Risk factors for bovine herpesvirus-1 seropositivity.

This paper reports the investigation of risk factors for bovine herpesvirus-1-seropositivity, based on a cluster-sample survey of the Belgian cattle population. This serosurvey was carried out in 1998 in 309 randomly selected unvaccinated herds of all types (dairy, mixed and beef) were all bovids (N = 11,284) were sampled. Older and male cattle had higher seroprevalence. Origin (homebred or purchased) and herd size interacted; for smaller herds (< or = 50 cattle on the premises), purchase status and larger herd size were risk factors, whereas these effects were not observed for larger herds.

Design-based analysis of surveys: a bovine herpesvirus 1 case study.

This paper critically assesses the design implications for the analysis of surveys of infections. It indicates the danger of not accounting for the study design in the statistical investigation of risk factors. A stratified design often implies an increased precision while clustering of infection results in a decreased precision. Through pseudo-likelihood estimation and linearisation of the variance estimator, the design effects can be taken into account in the analysis. The intra-cluster-correlation can be investigated through a logistic random effect model and a generalised estimating equation (GEE), allowing the investigation of the extent of spread of infections in a herd (cluster). The advantage of using adaptive Gaussian quadrature in a logistic random effect model is discussed. Applicable software is briefly reviewed. The methods are illustrated with data from a bovine herpesvirus 1 (BHV-1) serosurvey of Belgian cattle.

Transmission of Theileria parva in the traditional farming sector in the Southern Province of Zambia during 1997-1998.

The incidence of first contact with the protozoan Theileria parva was determined in two traditional cattle herds in the Southern Province of Zambia during a period of average rainfall in 1997 and 1998, following a drought in the previous two years. Compared to that period, there was a marked increase in the number of rainy season first contacts attributable to transmission by Rhipicephalus appendiceulatus adults. However, there were still more dry season contacts that resulted from nymphal transmission. These results highlight the important role that climate plays in the transmission of theileriosis in the Southern Province of Zambia.

Transmission of theileriosis in the traditional farming sector in the southern province of Zambia during 1995-1996.

The incidence of first contact with the protozoan Theileria parva was determined in three traditional cattle herds in the Southern Province of Zambia in 1995 and 1996. The majority of first contacts occurred during the dry season in June, July and August, at a time of nymphal activity and in the absence of Rhipicephalus appendiculatus adults, indicating that larva to nymph transmission plays a more prominent role than nymph to adult transmission under the prevailing conditions.

Spatial and temporal variation in Rhipicephalus appendiculatus size in eastern Zambia.

The size of Rhipicephalus appendiculatus collected at different altitudes in the Eastern Province of Zambia between February 1985 and May 1986 and between October 1994 and December 1996 showed distinct variation dependent on altitude and season. The ticks were smallest during the dry season and at the start of the rains, and specimens were larger as the rainy season progressed. Second-generation adults where on average smaller than first-generation ticks. At higher altitudes, where a one-generation-per-annum phenology dominates, ticks were larger than at intermediate altitudes, where two generations per year are common. Larger size, associated with increased survival, is also favoured in low-lying, drier areas. Selective mortality of smaller adult ticks in years with a delayed rainy season appears to play an important role in the variation in size between years.

A survey of the ixodid ticks parasitising cattle in the Eastern province of Zambia.

The results of tick surveys carried out in the Eastern province of Zambia between December 1982 and February 1996 were principally in agreement with the findings of earlier surveys conducted during the period 1965-72. Boophilus decoloratus has almost been replaced by Boophilus microplus. Hyalomma truncatum was found in small numbers throughout the province and Hyalomma marginatum rufipes was only rarely encountered in collections made from cattle. Rhipicephalus evertsi evertsi was largely confined to the southern part of the plateau and the valleys of the Zambezi tributaries. The main difference between the present survey and the previous one concerns the status of Rhipicephalus appendiculatus. This species is currently expanding its range in a westward direction, whereas it was virtually absent from the southern part of the province during the period 1965-72. The majority of specimens collected are morphologically intermediate between R. appendiculatus and the closely allied Rhipicephalus zambeziensis. The available evidence indicates that R. zambeziensis (sensu stricto) is absent from the province. The phenology of R. appendiculatus is aberrant in the province: at lower altitudes a second generation of adult ticks is recorded on the hosts at the start of the dry season.

Evaluation of an in vitro method to measure behavioural diapause in the tick Rhipicephalus appendiculatus (Acarina: Ixodidae) in the laboratory.

A new method has been developed to investigate behavioral diapause in adult Rhipicephalus appendiculatus ticks. It is based on a system of gauze columns in which the activity of the ticks can be monitored in the laboratory. An experiment was conducted, involving different photoperiodic conditions during the nymph-to-adult moulting and pre-questing periods, with the aim of comparing the results obtained by the in vitro method with those of the standard in vivo method. Comparable results were obtained with both methods.

A study of the diapausing behaviour of Rhipicephalus appendiculatus and R. zambeziensis under quasi-natural conditions in Zambia.

The diapausing behaviour of Rhipicephalus appendiculatus was studied under quasi-natural conditions in the Eastern Province of Zambia. Newly-moulted adults of ticks indigenous to the study area entered a behavioural diapause when exposed to daylengths below a critical photoperiod, provisionally a daylength between 11 h 20 min and 11 h 45 min. In the Eastern Province of Zambia the diapause was apparently not terminated by a long-day signal, but by a weakening of the photoperiodic maintenance of the diapause because of increasing age of the ticks. Adults of a reference stock indigenous to Kenya also entered a diapause when exposed to daylengths below the same threshold and maintained this diapause for the same length of time. Adults of a reference stock of Rhipicephalus zambeziensis indigenous to Zimbabwe did not enter a diapause when exposed to the same daylengths. The relevance of the findings is discussed in relation to the distributions of the two species.

Re-assessment of tick control after immunization against East Coast fever in the Eastern Province of Zambia.

East Coast Fever, caused by the protozoon Theileria parva and transmitted by the ixodid tick Rhipicephalus appendiculatus is one of the most important cattle diseases in east and central Africa, responsible for considerable direct losses and necessitating expensive control measures. Traditionally, the disease was controlled by means of intensive tick control. The Belgian Animal Disease Control Project was requested to study the disease epizootiology and vector ecology in order to formulate and implement a control program adapted to the requirements and capabilities of the cattle owners in the Eastern Province of Zambia. The weaknesses of a rigorous tick control program were demonstrated. It was decided to initiate an immunization program in the enzootic areas. The overall calf mortality rate was lowered by 90% and it was shown that none of the other tick-borne diseases caused significant problems in the absence of tick control. The tick ecology studies had indicated that the climatic conditions in the area were so unfavourable that the important vector species (Amblyomma variegatum, Boophilus microplus and R. appendiculatus) would not attain problem levels. It was therefore recommended to suspend all tick control in the area. Control of East Coast Fever in the epizootic and disease-free areas is still a more complex issue. It appears unlikely that the latter will remain disease-free, because of the proximity of the enzootic areas and because of considerable cattle movement in the province. Given the advantages of control by immunization, it can be argued that a longterm solution should be based on this approach.(ABSTRACT TRUNCATED AT 250 WORDS)