Constraints to estimating the prevalence of trypanosome infections in East African zebu cattle.
BACKGROUND
In East Africa, animal trypanosomiasis is caused by many tsetse transmitted protozoan parasites including Trypanosoma vivax, T. congolense and subspecies of T. brucei s.l. (T. b. brucei and zoonotic human infective T. b. rhodesiense) that may co-circulate in domestic and wild animals. Accurate species-specific prevalence measurements of these parasites in animal populations are complicated by mixed infections of trypanosomes within individual hosts, low parasite densities and difficulties in conducting field studies. Many Polymerase Chain Reaction (PCR) based diagnostic tools are available to characterise and quantify infection in animals. These are important for assessing the contribution of infections in animal reservoirs and the risk posed to humans from zoonotic trypanosome species. New matrices for DNA capture have simplified large scale field PCR analyses but few studies have examined the impact of these techniques on prevalence estimations.


RESULTS
The Whatman FTA matrix has been evaluated using a random sample of 35 village zebu cattle from a population naturally exposed to trypanosome infection. Using a generic trypanosome-specific PCR, prevalence was systematically evaluated. Multiple PCR samples taken from single FTA cards demonstrated that a single punch from an FTA card is not sufficient to confirm the infectivity status of an individual animal as parasite DNA is unevenly distributed across the card. At low parasite densities in the host, this stochastic sampling effect results in underestimation of prevalence based on single punch PCR testing. Repeated testing increased the estimated prevalence of all Trypanosoma spp. from 9.7% to 86%. Using repeat testing, a very high prevalence of pathogenic trypanosomes was detected in these local village cattle: T. brucei (34.3%), T. congolense (42.9%) and T. vivax (22.9%).


CONCLUSIONS
These results show that, despite the convenience of Whatman FTA cards and specific PCR based detection tools, the chronically low parasitaemias in indigenous African zebu cattle make it difficult to establish true prevalence. Although this study specifically applies to FTA cards, a similar effect would be experienced with other approaches using blood samples containing low parasite densities. For example, using blood film microscopy or PCR detection from liquid samples where the probability of detecting a parasite or DNA molecule, in the required number of fields of view or PCR reaction, is less than one.
Cox
Andrew P
AP
Centre for Infectious Diseases, School of Biomedical Sciences, College of Medicine and Veterinary Medicine, University of Edinburgh, 1 Summerhall Square, Edinburgh, EH9 1QH, UK. andrew.cox@lshtm.ac.uk.
Tosas
Olga
O
Tilley
Aimee
A
Picozzi
Kim
K
Coleman
Paul
P
Hide
Geoff
G
Welburn
Susan C
SC
Vet Parasitol. 2000 Mar 28;89(1-2):129-32
10729652
Am J Trop Med Hyg. 2007 Jun;76(6):1132-7
17556624
Int J Parasitol. 2001 May 1;31(5-6):610-4
11334950
Acta Trop. 2006 May;98(2):183-8
16723098
Exp Parasitol. 2003 May-Jun;104(1-2):26-32
12932756
Ann N Y Acad Sci. 1998 Jun 29;849:21-31
9668446
J Parasitol. 1990 Oct;76(5):734-6
2213419
Exp Parasitol. 2008 Jan;118(1):41-6
17643434
Mol Biochem Parasitol. 1997 Feb;84(2):267-70
9084048
Int J Parasitol. 1992 Nov;22(7):909-18
1459784
Parasitology. 2009 Oct;136(12):1491-500
19555522
Diagn Microbiol Infect Dis. 2008 Aug;61(4):440-5
18486402
Parasitology. 1994 Apr;108 ( Pt 3):313-22
8022657
Trop Anim Health Prod. 1987 Aug;19(3):165-72
3660451
Ann Trop Med Parasitol. 2004 Dec;98(8):785-92
15667711
Exp Parasitol. 2005 Sep;111(1):24-9
16054487
Trends Parasitol. 2001 Jan;17(1):19-24
11137736
J Clin Microbiol. 1995 Apr;33(4):944-51
7790466
Trans R Soc Trop Med Hyg. 1966;60(2):175-82
5950928
Ann Trop Med Parasitol. 2004 Jan;98(1):21-7
15000727
Parasit Vectors. 2010 May 26;3:47
20504326
Vet Parasitol. 2005 Mar 10;128(1-2):1-9
15725527
Parasitology. 1992 Feb;104 Pt 1:67-74
1319564
Parasitol Today. 1996 Feb;12(2):50-5
15275254
Clin Microbiol Rev. 1999 Jan;12(1):112-25
9880477
Trends Parasitol. 2004 Jul;20(7):311-5
15193561
Infect Genet Evol. 2008 Jul;8(4):439-44
17826361
Trends Parasitol. 2001 Jan;17(1):25-8
11137737
Parasitology. 1994 Jul;109 ( Pt 1):95-111
7914692
Trends Parasitol. 2006 Mar;22(3):123-8
16458071
Trans R Soc Trop Med Hyg. 1967;61(5):684-7
6055560
Parasitology. 2009 Dec;136(14):1943-9
19691861
J Vet Med Sci. 2009 Apr;71(4):525-7
19420862
Parasit Vectors. 2008 Dec 24;1(1):46
19108737
Infect Genet Evol. 2008 Jan;8(1):26-33
17964224
PLoS One. 2010 Jan 07;5(1):e8628
20062795
Vet Parasitol. 2002 May 2;105(3):179-89
11934458