History of emerging infectious diseases

History of emerging infectious diseases

The discovery of disease-causing pathogens is an important activity in the field of medical science, as many viruses, bacteria, protozoa, fungi, helminthes and prions are identified as a confirmed or potential pathogen. A Center for Disease Control program begun in 1995 identified over a hundred patients with life threatening illnesses which were considered to be of an infectious cause, but could not be linked to a known pathogen.cite journal |author=Day M |title=Stalking the diseases with no name |journal=The New Scientist |year=1997 | date=June 7, 1997 | url=http://www.newscientist.com/article/mg15420850.600-stalking-the-diseases-with-no-name.html] The association of pathogens with disease can be a complex and controversial process, in some cases requiring decades or even centuries to achieve.

Factors impairing identification of pathogens

Factors which have been identified as impeding the identification of pathogens include the following:

:1. Lack of animal models: Experimental infection in animals has been used as a criterion to demonstrate an organism's disease-causing ability, but for some pathogens (such as "Vibrio cholerae", which cause disease only in humans) animal models do not exist. In cases where animal models were not available, scientists have sometimes infected themselves or others to determine an organism's disease causing ability.cite web | last =Barry | first =Marshall | title =Autobiography | publisher =Nobel Foundation | date =2005 | url =http://nobelprize.org/nobel_prizes/medicine/laureates/2005/marshall-autobio.html | accessdate = 2007-01-28 | authorlink = Barry Marshall]

:2. Pre-existing theories of disease: Before a pathogen is well-recognized, scientists may attribute the symptoms of infection to other causes, such as toxicological, psychological, or genetic causes. Once a pathogen has been associated with an illness, researchers have reported difficulty displacing these pre-existing theories.cite book | last = Marshall | first = Barry | title = Helicobacter Pioneers: Firsthand Accounts from the Scientists who Discovered Helicobacters, 1892-1982 | publisher = Blackwell Publishing Limited | date = May 1, 2002 | isbn = 978-0867930351] cite journal |author=Bentivoglio M, Pacini P |title=Filippo Pacini: a determined observer |journal=Brain Res. Bull. |volume=38 |issue=2 |pages=161–5 |year=1995 |pmid=7583342 |doi=]

:3. Variable pathogenicity: Infection with pathogens can produce varying responses in hosts, complicating the process of showing a relationship between infection and the pathogen. cite journal |author=Smith JW, Wolfe MS |title=Giardiasis |journal=Annu. Rev. Med. |volume=31 |issue= |pages=373–83 |year=1980 |pmid=6994619 |doi=10.1146/annurev.me.31.020180.002105] In some infectious diseases, the severity of symptoms has been shown to be dependent on specific genetic traits of the host. cite journal |author=Gabriel SE, Brigman KN, Koller BH, Boucher RC, Stutts MJ |title=Cystic fibrosis heterozygote resistance to cholera toxin in the cystic fibrosis mouse model |journal=Science |volume=266 |issue=5182 |pages=107–9 |year=1994 |pmid=7524148 |doi= ] cite journal |author=El-Omar EM, Carrington M, Chow WH, "et al" |title=Interleukin-1 polymorphisms associated with increased risk of gastric cancer |journal=Nature |volume=404 |issue=6776 |pages=398–402 |year=2000 |pmid=10746728 |doi=10.1038/35006081 ]

:4. Organisms that look alike but behave differently: In some cases a harmless organism exists which looks identical to a disease causing organism with a microscope, which complicates the discovery process.cite news | last = Kolata | first Gina | title = Detective Work and Science Reveals a New Lethal Bacteria | publisher = New York Times | date = January 6, 1998 | url = http://query.nytimes.com/gst/fullpage.html?res=9B00E7DD1230F935A35752C0A96E958260&sec=health&spon=&partner=permalink&exprod=permalink | accessdate = 2007-08-08 ]

:5. Lack of research effort: Slow progress has been attributed to the small numbers of researchers working on a pathogen.cite journal |author=Tan KS |title=Blastocystis in humans and animals: new insights using modern methodologies |journal=Vet. Parasitol. |volume=126 |issue=1-2 |pages=121–44 |year=2004 |pmid=15567582 |doi=10.1016/j.vetpar.2004.09.017]

