Httponlinelibrary.wiley.comdoi10.1046j.1469 0691.2002.00485.Xpdf

7/27/2019 Httponlinelibrary.wiley.comdoi10.1046j.1469 0691.2002.00485.Xpdf

1/7

U P D A T E

Toxoplasma gondii: transmission, diagnosis and preventionD. Hill and J. P. Dubey

Parasite Biology, Epidemiology, and Systematics Laboratory, Animal and Natural Resources Institute,

Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 207052350, USA

Toxoplasmosis, caused by the protozoan parasite Toxoplasma gondii, is one of the mostcommon parasitic infections of man and other warm-blooded animals. It has been foundworld-wide from Alaska to Australia. Nearly one-third of humanity has been exposed tothis parasite. In most adults it does not cause serious illness, but it can cause blindnessand mental retardation in congenitally infected children and devastating disease inimmunocompromised individuals.

Keywords Toxoplasma gondii, protozoan, apicomplexan, cats, congenital infection

Accepted 25 February 2002

Clin Microbiol Infect 2002; 8: 634640

Toxoplasma gondii infection is widespread inhumans, although its prevalence varies widelyfrom place to place. In the United States and theUnited Kingdom, it is estimated that 1640% of thepopulation are infected, whereas in Central andSouth America and continental Europe, estimatesof infection range from 50 to 80% [1]. Most infec-tions in humans are asymptomatic but at times theparasite can produce devastating disease. Infec-tion may be congenitally or postnatally acquired.Congenital infection occurs only when a womanbecomes infected during pregnancy. Congenitalinfections acquired during the first trimester aremore severe than those acquired in the second andthird trimester [2,3]. While the mother rarely hassymptoms of infection, she does have a temporaryparasitemia. Focal lesions develop in the placentaand the fetus may become infected. At first there isgeneralized infection in the fetus. Later, infection iscleared from the visceral tissues and may localizein the central nervous system. A wide spectrum ofclinical diseases occurs in congenitally infectedchildren [2]. Mild disease may consist of slightlydiminished vision, whereas severely diseased

children may have the full tetrad of signs: retino-choroiditis, hydrocephalus, convulsions and intra-cerebral calcification. Of these, hydrocephalus isthe least common, but most dramatic, lesion oftoxoplasmosis. By far the most common sequel ofcongenital toxoplasmosis is ocular disease [2,3].

The socio-economic impact of toxoplasmosis inhuman suffering and the cost of care of sick chil-dren, especially those with mental retardation andblindness, are enormous [4,5]. The testing of allpregnant women for T. gondii infection is routinein some European countries, including France andAustria. The cost-benefit of such mass screening isbeing debated in many other countries [3].

Postnatally acquired infection may be localizedor generalized. Humans become infected byingesting tissue cysts in undercooked or uncookedmeat or by ingesting food and water contaminatedwith oocysts from infected cat faeces. Oocyst-transmitted infections may be more severe thantissue cyst-induced infections [1,610]. Enlargedlymph nodes are the most frequently observedclinical form of toxoplasmosis in humans (Table 1).Lymphadenopathy may be associated with fever,fatigue, muscle pain, sore throat and headache.Although the condition may be benign, its diag-nosis is vital in pregnant women because of therisk to the fetus. In an outbreak in British Colum-bia, of 100 people who were diagnosed with acuteinfection, 51 had lymphadenopathy and 20 hadretinitis [7,8].

Encephalitis is the most important manifesta-tion of toxoplasmosis in immunosuppressed

2002 Copyright by the European Society of Clinical Microbiology and Infectious Diseases

Corresponding author and reprint requests: D. Hill, ParasiteBiology, Epidemiology, and Systematics Laboratory, Animaland Natural Resources Institute, Agricultural ResearchService, U.S. Department of Agriculture, Building 1044,BARC-East, 10300 Baltimore Avenue, Beltsville, Maryland207052350, USATel: 301 504 8770Fax: 301 504 6293E-mail: [email protected]


2/7

patients as it causes the most severe damage to thepatient [1]. Infection may occur in any organ.Patients may have headache, disorientation, drow-siness, hemiparesis, reflex changes and convul-sions, and many become comatose. Encephalitiscaused by T. gondii is now recognized with great

frequency in patients treated with immunosup-pressive agents.

