Susceptibility, Immunity, and Persistent Infection Drive Endemic Cycles of Coxiellosis on Dairy Farms

Coxiella (C.) burnetii, a zoonotic bacterium, is prevalent in dairy farms. Some cows develop a persistent infection and shed C. burnetii into milk and occasionally by amniotic fluid at calving. Serological diagnosis of Q fever in humans is performed by phase (Ph)-specific antibody tests; PhII antibodies usually indicate an acute infection, while the development of a chronic infection is characterised by elevated PhI antibody titres. Phase-specific tests have now been established for diagnosis of coxiellosis in cattle. Additionally, an interferon-γ (IFN-γ) recall assay has been implemented to assess cellular immunity to C. burnetii in cattle. Milk samples from all lactating cows (n = 2718) of 49 Bavarian dairy farms were collected through a convenience sample and analysed for phase-specific antibodies. Antibody profiles were evaluated by age. Based on the seropositivity of first-lactation cows, three distinct herd profiles were observed: an 'acute' state of herd infection was characterised by a PhI-/PhII+ pattern. The detection of PhI antibodies (PhI+/PhII+) characterised the 'chronic' state, and seronegative results defined the 'silent' state of herd infection. If antibodies had not been detected in multiparous cows, the herd was considered as probably free of coxiellosis. The analysed cattle herds were noted to have an 'acute' (n = 12, 24.5%), 'chronic' (n = 18, 36.8%), or 'silent' state of herd infection (n = 16, 32.6%). Only three farms (6.1%) were classified as 'free' of C. burnetii. The detection of these herd states over a time period of 4 years in one farm indicated that the described states occur in a cyclical manner. Frequently, a wave-like profile was seen, i.e., a circumscribed seronegative age group was flanked by seropositive age groups. In seronegative animals, IFN-γ reactivity was demonstrated. Seroconversion after vaccination was observed by day 7 post-vaccination in chronically infected herds, whereas in the case of silent infection, it started by day 14. These data indicated a pre-existing immunity in seronegative animals in chronically infected herds. Additionally, IFN-γ reactivity was detected in seronegative calves (>3 months) and heifers from chronically infected farms compared to a negative farm. An infection prior to 3 months of age resulted in cellular immunity in the absence of detectable antibodies. An infection around calving would explain this. The aforementioned circumscribed seronegative age groups are, therefore, explained by an infection early in life during active shedding at calving. Based on these results, an endemic cycle of coxiellosis is proposed: Susceptible young heifers get infected by persistently infected cows. Subsequently, shedding of C. burnetii at calving results in infection and then in cellular immunity in offspring. When these calves enter the cow herd two years later, a maximum of herd immunity is achieved, shedding ceases, and new susceptible animals are raised. In an acutely infected dairy farm, the PhI+/PhII+ serological pattern prevailed in second-lactation cows. In this study, stored sera collected since birth were analysed retrospectively. From the earliest seroconversion, the peak of seroconversion took about 33 months. These data suggested a slow spread of infection within herds. The classification of dairy cow herds is a promising basis for further analysis of the clinical impact of coxiellosis.

The immune response to a Coxiella burnetii vaccine in sheep varies according to their natural pre-exposure

Q fever in humans is caused by Coxiella (C.) burnetii. In 2008 and 2012, cases of Q fever in humans were linked to an infected flock of approximately 650 ewes. Since 2013 gimmers (G'13, G'14, G'15 etc.) were primary vaccinated (two doses) with an inactivated C.burnetii vaccine without any revaccination. In 2013, 30 ewes were primary vaccinated (A'13). Shedding was annually monitored by qPCR-testing of vaginal and nasal swabs collected at lambing. Animals were tested for Phase I- (PhI) and PhII-antibodies (Ab) and for PhII-specific-interferon-γ (IFN-γ) before and after vaccination. The effect of a revaccination was determined in 2018 and 2023. Groups of randomly selected gimmers primary vaccinated in 2015, 2016 and 2017 and a mixed group of older animals (A'13, G'13 and G'14) were revaccinated once in 2018. The trial was repeated in 2023 on groups primary vaccinated in 2019-2023. Major shedding after the outbreak in 2012 ceased in 2014. Thereafter C.burnetii was only sporadically detected at low-level in 2018, 2021 and 2023. Sheep naturally exposed to C.burnetii during the outbreak in 2012 (A'13, G'13) mounted a strong and complete (PhI, PhII, IFN-γ) recall immune response after vaccination. A serological PhI+/PhII+ pattern dominated after vaccination. In contrast, since 2014 a weaker immune response (PhII-titre, IFN-γ) and a dominance of the PhI-/PhII+ pattern was observed in vaccinated gimmers. The number of serologically non-responding gimmers to vaccination increased to 25.0 % in G'16/G'17 and 40.4 % in G'19/G'20. But revaccination even three (G'15 in 2018) and four (G'19 in 2023) years after primary vaccination resulted in a strong and complete immune response. No difference of the immune response nor to more recently primary vaccinated animals (G'23 in 2023) nor to those animals that were present during the outbreak (A'13/G'13/G'14 in 2018) was observed.

