Introduction

Parasitological studies of large patient populations are rare in the United States compared to third-world countries where endemic parasitosis are more readily reported.¹ We, at the Parasitology Center, Inc. (PCI), in Scottsdale, Arizona, routinely monitor and report on the patterns and trends of human parasitosis in the US.

• The seasonal prevalence of 19 species of intestinal parasites infecting 916 of 2,896 (32%) examined patients from 48 states in the year 2000 was reported.² In that study, 314 of 859 examined patients (36%) from Cali­fornia were infected.² Infections with helminth parasites such as Ascaris lumbricoides and non-major protozoans were rare and are not included in the present study. The seven reported species of protozoan parasites constituted 91.5% of 18 species of parasites reported in the United States. Multiple infections with two-to-four parasitic species constituted 10% of the infected cases.²
• We have also researched the epidemi­ology of Blastocystis hominis in 48 states and the District of Columbia in 2002-2004 and included trends in annual, seasonal, geographical and host distribution, and symptomology by age, sex and season.³ In that report, 16% of 10,582 fecal specimens from 5,291 patients tested positive for B. hominis; in California, 263 of 1,328 examined patients (20%) were also positive for B. hominis.
• In a similar three-year epidemiological study of 9,856 fecal specimens from 4,928 patients from all states and the District of Columbia that we tested between 2003 and 2005, 279 (6%) were positive for Cryptosporidium parvum infections.

Studies of this magnitude are not known in the US. Few other studies of relatively large patient populations in the US^4,5 or in more geographically limited populations addressing B. hominis only, e.g., California⁶ or Ontario⁷ have been reported. The present investigation is the first to cover the span of 18 years. Evaluating the patterns and trends of parasitic infections in studies of such a long duration is a clinically useful tool for understanding the epidemiological characteristics, disease burden, improving the reporting of cases, planning prevention, therapeutic, and other public health measures to be considered in the management of these infections. Nevertheless, an overview of studies of shorter duration from comparable urban/suburban area in developing and developed nations are included.

Materials and Methods

A total of 7766 specimens from 3883 patients (two specimens per patient) were collected, preserved, and transported to Parasitol­ogy Center, Inc. (PCI) in Proto-fix™ (Alpha-Tec Systems, Inc. Vancouver, Wash.) or SAF (sodium acetate-acetic acid-formalin mixture) in plastic vials provided in mailable kits. Patients were referred to PCI by 187 doctors in Los Angeles County from January 1996 through December 2013. The number of referring practitioners varied from year to year. Specimens were processed and stained with CONSED™ according to the manufacturer’s (Alpha-Tec Systems) directions. This procedure was used in thousands of specimens at PCI, evaluated, and described previously.⁸

Briefly, specimens are filtered, mixed with CONSED and ethyl acetate, vortexed, centrifuged, and decanted. The resulting fecal plug was mixed with CONSED diluting reagent, transferred to, and mounted on a slide for microscopic examination as wet mounts. All samples were evaluated by the same observer blinded to patient information. The reliability of diagnosis is indicated by the consistency of detection of different parasites at different levels of infection during the same period of time. Differences in the number of patient samples tested in different years reflect changing patterns of patient traffic from Los Angeles County over time. Positive results were quantified (number of organisms per high power field on a scale of 1 – 4) from duplicate samples from each patient. About 10% of infected patients had two-to-four parasitic species each which is the same prevalence rate of concurrent infections noted in our earlier study.² A prevalence rate based on the number of infected/examined patients would be approximately 10% lower but would not reflect the activity of individual parasitic species. The number of samples tested underwent a gradual decrease over the years corresponding to changing pat­terns of patients and practitioners’ traffic in the Los Angeles area while increasing elsewhere in the US and internationally.