19th century discoveries

"Vibrio cholerae" (1849-1884)

Cholera is a sometimes fatal disease caused by infection with the bacteria "Vibrio cholerae", cite book | last=Ryan | First=KJ | title=Sherris Medical Microbiology, 4th ed | Publisher=McGraw Hill | date=2004 | pages=276-7 | isbn=0838585299] which is transmitted through contaminated water. Once ingested, the bacteria colonizes the intestinal tract of the host and produces a toxin which causes body fluids to flow across the lining of the intestine. Death can result in 2-3 hours from dehydration if no treatment is provided.

Prior to the discovery of an infectious cause, the symptoms of Cholera were thought to be caused by an excess of bile in the patient. Citation | last = Shapin | first = S | title = Sick City:Maps and Morbidity in the Time of CHolera | journal = New Yorker | date = 11/6/2006 | url=http://www.newyorker.com/archive/2006/11/06/061106crbo_books ] The disease Cholera gets its name from the Greek word "choler" meaning bile. This was consistent with medical thought at the time, which held that four liquids or "humors" controlled health, and lead to such medical practices as bloodletting as a method of curing illnesses. The bacteria was first reported in 1849 by M. Gabriel Pouchet, who discovered it in stools from patients with Cholera. But he did not appreciate its significance. cite book | last = Bulloch | first = William | title = The History of Bacteriology | Publisher = Dover publications | date=1979 | pages=167 | isbn=978-0486237619] The first scientist to understand the significance of "Vibrio cholerae" was an Italian anatomist Filippo Pacini, who published detailed drawings of the organism in "Microscopical observations and pathological deductions on cholera" in 1854. He would go onto publish additional papers in 1866, 1871, 1876 and 1880, all of which were ignored by the scientific community. He correctly described how the bacteria caused diarrhea, and developed treatments that were found to be effective. But his findings did not influence medical opinion. In 1874, scientific representatives from 21 countries voted unanimously to resolve that Cholera was caused by environmental toxins from "miasmatas", or clouds of unhealthy substances which float in the air. cite journal |author=Howard-Jones N |title=Robert Koch and the cholera vibrio: a centenary |journal=British medical journal (Clinical research ed.) |volume=288 |issue=6414 |pages=379–81 |year=1984 |pmid=6419937 |doi= ] In 1884, Robert Koch re-discovered "Vibrio cholerae" as a causal element in Cholera. Some scientists opposed the new theory, and even drank Cholera cultures to disprove it:

Koch announced his discovery of the cholera vibrio in 1884. His conclusions were based upon the constant finding of the peculiar "comma bacillus" in the stools of cholera patients, and the failure to demonstrate this organism in the feces of other persons. It was not possible to reproduce typical cholera in laboratory animals. At the time the "germ theory" of disease had not yet obtained general acceptance, and Koch's announcement was received with considerable skepticism, particularly after it was found that similar "comma bacilli" could be found at times in the feces of persons not suffering from cholera, and often in all sorts of other environments - well and river waters, cheese, etc. We now know that these were saprotrophic species of Vibrio, which may be differentiated from the cholera vibrio by cultural and immunological methods. But the correctness of Koch's opinion was dramatically demonstrated by von Pettenkofer and Emmerich who, doubting the etiological relationship of Koch's organisms, deliberately drank cultures of it. Von Pettenkofer developed merely a transient diarrhea, but Emmerich suffered from a typical and severe attack of cholera.
by A.T. Henrici, The Biology of Bacteria, DC Heath and Company, 1939. ASIN B00085GABK,

Von Pettenkofer considered his experience proof that Vibrio cholerae was harmless, as he did not develop Cholera from consuming the culture. Between 1849 when Pouchet discovered Vibrio cholerae and 1891, over a million people died in Cholera epidemics in Europe and Russia. cite book | last = Stitt | first = Edward | title = The Diagnosis and Treatment of Tropical Diseases | publisher = P. Blakiston's Son, Philadelphia| date = 1922 ] In 1995, researchers published a study in "Science Magazine" explaining why some persons are able to be infected with Cholera without symptoms, possibly explaining why Pettenkofer did not get sick. The study showed that a series of genetic mutations in some people provide resistance to Cholera toxin. However, these mutations come with a price. If too many of them occur in a person, they will develop Cystic Fibrosis, an incurable and often fatal genetic disorder.

