Sero-surveillance of hypodermosis in a herd under therapeutic control. Effect of a low level of infestation.

A cattle herd from the experimental farm of INRA in Nouzilly has been treated for hypodermosis since October 1990. Additionally, a regional eradication scheme has been implemented in this area since autumn 1992. Bi-monthly warble counts were performed between March and July each year on an average of 200 animals. No warble was recorded in this herd from 1991 to 1994 with the exception of two dairy cows in 1993. In autumn 1994, therapeutic control measures were stopped. Serological surveys were performed in the autumn of each year from 1991 until 1995. Anti-Hypoderma antibodies were found in 25%, 27.2%, 4.3%, 3.2% and 0% of the animals respectively. An experimental low infestation was conducted in the summer of 1994. During the spring 1994, third instars of Hypoderma bovis were collected from naturally infested animals. From a total of 13 pupae, six adults (four males and two females) emerged and were released in the herd of Nouzilly on 24 June and 4 July. In October 1994 serological investigations revealed two animals seropositive for hypodermosis. This number increased to six in January 1995. The antibody kinetics of these six animals remained parallel throughout the next 6 months: the titres increased up to April and started to fall in May to return to negative values in August. Manual examinations of the animals at weekly intervals between April and July revealed the presence of four warbled animals with one, one, two and three warbles respectively. The two other seropositive animals remained warble free. One other animal showed antibody titre fluctuations between negative and low positive values, but was warble free in the spring. In October 1995 all the animals of the herd were seronegative. The interpretation and the value of a sensitive immunodiagnosis in a large eradication programme are discussed and compared with warble counts, especially in the case of a low level of infestation.