Issues in Diagnosis

We accept the possible presence of Blastocystis and Cryptosporidium organisms from animal sources in human infections with B. hominis and Cryptosporidium parvum as reviewed in Tan⁹ and Garcia,¹⁰ respectively. Fletcher., et al¹¹ provided a compre­hensive, well referenced diagnostic coverage of intestinal protozoan infections in developed countries. Human, mammalian, avian, and reptilian isolates of B. hominis have been assigned to 13 subtypes. Blastocystis subtype 3 is most commonly associated with illness in human prevalence studies. The term Blastocystis hominis normally refers to approximately 10 different genetic populations that are indistin­guishable microscopically. That term is used for parasites isolated from humans while Blastocystis spp. is used for isolations from animal hosts.¹¹

Most animals are not infected with human pathogenic cryptosporidiosis. However, zoonotic transmission from direct contact with infected animals or their feces can occur through indirect sources, including drinking of contaminated water. The invasive Entamoeba histolytica trophozoites is less common than the morphologically identical non-pathogenic E. dispar and E. moshkovskii but distinguish­able from them by isoenzyme analysis. Giardia intestinalis infections are detected microscopically by us and also by various antigen assays demonstrating seven genetically distinct genotypes (A-G). Assemblages A and B infect humans as well as other mammalian species and are considered zoonotic. See Fletcher., et al.¹¹ for a discussion of above diagnostic issues.

Results: Prevalence

A total of 1629 parasitic infections from 3883 patients (41%) were identified between 1996 – 2013 in the Los Angeles area. Blastocystis hominis was the most frequent parasitological finding. It was identified in 19% of samples and represented 45% of all parasi­tological infections (Table 1). The next most common protozoan was E. histolytica/dispar. It was found in 6% of samples constituting 15% of all infections. The prevalence of E. hartmanni, C. parvum and E. coli was 6%, 5%, and 4%, respectively. Dientamoeba fragilis and Giardia intestinalis were found in 4% and < 1% of the samples examined, in the same order. These parasites constituted 91.5% of 18 species of intestinal parasites reported from 5792 fecal specimens tested from throughout the US in 2000.²

Discussion: Prevalence in the US

The overall prevalence of infection with all investigated protozoans was 41% of double fecal samples examined between 1996 and 2013 from 3,883 patients in the Los Angeles County. Infections with B. hominis made up roughly half (45%) of all protozoans studied and noted in Table 1. Los Angeles is an urban/suburban area, and a 41% prevalence rate is markedly higher than the 32% and 36% prevalence rates reported earlier in the United States and California, respectively.² The testing procedures employed in this study produced a prev­alence rate of 32.6% (3,373 infected of 10,358 examined patients throughout the United States) between 1996 and 1998.⁸ An almost identical prevalence of 32% was reported in our comprehensive study of the prevalence of intestinal parasites in 5,792 fecal specimens from 2,896 patients in the United States.² Our results reflect our most efficient methods of parasite detection,⁸ which show con­siderably higher prevalence rates than others across the country. For instance, Kappus⁴ reported US prevalence of 20% (from 216,275 stool specimens) compared with 19.7% (from 178,786 stool specimens) reported by state diagnostic laboratories in 1987. Similarly, Garcia and Brukner¹² reported a prevalence of 20.6% from 2,360 US patients. Differences in test populations or in the composition of the component parasite may be involved. Church., et al.⁵ reported a low prevalence of 6.4-7.2% of 2,604 fecal specimens from Colorado, Montana, New Mexico, and Utah, infected with parasites between August 2006 and April 2007. Amin² reported 19%, 50%, 39%, and 29% from the same states in the same order. Church., et al.⁵ attributed their low prevalence rates, in part, to their inability to detect infections with C. parvum and C. cayetanensis; Quest Diagnostics, Denver, tested their specimens.

