A survival analysis of trypanosomosis diagnostic-test performance under natural infection challenge

Little is known about the time-to-first detection and the time difference ( TD) between first parasitological and first serological diagnosis of Trypan osoma spp. infections under natural infection challenge in cattle. The obje ctive of our study was to estimate these measures of "longitudinal aspects" of diagnostic performance and to investigate potential biological factors. Emphasis was on diagnosis at the genus level (Trypanosoma spp.). Twelve N' Dama, 12 Gobra zebu and 12 N'Dama x Gobra (Fl) crossbred cattle (all animal s non-infected at the start of the experiment, six male and six female anim als in each cohort) were exposed to natural high tsetse challenge in the Ni amina East area in The Gambia [Acta Trop. 71 (1998) 57]. The animals were i nvestigated parasitologically (detection of trypanosomes by buffy-coat tech nique), serologically (detection of T brucei, T congolense and T vivax anti gen by enzyme-linked immunosorbent assay (ELISA)) and clinically (packed-ce ll volume, PCV) over a period of 180 days. The time-to-first detection of t rypanosomes, trypanosomal antigen (cut-off as suggested by test supplier) a nd drop in PCV (subject-based cut-off values) were recorded as outcomes of interest. Thus, incidence was both parasitologically (I-p), serologically ( I-s) and clinically (I-c). Recurrent events were not considered. The TD bet ween first parasitological and first serological detection was established as I-s time minus I-p time. The effect of breed and sex on the time-to-firs t detection and on TD was investigated using Cox (proportional hazard) regr ession and ANOVA, respectively. We found that time-to-first parasitological detection of trypanosomosis in N'Dama animals was significantly longer than in the two other breeds (Cox r egression, P = 0.002). A similar but less-strong (P = 0.063) effect of bree d on time-to-first detection of trypanosomal. antigen was found, whereas no breed effect was observed for clinical detection (P = 0.432). Sex had no e ffect in all detection systems. The TD varied between -56 and 115 (mean 28) . Marked differences among breeds and between sexes were not observed (ANOV A, P = 0.8). We suggest that incidence studies are more suitable for detect ing risk factors for animal trypanosomosis than prevalence-based (cross-sec tional) studies because the latter often result in misinterpretation of fac tors that increase the survival time with infection as risk factors. (C) 20 01 Elsevier Science B.V. All rights reserved.