Toxoplasmosis ranks high on the list of diseaseswhich lead to death in patients with acquiredimmunodeficiency syndrome (AIDS); approxi-

mately 10% of AIDS patients in the USA and upto 30% in Europe are estimated to die from tox-oplasmosis [11] Although in AIDS patients anyorgan may be involved, including the testis, der-mis and the spinal cord, infection of the brain ismost frequently reported. Most AIDS patientssuffering from toxoplasmosis have bilateral,severe and persistent headache which respondspoorly to analgesics. As the disease progresses, theheadache may give way to a condition character-ized by confusion, lethargy, ataxia and coma. Thepredominant lesion in the brain is necrosis, espe-

cially of the thalamus [12].Contamination of the environment by oocysts iswidespread as oocysts are shed by domestic catsand other members of the Felidae [1,13] (Figure 1).Domestic cats are probably the major source ofcontamination since oocyst formation is greatest indomestic cats. Cats may excrete millions of oocystsafter ingesting only one bradyzoite or one tissuecyst, and many tissue cysts may be present in oneinfected mouse [14,15]. Generally, only about 1%of cats in a population are found to be sheddingoocysts at any given time. Oocysts are shed for

Table 1 Frequency of symptoms in people with postna-tally acquired toxoplasmosis

Patients with symptoms (%)

SymptomsAtlanta outbreaka

(35 patients)Panama outbreakb

(35 patients)

Fever 94 90Lymphadenopathy 88 77Headache 88 77Myalgia 63 68Stiff neck 57 55Anorexia 57 NRc

Sore throat 46 NRArthralgia 26 29Rash 23 0Confusion 20 NREarache 17 NRNausea 17 36Eye pain 14 26Abdominal pain 11 55

aFrom Teutsch et al. [9].bFrom Benenson et al. [6].cNot reported.

Figure1 Life cycle of Toxoplasmagondii.

Hill and Dubey T. gondii transmission, diagnosis and prevention 635

2002 Copyright by the European Society of Clinical Microbiology and Infectious Diseases, CMI, 8, 634640


3/7

only a short period (12 weeks) in the life of the cat[1], however, the enormous numbers shed assurewidespread contamination of the environment.Under experimental conditions, infected cats canshed oocysts after reinoculation with tissue cysts

[16]. It is not known whether repeated shedding ofoocysts occurs in nature, but this would greatlyfacilitate oocyst spread. Sporulated oocysts sur-vive for long periods under most ordinary envir-onmental conditions. They can survive in moistsoil, for example, for months and even years [1].Oocysts in soil do not always stay there, as inver-tebrates such as flies, cockroaches, dung beetlesand earthworms can mechanically spread theseoocysts and even carry them onto food. Congenitalinfection can occur in cats, and congenitallyinfected kittens can excrete oocysts, providing

another source of oocysts for contamination. Infec-tion rates in cats reflect the rate of infection in localavian and rodent populations because cats arethought to become infected by eating these ani-mals. The more oocysts there are in the environ-ment, the more likely it is that prey animals willbecome infected, and this results in increasedinfection rates in cats. Oocysts can be detectedby examination of cat feces, though for epidemio-logical surveys, detection ofT. gondii oocysts in catfeces is not practical. Concentration methods (e.g.flotation in high-density sucrose solution) areoften used because the number ofT. gondii oocystsin cat feces may be too few to be detected by directsmear. For definitive identification, T. gondiioocysts should be sporulated and then bioassayedin mice to distinguish them from other relatedcoccidians [1]. Determining serological prevalenceis a better measure of exposure of cats to T. gondiiinfection than detection of oocysts. It is a fairassumption that cats that are seropositive havealready shed T. gondii oocysts.