Prevalence in Developing Countries

In comparable urban/suburban areas in Africa, Asia, and South America, the prevalence of parasitic infections was mostly similar to ours (Table 1) but occasionally somewhat lower or markedly higher. It was 21.4% of 5,990 patients in Madhya Pradesh, India,¹³ 23.14% of 350 patients in Dhakka University, Bangladesh,¹⁴ 29.26% of 287 patients in Muzaffaraband, Pakistan (Chaudhry., et al.,¹⁵ who reviewed prevalences in 14 Pakistani cities from 40,096 subjects), 33% of 199 patients in Chennai, India,¹⁶ 33.4% of 1,127 patients in Izmir, Turkey,¹⁷ 42.9% of 2,400 patients in Kumasi, Ghana,¹⁸ 47% of 293 patients in Varamin, Iran,¹⁹ 47.2% of 1267 patients in Leon, Nicaragua,²⁰ 50.5% of 93 patients in Central Nigeria,²¹ 62% of 195 patients (protozoans only) in Tamil Nadu, India,²² and 75.1% of 1,227 patients in Bioko, Equatorial Guinea.²³ On rare occasions, the overall prevalence of intestinal infection was very low reaching 5.92% of 5,743 patients in Eghbalieh City, Iran.²⁴ We believe such differences in prevalence to be attributable to demography, diet, environmental exposure, social habits, urbanization, and zoonotic relationships as can be discerned from the above articles.

Prevalence in Developed Countries

Fletcher., et al.¹² summarized 33 prevalence studies from cities in developed countries, 23 in Europe and 10 from the US, Canada, Australia, and Korea. The prevalence rate of B. hominis varied between 1% in Danish counties, Denmark, to 16.9% in Sydney, Australia. For C. parvum: between 0.4% in Melbourne, Australia, to 9.1% in Helsinki, Finland. For E. histolytica/dispar. between 0.4% in Helsinki to 3.5% in Noumea, New Caledonia. For G. intestinalis: between 0.3% in Melbourne, Australia, to 29% in Helsinki. Entamoeba coli was reported only once from Brussels, Belgium, at a rate of 5.4% and D. fragilis 6 times between 0.4% in Melbourne, Australia, and 14.6% in Holland.

Prevalence of Individual Protozoan Species

In the United States, B. hominis was the most dominant protozoan parasite. Its reported prevalence was 19% (45% of all infections) (Table 1), 23% in 2000,² 20 – 30%,²⁵ and 12.2%.¹² It was also the most dominant parasite species reported by Kappus⁴ and Church., et al⁵ but at surprisingly lower prevalence of only 2.6% and 4.3%, respectively. In developed countries, B. hominis appears to also be the dominant intestinal parasite, e.g. Izmir, Turkey,¹⁷ Amsterdam, Holland (24.2%),²⁶ Sydney, Australia (18 – 21%),²⁷ Thessaloniki, Greece (5.3 – 16.8%),²⁸ Stockholm, Sweden (4.0%),²⁹ Berlin, Germany (7.6%),³⁰ Helsinki, Finland (Table 1; 13%),¹¹ Rome, Italy (7.5 – 14.1%),³¹ and Brussels, Belgium (Table 1; 9.8%).¹¹ In most developing countries in Asia, Africa, and South America, however, E. histolytica/dispar and/or G. intestinalis appear to be the dominant parasites, e.g., Jordan,³² India,^11,13,33 Saudi Arabia,³⁴ Lebanon,³⁵ Nicaragua,²⁰ Ghana,¹⁸ Equatorial Guinea,²³ Pakistan,¹⁵ Iran,¹⁹ and Bangladesh.¹⁴ Fletcher., et al¹¹ concluded that “while some enteric protozoa, such as Entamoeba sp., Cryptosporidium, and Giardia are isolated frequently from diarrheal patients in developing regions such as Asia and sub-Saharan Africa, others, such as Blastocystis spp. and Dientamoeba fragilis are isolated mainly in developed countries.” We concur.