Long-term control of Coxiellosis in sheep by annual primary vaccination of gimmers

Coxiella (C.) burnetii, a Gram-negative intracellular bacterium, causes Q fever in humans and Coxiellosis in animals. Ruminants are a primary source of human infection with C.burnetii. In 2013, vaccination was implemented in a sheep flock with 650 ewes associated with two outbreaks of Q fever in humans in 2008 and 2012. Only gimmers (yearlings) received two doses of a commercial C.burnetii phase I whole cell vaccine three weeks apart (primary vaccination) without any revaccination. Vaginal and nasal swabs collected shortly after lambing were tested by qPCR. Additionally, a group of non-vaccinated sentinels was serologically monitored for phase I (PhI), II (PhII) antibodies and for Interferon γ (IFN-γ) after stimulation of whole blood cells with PhII-antigen with and without an IL-10-neutralizing monoclonal antibody. In 2021, 679 sera collected in 2014-2021 were retested retrospectively with three commercial ELISA kits and one batch of an in-house PhI/PhII-ELISA. A low-level shedding of C.burnetii (<10³ mean C.burnetii/swab) was observed until 2014. In 2021 C.burnetii was detected in two animals (<10^3.1C.burnetii/swab), but vaginal swabs collected at two subsequent lambing seasons remained negative. Seroconversion of sentinels was detected until 2017. However, the retrospective analysis of sentinels in 2021 revealed additional single seropositive animals from 2018 to 2021. IFN-γ reactivity was observed during the whole study period; it peaked in 2014 and in 2018 and decreased thereafter. The sporadic detection of C.burnetii and the immune responses of sentinels suggested that a subliminal infection persisted despite vaccination. Nevertheless, vaccination of gimmers prevented the development of a major outbreak, it controlled the infection and reduced the risk of human infection.

An intercomparison study of ELISAs for the detection of porcine reproductive and respiratory syndrome virus – evaluating six conditionally dependent tests

Latent class analysis is a widely used statistical method for evaluating diagnostic tests without any gold standard. It requires the results of at least two tests applied to the same individuals. Based on the resulting response patterns, the method estimates the test accuracy and the unknown disease status for all individuals in the sample. An important assumption is the conditional independence of the tests. If tests with the same biological principle are used, the assumption is not fulfilled, which may lead to biased results. In a recent publication, we developed a method that considers the dependencies in the latent class model and estimates all parameters using frequentist methods. Here, we evaluate the practicability of the method by applying it to the results of six ELISA tests for antibodies against the porcine reproductive and respiratory syndrome (PRRS) virus in pigs that generally follow the same biological principle. First, we present different methods of identifying suitable starting values for the algorithm and apply these to the dataset and a vaccinated subgroup. We present the calculated values of the test accuracies, the estimated proportion of antibody-positive animals and the dependency structure for both datasets. Different starting values led to matching results for the entire dataset. For the vaccinated subgroup, the results were more dependent on the selected starting values. All six ELISA tests are well suited to detect antibodies against PRRS virus, whereas none of the tests had the best values for sensitivity and specificity simultaneously. The results thus show that the method used is able to determine the parameter values of conditionally dependent tests with suitable starting values. The choice of test should be based on the general fit-for-purpose concept and the population under study.

[The PRRSV-serumneutralization test detects gaps in herd immunity]

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) appears in two genotypes (EU and US), for both genotypes attenuated live-vaccines are available. A cross-sectional study in 38 Bavarian sow herds was performed to assess the level of neutralizing antibodies. Per herd 38 blood samples were collected (10 weaned piglets, 10 gilts and 6 sows of 1./2., 3J4. and 5/6. parity, respectively). Sera were tested by ELISA, serumneutralization test (SNT) against EU- and US-vaccine virus, and pooled sera were tested by real-time RT-PCR. Herds were classified by the last vaccination of sows as "Vacc EU" "Vacc US"and "nv (non-vaccinated) and by detection of PRRSV-US and vaccination of piglets were not included as variables. Sows of group (2) Vacc EU/EU- showed the highest EU-SNT-titers irrespective of parity. Groups (5) Vacc US/EU+ and (1) Vacc EU/EU+ followed in descending order. Significantly lower SNT-titers in (1) Vacc EU/EU+ were especially observed in sows of 1/2. Parity (Kruskal-Wallis, p < 0.05). Very low SNT-titers were observed in the three remaining groups (3) nv/EU+, (4) nv/EU- and (6) Vacc US/EU-. In US-vaccinated herds detection of PRRSV-EU coincided with strong ELISA-reactivity in all animal groups. In EU-vaccinated herds this was only observed for weaned piglets. Sows showed a strong ELISA-reactivity irrespective of detection of PRRSV-EU. The value of the ELISA is restricted to the certification of PRRSV-free herds. The EU-SNT reflects the level of herd immunity at least against vaccine virus; it indicates gaps in herd immunity.

