Most human parasites are, in some fashion, major (important, serious, or significant), to the individual infected. Some human parasites are major or important by virtue of being extremely common, although they are not particularly harmful, e.g. E. vermicularis, which is almost a commensal. Some parasites are major in view of their serious effect on the individual, although they are quite uncommon e.g. A. cantonensis[i]. Between the two ends of this spectrum of severity, are parasitic infections that, whilst not immediately fatal, cause some degree of morbidity. Whilst not as dramatic as death, this morbidity may cause major effects on the individual and society, and even the human genome, e.g. morbidity from parasites may decrease productivity in adults and reduce learning ability in children. The bottom billion[ii] of the world’s population have a disproportionally high parasitic burden, which makes it even more difficult for them to escape from poverty, and hence exposure to faecal matter, insects, contaminated water etc. They also struggle to pay for treatment of disease.

The convergence of a high frequency of infection, and moderate severity of morbidity (and some mortality), makes some parasites particularly worthy of the status of ‘major parasitic infections of humans’

I will present the following parasitic infections: Malaria, Schistosomiasis, Hookworms, Leishmaniasis and Trypanosomiasis. I will justify why I consider them to be major and outline the reasons why Australia is free of most, if not all of these infections.

 

Malaria

Malaria is the most major of all parasitic infections of man, because it kills and injures the most people, and has done since antiquity. The World Health Organisation reports “According to the latest estimates, there were about 219 million cases of malaria in 2010 (with an uncertainty range of 154 million to 289 million) and an estimated 660 000 deaths (with an uncertainty range of 490 000 to 836 000).” [iii] Other sources estimate deaths of about 1.2 million persons per year.[iv] This highlights the difficulties of getting accurate figures for parasite related deaths.

 

Malaria is caused by a blood parasite, transmitted from one human to another by the feeding of the female Anopheles mosquitoes. Infection with the parasite leads to an acute feverish illness, involving many body organs and can lead to death, or can lead to chronic or intermittent infections. Chronic infection decreases the quantity and quality of life of the sufferer6.

 

Malaria has been problematic since antiquity. “It is now generally held that malaria arose in our primate ancestors in Africa and evolved with humans, spreading with human migrations first throughout the tropics, subtropics, and temperate regions of the Old World and then to the New World with explorers, missionaries, and slaves. The characteristic periodic fevers of malaria are recorded from every civilized society from China in 2700 BC through the writings of Greek, Roman, Assyrian, Indian, Arabic, and European physicians up to the 19th century.” [v]

 

Malaria has changed the course of wars, agriculture, and even the genetic code of humans; favouring mutations that are protective against malaria such as sickle cell, glucose 6 dehydrogenase deficiency, and thalassemia[vi]. Thus the current burden of malaria “is composed not only of the direct effects of malaria but also of the great legacy of debilitating, and sometimes lethal, inherited diseases that have been selected under its impact in the past. These inherited illnesses will probably be among us for many generations to come.”6

 

The harmful effects of malaria spurred exploration and even progress in the field of botany. Once the cinchona plant was identified, a great deal of effort was expended to procure seeds, and then grow the trees outside their native home of South America.[vii]

 

The search for ways to ameliorate the problem of mosquitoes spreading malaria led to the proliferation of insecticides such as dichlorodiphenyltrichloroethane (DDT). The harmful effects of DDT on birds, was documented in the book, Silent Spring.[viii] This book brought environmental issues to the notice of many. “Highly controversial, but may be the most important book in the formation of the environmental movement in the 1960s”[ix].

 

The Centre for Disease Control (CDC) in USA reports that malaria costs are a significant burden to individuals and governments:

“Costs to individuals and their families include purchase of drugs for treating malaria at home; expenses for travel to, and treatment at, dispensaries and clinics; lost days of work; absence from school; expenses for preventive measures; expenses for burial in case of deaths. Costs to governments include maintenance of health facilities; purchase of drugs and supplies; public health interventions against malaria, such as insecticide spraying or distribution of insecticide-treated bed nets; lost days of work with resulting loss of income; and lost opportunities for joint economic ventures and tourism. Direct costs (for example, illness, treatment, premature death) have been estimated to be at least US$ 12 billion per year. The costs are many times more than that in lost economic growth.”[x]

 