In the present study, the next highest prevalence to B. hominis were 6%, 5%, 5%, and 4% noted for E. histolytica/dispar, E.hartmanni, C. parvum, and E. coli, respectively. Only 20 patients (< 1%) were infected with G. intestinalis (Table 1). This ranking was not consistent in some other studies in the United States. For example, in the Rocky Mountain states, the prevalence of Endolimax nana and G. intestinalis ranked second (1.5%) and third (1.4%) to Blastocystis infections.⁵ The prevalence of E. histolytica/dispar of 6% (Table 1) is markedly higher than the 0.9% reported in a large 1987 survey and the estimated prevalence of 4% in the United States.³⁶ The prevalence of C. parvum of 5% (Table 1) is higher than 0.6-4.3% reported elsewhere in North America but less than the 3-20% known from other areas of the world (Asia, Australia, Africa, and Central and South America).¹⁰ Cryptosporidium parvum appears to be underdiagnosed in the western hemisphere; its seroprevalence in Europe and North America is usually between 25% and 35% and may reach 64% in South America.³⁷ Cryptosporidium oocysts were observed in 27% of drinking water sampled from 66 surface water treatment plants in 14 US states and one Canadian province.³⁸ Differences in the prevalence and composition of the intestinal parasite fauna in different geogra­phies are probably attributable to demography, diet, environmental exposure, social habits, urbanization, and zoonotic relationships as well as to the structure of the edaphic conditions and weather affecting the extra-human stages of the parasites particular to each location.

Discussion of Annual Prevalence

The total number of samples submitted from the Los Angeles area was highest in 1996 – 1997 but declined, then stabilized afterward, corresponding with changing patterns of patient traffic. The prevalence of most parasitic infections was highest during 1996-97 (63%) then gradually declined reaching 21% in 2012—2013 agreeing with that of B. hominis, the most common parasite detected, being 21% and 7%, in the same order. The prevalence of all other protozoans, except C. parvum, was highest in 1996-1997. The prevalence of C. parvum progressively increased from 2% in 1996-1997, when prevalence of all other infections was at a minimum, to a high of 11% in 2008-2009 then declined afterwards. Giardia intestinalis was the only protozoan that was consistently identified in 1% or less of the samples throughout the study period.

Decline in Annual Prevalence

The general decline in the prevalence of all parasites and especially of the dominant B. hominis over the years was similar to declines over time reported in other studies. In 10,582 fecal specimens from US general population,³ reported declining B. hominis prevalence rates of 23%, 20%, 15%, and 11% between 2000 and 2004, respectively. A similar study of C. parvum from 9,856 fecal specimens from US general population between 2003 and 2005 noted an almost even prevalence of 5 – 6%.³⁹ Annual prevalence rates of microsporidiosis from fecal specimens of 8,550 HIV-infected patients in Southern California demonstrated a decline from 8.8% in 1993, 9.7% 1994, 6.6% in 1995, and 2.9% in 1996 which was attributed to “the use of multi-drug antiretroviral regimens and the use of protease inhibitors, a new class of antiretroviral agents, the first of which was licensed in 1995.”⁶ In Izmir, a Turkish Mediterranean coastal city with a climate similar to that of Los Angeles, the prevalence of intestinal parasites was 42.5% in 2003⁷ and 65% five years earlier in the same area.⁴⁰ Of 18,563 hospital patient records studied in Qatar, the prevalence of intestinal parasites decreased from 13.4% in 2005 – 2008 to 6.6% in 2009-2011.⁴¹ During this period, the prevalence of B. hominis, G. intestinalis, and pathogenic amoeba decreased from 4.3% to 2.9%, 1.9% to 1.4%, and 0.29% to 0.25%, respectively. This decline was attributed, in part, to improved screening of foreign workers. In Madhya, Pradesh, India, the prevalence of intestinal parasites (mostly Giardia and E. histolytica/dispar), during 2003, 2006, 2007, 2008, 2009, 2010, and 2011 was 59.5%,⁴² 24.1%, 22.3%, 20.3%, 19.9%, and 20.4%, 21.4%,¹³ respectively. In Lebanon, Araj., et al³⁵ reported decreasing prevalence from 14% to 12% in E. histolytica/dispar and from 16% to 6% in G. intestinalis between 1997-1998 and 2007-2008 in 14,771 and 7,477 fecal specimens tested, respectively. We can attribute the overall decline in prevalence of parasitic proto­zoan infections over time to improved health education, better preventive measures, and more effective drug therapies.