20th century discoveries

"Giardia lamblia" (1681-1975)

Giardiasis is a disease caused by infection with the protozoan "Giardia lamblia". Infection with "Giardia" can produce diarrhea, gas, and abdominal pain in some people. If untreated, infection can be chronic. In children, chronic "Giardia" infection can cause stunting (stunted growth) and lowered intelligence, cite journal |author=Ali SA, Hill DR |title=Giardia intestinalis |journal=Curr. Opin. Infect. Dis. |volume=16 |issue=5 |pages=453–60 |year=2003 |pmid=14501998 |doi=10.1097/01.qco.0000092817.64370.ab ] Infection with "Giardia" is now universally recognized as a disease, and treated by physicians with anti-protozoal drugs. Since 2002, "Giardia" cases must be reported to the Center for Disease Control, according to the CDC’s Reportable Disease Spreadsheet. [cite web
title = CDC Reportable Disease Spreadsheet
url = http://www.cdc.gov/epo/dphsi/phs/files/NNDSS_history_spreadsheet_2007_for_web.pdf
accessdate = 2007-09-08
] The United States National Institutes of Health [http://www3.niaid.nih.gov/labs/aboutlabs/lpd/gastrointestinalParasitesSection/nash.htm Gastrointestinal Parasites Lab] studies "Giardia" almost exclusively.

However, "Giardia" experienced an extraordinarily long term of emergence, from its discovery in 1681, until the 1970s when it was fully accepted that infection with "Giardia" was a treatable cause of chronic diarrhea:

"Giardia lamblia" was first discovered by Leeuwenhoeck (1681) who found the parasite in his own {diarrheal} stools. It was long considered to be a harmless commensal organism, but in recent years has been recognized as a cause of intestinal disease often acquired by travelers to foreign countries, persons drinking contaminated water in this country, children in day care nurseries and homosexual males. It is the most common pathogenic intestinal parasite in the United States, being found in 4% of stool specimens submitted to state public health laboratories for parasite examination. Attesting to its increasing importance in the United States, a symposium on Giardiasis, sponsored by the Environmental Protection Agency, was held in the Fall of 1978..
by JW Smith, "Giardiasis", 1980

Some of the first evidence in modern times of "Giardia's" pathogenicity came during World War II when soldiers were treated for malaria with the antiprotozoal Quinacrine, and their diarrhea disappeared, as did the "Giardia" from their stool samples. In 1954, Dr. R.C. Rendtorff performed experiments on prisoner volunteers, infecting them with "Giardia". cite journal |author=RENDTORFF RC |title=The experimental transmission of human intestinal protozoan parasites. II. "Giardia lamblia" cysts given in capsules |journal=American journal of hygiene |volume=59 |issue=2 |pages=209–20 |year=1954 |pmid=13138586 |doi=] In the experiment, although some prisoners experienced changes in stool habits, he concluded that these could not be conclusively linked to "Giardia" infection, and also indicated that all prisoners experienced spontaneous clearance of "Giardia". His experiments were described in the EPA Symposium on Waterborne Transmission of Giardiasis in 1979:

We also included "Giardia lamblia", which at that time was not generally believed to be an invasive pathogenic parasite of man. Giardia was thought in the 1950s to cause occasional problems of diarrhea in children but its appearance was so common and, in adults so lacking in clinical symptomatology, that most considered it a non-pathogen. As a result we felt safe in exposing prisoners to Giardia... cite journal |author=RENDTORFF RC |title=The experimental transmission of human intestinal protozoan parasites. II. Giardia lamblia cysts given in capsules |journal=American journal of hygiene |volume=59 |issue=2 |pages=209–20 |year=1954 |pmid=13138586 |doi=]
by Dr. RC Rendtorff in an EPA Symposium on the Waterborne Transmission of Giardiasis - in 1979