Parasites such as malaria also have more indirect effects. “Acquired childhood parasitic infections, most commonly malaria, can also affect subsequent immune response to vaccination. Additional data suggest that antiparasite therapy can improve the effectiveness of several human vaccines. Emerging evidence demonstrates that both antenatal and childhood parasitic infections alter levels of protective immune response to routine vaccinations. Successful antiparasite treatment may prevent immunomodulation caused by parasitic antigens during pregnancy and early childhood and may improve vaccine efficacy.”[xi]

 

Australia is largely free of malaria. Locally acquired malaria is rare in Australia; “Australia was certified as being malaria free by the World Health Organization (WHO) in 1981. However, several hundred imported cases of malaria are recorded in Australia each year”.[xii] Virtually the only malaria in Australia is brought in by travellers, who acquire their infection in malaria endemic countries. Travellers generally make significant effort to avoid this disease, e.g. mosquito avoidance, and chemoprophylaxis. If malaria cases do occur, infections generally do not spread within Australia, although there are notable brief exceptions. For example, in September 2002, a man with imported P. vivax malaria led to infection of mosquitoes and thereafter 10 adult campers were infected with P. vivax in the Daintree, North Queensland. In this case the disease was quickly eliminated[xiii]. Malaria is generally very obvious in Australian population as there is no acquired immunity, and it is readily treatable and recognized by the population to be so, hence cases are usually quickly recognized and treated before significant spread has occurred. Australia has extensive and expensive public health infrastructure, which keeps malaria at bay: public health staff and physicians, disease surveillance, excellent diagnostic and treatment facilities. Most importantly, the required vector mosquito is absent from most of Australia[xiv].

 

 

Schistosomiasis

Shistosomiasis is a blood fluke or trematode that is usually caught from exposure to the infective forms (cerceriae) in contaminated water. “Female worms lay eggs which actively penetrate tissues to be excreted in urine/faeces or they become trapped in organs where they cause granuloma formation”[xv]

Schistosomiasis is major because it kills and debilitates many persons. CDC reports that “Schistosomiasis ranks second only to malaria as the most common parasitic disease, and is the most deadly Neglected Tropical Disease (NTD), killing an estimated 280,000 people each year in the African region alone. Seven hundred million people are at risk in 74 countries, and 240 million are already infected.”[xvi]

 

NTD’s are a group of parasitic and bacterial diseases that cause substantial illness for more than one billion people globally.

 

Even those who do not die from schistosomiasis may experience significant problems if they have early and prolonged infection with this parasite. Recent analysis of studies have found that the lifetime burden of schistosomiasis is much greater than was previously appreciated, and chronic infection is responsible for conditions such as: “anaemia, chronic pain, exercise intolerance, and under nutrition (growth stunting), … infection associated cognitive deficit …and infection associated deficit in fieldwork performance”. [xvii]

 

Studies have shown an increased risk of HIV in young African women infected with female urogenital schistosomiasis, [xviii],[xix] and maternal infection with schistosomiasis during pregnancy can lead to decreased response to the BCG (TB vaccination) in the child.[xx]

 

Ineffective but enthusiastic injectible treatment for schistosomiasis in Egypt, contributed substantially to an outbreak of Hepatitis C which is causing liver failure and cancers, and is expected to peak at an estimated 6 million persons [xxi]

 

Schistosomiasis is rare in Australia and always imported. Occasional travellers bring this parasite home as an unwelcome souvenir of their travels – especially from Lake Malawi, Africa. The infection causes some mental distress, but is seldom severe, although there was one case of a Brisbane footballer who had an egg lodge in the spine and was temporarily paralysed.[xxii] Returning travellers have ready access to diagnosis, and reasonably effective treatment, (biltricide). Schistosomiasis cannot complete its life cycle in Australia because we do not have the snails present in this country that are required to complete the parasites lifecycle. (Even if we did have the snails it would be unlikely to be a problem here as our well maintained and functioning sewage systems, meaning infected urine could not contaminate the waterways where snails could theoretically be found.)

