Trypanosoma cruzi

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Trypanosoma cruzi. Miguel Angel Basombrio/+, ... infection. The number of bites from Trypanosoma cruzi .... 1991 Med Vet Entomof 5: 325-333) was based on ...
Mem lost Oswald0 Cruz, Rio de Janeiro, Vol. RESEARCH

97(4):

421-424,

Jul./Aug.

1996

421

number of infected guinea pigs (B):

NOTE

NBNI = +

Number of Vector Bites Determining the Infection of Guinea Pigs with

Trypanosoma

cruzi

NBNI =A N.1 (for I >O)

Miguel Angel Basombrio/+, David Carla*, Silvia CatalP*, Maria A Segura, Maria C Mora, Laura G6mez, Julio Nasser

(formula 1)

Laboratorio de Patologia Experimental, Facultad de Ciencias de la Salud, Universidad National de Salta, Calle Buenos Aires 177, 4400 Salta, Argentina ‘Facultad de Ciencias Exactas, Fisicas y Naturales, Universidad National de Cordoba, Cordoba, Argentina Key words: guinea pigs - Trypanosoma cvuzi - bites infection

The number of bites from Trypanosomacruzi infected Triatoma infestansreceived by a mammal is one of the main determinantsof its risk of becoming infected by this parasite. Although T. cruzi is not introduced by the bite itself, it ismostly during the bite that T. cruzi contaminatedfecesare depositedon the skin (C Chagas1909 Mem lnst Oswald0 Cruz 1: 159-218, E Brumpt 1912 Bull Sot Path Exot 5: 723-724). Domestic

guinea pig corrals

or “cuyeras”

Sinceoften not all guineapigsare infected, factor B can be expressedasthe number of animalsin the corral (N) multiplied by the proportion of animalsinfected (I), so that:

are

After the first animal becomesinfected, some of the bites are “wasted” on infected animals and shouldbe sub&acted from NBNI. Since1increases with time, thesebites are estimatedasone half of thosereceived by the animalspresentinginfection at the end of the exposure period. This total number of redundantbites is then divided by N, to estimate those corresponding to one guinea pig, so that % A.1 NBNI= A-p N.1 N and using a commondenominator: A - % AI2 NBNI = NI (formula 2) Regardingfactor A, severalmeasurementscan aIlow its estimation with increased(although not complete)accuracy. Taking into account the number of insectsin the corral at the time of tensing (n), the numberof daysvectors andhostshave lived together (d) and the daily proportion of fed vectors (PFV), factor A could be estimatedasn x PFV x d. Furthermore, not all bugsare infected and it is possibleto examine them and determine the proportion of vectors infected (PVI). Thus, it wa; possibleto dissectformula 2, incorporating all the determinantsjust mentioned:

known asfoci of T. cruzi spread(RA Torrico 1950 Bol of Sanit Panam 29: 827-840) and have also been used for experimental purposes (MA Basombrioet al. 1987Am JTrop MedHyg37: 5762). When guinea pigs are placed in T infestansA = PFV n . PVI d colonized corrals, the number of bites necessary N.1 for infecting a guineapig (NBNI) shouldbe equal Measurementsfor each of thesefactors could to the number of bites taking place in the corral during the time of exposure (A), divided by the be obtained in the field post of our laboratory. After an initial, pilot determination previously reported within an ecologicstudy (S Catalaet al. 1992 Am J Trop Med Hyg 47: 20-26) we presenthere the results of 11 setsof data from 4 separateexperimentsperformed in uniform, comparableconThis research was supported by the UNDP/ Workd Bank/ WHO Special Programme for Research and Training in ditions. The postis placedin Cobos(24’47 S 65’06 Tropical Diseases, by CONICET (Consejo National de W), Province of Salta, Argentina, a Chagas’diseaInvestigaciones Cientilicas y Ttcnicas), and by Consejo seendemicrural area,now under insecticide conde lnvestigacion, Universidad National de Salta. trol. Average temperaturesrangefrom 9°C in win‘Corresponding author. Fax: 54-87255333 ter to 30” C in summerand humidity from 60 to Received 24 July 1995 100%. A systembasedon standardizedguinea pig

Accepted 8 May 1996

422

Number of Vector Bites and T. cruzi Infection

l

MA Basombrio et al.