Seasonal Prevalence

The prevalence of all infections in our study populations did not show a dramatic seasonal periodicity.^2,3,39 However, in the present investigation, the seasonal prevalence of all protozoan infections was highest in February (51%) and lowest in August (34%) corresponding with the pattern in B. hominis. In 2000, we found that the seasonal prevalence of all infections from 50 states and the District of Columbia was highest in September and October (42% and 43%) and lowest in February (22%).² In our studies of 2000,² and 2002-2004,³ the seasonal prevalence of B. hominis in the general US populations also did not show any marked seasonality but was highest in September (23%) and lowest in February (13%). The seasonal prevalence of C. parvum in the Los Angeles area (this paper) was more or less stable throughout the year (3% in August to 7% in September). The prevalence of microsporidiosis in 8,439 fe­cal specimens from persons with diarrhea and human immunodeficiency syndrome in southern California showed no seasonal variation.⁶ The prevalence of C. parvum from throughout the US was low (3 – 6%) during the colder months of the year (October to March) but higher during the warmer months of April and May, reaching 9% of 9,856 fecal specimens examined between 2003-2005.³⁹

“Peaks in C. parvum prevalence appear to correspond with warmer seasons in temperate and tropical climates especially when associated with rain. During the rainy seasons, the run-off from cattle farms readily contaminate surface waters feeding into water treatment plants as happened during the March-April 1993, Milwaukee outbreak.³⁹ This pattern of seasonal waterborne fecal contamination has also been reported by other observers throughout the world, e.g. US,⁴³ New Orleans,⁴⁴ Peru,⁴⁵ England,⁴⁶ Korea,⁴⁷ Uganda,⁴⁸ Jordan,⁴⁹ Guatemala,⁵⁰ Indonesia,⁵¹ and Zambia.⁵² Cryptosporidium infections from 2000 HIV-positive patients in Benin City, Nigeria were reported to also be associated with the rainy season.⁵³ The infectious stages of these soil-based intestinal parasites released into the environment are clearly vulnerable to seasonal variations in temperature, rainfall and humidity before they encounter other hosts. Transmission depends on the production of and host encounters with parasite stages in the environment. Seasonal variations in host immune system being weaker in the winter may also be involved.⁵⁴

Seasonality of Soil-Based Infections

The seasonal prevalence of other soil-based infections, e.g., E. histoiytica/dispar and G. intestinalis was also highest in the rainy season and lowest in winter or summer in 350 fecal specimens tested in Dhakka University Medical Center, Bangladesh.¹⁴ Similar results were reported for the same two protozoans from 23,278 fecal specimens in Qassim region of Saudi Arabia and from 22,970 stools in the Nablus area, Jordan by Imam., et al³⁴ and Ali-Shtayeh., et al,³² respectively. Patel,³³ however, reported “no significant” seasonal differ­ences in the prevalence of E. histolytica/dispar, G. intestinalis, and E. coli from 36,000 reports of inpatients and outpatients of Bombay Hospital, India, analyzed between 1966 and 1975.

Non-water sources of seasonal Cryptosporidium infections (food stuffs, drinks, animal to person, person to person contact, exposure to contaminated recreational water, among others) are known in arid desert countries mostly in the coldest season and may reach as high as 50% of total exposures.⁵⁵ This pattern is best illustrated in the desert country of Kuwait where 77% of the infections occur between November and April.⁵⁶ Seasonal prevalence of other protozoans investigated were more or less stable throughout the year.