Currently, there are no tests which can discri-minate between oocyst ingestion and tissue cystingestion as the infection route. Available evidencefor the oocyst infection route is based upon epi-demiological surveys. For example, in certainareas of Brazil, approximately 60% of 68-year-old children have antibodies to T. gondii linked tothe ingestion of oocysts from an environmentheavily contaminated with T. gondii oocysts [17].Infections in aquatic mammals indicate contami-nation and survival of oocysts in sea water [18].The largest outbreak of clinical toxoplasmosis inhumans was epidemiologically linked to drinking

water from a municipal water reservoir in BritishColumbia, Canada [19]. This water reservoir wasthought to be contaminated with T. gondii oocystsexcreted by cougars (Felis concolor) [20,21].

Although T. gondii has been isolated from soil,

there is no simple method for oocyst isolation fromsoil that is useful on an epidemiological scale.Although attempts to recover T. gondii oocystsfrom water samples in the British Columbia out-break were unsuccessful, methods to detectoocysts were reported [19]. At present, there areno commercial reagents available to detect T. gon-dii oocysts in the environment.

Infection in humans often results from ingestionof tissue cysts contained in undercooked meat[1,2224]. T. gondii infection is common in manyanimals used for food, including sheep, pigs and

rabbits. Infection in cattle, horses and water buf-faloes is less prevalent than infection in sheep orpigs. Toxoplasma gondii may survive in food ani-mals for years in tissue cysts.

Virtually all edible portions of an animal canharbor viable T. gondii. In one study, viable T.gondii was isolated from 17% of 1000 adult pigs(sows) from a slaughter plant in Iowa [25]. T. gondiiinfection is also prevalent in game animals.Among wild game, T. gondii infection is mostprevalent in black bears and in white-tailed deer.Approximately 80% of black bears are infected inthe USA [26], and about 60% of raccoons haveantibodies to T. gondii [26,27]. Because raccoonsand bears scavenge for their food, infection inthese animals is a good indicator of the prevalenceof T. gondii in the environment.

The number ofT. gondii tissue cysts in meat fromfood animals is very low. It is estimated that as fewas one tissue cyst may be present in 100 g of meat.Therefore, without using a concentration method,it is not practical to detect this low level ofT. gondiiinfection. Therefore, digestion of meat samples intrypsin or pepsin is used to concentrate T. gondii inmeat [28]. Digestion in trypsin or pepsin rupturesthe T. gondii tissue cyst wall, releasing hundreds ofbradyzoites. The bradyzoites survive in the digestsfor several hours. Even in the digested samples,only a few T. gondii tissue cysts are present andtheir identification by direct microscopic examina-tion is not practical. Therefore, the digested mate-rial is bioassayed in mice [28]. The mice inoculatedwith digested material have to be kept for 68 weeks before T. gondii infection can be detectedreliably, and therefore this procedure is not


636 Clinical Microbiology and Infection, Volume 8 Number 10, October 2002


4/7

practical for mass scale samples. The detection ofT. gondii DNA in meat samples by polymerasechain reaction (PCR) has been reported [29], butthere are no data on the specificity and sensitivityof this method to detect T. gondii in meat samples.

A highly sensitive method using a real-time PCRand fluorogenic probe was found to detect T.gondii DNA from as few as four bradyzoites [30].

Cultural habits of a population may affect theacquisition of T. gondii infection from ingestedtissue cysts in undercooked meat. For example,in France the prevalence of antibody to T. gondii isvery high in humans. Though 84% of pregnantwomen in Paris have antibodies to T. gondii, only32% in New York City and 22% in London havesuch antibodies [1]. The high incidence ofT. gondiiinfection in humans in France appears to be related

in part to the French habit of eating some meatproducts raw or undercooked. In contrast, the highprevalence of T. gondii infection in Central andSouth America is probably due to high levels ofcontamination of the environment by oocysts [1].As stated above, the relative frequency of acquisi-tion of toxoplasmosis from eating raw meat andthat due to ingestion of oocysts from cat feces isimpossible to determine, and as a result, state-ments on the subject are at best controversial.

