A Palyam serogroup virus of the
Kasba virus is transmitted by haematophagous arthropod vectors such as
The lack of baseline data on the seroprevalence of Kasba virus in small ruminants in South Korea has resulted in a poor overall understanding of the epidemiology of the diseases which it causes. The objective of this study was to estimate the seroprevalence of antibodies to Kasba virus in sheep and goats in South Korea. In addition, individual, management, and regional risk factors associated with seropositivity were evaluated, including flock size, history of reproductive problems, vector control, presence of ruminant farms, lakes or rice paddies within a 1 km radius, type of land use, and location. A retrospective serosurvey was also conducted to determine the presence of Kasba virus antibodies in archived sheep and goat sera and to improve our understanding of the epidemiological situation in South Korea.
Seroprevalence of Kasba virus in sheep and goats in South Korea (2011–2012)
Flock-level seroprevalence | Animal-level seroprevalence | |||||
---|---|---|---|---|---|---|
Province latitude–longitude | Positive* | Tested | % | Positive* | Tested | % |
Incheon (N) 36°55′–37°58′ (E) 124°36′–126°47′ | 1 | 10 | 10.0 | 2 | 25 | 8.0 |
Ulsan (N) 35°19′–35°43′ (E) 128°58′–129°27′ | 1 | 12 | 8.3 | 3 | 27 | 11.1 |
Gyeonggi (N) 36°53′–38°17′ (E) 126°22′–127°51′ | 10 | 105 | 9.5 | 31 | 281 | 11.0 |
Gangwon (N) 38°09′–39°25′ (E) 126°46′–128°22′ | 1 | 38 | 2.6 | 2 | 84 | 2.4 |
Chungbuk (N) 37°15′–36°00′ (E) 127°16′–128°38′ | 2 | 34 | 5.9 | 6 | 91 | 6.6 |
Chungnam (N) 35°58′–37°03′ (E) 125°32′–127°38′ | 1 | 60 | 1.7 | 2 | 87 | 2.3 |
Jeonbuk (N) 35°18′–36°09′ (E) 125°58′–127°54′ | 3 | 33 | 9.1 | 8 | 93 | 8.6 |
Jeonnam (N) 33°54′–35°30′ (E) 125°04′–127°54′ | 2 | 44 | 4.5 | 30 | 105 | 28.6 |
Gyeongbuk (N) 35°34′–37°33′ (E) 127°48′–131°52′ | 2 | 43 | 4.7 | 8 | 85 | 9.4 |
Gyeongnam (N) 34°39′–35°54′ (E) 127°35′–129°28′ | 3 | 44 | 6.8 | 10 | 87 | 11.5 |
Jeju (N) 33°06′–34°00′ (E) 126°08′–126°58′ | 2 | 18 | 11.1 | 13 | 38 | 34.2 |
Total (N) 33°06′–39°25′ (E) 124°36′–131°52′ | 28 | 441 | 6.3 | 115 | 1003 | 11.5 |
* – number of seropositive flocks or animals (individuals)
In this study, the following individual exposure variables were evaluated in the univariate analyses: animal species; population size of the flock; flock structure; history of reproductive problems; vector control; presence of neighbouring ruminant farms, lakes, or rice paddies; land use; and geography (the region of the country in which the farm was located). For the study of the risk factors associated with Kasba virus infection, the epidemiological questionnaire variables were first submitted to an exploratory data analysis using the chi-square test (X2, univariate). These factors were considered variables with a significance level greater than or equal to 95% (P ≤ 0.05). The SPSS Statistics package in version 25 (IBM Corp., Armonk, NY, USA) was used for all data analyses.
At the national level between 2011 and 2012, 28 out of 441 sheep and goat flocks (6.3%, 95% CI: 4.4–9.0%) and 115 out of 1003 animals (11.5%, 95% CI: 9.6–13.6%) were positive for neutralising antibodies against Kasba virus, as shown in Table 1 and Fig. 1. The agreement in serological status between animals sampled within the same flock as measured by the intraclass correlation coefficient was high at 0.65. This finding indicates a tendency toward most animals in any particular flock being seropositive or most being seronegative.