 

Hookworm

The World Health organization states, “Human hookworm infection is a soil-transmitted helminth infection caused by the nematode parasites Necator americanus and Ancylostoma duodenale. It is a leading cause of anaemia and protein malnutrition, afflicting an estimated 740 million people in the developing nations of the tropics…the presence of between 40 and 160 adult hookworms in the human intestine results in blood loss sufficient to cause anaemia and malnutrition. The term “hookworm disease” refers primarily to the iron-deficiency anaemia with reduced host haemoglobin, serum ferritin, and protoporphyrin that results from moderate and heavy infections and is in direct correlation with the number of parasites (as measured by quantitative egg counts). In children, chronic hookworm infection has been shown to impair physical and intellectual development, reduce school performance and attendance, and adversely affect future productivity and wage-earning potential.” [xxiii]

 

“In 1962, Norman Stoll, the distinguished Rockefeller Institute scientist who helped to establish human parasitology research in North America, described the unique health impact of hookworm as follows:

As it was when I first saw it, so it is now, one of the most evil of infections. Not with dramatic pathology as are filariasis, or schistosomiasis, but with damage silent and insidious. Now that malaria is being pushed back, hookworm remains the great infection of mankind. In my view it outranks all other worm infections of man combined…in its production, frequently unrealized, of human misery, debility, and inefficiency in the tropics” [xxiv]

 

 

Hookworm is thought to have been problematic throughout recorded history, in the form of epidemics of anaemia. “The striking pallor of miners is frequently mentioned… appearing for example in the works of Lucretius and Lucan (50 BC ad 50AD).”25 The disease came to be recognised as a problem in 1882 amongst Italian miners building the St Gotthard Tunnel in Switzerland. One miner who died of anaemia was found with 1,500 worms in his body. Once the disease was recognized, and methods to confirm hookworm eggs in the faeces were perfected, the disease was found in miners and non-miners all over Europe.[xxv]

 

Anaemia is a particular problem in pregnant women. “Evidence indicates that increasing hookworm infection intensity is associated with lower haemoglobin levels in pregnant women in poor countries.”[xxvi]

 

Despommier tells the story of how hookworm had a significant impact on the southern states of the USA. In the early 1900’s the US, Philanthropist John D. Rockefeller commissioned a study to determine why the southern states of the USA seemed economically depressed. He sent a team of experts to investigate and they found many of residents in the south were anaemic, and there was a correlation between soil type and anaemia. Subsequently they found that hookworm was causing the problem. “Some counties in 7 states recorded 50% to 100% of the population with positive screens for hookworm in a monumental surveillance and treatment campaign.”[xxvii] The team did experiments to determine how far the hookworm larva could travel away from the stool (it was 4 feet) and determined that if they built outhouses and dug a hole 6 feet deep, the hookworm could no longer infect the farmers. Thus in preventing hookworm, they also prevented many other diseases spread by contact with human manure.[xxviii]

 

There is a theory currently under investigation that “Helminths interact with both host innate and adoptive immunity to stimulate immune regulatory circuitry and to dampen effector pathways that drive aberrant inflammation. The first prototype worm therapies directed against immunological diseases are now under study in the United States and various countries around the world. Additional studies are in the advanced planning stage.“ [xxix] One study looked at the use of Necator americanus in patients with Crohn’s disease and demonstrated some success.[xxx] This suggests that the effects of hookworm may some day be used for good instead of evil.

 

Hookworm is not common in most of Australia, but it still exists in areas where modern hygiene and sewage disposal are not rigorously followed: “Although virtually eradicated from the non-Indigenous Australian population by the 1960s, hook worm infection has persisted in Indigenous Australians in remote communities and in immigrants from endemic countries.”[xxxi]

 

Prociv said, “At the community level, parasitic infections of the gut represent not so much a primary problem as a symptom of a more fundamental societal malaise. They will disappear from Aboriginal communities only with a sustained improvement in living conditions (including nutrition, sanitation and personal hygiene), driven by the people themselves when armed with effective public health knowledge. This is a challenge that continues to dwarf the capabilities of all our governments and politicians, at all levels”[xxxii]

 

Leishmania

 

Leishmaniasis is a disease caused by a protozoa of the genus Leishmania. It is a major human parasite because (like most parasites), it has been problematic to humanity for a long time, and causes both systemic and cutaneous disease along with many indirect effects.