In 1954-55, an outbreak of "Giardia" infection occurred in Oregon (US), sickening 50,000 people. cite journal |author=Veazie L |title=Epidemic giardiasis |journal=N. Engl. J. Med. |volume=281 |issue=15 |pages=853 |year=1969 |pmid=5809527 |doi= ] This was documented in a communication by Dr. Lyle Veazie, which wasn't published until 15 years later in the "New England Journal of Medicine." In the communication, Veazie notes that he was unable to find a publisher for his account of the epidemic. The communication was re-published in the "EPA Symposium on Waterborne Transmission of Giardiasis" in 1979, and that version included the following quote from the Director of the Oregon State Board of Health, suggesting that diarrhea from "Giardia" was still being attributed to other causes by health authorities in 1954:

While an unidentified virus seems the most likely etiologic agent, the unusual prevalence of "Giardia lamblia" cysts in stools of patients seems worthy of record.
by Oregon State Board of Health commenting on 1954-55 outbreak of "Giardiasis", as quoted in Veazie, 1979.

"Helicobacter pylori" (1892-1982)

Infection with the bacteria "Helicobacter pylori" is the cause of most stomach ulcers. The discovery is generally credited to Australian gastroenterologists Dr. Barry Marshall and Dr. J Robin Warren, who published their findings in 1983. The pair received the Nobel Prize in 2005 for their work. Before this, nobody really knew what caused stomach ulcers, though a popular belief was that the "stress" played a role. Some researchers suggested that ulcers were a psychosomatic illness. cite journal |author=Paulley JW |title=Cultural influences on the incidence and pattern of disease |journal=Psychotherapy and psychosomatics |volume=26 |issue=1 |pages=2–11 |year=1975 |pmid=1178798 |doi= ] cite journal |author=Kellner R |title=Psychotherapy in psychosomatic disorders |journal=Arch. Gen. Psychiatry |volume=32 |issue=8 |pages=1021–8 |year=1975 |pmid=1156110 |doi= ] cite journal |author=Aitken C, Cay E |title=Clinical psychosomatic research |journal=International journal of psychiatry in medicine |volume=6 |issue=1-2 |pages=29–41 |year=1975 |pmid=773858 |doi= ]

In "H Pylori Pioneers", Dr. Marshall noted that other physicians had produced evidence of "H. pylori" infection as early as 1892. Marshall writes that earlier reports were disregarded because they conflicted with existing belief. The first description of "H. Pylori" came in 1892 from Giulio Bizzozero, who identified acid-tolerant bacteria living in a dog’s stomach. Later, a theory would be developed that no bacteria could live in the stomach. Although the theory has no scientific basis, it would become a stumbling block for scientists, discouraging them for searching for infective causes of stomach ulcers. In 1940, two physicians, Dr. A. Stone Freeberg and Dr. Louis E. Barron published a paper describing a spiral bacteria found in about half of their gastroenterology patients who had stomach ulcers. Dr. John Lykoudis, a Greek physician, was one of the first physicians to treat stomach ulcers as an infectious disease. Between 1960 and 1970, he treated over 10,000 ulcer patients in Athens with antibiotics.cite journal |author=Rigas B, Feretis C, Papavassiliou ED |title=John Lykoudis: an unappreciated discoverer of the cause and treatment of peptic ulcer disease |journal=Lancet |volume=354 |issue=9190 |pages=1634–5 |year=1999 |pmid=10560691 |doi=] Lykoudis tried to publish a paper on his findings, but they conflicted with traditional theory, and his work was never published. Lykoudis' experience was followed in 1975 by a further publication in Gut magazine that included spiral bacteria living on the borders of duodonal ulcers.cite journal |author=Steer HW, Colin-Jones DG |title=Mucosal changes in gastric ulceration and their response to carbenoxolone sodium |journal=Gut |volume=16 |issue=8 |pages=590–7 |year=1975 |pmid=810394 |doi=] The medical significance of Steer’s findings was disregarded, but he “continued to publish papers on "H. Pylori", mostly as a hobby."