References

1. Amin OM. Prevalence and host relationships of intestinal protozoan infections during the summer of 1996. Explore 8.2 (1997): 29-35.
2. Amin OM. Seasonal prevalence of intestinal parasites in the United States during 2000. American Journal of Tropical Medicine and Hygiene 66.6 (2002): 799-803.
3. Amin OM. The epidemiology of Blastocystis hominis in the United States. Research Journal of Parasitology 1.1 (2006): 1-10.
4. Kappus KD, et al. Intestinal parasitism in the United States: update on a continuing problem. American Journal of Tropical Medicine and Hygiene 50.6 (1994): 705-713.
5. Church C., et al. Intestinal infections in humans in the Rocky Mountain region, United States”. Journal of Parasitology 96.1 (2010): 194-196.
6.  Conteas CN, et al. Examination of the prevalence and seasonal variation of intestinal microsporidiosis in the stools of persons with chronic diarrhea and human immunodeficiency virus infections. American Journal of Tropical Medicine and Hygiene 58.5 (1998): 559-561.
7. Senay H, Macpherson D. Blastocystis hominis epidemiology and natural history. Journal of Infectious Diseases 162.4 (1990): 987-990.
8. Amin OM. Evaluation of a new system for the fixation, concentration, and staining of intestinal parasites in fecal specimens, with critical observations on the trichrome stain. Journal of Microbiological Methods 39.2 (2000): 127-132.
9. Tan KSW. New insights on classification, identification, and clinical relevance of Blastocystis spp. Clinical Microbiology Reviews 21.4 (2008): 639-666.
10. Garcia LS. Diagnostic medical Parasitology, 2001. American Society Microbiology Press, Wash, DC. (2001): 1092.
11. Fletcher SM, et al. Enteric Protozoa in the developed world. Clinical Microbiology Reviews 25.3 (2012): 420-449.
12. Garcia LS, et al. Clinical relevance of Blastocystis hominis (letter). Lancet 323.8388 (1984): 1233-1234.
13. Marothi Y, Singth B. Prevalence of intestinal parasites at Ujjain, Madhya Pradesh, India: Five-year study. African Journal of Mi­crobiology Research 5.18 (2011): 2711-2714.
14. Khanum H, et al. Ocurence of intestinal parasites among the teachers, students and staffs of Dhaka University. Journal of the Asiatic Society of Bangladesh, Science 39.2 (2013): 239-246.
15. Chaudhry ZH, et al. Epidemiological factors affecting prevalence of intestinal parasites in Children of Muzaffarabad District. Paki­stan Journal of Zoology 36.4 (2004): 267-271.
16. Fernandez MC, et al. A comparative study of the intestinal parasites prevalent among children living in urban and rural settings in and around Chennai. Journal of Communication Disorders 34.1 (2002): 35-39.
17. Aksoy U, et al. Demographic status and prevalence of intestinal parasitic infections in schoolchildren in Izmir, Turkey. Turkish Journal of Pediatrics 49.3 (2007): 278-282.
18. Walana W, et al. Prevalence of intestinal protozoan infestation among primary school children in urban and peri-urban communi­ties in Kumasi, Ghana. Science Journal of Public Health 2.2 (2014): 52-57.
19. Aminzadeh Z, et al. Prevalence of intestinal parasites and related factors in primary school children in Varamin”. Iranian Journal of Pediatrics Society 1.2 (2007): 55-58.
20. Tellez A, et al. Prevalence of intestinal parasites in the human population of Leon, Nicaragua. Acta Tropica 66.3 (1997): 119-125.
21. Ikeh E, et al. Intestinal parasitism in rural and urban areas of North Central Nigeria: An update. Internet Journal of Microbiology 2.1 (2005): 1-11.
22. Rayan P, et al. Geographical location and age affects the incidence of parasitic infestations in school children. Indian Journal of Pa­thology and Microbiology 53.3 (2010): 498-502.
23. Roche J, Benito A. Prevalence of intestinal parasite infections with special reference to Entamoeba histolytica on the Island of Bioko (Equatorial Guinea). American Journal of Tropical Medicine and Hygiene 60.2 (1999): 257-262.
24. Sadeghi H, et al. Prevalence of intestinal parasites in a population in Eghbalieh City from Qazvin Province, Iran. Journal of Parasitic Diseases 39.2 (2013): 126-129.
25. Lee MJ. Parasites, yeasts and bacteria in health and disease. Journal of Advances in Medicine       8 (1995): 121-130.
26. Bart A, et al. Diagnosis and subtype analysis of Blastocystis sp. in 442b patients in a hospital setting in the Netherlands. BMC Infec­tious Diseases 13 (2013): 389.
27. Stark D, et al. Prevalence of enteric protozoa in human immunodeficiency virus (HIV) positive and HIV negative men who have sex with men from Sydney, Australia. American Journal of Tropical Medicine and Hygiene 76.3 (2007): 549-552.
28. Papazahariadou MG, et al. Prevalence of gastrointestinal parasites in the Greek population: local people and refugees. Annals of Gastroenterology 17.2 (2004): 194-198.