There is little, if any, danger ofT. gondii infectionby drinking cows milk and, in any case, cowsmilk is generally pasteurized or boiled, but infec-tion has followed drinking unboiled goats milk[1]. Raw hens eggs, although an important sourceof Salmonella infection, are extremely unlikely totransmit T. gondii infection. Transmission by sex-ual activity including kissing is probably rare andepidemiologically unimportant [1].

Transmission of T. gondii may also occurthrough blood transfusions and organ transplants.Of these routes, transmission by transplantation ismost important. Toxoplasmosis may arise in twoways in people undergoing transplantation: (i)from implantation of an organ or bone marrowfrom an infected donor into a non-immune, immu-nocompromised recipient and (ii) from inductionof disease in an immunocompromised, latentlyinfected recipient. Tissue cysts in the transplantedtissue or in the latently infected transplant patientare probably the source of the infection. In bothcases, the cytotoxic and immunosuppressive ther-apy given to the transplant recipient is the cause ofthe induction of the active infection and disease[1,31].

Diagnosis of toxoplasmosis in humans is madeby biological, serological, histological, or molecu-lar methods, or by some combination of the above.Clinical signs of toxoplasmosis are non-specificand are not sufficiently characteristic for a definite

diagnosis. Toxoplasmosis in fact mimics severalother infectious diseases.

Detection ofT. gondii antibody in patients mayaid diagnosis. There are numerous serologicalprocedures available for the detection of humoralantibodies; these include the SabinFeldman dyetest, the indirect hemagglutination assay, the indir-ect fluorescent antibody assay (IFA), the directagglutination test, the latex agglutination test(LAT), the enzyme-linked immunosorbent assay(ELISA), and the immunosorbent agglutinationassay test (IAAT). The IFA, IAAT and ELISA have

been modified to detect immunoglobulin M (IgM)antibodies [3,13]. The IgM antibodies appearsooner after infection than the IgG antibodiesand disappear faster than IgG antibodies afterrecovery [3].

The finding of antibodies to T. gondii in oneserum sample only establishes that the host hasbeen infected at some time in the past. It is best tocollect two samples from the same individual, thesecond collected 24 weeks after the first. A 16-foldhigher antibody titer in the second sample indi-cates an acute infection. A high antibody titersometimes persists for months after infection. Arise in antibody titer may not be associated withclinical symptoms because, as indicated earlier,most infections in humans are asymptomatic.The fact that titers persist in infected people afterclinical recovery complicates the interpretation ofthe results of serological tests. Establishing recencyof infection in pregnancy is of clinical importancewith respect to medical intervention to minimizedamage to the fetus, and there is not one test thatcan achieve this at the present time.

Toxoplasma gondii can be isolated from patientsby inoculation of laboratory animals and tissuecultures with secretions, excretions, body fluids,tissues taken by biopsy, and tissues with macro-scopic lesions taken post mortem. Using suchspecimens, one may not only attempt isolationof T. gondii, but one may search for T. gondiimicroscopically or for toxoplasmal DNA usingPCR [32]. Recent studies have shown that mono-plex and multiplex PCR can be useful for specifi-cally identifying T. gondii (using the B1 gene as thetarget sequence) from tissue biopsies, cerebrosp-




5/7

inal fluid or vitreous body from patients withundiagnosed uveitis, fetal blood and amnioticfluid [3337].

As just noted, diagnosis can be made by findingT. gondii in host tissue removed by biopsy or atnecropsy. A rapid diagnosis may be made bymicroscopic examination of impression smearsof lesions. After drying for 1030 min, the smearsare fixed in methyl alcohol and stained with one ofthe Romanowsky strains, the Giemsa stain beingvery satisfactory. Well-preserved T. gondii are cres-cent-shaped (Figure 2). In sections, the tachyzoitesusually appear round to oval. Electron microscopycan aid diagnosis. T. gondii tachyzoites are alwayslocated in vacuoles. Tissue cysts are usually sphe-rical, lack septa, and the cyst wall can be stainedwith a silver stain. The bradyzoites are stronglypositive on periodic acid Schiff (PAS) staining.Immunohistochemical staining of parasites withfluorescent or other types of labelled T. gondiiantisera can aid in diagnosis.