Geographical regions and the seroprevalence of Kasba virus in South Korea
In the univariate analysis presented in Table 2, the management risk factor attributes showed that a history of reproductive problems on the farm was associated with an increase in the probability of seropositivity (odds ratio (OR) = 1.708, 95% CI :1.116–2.615, P value = 0.013). Preventive measures, such as the routine application of insecticide on the farm, decreased the odds of seropositivity for Kasba virus (OR = 0.611, 95% CI : 0.388–0.961, P = 0.031). No significant differences in risk were observed for the presence of ruminant farms, lakes or rice paddies within a 1 km radius. The regional risk factor attributes showed that the location of the farm being near an agricultural area was a significant risk factor (OR = 2.021, 95% CI : 1.098–3.718, P = 0.022). Additionally, associations between the probability of positivity and the region of the country where the farm was located were identified. We observed significantly increased odds of positivity in southern provinces compared with northern provinces (OR = 1.773, 95% CI : 1.184–2.654, P = 0.005) and in western provinces compared with eastern provinces (OR = 1.884, 95% CI : 1.211–2.931, P = 0.004) at the individual level. The retrospective serosurvey results showed that the virus was widely distributed in sheep and goats in South Korea, with seroprevalences ranging from 6.8% in 2004 to 13.7% in 2008 at the animal level (Table 3). Information on the distribution of neutralising antibody titres against Kasba virus in sera from sheep and goats in South Korea is presented in Table 3.
Univariate analysis of Kasba virus exposure variables relative to seropositivity outcomes in sheep and goats
Univariate analysis | ||||||
---|---|---|---|---|---|---|
Variable | Positive (n=139) | Negative (n=776) | OR (95% CI) | P value | OR (95% CI) | P value |
Animal species | ||||||
Goat | 60 | 450 | Reference | |||
Sheep | 55 | 438 | 0.942 (0.638–1.389) | 0.762 | ||
Age class | ||||||
Juvenile | 20 | 242 | Reference | 0.575 (0.334–0.989) | 0.044 | |
Subadult | 44 | 291 | 1.830 | 0.031 | 1.052 | 0.816 |
(1.050–3.188) | (0.683–1.621) | |||||
Adult | 51 | 355 | 1.738 (1.011–2.990) | 0.044 | Reference | |
Population sizes of the flocks | ||||||
<6 head | 28 | 280 | Reference | 0.707 (0.428–1.166) | 0.173 | |
6-10 head | 43 | 297 | 1.448 (0.875–2.395) | 0.148 | 1.023 (0.653–1.604) | 0.920 |
>10 head | 44 | 311 | 1.415 (0.858–2.334) | 0.173 | Reference | |
Flock structure | ||||||
Goats and/or sheep alone | 66 | 502 | Reference | |||
With other ruminants | 49 | 386 | 0.966 (0.652–1.430) | 0.861 | ||
Reproductive problems | ||||||
No | 79 | 701 | Reference | |||
Yes | 36 | 187 | 1.708 (1.116–2.615) | 0.013 | ||
Vector control | ||||||
No | 88 | 591 | Reference | |||
Yes | 27 | 297 | 0.611 | 0.031 | ||
(0.388–0.961) | ||||||
Presence of ruminant farms | ||||||
within a 1-km radius | ||||||
No | 60 | 461 | Reference | |||
Yes | 55 | 427 | 0.990 (0.671–1.460) | 0.958 | ||
Presence of lakes or rice paddies within | ||||||
a 1-km radius | ||||||
No | 63 | 512 | Reference | |||
Yes | 52 | 376 | 1.124 (0.761–1.661) | 0.558 | ||
Land use | ||||||
Urban | 15 | 199 | Reference | 0.540 (0.297–0.981) | 0.041 | |
Agricultural | 54 | 387 | 2.021 (1.098–3.718) | 0.022 | Reference | |
Woodland and seminatural | 46 | 302 | 1.851 (1.019–3.363) | 0.041 | 1.092 (0.716–1.663) | 0.683 |
Location of the farm | ||||||
in South Korea | ||||||
Northern | 41 | 440 | Reference | |||
Southern | 74 | 448 | 1.773 (1.184–2.654) | 0.005 | ||
Location of the farm in South Korea | ||||||
Eastern | 86 | 543 | Reference | |||
Western | 29 | 345 | 1.884 (1.211–2.931) | 0.004 |
Retrospective serosurvey of sheep and goats sampled between 2003 and 2008 for antibodies against Kasba virus
Province | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
+ | Tested | + | Tested | + | Tested | + | Tested | + | Tested | + | Tested | |
Gyeonggi | 7 | 159 | 14 | 124 | 9 | 73 | 5 | 79 | 13 | 129 | 8 | 148 |
Gangwon | 10 | 130 | 9 | 79 | 8 | 129 | 2 | 95 | 8 | 55 | 3 | 87 |
Chungbuk | 8 | 83 | 15 | 84 | 8 | 94 | 5 | 114 | 9 | 126 | 2 | 120 |
Chungnam | 6 | 57 | 10 | 86 | 3 | 51 | 9 | 120 | 5 | 175 | 6 | 88 |
