Table 4. The calculation of apparent prevalence and apparent incidence and the tracking of the animals in the next testing in bi-annually phase for three farms (2004–2010).

Year	2004		2005		2006		2007		2008		2009		2010
Test phase	1	2	3	4	5	6	7	8	9	10	11	12	13	14
Herd A
Total positive cows a	14	25	34	28	34	32	21	23	24	17	14	9	7	0
Animals tested	315	330	364	349	354	364	338	332	337	347	341	347	296	239
Apparent prevalence	4.4	7.6	9.3	8.0	9.6	8.8	6.2	6.9	7.1	4.9	4.1	2.6	2.4	0.0
New casesb	14	16	18	12	13	13	7	10	11	6	5	2	0	0
Cow-years at riskc		239		293		284		272		276		280		198
Apparent incidenced		0.13		0.10		0.09		0.06		0.06		0.02		0
Herd B
Total positive cows	9	8	6	3	6	5	4	3	3	2	5	5	1	0
Animals tested	106	122	128	128	113	113	115	114	111	109	113	109	82	1
Apparent prevalence	8.5	6.6	4.7	2.3	5.3	4.4	3.5	2.6	2.7	1.8	4.4	4.6	1.2	0.0
New cases	9	1	2	0	5	4	1	0	1	1	4	1	0	0
Cow-years at risk		72		99		95		94		93		83		37
Apparent incidence		0.14		0.02		0.10		0.01		0.02		0.06		0
Herd C
Total positive cows	0	17	26	23	19	22	18	20	18	15	13	8	7	0
Animals tested	0	121	145	149	178	161	145	155	157	145	142	117	102	0
Apparent prevalence	NA	14.0	17.9	15.4	10.7	13.7	12.4	12.9	11.5	10.3	9.2	6.8	6.9	NA
New cases	0	17	9	7	5	9	5	4	4	5	2	2	1	0
Cow-years at risk		13		114		123		110		117		108		33
Apparent incidence		1.27		0.14		0.11		0.08		0.08		0.04		0.03

^aTest positive cows by considering enzyme-linked immunosorbent assay (ELISA) testing, fecal testing and tissue testing.

^bNumber of cows tested positive for the first time

^cObservation time (in years) from entry in the study (at the first testing) until each cow tested positive or left the study (by culling, i.e. the infection status of the cow is right censored)

^dNew cases per year / cow-years at risk