Sulphadiazine and pyrimethamine (Daraprim)are two drugs widely used for treatment of tox-

oplasmosis [38]. While these drugs have a bene-ficial action when given in the acute stage of thedisease process when there is active multiplicationof the parasite, they will usually not eradicateinfection. It is believed that these drugs havelittle effect on subclinical infections, but thegrowth of tissue cysts in mice has been restrainedwith sulphonamides. Certain other drugs, dia-minodiphenylsulphone, atovaquone, spiramycinand clindamycin, are also used to treat toxoplas-mosis in difficult cases.

To prevent infection of human beings by T.gondii, the hands of people handling meat shouldbe washed thoroughly with soap and water beforethey begin other tasks [1,23]. All cutting boards,sink tops, knives and other materials coming incontact with uncooked meat should be washedwith soap and water also. Washing is effectivebecause the stages of T. gondii in meat are killedby contact with soap and water [1].

T. gondii organisms in meat can be killed byexposure to extreme heat or cold. Tissue cystsin meat are killed by heating the meat throughout

Figure 2 Stages of Toxoplasma gondii. Scale bar in (A) to (D) 20mm, in (E) to (G) 10mm. (A) Tachyzoites in impressionsmear of lung. Note crescent-shaped individual tachyzoites (arrows), dividing tachyzoites (arrowheads) compared withsize of host red blood cells and leukocytes; Giemsa stain. (B) Tissue cysts in section of muscle. The tissue cyst wall is verythin (arrow) and encloses many tiny bradyzoites (arrowheads); haematoxylin and eosin stain. (C) Tissue cyst separatedfrom host tissue by homogenization of infected brain. Note tissue cyst wall (arrow) and hundreds of bradyzoites(arrowheads); unstained. (D) Schizont (arrow) with several merozoites (arrowheads) separating from the main mass;impression smear of infected cat intestine, Giemsa stain. (E) A male gamete with two flagella (arrows); impression smear ofinfected cat intestine, Giemsa stain. (F) Unsporulated oocyst in faecal float of cat feces; unstained. Note double-layeredoocyst wall (arrow) enclosing a central undivided mass. (G) Sporulated oocyst with a thin oocyst wall (large arrow), twosporocysts (arrowheads). Each sporocyst has four sporozoites (small arrow) which are not in complete focus; unstained.




6/7

to 67 8C [39] or by cooling to 13 8C [40]. Toxo-plasma in tissue cysts are also killed by exposure to0.5 kilorads of gamma irradiation [41]. Meat of anyanimal should be cooked to 67 8C before consump-tion, and tasting meat while cooking or while

seasoning should be avoided.Pregnant women, especially, should avoid con-

tact with cats, soil and raw meat. Pet cats shouldbe fed only dry, canned, or cooked food. The catlitter box should be emptied every day, a task to beavoided by pregnant women. Gloves should beworn while gardening. Vegetables should bewashed thoroughly before eating because theymay have been contaminated with cat feces.Expectant mothers should be aware of the dangersof toxoplasmosis [42]. At present there is no vac-cine to prevent toxoplasmosis in humans.

In conclusion, infection by the protozoan para-site T. gondii is widely prevalent in humans andanimals. Although it causes asymptomatic infec-tion in immune competent adults, T. gondii cancause devastating disease in congenitally infectedchildren and those with depressed immunity. Toprevent human infection, all meat should becooked well before consumption. Gloves shouldbe worn while gardening, and sandboxes used bychildren should be covered when not in use toprevent exposure to soil contaminated with T.gondii oocysts excreted in cat feces. Extreme careshould be used in handling litterboxes used bycats; and pregnant women, children and immu-nocompromised individuals should avoid litter-boxes altogether.

R E F E R E N C E S

1. Dubey JP, Beattie CP. Toxoplasmosis of Animals andMan. Boca Raton, FL: CRC Press, 1988.

2. Desmonts G, Couvreur J. Congenital toxoplasmosis.A prospective study of 378 pregnancies. N Engl J

Med 1974; 290: 111016.3. Remington JS, McLeod R, Desmonts G. Toxo-

plasmosis. In: Remington JS, Klein JO, eds. Infect-ious Disease of the Fetus and Newborn Infant.Philadelphia: W.B. Saunders Company, 1995;140267.