"H. pylori" can infect the stomach of some people without causing stomach ulcers. In investigating asymptomatic carriers of "H. pylori", researchers identified a genetic trait called Interleuik-1 beta-31 which causes increased production of stomach acid, resulting in ulcers if patients become infected with "H. pylori". Patients without the trait do not develop stomach ulcers in response to "H. pylori" infection, but instead have increased risk from stomach cancer if they become infected. Investigation into other gastrointestinal infections has also shown that the symptoms are the result of interaction between the infection and specific genetic mutations in the host.

Pathogenic variants of "Escherichia coli" (1947-1983)

There are different types of E. coli, some of which are found in humans and are harmless. Enterotoxigenic Escherichia coli (ETEC) is a type found to cause illness in humans, possessing gene that allows it to manufacture a substance toxic to humans. Cattle are immune to its effects but when people eat food contaminated with cattle feces, the organism can cause disease. Reports of pathogenic E. coli appear in medical literature as early as 1947. cite journal |author=Ruchman I, Dodd K |title=The Isolation of a Strain of Escherichia coli Pathogenic for the Rabbit's Eye from a Patient with Diarrhea |journal=J. Bacteriol. |volume=53 |issue=5 |pages=653–6 |year=1947 |pmid=16561320 |doi= ] Publications regarding variants of E. coli which cause disease appeared regularly in medical journals throughout the 1950s, 60s, and 70s, cite journal |author=MACQUEEN RL |title=Isolation of Bact. coli O.26.B.6 from a child with recurrent diarrhoea |journal=British medical journal |volume=1 |issue=4877 |pages=1475–6 |year=1954 |pmid=13160505 |doi= ] cite journal |author=McCLURE WB |title=A severe nursery epidemic of diarrhoea associated with Esch. coli type 111 B4 |journal=Canadian Medical Association journal |volume=72 |issue=2 |pages=83–5 |year=1955 |pmid=13230990 |doi= ] cite journal |author=GRONROOS JA |title=Investigations on Escherichia coli O groups 1-25, 44 and 78 and serotypes 26:B6, 55:B5, 86:B7, 111:B4, 125:B15 and 126:B16; occurrence in faeces of healthy and diarrhoeal infants |journal=Annales medicinae experimentalis et biologiae Fenniae |volume=35 |issue=Suppl 2 |pages=1–35 |year=1957 |pmid=13444875 |doi= ] cite journal |author=COWART GS, THOMASON BM |title=IMMUNOFLUORESCENT DETECTION OF ESCHERICHIA COLI; INCIDENCE OF CERTAIN SEROGROUPS SUSPECTED OF BEING PATHOGENIC |journal=Am. J. Dis. Child. |volume=110 |issue= |pages=131–6 |year=1965 |pmid=14320761 |doi= ] cite journal |author=Linde K, Köditz H, Hasenjäger H |title= [Treatment of infantile diarrhea caused by pathogenic Escherichia coli strains with oral large doses of benzyl procaine penicillin] |language=Polish |journal=Pediatria polska |volume=41 |issue=8 |pages=905–12 |year=1966 |pmid=5341176 |doi= ] with fatalities being reported in humans and infants starting in the 1970s. cite journal |author=Glantz PJ |title=Unclassified Escherichia coli serogroup OXI isolated from fatal diarrhea of rabbits |journal=Can. J. Comp. Med. |volume=34 |issue=1 |pages=47–9 |year=1970 |pmid=4246003 |doi= ] cite journal |author=Drucker MM, Polliack A, Yeivin R, Sacks TG |title=Immunofluorescent demonstration of enteropathogenic Escherichia coli in tissues of infants dying with enteritis |journal=Pediatrics |volume=46 |issue=6 |pages=855–64 |year=1970 |pmid=4099362 |doi= ] cite journal |author=Smith HW, Gyles CL |title=The relationship between two apparently different enterotoxins produced by enteropathogenic strains of Escherichia coli of porcine origin |journal=J. Med. Microbiol. |volume=3 |issue=3 |pages=387–401 |year=1970 |pmid=4919579 |doi= ] Despite the earlier reports, pathogenic E. coli did not rise to public prominence until 1983, when a Center for Disease Control researcher published a paper identifying ETEC as the cause of a series of outbreaks of unexplained hemorrhagic gastrointestinal illness. cite journal |author=Riley LW, Remis RS, Helgerson SD, "et al" |title=Hemorrhagic colitis associated with a rare Escherichia coli serotype |journal=N. Engl. J. Med. |volume=308 |issue=12 |pages=681–5 |year=1983 |pmid=6338386 |doi= ] Despite the earlier publication of pathogenic variants of E. coli, researchers encountered significant difficulties in establishing ETEC as a pathogen.