 

“ The earliest known report is from the 7th century BC, during the reign of the Assyrian King Ashurbanipal. This king, although known for keeping defeated enemies in dog kennels, was also a scholar and these reports were likely transcribed from sources as far back as 2500 BC. Leishmaniasis was also described as Balkh sores by Muslim physicians in the tenth century AD, Kala-azar (literally black-fever) by Indian physicians and, in an odd choice of decoration, depictions of lesions and deformed faces have shown up on Incan pottery from the 1st century AD.”[xxxiii]

 

Leishmania causes death. As with all neglected diseases, precise data on the number of deaths is not readily available. “We reach a tentative estimate of 20,000 to 40,000 leishmaniasis deaths per year. Although the information is very poor in a number of countries, this is the first in-depth exercise to better estimate the real impact of leishmaniasis.”[xxxiv]

 

Leishmania causes delay in economic development. WHO says it nicely: “Epidemics of leishmaniasis have significantly delayed the implementation of numerous development programmes, particularly in the Amazon basin, the tropical regions of the Andean countries, Morocco and Saudi Arabia. The disease has thus become a serious impediment to socioeconomic development. Until recently, the public health impact of leishmaniasis was grossly underestimated. During the past 10 years, the endemic regions have been spreading, and there has been a sharp increase in the number of recorded cases of the disease. In addition, a substantial number of cases are not recorded, as notification of the disease is compulsory in only 32 of the 88 affected countries. An estimated 2 million new cases (1.5 million cases of cutaneous leishmaniasis and 500 000 of visceral leishmaniasis) occur annually, with about 12 million people currently infected. As with many diseases of poverty that cause high morbidity but low mortality, the true burden of leishmaniasis remains largely invisible. This is partly because those most affected live in remote areas, and partly because the social stigma associated with the deformities and disfiguring scars caused by the disease keeps patients hidden. Leishmaniasis-related disabilities impose a great social burden, especially on women, and reduce economic productivity”.[xxxv]

 

Mucocutaneous Leishmania can cause extensive facial disfigurement as noted above, which makes the disease more ‘major’ at an emotional level; it is feared in some ways like leprosy. Most members of the human species have a low tolerance of facial disfigurement, so sufferers are not able to live normally in the community, compared to someone for example who had a lower leg deformity.

 

Leishmania is major as it is making the HIV epidemic worse around the world, especially in resource poor countries. “The HIV/AIDS pandemic has modified the natural history of leishmaniasis. HIV infection increases the risk of developing Visceral Leishmaniasis by 100 to 2,320 times in areas of endemicity, reduces the likelihood of a therapeutic response, and greatly increases the probability of relapse. At the same time, Visceral Leishmaniasis promotes the clinical progression of HIV disease and the development of AIDS-defining conditions. Both diseases exert a synergistic detrimental effect on the cellular immune response because they target similar immune cells”[xxxvi]

 

Leishmania is even making things more difficult for intravenous drug users in Europe. “Leishmania spp were detected in 65 (52%) of 125 syringes collected in southern Madrid, Spain, in 1998, and in 52 (34%) of 154 collected in southwestern Madrid in 2000–01.”[xxxvii]

 

Leishmania is also a problem as it is difficult to treat, especially for the poor, and resistance is developing to the commonly used antimonial compounds, [xxxviii] and there is no vaccine. [xxxix]

 

Leishmania is likely absent from Australia because we do not have very much – as WHO puts it -“malnutrition, displacement, poor housing, illiteracy, gender discrimination, weakness of the immune system and lack of resources. Leishmaniasis is also linked to environmental changes, such as deforestation, building of dams, new irrigation schemes and urbanization, and the accompanying migration of non-immune people to endemic areas.”

 

Leishmania parasites were also, until recently, considered to be completely absent from Australia as we did not have a suitable phlebotomine sandfly vector, however recently a strain of Leishmania has been found in kangaroos that uses a novel vector (a midge) that feeds on the kangaroo’s tail.33 This is not currently known to be a threat to humans.

 

Trypanosomiasis

Trypanosomiasis is another blood protozoa of humans, transmitted by various insects. There are two major variations of this parasite‘s life cycle: African and South American. In Africa they are transmitted by tsetse flies (genus Glossina) and in South America by triatomine bugs. Evidence points to all trypanosomes having a common ancestor, and the Salivarian group (including T. b. gambiense and T. b. rhodesiense) diverged from the Stercoraria group (including T. cruzi) by some theorists around “100 million years ago, when Africa became isolated from the other continents.” [xl] For this reason I will discuss trypanosomiasis as a single infection. Trypanosomes are major because they cause morbidity and mortality, along with significant indirect effects.