29. Svenungsson B, et al. Enteropathogens in adult patients with diarrhea and healthy control subjects: a 1-year proispective study in a Swedish clinic for infectious diseases. Clinical Infectious Diseases 30.5 (2000): 770-778.
30. Jansen A, et al. Aetiology of community-acquired, acute gastroenteritis in hospitalized adults: a prospective cohort study. BMC Infectious Diseases 8 (2008): 143.
31. Masucci L, et al. Intestinal parasites isolated in a large teaching hospital, Italy, 1 May 2006 to 31 December, 2008. Euro Surveillance 16.24 (2011): 19891.
32. Ali-Shtayeh MS, et al. Prevalence and seasonal fluctuations of intestinal parasitic infections in the Nablus area, West Bank of Jordan. Annals of Tropical Medicine and Parasitology 83.1 (1989): 67-72.
33. Patel JC. Ten year study of stool samples with particular reference to intestinal parasites. Journal of Postgraduate Medicine 32.4 (1986): 219-224.
34. Imam A, et al. Frequency and seasonality of intestinal parasitism in Qassim region, Saudi Arabia. Pakistan Journal of Medical Sci­ences 28.5 (2012): 913-916.
35. Araj GF., et al. Trends and prevalence of intestinal parasites at a tertiary care center in Lebanon over a decade. Le Journal Medical Libanais 59.3 (2011): 143-148.
36. Kelsall BL, Ravdin JI. Amebiasis: Human infection with Entamoeba histolytica”. Tsieh S, ed. Progress in Clinical Parasitology. Ann Arbor, MI: CRC Press. 4 (1994): 27-54.
37. Current WL, Garcia LS. Cryptosporidiosis. Clinics in Laboratory Medicine 11 (1991): 873-895.
38. LeChevallier MW, et al. Giardia and Cryptosporidium spp. in filtered drinking water supplies. Applied and Environmental Microbiol­ogy 57.9 (1991): 2617-2621.
39. Amin OM. Epidemiology of Cryptosporidium parvum in the United States. Parasitology Unit Journal 1 (2008): 15-22.
40. Akisu C., et al. Investigation of intestinal parasites in school children living under low socio-economic conditions in Izmir. Acta Parasitology 24 (2000): 52-54.
41. Abu-Madi MA., et al. Intestinal parasitic infections among long-term-residents and settled immigrants in Qatar in the period 2005 to 2011. American Journal of Tropical Medicine and Hygiene 88.6 (2013): 1185-1195.
42. Rao VG, et al. Intestinal parasitic infections, anaemia and undernutrition among tribal adolescents of Madhya Pradesh. Indian Journal of Community Medicine 27 (2003): 26-29.
43.  Hlavsa MC, et al. Cryptosporidiosis survillance- United States 1999-2002. Morbidity and Mortality Weekly Report. Surveillance Summaries 54.1 (2005): 1-8.
44. Inungu JN, et al. Risk factors, seasonality, and trends of cryptosporidiosis among patients infected with human immunodeficiency virus. American Journal of Tropical Medicine and Hygiene 62.3 (2000): 384-387.
45. Bern C, et al. Epidemiologic differences between cyclo-sporiasis and cryptosporiasis in Peruvian children. Emerging Infectious Diseases 8.6 (2002): 581-585.
46.  Howe AD, et al. Cryptosporidium oocysts in a water supply associated with a cryptosporidiosis outbreak. Emerging Infectious Disease 8.6 (2002): 619-624.
47. Chai JY, et al. High prevalence and seasonality of cryptosporidiosis in a small rural village occupied predominantly by aged people in the Republic of Korea. American Journal of Tropical Medicine and Hygiene 65.5 (2001): 518-522.
48. Tumwine JK., et al. Cryptosporidium paravum in children with diarrhea in Mulago hospital, Kampala, Uganda. American Journal of Tropical Medicine and Hygiene 68.6 (2003): 710-715.
49. Mahgoub ES., et al. Cryptosporidiosis in children in a north Jordanian paediatric hospital. Eastern Mediterranean Health Journal 10 (2004): 494-501.
50. Bern C, et al. The contrasting epidemiology of Cyclospora and Cryptosporidium among outpatients in Guatemala. American Journal of Tropical Medicine and Hygiene 63.5-6 (2000): 231-235.
51. Katsumata T, et al. Cryptosporidiosis in Indonesia: a hospital based study and a community- based survey. American Journal of Tropical Medicine and Hygiene 59.4 (1998): 628-632.
52. Nchito M, et al. Cryptosporidiosis in urban Zambian children: an analysis of risk factors. American Journal of Tropical Medicine and Hygiene 59.3 (1998): 435-437.
53. Akinbo FO, et al. Seasonal variation of intestinal parasitic infections among HIV-positive patients in Benin City, Nigeria. Ethiopian Journal of Health Sciences 21.3 (2011): 191-194.
54. Altizer S, et al. Seasonality and dynamics of infectious diseases. Ecology Letters 9.4 (2006): 467-484.
55. Anonymous. Cryptosporidium and water- the numbers game. Hlth Stream. 16 (1999): 1- 4.
56. Gaitei W, et al. Cryptosporidiosis: prevalence, genotype analysis, and symptoms accociated with infections in children in Kenya. American Journal of Tropical Medicine and Hygiene 75.1 (2006): 78-82.

