOD from NTDs with an estimate of 3.32 million DALYs according to Hotez et al. (2014) [7]. Most tropical countries are at risk of endemic leishmaniasis with spread being likely, considering that the primary vector is the phlebotomine sandfly, which has over 30 species and does not respect international country borders [18]. A large problem in Leishmania spp. endemic countries is co-infection with other NTDs, such as HIV, which act together to produce immunocompromise and increased levels of morbidity associated with leishmaniasis [18]. Much like Anopheles spp. malaria vectors, phlebotomine sandfly distribution is limited to areas above a certain temperature (in this case 15.6 °C), and with global warming ultimately pushing the latitudinal tropical borders further apart, the risk of spread of leishmaniasis via sandflies to naïve countries is a real possibility [19].
The most common form of leishmaniasis is cutaneous and presents with skin sores, which can be painful and ultimately result in debilitating social isolation and decreased quality of life (sometimes leading to mucosal leishmaniasis) similar to lymphatic filariasis [20]. Visceral leishmaniasis, also significantly present in the tropics, has a much higher morbidity rate than cutaneous resulting in multiple organ failure as well as anaemia, leukopenia, and thrombocytopenia [20]. Regarding the spread of leishmaniasis, depending on the region of the world and strain of Leishmania spp. the parasitic life cycle can be propagated without the presence of humans, with rodents and dogs being able to act as reservoirs for the disease [20,21]. In regions where anthroponotic spread is prevalent, treating individuals will help break the cycle of spread; however, with changing climates, increasing human traffic, and increases in global population density, the prevalence of less common forms of leishmaniasis without the need for anthropogenic spread could increase [20,21]. Globally Emerging Zoonotic Parasitic Diseases Essay.
4. Schistosomiasis
Although schistosomiasis has a similar global BOD, as estimated by Hotez et al. (2014) at 3.31 million DALYs, it has a significantly higher proportion of YLL % and higher global prevalence [7]. Schistosoma japonicum, S. haematobium, and S. mansoni are the primary pathogens of humans contributing to the majority of the global BOD. The geographic distribution lies primarily in Africa/Caribbean/Middle East for S. mansoniand S. haematobium, and in Asia for S. japonicum. In addition, the salient human schistosomes S. intercalatum and S. mekongi are primarily found in Central-West Africa and Southeast Asia, respectively [22,23]. The vectors for all Schistosoma spp. are types of snails, which are infected by Schistosoma spp. miracidia that hatch in water following human defecation or urination (for S. haematobium). These further develop within the snail into cercariae (the infective forms), which are then released back into the water and directly infect the definitive hosts when they comes into contact with them [22,23]. Of significance is the ability of S. mekongi and S. japonicum to live in animal reservoirs including many agricultural/domestic animals such as dogs, cats, pigs, goats, and horses [22,23].
With the increasing reliance on local agricultural produce and a lack of adequate infrastructure and sanitation in parts of the globe most affected by schistosomiasis, the threat of spread of the disease should be considered with surveillance/monitoring of infection in populations, snail xenodiagnoses, and adequate treatment regimens [22,24]. There are two main stages of schistosomiasis—the chronic and acute stages—with the acute stage usually presenting in pathogen-naïve persons as a cluster of symptoms known as Katayama syndrome [22,23].Globally Emerging Zoonotic Parasitic Diseases Essay. Over time, the worms take refuge in the mesenteric venules/hepatic portal system, causing crippling pathological sequelae ranging from organomegaly/cirrhosis to gastrointestinal bleeding (haematuria for S. haematobium) and anaemia [22,23]. A hallmark feature of schistosomiasis often overlooked by BOD estimates is the long-lasting effects of the illness on persons even long after de-worming [22,25]. Growth stunting, cognitive impairment, and permanent organ damage/increased risks of numerous cancers all add up to nearly double the global BOD for an already underestimated disease, according to Colley et al. (2014) [22,23,25].
5. Lymphatic Filariasis (LF)
At the bottom of the top five list by global BOD, as calculated by Hotez et al. (2014), is LF—accountable for approximately 2.78 million DALYs, mostly through YLDs [7]. Although the three pathogenic helminths responsible for LF, Brugia malayi, Brugia timori, and Wuchereria bancrofti, are technically nematodes, the fact that they are transmitted by mosquitos at the larval stage warrants their separate treatment to the majority of soil-transmitted nematodes [26]. The disease is found within the tropics, specifically in East/Southeast Asia, Oceania, Africa, and South America, with many different mosquito species able to function as vectors including Culex spp., Anophelesspp., Aedes spp., Mansonia spp., and Coquillettidia juxtamansonia [26,27]. This wide range of possible vectors brings with it a very salient risk of future LF outbreaks and spread beyond the currently endemic areas if proper public health systems and prophylactic measures are not taken to control spreading vectors (due to urbanisation and climate change) and prevent infection of at-risk persons [27].
The main pathophysiology of LF is linked to the L3 stage of the worm larvae, which further develop into adults and reside in the human lymphatics leading to lymphoedema, hydrocele, and an immunocompromised status resulting in increased opportunistic infections (which leave fibrotic tissue and thus elephantiasis), the majority of which account for the global BOD [26,27]. LF, unlike the other diseases mentioned in this review, quite rarely presents acutely, with the vast majority of sufferers living with chronic LF. Globally Emerging Zoonotic Parasitic Diseases Essay.However, the majority of the burden stems from the decrease in quality of life through psychiatric, social, and chronic suffering complications associated with the infection [7,26]. This is a double-edged sword, as although the rate of morbidity of LF is not high, this in turn means that governments and relevant stakeholders are less likely to prioritise LF treatment when other more apparent diseases, such as malaria and even leishmaniasis or schistosomiasis are present.
6. Control Methods for Cryptosporidiosis
Having considered five of the NTDs responsible for the greatest global BOD that should be prioritised by global stakeholders, the WHO, and nation-states, it is vital to consider the pragmatism of implementing any control or prevention strategies to curb the associated BOD. For example, LF elimination programs that originated as early as the year 2000 are already underway with mass drug administration and DEC-medicated salt as major programs for elimination [27]. The other zoonoses mentioned (leishmaniasis, schistosomiasis and INIs) currently all fall within the WHO’s NTD framework and coordinated multinational approaches to control these are either in place (de-worming strategies for INIs) or are planned for the near future [7,11]. What is key about the existing NTD framework, is that it inherently acts to prioritise certain diseases within the public health sphere for monitoring and surveillance, without which even the most effective treatment/control strategies would not be effective [11]. As such, cryptosporidiosis control and strategy development should be prioritised within the existing WHO NTD framework.
As outlined above, the detection of cryptosporidial infection is difficult for a number of reasons, many of which are caused by or compounded by low-resource settings in the most afflicted countries. The following domains of infection control will be outlined for cryptosporidiosis with recommendations made at the end (Table 2): (i) Monitoring/surveillance; (ii) treatment and prevention. Globally Emerging Zoonotic Parasitic Diseases Essay.