 

WHO says of the African variety “Sleeping sickness threatens millions of people in 36 countries in sub-Saharan Africa. Many of the affected populations live in remote areas with limited access to adequate health services, which hampers the surveillance and therefore the diagnosis and treatment of cases. In addition, displacement of populations, war and poverty are important factors leading to increased transmission and this alters the distribution of the disease due to weakened or non-existent health systems. In 1998, almost 40 000 cases were reported, but estimates were that 300 000 cases were undiagnosed and therefore untreated. In 2009, after continued control efforts, the number of cases reported has dropped below 10 000 (9878) for first time in 50 years. This trend has been maintained in 2010 with 7139 new cases reported. The estimated number of actual cases is currently 30 000.[xli]

 

Chagas’ disease, the South American variety infects more persons than the African variety. As WHO says, “About 7 million to 8 million people are estimated to be infected worldwide, mostly in Latin America. The cost of treatment for Chagas’ disease remains substantial. In Colombia alone, the annual cost of medical care for all patients with the disease has been estimated to be about US$ 267 million.”41 “20 to 30% of these will develop symptomatic, potentially life-threatening Chagas’ disease.”[xlii]

 

The African parasites are more difficult to treat: WHO says “..drugs are toxic and complicated to administer. [xliii]

T. cruzi is easy to treat in the early stages. It is “treated with benznidazole and also nifurtimox. Both medicines are almost 100% effective in curing the disease if given soon after infection at the onset of the acute phase.”[xliv] Persons not treated can develop heart failure, which is debilitating and difficult to treat.

 

It is postulated that trypanosomiasis played a role in human evolution, along with the evolution of other species of African wildlife – as noted by Stevens “It is tempting to speculate that the long evolutionary history of humans with Salivarian trypanosomes explains our present innate resistance to infection with all but two subspecies of T. brucei”.40 Tsetse flies seldom fly or rest more than 3 metres from the ground. Tree dwelling primates are susceptible to trypanosomiasis while humans are resistant to most trypanosomes, with the exception of the more recent ones; T. b. gambiense and T. b. rhodesiense. The fact that humans are resistant to all other African trypanosome species suggests that human African trypanosomiasis is a recent event in human history. [xlv]

 

The trypanosomes that infect domestic livestock in Africa have had an indirect effect on humans. Non-African domesticated livestock animals suffer a type of trypanosomiasis, referred to as N’gana (a Zulu word for powerless/ useless) ..making the raising of this type of stock difficult in tsetse fly affected areas. The long coexistence of both tsetse flies and African game animals may explain why most African wildlife species are tolerant of trypanosomiasis: they become infected by the parasite but show no ill effects. In contrast, other animals had no time to develop resistance to trypanosome infections. The Egyptians were reported to be unable to farm domestic livestock until the regulation of the Nile made local habitat less attractive for the tsetse fly. It is also thought that the horse was difficult to breed in the old Kingdom in Egypt because of its susceptibility to trypanosomiasis. There is also evidence that the presence of sleeping sickness affected where humans settled, as humans endeavoured to avoid areas infested with tsetse flies.[xlvi]

 

Human trypanosomiasis is rare in Australia. Travellers sometimes import the parasites, but there are no local cases or transmission here because we do not have the necessary arthropod hosts (tsetse flies and triatomine bugs).

 

 

In summary, the five major parasites of humans that I have discussed in this essay; Malaria, Schistosomiasis, Hookworm, Leishmania and Trypanosomiasis have similarities that warrant consideration as major parasites. They are all fairly common, especially in the poorer nations of the world. The parasites cause direct effects such as death and disability. However they also cause indirect effects, such altering the human immune system, predisposing to cancers, and HIV, interfering with learning, and even affecting the human genome, human settlement patterns, and retarding economic development. These parasites are virtually absent from Australia due to our high standard of living, and extensive public health infrastructure. In some cases, there is a lack of appropriate vectors.

One Response to “5 major parasite infections of humans”

1. Dr Pullen says:

   September 5, 2013 at 10:38 pm

   Malaria cannot be overstated as a worldwide health risk, and one so far short of draining wetlands, with the devastating ecologic consequences, have made relatively little impact on. Take your travel meds!

   Reply