The impact of Q fever-phase-specific milk serology for the diagnosis of puerperal and chronic milk shedding of C. burnetii in dairy cows

C. burnetii infection might be associated with puerperal shedding; additionally, the chronic shedding of this pathogen in milk has been observed in individual animals. A longitudinal survey was performed in an endemically infected dairy cow herd with 100 cows in order to compare phase-specific milk-serology with pathogen shedding. From March 2010 through December 2011, 870 individual milk samples from 212 cows were analysed using both quantitative (q) PCR and phase-specific antibody-ELISA. The mean milk-shedding/cow was calculated for 137 cows with > or = 3 milk samples per cow. In addition, 110 puerperal swabs were collected after August 2010. The cows yielding three successive qPCR-positive milk samples or > 3 qPCR-positive milk samples, irrespective of the sequence of positive/negative results, were classified as chronic shedders (CS). Milk shedding was observed during the entire study, but a major period of puerperal shedding occurred from February through October 2011; 35/52 swabs tested positive, whereas only 3/58 swabs collected outside this period were positive. The PhI/PhII(+)-pattern in primiparous cows (< 36 months old) was consistent with puerperal shedding in the herd, but not at the individual level. This pattern was observed in older cows, irrespective of the period of puerperal shedding. Four primiparous CS-cows showed low-level mean shedding < 100 C.b./ml milk, and the PhI-titre increased from negative or weakly positive to more than 500 at the end of the first lactation. Puerperal shedding during the second parturition was observed in three of these cows. Six multiparous CS-cows with mean shedding exceeding 100 C.b./ml milk were characterised with stable PhI-titres of > or = 500. The three available puerperal swabs tested negative. Only one multiparous CS-cow showed low-level shedding and a PhI-titre below 500 for the entire study. In conclusion, the PhI-/PhII(+)-pattern in primiparous cows indicated puerperal shedding at the herd level, and a PhI-titre > or = 500 is a suitable screening method for the detection of chronic shedding in milk.

Epidemiologically non-feasible singleton reactors at the final stage of BoHV1 eradication: serological evidence of BoHV2 cross-reactivity

A voluntary marker-independent Bovine Herpesvirus 1 (BoHV1) eradication program started in 1986; in 1998 it changed to a compulsory one. Certification of free regions in European member states is based on Article 10 of directive 64/432/EEC. According to this rule Bavaria is listed as free of BoHV1 since October 2011. Surveillance of BoHV1-free dairy cattle farms is currently performed with quarterly bulk-milk testing. Non-negative bulk-milk results must be confirmed by blood tests in cattle older than nine months. An increased regional rate of non-negative bulk-milk samples and the subsequent detection of epidemiologically non-feasible singleton BoHV1-reactors by analysis of blood were observed at the final stage of eradication in southwest Bavaria. Nineteen case farms (734 animals) defined by singleton reactors born at least two years after certification of the farms as BoHV1-free, 23 negative control (NC) farms (NC I: 321 animals) from the same region, 11 NC-farms (NC II: 423 animals) from an already-certified Article 10 region in northeast Bavaria and two BoHV1-infected farms (264 animals) were analysed using BoHV1-, BoHV2- and Feline Herpesvirus 1 (FeHV1)-neutralisation tests (NTs), and three commercially available ELISAs supplied by Idexx Laboratories, B.V., The Netherlands: the CHEKIT™ Trachitest 2nd Gen. test for milk or serum (Trachitest), Herdchek™ gB- (gB-ELISA) and Herdchek™ gE-ELISA (gE-ELISA). Significantly increased levels of BoHV2 antibodies were observed on case farms compared to NC I or II farms. Additionally, reactivity by gB-ELISA and the Trachitest was significantly increased for animals with BoHV2 neutralising antibodies. Singleton BoHV1-reactors tested negative by gE-ELISA even if an elevated cut-off of 0.95±0.05 was applied. At this cut-off, the gE-ELISA was as sensitive and specific as the gB-ELISA. Comparative titration of milk samples from seropositive animals from a BoHV1-infected dairy cattle farm and from singleton BoHV1-reactors performed in CHEKIT™ Trachitest 2nd Gen. Milk revealed that the slopes of both groups were distinct; therefore, optimised cut-offs for bulk-milk testing to exclude singleton BoHV1-reactors are proposed.