Dr. Omar M. Amin, PhD, DNM, MSc, holds a MSc in medical entomology, and a PhD in parasitology and infectious diseases with post-doctoral experience at Old Dominion University in Virginia and at the CDC. His employment has included work at NAMRU-3 (US Naval Medical Research Unit # 3) in Cairo, Egypt, and at the University of Wisconsin. His research in the Persian Gulf was supported by Fulbright Research Scholarships, and he has published more than 280 articles on parasites and diseases throughout the world. He has described 85 new species and a new clinical disorder (neuro-cutaneous syndrome (NCS), a toxicity disorder), has given more than 450 presentations, and is an active board member of a dozen professional societies. Dr. Amin is founder and director of the Parasitology Center, Inc. (PCI), Scottsdale, Arizona, USA with subunits in Mexico, West Africa (Mali), and London, UK.

E-mail: omaramin@aol.com
Web site: www.parasitetesting.com

© All rights reserved by Omar M Amin and Karim O Amin.

*This article is based on the November, 2014 proceedings of presentations made at the PCI-Europe workshops at the Royal Society of Medicine Conference Center, the Bowskill Clinic, and the Kings College in London, UK on March 5-8, 2014. Amin OM, Amin K. An eighteen–year study of intestinal protooans in the Los Angeles area between 1996 and 2013. EC Gasteroenterology and Digestive System. 2017;2.6:494-504, provided here with permission of the copyright holder, Omar Amin, PhD.