periment 4, the number of guinea pigs was eight and the vector population startedwith 696, mostly adult, non stratified T. infestuns bugs. In all experiments, initial PVI exceeded 95% in samples and was assumedto be 100% in populations. n: the number of bugswas determinedby periodic censuses. The yards were completely disassembled and all bugswere collected and taken to the laboratory. Developmental stagesand sexeswere recorded but only the total number of bugswas consideredin this work. PFV: the method usedto estimatethis factor (S Catala et al. 1991 Med Vet Entomof 5: 325-333) was basedon the finding of transparent urine in the rectal ampouleof bugs fed during the last 24 hr. Since PFV is temperature-dependent(Catala et al. foe. cit.), the averagetemperaturerecorded durCorrals designed to perform entomological studies of Triaroma ing eachperiod betweencensuseswasusedto adinfesfans populations and to obtain vectorial transmission of just PFV in the following census. Ttypanosoma cruzi to guinea pigs under natural climatic conPVI: fecesfrom bugswere examinedunderthe miditions. croscopefor the presenceof T. cruzi. For both PFV and PVI determinations,samplesof 140 bugs (20 In experiments I,2 and 3, five guineapigs and for each developmentalstage)were used. an original seeding population 696 T. cruzi bearN and I: the number and proportion of guineapigs ing T. in&stuns bugs were initially placed in a carrying T. cruzi infection wasrecorded. Infection corral. During 4 to 7 months, this population was wasdeterminedby the microhematocritmethod (H censusedat intervals, but care was taken not to Freilij et al. 1986J Clin Microbial 18: 327-330) introduce changesin either bugs or guinea pigs. usingthree capillariesper animal, followed by xeThe original bug seed consistedof 42 adult (I2 nodiagnosiswith 20 nymphs and a serologic test male and 30 female) and 654 nymphal stages (direct agglutination or ELISA). (57N5, 72N4, 290N3, l9lN2 and 44Nl). In exThe Table presentsdata of I I censusesfrom 4 corrals, madeof loosebrick, measuring1 m2x 40 cm height, and isolatedby two layers of mosquito nets(Fig.) was used.

TABLE Field samples for estimation ofthe number of Triatomain/&tans bites necessary for the infection of one guinea pig with Trypanosoma cruzi Census No.

Date of census

PFV”’

,h

PVlh

d

I 2 3

07-I I-91 04-12-91 05-03-92

0.22 0.22 0.24

653 463 965

0.516 0.486 0.850

2 2 2

4 5 6

03-09-92 05- I l-92 21-12-92

0.16 0.17 0.21

696 470 708

3 3

7 8

05-08-93 l6- 12-93

0.02 0.10

4 4 4

9 IO II

IO-I l-94 23-12-94 30-03-95

0.20 0.26 0.27

EX-

periment I I I

I

N

ENB’

7 34 I21

0.00 0.20 0.40

5 5 5

5519 1650 10957

0.65 I 0.792 0.770

7 70 II4

0.00 0.20 0.60

5 5 5

>508 4341 3567

545 1050

0.770 0.850

35 166

0.00 0.60

5 5

>294 4049

696 2483 2793

0.685 0.633 0.705

70 III 205

0.37 0.50 I .oo

8 8 4

2071 9922 213624

a: PFV proportion of fed vectors per day, n: number of bugs, PVI: proportion of vectors infected, d: days of exposure, I: proportion of infected guinea pigs, N: number of guinea pigs, NBNI: number of bites necessary for one infection (calculated with formula 2); b: the values for n and PVI are the average of values obtained from the initial (seeding) bug population, of previous censuses and of last census; c: ENB (estimated number of bites) is equal to NBNI for all cases where l>O. In censuses I, 4, and 7 I=0 and NBNI= a~. ENB represents in these cases the number of bites per guinea pig, which is less than NBNI. The values for PFV are the average of the values obtained in previous and last censuses

Mem lnst Oswaldo Cruz, Rio de Janeiro, Vol. 91(4),

separateexperiments.It can be seenthat, in 3 censuses(No. 1,4 and 7), the numberof bites (15 19) was too small to produce any infection (I=O). In censusNo. 11, the number of bites was 13624, a number equalor higher than necessaryto infect all animalsin the yard (I=l). In 7 tenses,the number of bites that produced the infection of one guinea pig fell between 1650and 10957(X = 5222; SE= 1402).Thisrangeis slightly higher,but not substantially distinct from a previous, preliminary determination by our group for a larger, partially sampledguinea pig corral (NBNI = 1461; Catala et al. Zoc.cit.) or to estimatesfor human infection by 7’. cruzi in triatomine infested human dwellings (NBNI = 1000 to 2500, JE Rabinovich et al. 1990Bull WHO 68: 737-746). Somepossibleconfounding factors have to be considered,which possibly reducethe accuracy of these estimates. Underestimation of NBNI may stem from infections not related to bites, such as may occur by contamination of drinking water or food with T. infestansfecesor by direct ingestion of infected bugsby the guinea pigs. This altemative wassuspectedin this systemby a rare episode

Jul./Aug.

1996

423

of suddeninfection of all animals in a corral with very high density of bugs and an open drinking water dish. In this study, we have attempted to reduce this possibility by using improved water dispensers(Fig.) and avoiding an excessive number of bugs.Insectsfailing to collect transparenturine after unsuccessfulbites may also result in underestimation of NBNI. On the other hand, overestimation of NBNI can result from bug populations with an excessof younger developmental stages with low vectorial capacity, such as occurs early in the summer.Finally, a crucial and highly variable factor in transmission,e.g., the concentration of infecti.Je, metacyclic T. cruzi stagesinbug’s feces,hasnot been consideredas such in theseestimates.Bug infectivity has’ steadbeenapproached in this study by PVI, a para “k, eter which is lessinfluential in transmission.In spite of thesepossible inaccuracies,this systemallows the measurement of factors which decisively correlate the behaviour of T. infestansand the transmissionof T. cruzi. Acknowledgment: to the reviewer of this manuscript and Lit. Eusebio Cleto del Rey for their helpful discussion and contribution.