Jeonbuk | 8 | 79 | 14 | 156 | 3 | 179 | 16 | 81 | 16 | 150 | 10 | 119 |
Jeonnam | 14 | 91 | 15 | 108 | 9 | 135 | 10 | 139 | 24 | 73 | 9 | 108 |
Gyeongbuk | 6 | 58 | 12 | 94 | 16 | 103 | 13 | 96 | 9 | 89 | 16 | 89 |
Gyeongnam | 10 | 94 | 13 | 64 | 19 | 71 | 5 | 94 | 18 | 72 | 6 | 134 |
Jeju | 19 | 112 | 19 | 90 | 9 | 72 | 9 | 79 | 16 | 79 | 6 | 79 |
Subtotal | 88 | 863 | 121 | 885 | 84 | 907 | 74 | 897 | 118 | 948 | 66 | 972 |
To the best of our knowledge, no national epidemiological study of Kasba virus in sheep has been performed in South Korea; however, Yang
We found that neutralising antibodies against Kasba virus were present in 11.5% of the sheep and goat populations and on 6.3% of sheep and goat farms. Kasba virus seroprevalence was estimated to be 15.2%, indicating that exposure to Kasba virus is prevalent among sheep and goats in South Korea, despite the paucity of reported Kasba virus outbreaks. This finding is probably due to the often unapparent clinical signs and underdiagnosis of Kasba virus when abortions and other reproductive problems occur in cattle (10).
The serological prevalence of Kasba virus infection in sheep and goats was significantly different between flocks with history of reproductive problems and those without, flocks farmed where vector control was used and where it was not, animals reared on land of different use types, and animals from different geographical locations. The reasons for these differences are unclear. Similar studies or publications that would be useful for the interpretation of these results are limited. One possibility is that specific ecological and climatic factors might promote increased exposure to infected reservoirs in tropical rainforest regions. Alternatively, viral persistence or transmissibility may be greater in regions with elevated temperatures and rainfall. In addition, where one of the aetiological agents that had caused reproductive problems on farms in the past could have been Kasba virus, some difference may derive from it not having been recognised there as the cause of the disorder. Based on our results, vector control was a significant protective factor; therefore, control of vectors during the summer would seem to need to be better implemented in provinces with elevated seroprevalences.
D’Aguilar virus (DAGV), a member of the serogroup of Palyam viruses, is antigenically related to and serologically cross-reacts with Kasba virus. This virus has been repeatedly isolated in eastern Asia (25), and a previous study also reported that Kasba virus showed a cross-reaction with DAGV in cross-neutralization tests (19). Unfortunately, because the distribution of DAGV was not studied and DAGV has not been isolated in South Korea to date, differentiation between DAGV and Kasba virus based on cross-reactivity could not be evaluated in this study. This lack of differentiation is a limitation of the present study because the DAGV seroprevalence in this region remains unclear.
The determination of seropositivity rates often leads to an understanding of virus circulation dynamics and is useful in the formulation of disease control measures. The results of this seroprevalence study may serve as a basis for future epidemiological studies on Kasba virus infection in South Korea. For seroepidemiological studies to investigate virus exposure, immune responses to vaccination and those to infection must be distinguished. In the present study, we were unable to distinguish between vaccinated and unvaccinated animals because no diagnostic method is available to differentiate vaccinated and naturally infected animals. Therefore, in this study, farms with history of vaccination were excluded from the sampling frame to avoid the detection of antibodies due to vaccine-induced immunity. If a diagnostic test to differentiate these animals is developed and available, more detailed epidemiological information will be obtained by conducting serological surveys that include samples from vaccinated animals. The continuation of this research is crucial for deeper understanding of the epidemiology of this disease in the country and progress toward a full epidemiological assessment.