The data presented here show that C. burnetii
can easily spread from an infected goat herd into the farm environment and persist there for at least 1 year. Evidence of environmental contamination was found on all 7 farms described in this study. Most of the organisms were found in areas of the farms where the goats give birth. Based on the replication of C. burnetii
to very high levels in the placenta of infected goats (6
) and the detection of very large amounts of C. burnetii
in goat placental tissue collected at these farms (), it is not surprising that the largest amounts of organism were found in places where birth products would be deposited. C. burnetii
was also detected in other areas of the farms where goats have access, but the numbers of organisms were much lower than they were in the birthing areas.
Previous studies have demonstrated that infectious C. burnetii
can travel long distances on the wind, perhaps as far as 18 km (9
). However, Q fever cases occurring at such long distances from the animal source appears to be uncommon, and other studies have seen cases dropping to near zero at distances greater than 400 m from the infected animals (19
). In this study, samples were taken in the farm environment at various distances from the animal birthing areas. This allowed a spatial analysis of C. burnetii
on these farms at a fine scale. The numbers of organisms detected dropped off fairly quickly with the distance from the goat birthing areas. On the farms analyzed in the greatest detail (farms 5 and 7), only small quantities of C. burnetii
were found at distances greater than 50 m from goat housing/birthing areas. High concentrations of C. burnetii
DNA were found only in the goat pens and in the farm houses. Many factors will influence transfer of C. burnetii
from animals to the environment, but it is expected that hot and dry conditions will favor wind-borne spread of C. burnetii
aerosols. The relatively short distances traveled by C. burnetii
on these farms suggest that conditions at these two farms did not favor wind-borne spread.
Although wind did not appear to be able to transport large amounts of C. burnetii
from the goat pens to the perimeter of the farms, C. burnetii
DNA was found in 67% of air samples taken at farm 5 in May 2012. This high percentage was surprising given that many of the samples were taken at the perimeter of the farm away from birthing areas and that shedding from the goats had declined between 2011 and 2012 (our unpublished data). Despite the high percentage of positives, the numbers of genomes detected in most of the samples were near the limits of detection, and most of the PCRs were detecting close to a single organism. Because such a large volume of air (500 liters) can be concentrated on a small filter, the air sampling technique is very sensitive for detection of C. burnetii
in the farm environment. Air sampling for this study was performed only at this farm, which had a history of goat abortion and Q fever cases. It is not clear whether air sampling would detect such low levels of C. burnetii
DNA even on farms with no recent history of C. burnetii
contamination. A recent study from the Netherlands did compare aerosol samples from farms with known Q fever abortion waves to samples from farms without a Q fever history and found a higher percentage of positive samples on the farms with Q fever history (13
). However, 66.7% of samples from farms with no Q fever history were positive, and the differences between infected and noninfected farms were not statistically significant (13
). The data suggest that low levels of aerosols containing detectable amounts of C. burnetii
DNA may be common on any farm environment.
With such low levels of C. burnetii detected by PCR, contamination is a concern. However, several points argue against contamination. First, samples taken outside the pasture area were mostly negative, whereas all of the samples taken in the goat birthing area were positive. Second, of the 5 samples taken inside the house, only one was positive, as would be expected from the relatively static air found inside the home. Finally, the last sample taken was at the perimeter of the pasture and it was negative. Therefore, the results show that a low level of airborne C. burnetii exists on the farm, but these organisms did not seem to travel long distances and there is no evidence regarding the viability of the airborne organisms on these farms.
If wind did not play a major role in spreading the bacteria, then it might be expected that the goats themselves would be an effective means to spread C. burnetii
through the environment. Vaginal swabs taken from goats on these farms indicated that high levels of C. burnetii
were being shed. It is possible that shedding by these infected goats can contaminate the environment over a large area. However, C. burnetii
was primarily concentrated in and around animal pens and birthing areas. The trafficking patterns of the goats on these farms are not known, so it is possible that the high burden of C. burnetii
in goat pens compared to that in pastures reflects the fact that goats may have spent most of their time in the pens. In addition to the goat pens, the other areas on the farms that had a high burden of C. burnetii
were places that had human activity. The relatively strong PCR signals in samples from houses on farms 5 and 7 and the truck on farm 7 suggest that human activity played an important role in environmental contamination. On farm 5, the organisms in the house persisted into 2012 despite efforts by the homeowners to decontaminate the area. Although the effectiveness of the decontamination efforts was not evaluated, the high levels of organism in the goat pens and birthing areas and the persistence of high levels of C. burnetii
in the home on farm 5 suggest that there was repeated introduction of C. burnetii
into the home, most likely by people tracking C. burnetii
back into the house after farm work. In a large environmental study published in 2010, many C. burnetii
-positive samples were found at locations that were not near animals (10
). A likely explanation for the introduction of C. burnetii
into these locations is foot traffic from people that had recently visited areas directly contaminated by infected livestock. Such spreading by human foot traffic emphasizes the need for careful decontamination of shoes and clothing after working in or visiting an area with C. burnetii
Six new isolates of C. burnetii
were established from samples associated with this outbreak. All six of the isolates were estimated to have approximately 60 copies of IS1111a
, contain the QpRS plasmid, and fall into the ST8 genotype. The method used for the genotyping was an SNP-based rapid typing method (17
) that can define sequence types that are compatible with previously reported MLST sequence types (18
). The ST8 genotype has been found previously in numerous human heart valve samples and in other goats from the western United States (18
). Although the samples from this outbreak used for genotyping were all from animals and environmental samples, the close association with human acute Q fever cases suggests that the ST8 genotype also caused the acute Q fever cases associated with this outbreak with a range of symptoms and severity of disease.
This analysis demonstrates that C. burnetii can spread to the environment from infected goats. C. burnetii found in the environment can remain viable and infectious. In the farms examined, the organisms did not appear to spread for great distances and were confined to the vicinity of goat birthing areas. Because most of the organisms were found in goat birthing areas, it is likely that release of C. burnetii during parturition is the primary means for contamination of the environment, and ongoing shedding from infected goats plays a minor role. Although C. burnetii was able to be detected in the air on the farm, the very low numbers of C. burnetii isolates found away from the animal housing areas suggest that wind-borne spread under these conditions was not significant. The pattern of positive samples suggests that humans on the farms serve as an important vehicle for transportation of viable C. burnetii from the goat pens to the farm house, other parts of the farm, and perhaps beyond.