The Effect of Wind on Coxiella burnetii Transmission Between Cattle Herds: a Mechanistic Approach.

There is a consensus that wind plays a key role in the transmission of Coxiella burnetii, the causative agent of Q fever, between ruminants and from ruminants to humans. However, no observational study so far has focused on the mechanisms associated with this airborne transmission. This study applied a mechanistic epidemiological approach to investigate the processes underlying the wind effect and to assess its influence on the risk for a dairy herd to become C. burnetii infected. Ninety-five dairy cattle herds located in the Finistère department (western France) were subjected to samplings of bulk tank milk and indoor dust every 4 months over a 1-year period to determine their C. burnetii status using PCR tests. A total of 27 incident herd-periods (negative-tested on both PCR tests and becoming positive-tested at least once at the subsequent sampling time) and 71 negative herd-periods were retained for analysis. Using logistic regression, we assessed the effect of (i) the cumulated number of bacteria in herds located under the main wind direction and (ii) the mean wind speed in this area, on a given herd's risk of becoming incident. Compared to herds in areas with low wind speed (≤5.5 m/s), the risk was significantly higher (OR = 3.7) in herds in areas with high wind speed (>5.5 m/s) and high bacterial load (>10), whereas it was not significantly different from unity in other situations. In agreement with our assumptions, C. burnetii transmission to a previously infection-free herd occurs only when (i) the wind transporting from infected sources and (ii) the load in the contaminated particles/aerosols generated are high enough to act jointly.

Q fever infection in dairy cattle herds: increased risk with high wind speed and low precipitation.

Ruminants are considered the main reservoir for transmission of Coxiella burnetii (Cb) to humans. The implementation of effective control measures against Cb in ruminants requires knowledge about potential risk factors. The objectives of this study were (i) to describe the spatial distribution of Q fever-infected dairy cattle herds in Sweden, (ii) to quantify the respective contributions of wind and animal movements on the risk of infection, while accounting for other sources of variation, and (iii) to investigate the possible protective effect of precipitation. A total of 1537 bulk milk samples were collected and tested for presence of Cb antibodies. The prevalence of test-positive herds was higher in the south of Sweden. For herds located in areas with high wind speed, open landscape, high animal densities and high temperature, the risk of being infected reached very high values. Because these factors are difficult to control, vaccination could be an appropriate control measure in these areas. Finally, the cumulated precipitation over 1 year was identified as a protective factor.

Decrease in milk yield associated with exposure to bluetongue virus serotype 8 in cattle herds.

Decreased milk yield and reduced fertility are the primary consequences of infection by bluetongue virus serotype 8 (BTV-8). These effects must be quantified to fully assess the economic benefit of vaccination. This can be estimated by measuring the effect of BTV-8 exposure on milk yield and fertility for all cows belonging to an infected herd. The objectives of this study were (1) to quantify the mean effect of exposure to BTV-8 on milk yield following natural challenge for cows in herds previously naïve, (2) to determine the duration of reduced milk yield before and after the date disease was first detected in the herd to estimate the cumulative loss of milk yield during this period, and (3) to evaluate the influence of the proportion of infected neighboring herds on the reduction in milk yield following exposure to BTV-8. The effects of exposure to BTV-8 during the French outbreak of 2007 were assessed using mixed linear models, which allow adjustment for factors known to influence milk yield. Exposure to BTV-8 was associated with a sharp decrease in milk yield over a period of 6 mo (2 mo before to 4 mo after the reported date of disease detection in the herd). The cumulative loss of milk yield was more than 3% of annual production. The relatively earlier reduction in milk yield in infected herds detected later in the outbreak period suggests that detection of clinical signs was delayed in these herds. Finally, the greatest decrease in milk yield was observed in herds detected early during the outbreak period and located in areas with the highest disease incidence. This may be due to a greater within-herd incidence or to a greater amount of virus injected by midges to individual cows in these herds.