Human Immunodeficiency Virus (1959-1984)

AIDS was first reported June 5, 1981, when the U.S. Centers for Disease Control and Prevention recorded a cluster of "Pneumocystis carinii" pneumonia (now still classified as PCP but known to be caused by "Pneumocystis jirovecii") in five homosexual men in Los Angeles.

cite web
author=CDC | publisher=CDC | year=1981
title=Pneumocystis Pneumonia — Los Angeles
accessdate = 2006-01-17

] The discovery of the virus took several years of research, and was announced in 1984 by Dr. Gallo of the National Cancer Institute, Dr. Luc Montagnier at the Pasteur Institute in Paris, and Dr. Jay Levy at the University of California, San Francisco.

However, the HIV virus existed long before the 1981 CDC report. Three of the earliest known instances of HIV infection are as follows:
#A plasma sample taken in 1959 from an adult male living in what is now the Democratic Republic of the Congo.

cite journal
author=Zhu, T., Korber, B. T., Nahmias, A. J., Hooper, E., Sharp, P. M. and Ho, D. D. | title=An African HIV-1 Sequence from 1959 and Implications for the Origin of the Epidemic
journal=Nature | year=1998 | pages=594–597 | volume=391 | issue=6667
pmid=9468138 doi|10.1038/35400

#HIV found in tissue samples from a 15 year old African-American teenager who died in St. Louis in 1969.

cite news
author=Kolata, G.
title=Boy's 1969 death suggests AIDS invaded U.S. several times
publisher=The New York Times
accessdate = 2006-06-19
#HIV found in tissue samples from a Norwegian sailor who died around 1976.

cite journal
author=Hooper, E.
title=Sailors and star-bursts, and the arrival of HIV
journal=BMJ | year=1997 | pages=1689–1691 | volume=315 | issue=7123


Two species of HIV infect humans: HIV-1 and HIV-2. HIV-1 is more virulent and more easily transmitted. HIV-1 is the source of the majority of HIV infections throughout the world, while HIV-2 is not as easily transmitted and is largely confined to West Africa.

cite journal
author=Reeves, J. D. and Doms, R. W
title=Human Immunodeficiency Virus Type 2
journal=J. Gen. Virol. | year=2002 | pages=1253–1265 | volume=83 | issue=Pt 6

] Both HIV-1 and HIV-2 are of primate origin. The origin of HIV-1 is the Central Common Chimpanzee ("Pan troglodytes troglodytes") found in southern Cameroon.

cite journal
author=Keele, B. F., van Heuverswyn, F., Li, Y. Y., Bailes, E., Takehisa, J., Santiago, M. L., Bibollet-Ruche, F., Chen, Y., Wain, L. V., Liegois, F., Loul, S., Mpoudi Ngole, E., Bienvenue, Y., Delaporte, E., Brookfield, J. F. Y., Sharp, P. M., Shaw, G. M., Peeters, M., Hahn, B. H.
title=Chimpanzee Reservoirs of Pandemic and Nonpandemic HIV-1
journal=Science | year=2006 | pages= | volume=Online 2006-05-25 | issue=
url= http://www.sciencemag.org/cgi/content/abstract/1126531

] It is established that HIV-2 originated from the Sooty Mangabey ("Cercocebus atys"), an Old World monkey of Guinea Bissau, Gabon, and Cameroon.