4. Roberts T, Frenkel JK. Estimating income losses andother preventable costs caused by congenitaltoxoplasmosis in people in the United States. J AmVet Med Assoc 1990; 196: 24956.

5. Roberts T, Murrell KD, Marks S. Economic lossescaused by foodborne parasitic diseases. ParasitolToday 1994; 10: 41923.

6. Benenson MW, Takafuji ET, Lemon SM et al.Oocyst-transmitted toxoplasmosis associated withingestion of contaminated water. N Engl J Med 1982;307: 6669.

7. Bowie WR, King AS, Werker DH et al. Outbreak oftoxoplasmosis associated with municipal drinking

water. Lancet 1997; 350: 1737.8. Burnett AJ, ShorttSG, Isaac-Renton J, King A,

Werker D, Bowie WR. Multiple cases of acquiredtoxoplasmosis retinitis presenting in an outbreak.Ophthalmology 1998; 105: 10327.

9. Teutsch SM, Juranek DD, Sulzer A et al. Epidemictoxoplasmosis associated with infected cats. N Engl

J Med 1979; 300: 6959.10. Smith JL. Documented outbreaks of Toxoplasmosis:

Transmission ofToxoplasma gondii to humans. J FoodProt 1993; 56: 6309.

11. Luft BJ, Remington JS. Toxoplasmic encephalitis inAIDS. Clin Infect Dis 1992; 15: 21122.

12. Renold C, Sugar A, Chave JP et al. Toxoplasmaencephalitis in patients with the acquired immuno-deficiency syndrome. Medicine 1992; 71: 22439.

13. Frenkel JK, Dubey JP, Miller NL. Toxoplasma gondiiin cats: fecal stages identified as coccidian oocysts.Science 1970; 167: 8936.

14. Dubey JP, Frenkel JK. Cyst-induced toxoplasmosisin cats. J Protozool 1972; 19: 15577.

15. Dubey JP. Oocyst shedding by cats fed isolatedbradyzoites and comparison of infectivity of bra-dyzoites of the VEG strain Toxoplasma gondii to catsand mice. J Parasitol 2001; 87: 2159.

16. Dubey JP. Duration of immunity to shedding ofToxoplasma gondii oocyts by cats. J Parasitol 1995; 81:

4105.17. Bahia-Oliveira LMG, Wilken de Abreu AM, Aze-

vedo-Silva J et al. Toxoplasmosis in southeasternBrazil: an alarming situation of highly endemicacquired and congenital infection. Int J Parasitol2001; 31: 1336.

18. Cole RA, Lindsay DS, Howe DK et al. Biological andmolecular characterizations of Toxoplasma gondiistrains obtained from southern sea otters (Enhydralutris nereis). J Parasitol 2000; 86: 52630.

19. Isaac-Renton J, Bowie WR, King A et al. Detection ofToxoplasma gondii oocysts in drinking water. ApplEnviron 1998; 64: 227880.

20. Aramini JJ, Stephen C, Dubey JP. Toxoplasma gondiiin Vancouver Island cougars (Felis concolor vancou-verensis): serology and oocyst shedding. J Parasitol1998; 84: 43840.

21. Aramini JJ, Stephen C, Dubey JP et al. Potentialcontamination of drinking water with Toxoplasma

gondii oocysts. Epidemiol Infect 1999; 122: 30515.22. Cook AJC, Gilbert RE, Buffolano W et al. Sources of

toxoplasma infection in pregnant women: Eur-opean multicentre case control study. Br Med J2000; 321: 1427.




7/7

23. Lopez A, Dietz VJ, Wilson M et al. Preventingcongenital toxoplasmosis. Morbidity MortalityWeekly Report 2000; 49: 5975.