Increase in the occurrence of abortions associated with exposure to the Bluetongue virus serotype 8 in naïve dairy herds.

The transplacental transmission capacity demonstrated for Bluetongue virus serotype 8 (BTV-8) in cattle probably is associated with an increased occurrence of abortions. The objectives of this study were to quantify the effect of BTV-8 exposure on the occurrence of abortions in previously naive dairy cow herds under natural infection conditions, and to determine a possible risk period during pregnancy associated with this increase. Two criteria were considered in order to estimate the occurrence of abortion: late return-to-service after a first artificial insemination (AI), and short gestations. A late return-to-service was defined as a return taking place 90 to 200 days after a first AI. These criteria were compared between cows in herds exposed during the 2007 epizootic in France and cows in herds that were not exposed. To determine the risk period during a pregnancy, variations in the occurrence of abortions were quantified according to the stage of the pregnancy during which the exposure took place. Survival analyses were used to estimate the risk of increased occurrence of abortion associated with BTV-8 exposure, adjusted by the principal factors known to influence the risk of abortion. Exposure to the BTV-8 virus under natural conditions in previously naive dairy herds notified after clinical suspicion during the 2007 epizootic was associated with an increase in the occurrence of abortions, regardless of the stage of pregnancy. The at-risk gestation period depended on the criteria used to detect abortions. The mean effect of BTV-8 exposure in the ensemble of detected outbreaks corresponded to an increase of 6.7% in late return-to-service. BTV-8 exposure during the first 3 mo of gestation was associated with a 15% increase in late return-to-service for cows with no return-to-service at 90 days, while this increase was 6% for exposure starting from the third month of gestation (in outbreaks detected in September). BTV-8 exposure from the third month of gestation was associated with a 1.9% increase of short gestations. The effect of exposure was more pronounced for outbreaks detected early in the epizootic compared with those detected later.

Quantification and at-risk period of decreased fertility associated with exposure to bluetongue virus serotype 8 in naïve dairy herds.

The detrimental effect of bluetongue virus serotype 8 (BTV-8) on fertility was quantified in seroconverting cows. Although the effect on individual cows provides information regarding the potential biological burden of infection, losses at a herd level are also dependent on the proportion of infected cows within the herd. The objectives of this study were to quantify the average effect of BTV-8 exposure in field conditions on the fertility of dairy cows in previously naïve herds, and to determine the at-risk period of decreased fertility related to the date of detection of the disease in the herd. The effect of BTV-8 exposure on fertility was assessed using the 90-d-return-to-service rates after the first artificial insemination (AI) calculated for cows in exposed herds (during the 2007 epizootic in France) and compared with that for cows in unexposed herds. Only herds with a confirmed detection that were reported after clinical suspicion were included. To determine the at-risk period of decreased fertility, variations of fertility in exposed herds were quantified according to the time interval between the date of AI for individual cows and the date that disease was detected in the herd. Survival analyses were used to assess the risk of decreased fertility associated with BTV-8 exposure, adjusting for the main factors known to influence fertility. The episode at risk for decreased fertility depended on the month of disease detection in the herd. For herds detected early in the epizootic, fertility was decreased for cows inseminated from 1 mo before to 1 mo after the date of disease detection in the herd. Depending on time interval between the date of AI of cows and the date of detection in the herd, the increase of return-to-service rate associated with BTV-8 exposure varied from 8 to 21 percentage points of 90-d return to service. The episode of decreased fertility is likely due to a combination of the effect of the infection at different stages of conception and early pregnancy and the delayed exposure of cows due to the spreading of the virus within herds. For herds detected during the second half of the epizootic, fertility was decreased for cows inseminated more than 2 mo before detection, which suggests a delay in the detection of clinical signs following virus introduction in the herd. No correlation was observed between the effect of BTV-8 exposure on fertility and the incidence of BTV-8 in the local geographical area. Given the duration of the period that cows were at risk for decreased fertility and the magnitude of the effect, the average BTV-8 exposure in naïve herds led to major losses.