Most experts believe that HIV probably transferred to humans as a result of direct contact with primates, for instance during hunting or butchery.

cite journal
author=Cohen, J.
title=Vaccine Theory of AIDS Origins Disputed at Royal Society
journal=Science | year=2000 | pages=1850–1851 | volume= 289| issue=5486


"Cyclospora" (1995)

Cyclospora is a gastrointestinal pathogen that causes fever, diarrhea, vomiting, and severe weight loss. Outbreaks of the disease occurred in Chicago in 1989 and other areas in the United States cite news | last = Goleman | first =Daniel | title = Tiny Clinic in Katmandy Solves Mystery | publisher = New York Times | date = January 3, 1995 | url = http://query.nytimes.com/gst/fullpage.html?res=990CEFD7163AF930A35752C0A963958260&sec=travel&spon=&partner=permalink&exprod=permalink | accessdate = 2007-08-08 ] . But investigation by the Center for Disease Control could not identify an infectious cause. The discovery of the cause was made by Mr. Ramachandran Rajah, the head of a medical clinic's laboratory in Katmandu, Nepal. Mr. Rajah was trying to discover why local residents and visitors were becoming ill every summer. He identified an unusual looking organism in stool samples from patients who were sick. But when the clinic sent slides of the organism to the Center for Disease Control, it was identified as Blue-Green algae, which is harmless. Many pathologists had seen the same thing before, but dismissed it as irrelevant to the patient's disease. . Later, the organism would be identified as a special kind of parasite, and treatment would be developed to help patients with the infection. In the United States, Cyclospora infection must be reported to the Center for Disease Control according to the [http://www.cdc.gov/epo/dphsi/phs/infdis2007r.htm CDC's Reportable Disease Chart]

Present day discoveries

The process of identifying new infectious agents continues. One study has suggested there are a large number of pathogens already causing illness in the population, but they have not yet been properly identified.

Gastrointestinal pathogens

Many recently emerged pathogens infect the gastrointestinal tract. For example, there are three gastrointestinal protozoal infections which must be reported to the Center for Disease Control.cite web
title =Nationally Notifiable Infectious Diseases - 2007
work =
publisher =Center for Disease Control
date =2007
url =http://www.cdc.gov/epo/dphsi/phs/infdis2007r.htm
accessdate = 2007-08-08
] They are Giardia, Cyclospora and Cryptosporidium, and none of them were known to be significant pathogens in the 1970s.

Figure 1 shows the prevalence of gastrointestinal protozoa in studies from the United States and Canada.cite journal |author=Lagacé-Wiens PR, VanCaeseele PG, Koschik C |title=Dientamoeba fragilis: an emerging role in intestinal disease |journal=CMAJ : Canadian Medical Association journal journal de l'Association medicale canadienne |volume=175 |issue=5 |pages=468–9 |year=2006 |pmid=16940260 |doi=10.1503/cmaj.060265] cite journal |author=Amin OM |title=Seasonal prevalence of intestinal parasites in the United States during 2000 |journal=Am. J. Trop. Med. Hyg. |volume=66 |issue=6 |pages=799–803 |year=2002 |pmid=12224595 |doi= ] The most prevalent protozoa in these studies are considered emerging infectious diseases by some researchers, because a consensus does not yet exist in the medical and public health spheres concerning their importance in the role of human disease. cite journal |author=Markell EK, Udkow MP |title=Association of "Blastocystis" hominis with human disease? |journal=J. Clin. Microbiol. |volume=28 |issue=5 |pages=1085–6 |year=1990 |pmid=2351728 |doi= ] cite journal |author=Zierdt CH |title="Blastocystis" hominis--past and future |journal=Clin. Microbiol. Rev. |volume=4 |issue=1 |pages=61–79 |year=1991 |pmid=2004348 |doi=] Researchers have suggested that their treatment may be complicated by differing opinions regarding pathogenicity, lack of reliable testing procedures, and lack of reliable treatments. cite journal |author=Yakoob J, Jafri W, Jafri N, Islam M, Asim Beg M |title=In vitro susceptibility of "Blastocystis" hominis isolated from patients with irritable bowel syndrome |journal=Br. J. Biomed. Sci. |volume=61 |issue=2 |pages=75–7 |year=2004 |pmid=15250669 |doi= ] As with newly discovered pathogens before them, researchers are reporting that these organisms may be responsible for illnesses for which no clear cause has been found, such as Irritable Bowel Syndrome.cite journal |author=Borody T, Warren E, Wettstein A, et al. | title = Eradication of Dientamoeba fragilis can resolve IBS-like symptoms.
journal=J Gastroenterol Hepatol | year= 2002 | volume=17 | issue=Suppl; pages=A103
] cite journal |author=Yakoob J, Jafri W, Jafri N, "et al" |title=Irritable bowel syndrome: in search of an etiology: role of "Blastocystis" hominis |journal=Am. J. Trop. Med. Hyg. |volume=70 |issue=4 |pages=383–5 |year=2004 |pmid=15100450 |doi= ] cite journal |author=Hussain R, Jaferi W, Zuberi S, "et al" |title=Significantly increased IgG2 subclass antibody levels to "Blastocystis" hominis in patients with irritable bowel syndrome |journal=Am. J. Trop. Med. Hyg. |volume=56 |issue=3 |pages=301–6 |year=1997 |pmid=9129532 |doi= ]