24. Tenter AM, Heckeroth AR, Weiss LM. Toxoplasmagondii: from animals to humans. Int J Parasitol 2000;30: 121758.

25. Dubey JP, Thulliez P, Powell EC. Toxoplasma gondiiin Iowa sows: comparison of antibody titers toisolation ofT. gondii by bioassays in mice and cats. JParasitol 1995; 81: 4853.

26. Dubey JP, Odening K. Toxoplasmosis and relatedinfections. In: Samuel WM, Pybus MJ, Kocan AA,eds. Parasitic Diseases of Wild Mammals. Ames, IA:Iowa State University Press, 2001 4785190.

27. Dubey JP, Weigel RM, Siegel AM et al. Sources andreservoirs of Toxoplasma gondii infection on 47swine farms in Illinois. J Parasitol 1995; 81: 7239.

28. Dubey JP. Refinement of pepsin digestion methodfor isolation of Toxoplasma gondii from infected

tissues. Vet Parasitol 1988; 74: 757.29. Warnekulasuriya MR, Johnson JD, Holliman RE.Detection of Toxoplasma gondii in cured meats. Int J

Food Microbiol 1998; 45: 2115.30. Jauregue LH, Higgins JA, Zarlenga DS et al.

Development of a real-time PCR assay for thedetection of Toxoplasma gondii in pig and mousetissues. J Clini Microbiol 2001; 39: 206571.

31. Frenkel JK. The Coccidia. Eimeria. Isospora, Toxo-plasma and Related Genera. In: Hammond DM,Long PL, eds. Toxoplasmosis: Parasite Life Cycle,Pathology and Immunology. Baltimore, MD: Univer-sity Park Press, 1973; 343410.

32. Grover CM, Thulliez P, Remington JS, Boothroyd

JC. Rapid prenatal diagnosis of congenital Toxo-plasma infection by using polymerase chain reac-tion and amniotic fluid. J Clin Microbiol 1990; 28:2297301.

33. Burg JL, Grover CM, Pouletty P, Boothroyd JC.Direct and sensitive detection of a pathogenicprotozoan, Toxoplasma gondii, by polymerase chainreaction. J Clin Microbiol 1989; 8: 178792.

34. Bretagne S, Costa JM, Foulet F et al. Prospectivestudy of Toxoplasma reactivation by polymerase

chain reaction in allogeneic stem-cell transplantrecipients. Transpl Infect Dis 2000; 2: 12732.

35. Costa JM, Munoz C, Kruger D et al. Quality controlfor the diagnosis ofToxoplasma gondii reactivation inSCT patients using PCR assays. Bone MarrowTranspl 2001; 28: 5278.

36. Dabil H, Boley ML, Schmitz TM et al. Validation ofa diagnostic multiplex polymerase chain reactionassay for infectious posterior uveitis. Arch Ophthal-mol 2001; 119: 131522.

37. Julander I, Martin C, Lappalainen M et al. Poly-merase chain reaction for diagnosis of cerebraltoxoplasmosis in cerebral fluid in HIV-positive

patients. Scand J Infect Dis 2001; 33: 53841.38. Guerina NG, Hsu HW, Meissner HC et al. Neonatalserologic screening andearly treatment forcongenitalToxoplasma gondii infection. The New EnglandRegional Toxoplasma Working Group. N Engl J

Med 1994; 330: 185863.39. Dubey JP, Kotula AW, Sharar AK et al. Effect of

high temperature on infectivity ofToxoplasma gondiitissue cysts in pork. J Parasitol 1990; 76: 2014.

40. Kotula AW, Dubey JP, Sharar AK et al. Effect offreezing on infectivity of Toxoplasma gondii tissuecysts in pork. J Food Protection 1991; 54: 68790.

41. Dubey JP, Thayer DW. Killing of different strains ofToxoplasma gondii tissue cysts by irradiation under

defined conditions. J Parasitol 1994; 80: 7647.42. Foulon W, Naessens A, Derde MP. Evaluation of

the possibilities for preventing congenital toxoplas-mosis. Am J Perinatol 1994; 11: 5762.



Documents

Httponlinelibrary.wiley.comdoi10.1046j.1469 0691.2002.00485.Xpdf