"Dientamoeba fragilis"

"Dientamoeba fragilis" is a single-celled parasite which infects the large intestine causing diarrhea, gas, and abdominal pain. An Australian study identified patients with symptoms of IBS who were actually infected with "Dientamoeba fragilis". Their symptoms resolved following treatment. A study in Denmark identified a high incidence "Dientamoeba fragilis" infection in a group of patients suspected of having gastrointestinal illness of an infectious cause.cite journal |author=Stensvold CR, Arendrup MC, Mølbak K, Nielsen HV |title=The prevalence of Dientamoeba fragilis in patients with suspected enteroparasitic disease in a metropolitan area in Denmark |journal=Clin. Microbiol. Infect. |volume=13 |issue=8 |pages=839–42 |year=2007 |pmid=17610603 |doi=10.1111/j.1469-0691.2007.01760.x] . The study also suggested special methods may be required to identify infection.


"Blastocystis" is a single-celled protozoan which infects the large intestine. Physicians report that patients with infection show symptoms of abdominal pain, constipation, and diarrhea. cite journal |author=Qadri SM, al-Okaili GA, al-Dayel F |title=Clinical significance of "Blastocystis" hominis |journal=J. Clin. Microbiol. |volume=27 |issue=11 |pages=2407–9 |year=1989 |pmid=2808664 |doi= ] cite journal |author=Grossman I, Weiss LM, Simon D, Tanowitz HB, Wittner M |title="Blastocystis" hominis in hospital employees |journal=Am. J. Gastroenterol. |volume=87 |issue=6 |pages=729–32 |year=1992 |pmid=1590309 |doi=] cite journal |author=Zdero M, Cabrera G, Ponce de León P, Nocito I, Echenique C |title= [Parasitosis in an adult population with chronic gastrointestinal disorders] |language=Spanish; Castilian |journal=Acta Gastroenterol. Latinoam. |volume=27 |issue=2 |pages=67–73 |year=1997 |pmid=9412130 |doi=] . One study found that 43% of IBS patients were infected with "Blastocystis" versus 7% of controls. An additional study found that many IBS patients from whom "Blastocystis" could not be identified showed a strong antibody reaction to the organism, which is a type of test used to diagnose certain difficult-to-find infections. Other researchers have also reported that special testing techniques may be necessary to identify the infection in some people. cite journal |author=Stensvold R, Brillowska-Dabrowska A, Nielsen HV, Arendrup MC |title=Detection of "Blastocystis" hominis in unpreserved stool specimens by using polymerase chain reaction |journal=J. Parasitol. |volume=92 |issue=5 |pages=1081–7 |year=2006 |pmid=17152954 |doi=] While some scientists believe the finding that IBS patients carry a protozoal infection to be significant, other researchers have reported their belief that the presence of the infection is not medically significant . Researchers report that the infection can be resistant to common protozoal treatments in laboratory culture study and in experience with patients , so identifying "Blastocystis" infection may not be of immediate help to a patient. A 2006 study of gastrointestinal infections in the United States suggested that "Blastocystis" infection has become the leading cause of protozoal diarrea in that country. "Blastocystis" was the most frequently identified protozoal infection found in patients in a 2006 Canadian study.


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