Sero surveillance of human brucellosis and leptospirosis among veterinary personnel in Sikkim, India: A descriptive cross -sectional study

One Health (OH) recognises the close interconnection between human health, animal health and the environment. Sikkim, a small northeastern Himalayan State of India, was declared the country’s first fully organic State in 2016.¹ While this approach promotes ecological sustainability, it also involves close interaction with livestock, biodiversity and wildlife, potentially influencing zoonotic disease transmission.

Brucellosis and leptospirosis are important zoonotic infections with occupational relevance, particularly among veterinary personnel due to frequent exposure to animals, animal products and contaminated environments. Animal brucellosis has been documented in Sikkim;² however, information on human exposure is lacking. Data on leptospirosis from this region remain limited. However, the recent documentation of a fatal Leptospira wolffii infection from Sikkim further substantiates the presence of leptospirosis in this region.³ The present study aimed to determine the seroprevalence of human brucellosis and leptospirosis among veterinary personnel in Sikkim.

A descriptive cross-sectional study was conducted in 2024 among personnel working at government veterinary centres across five districts of Sikkim. All eligible veterinary, para-veterinary, and administrative staff were approached during scheduled monthly meetings, and consenting individuals were enrolled and samples collected. Technical staff included veterinarians and para-veterinarians involved in clinical and field-level animal health activities, while non-technical staff comprised administrative and support personnel without direct animal contact.

Serological testing for Brucella antibodies was performed using the Rose Bengal Test (RBT) and Standard Agglutination Test (SAT) with antigens for Brucella melitensis (SAT-M) and Brucella abortus (SAT-A) (Tulip Diagnostics Pvt. Ltd., Verna, Goa). Detection of anti-Brucella and anti-Leptospira IgG and IgM antibodies was carried out using commercial ELISA kits (NovaLisa®, NovaTec, Germany), with interpretation based on manufacturer-defined optical density cut-offs.

A total of 308 participants were enrolled, comprising 218 males (71%) and 90 females (29%). The highest proportion of participants belonged to the 30–39 yr age group (38.3%, n=118), followed by 20–29 yr (27.9%, n=86), 40–49 yr (17.5%, n=54), and ≥50 yr (16.2%, n=50). The study population consisted predominantly of technical staff (59%, 182/308), while non-technical personnel accounted for 41% (126/308).

Overall, Brucella seroprevalence was 18.8% (58/308), with positivity detected in 34 males (11.0%) and 24 females (7.8%). No statistically significant association was observed between gender and Brucella seropositivity (χ²=2.02, P=0.15). Males had higher odds of seropositivity than females, though this was also not significant [Odds ratio (OR) 1.45; 95% confidence interval (CI) 0.82–2.57]. Brucella seropositivity across age ranged from 15 (17.4%) in the 20–29 yr age group to 11 (22.0%) among those aged ≥50 yr. Comparison across age categories did not show a significant difference in seropositivity (P=0.59) ( Table I).

Among Brucella-seropositive participants, SAT-M demonstrated the highest positivity (31%), followed by SAT-A (25.9%) and RBT (24%), whereas ELISA IgG positivity was 15.5% and ELISA IgM showed the lowest positivity (5.2%). The distribution of positive results across diagnostic test patterns differed significantly (χ²=50.86, df=10, P<0.001). District-wise Brucella seropositivity ranged from 7.6% in Namchi to 29.3% in Gangtok, but this variation was not statistically significant (P=0.09) ( Table II).

Overall, Leptospira seroprevalence was 20.5% (63/308). IgM positivity (15.9%) exceeded IgG positivity (4.5%), and Equivocal results were more frequent for IgM (28/308; 9.1%) compared with IgG (13/308; 4.2%). None of the IgM-positive participants reported symptoms at the time of testing. Leptospira seropositivity was significantly higher among males (52/218; 23.9%) than females (11/90; 12.2%) (χ²= 5.30, P=0.02). Male gender was associated with higher odds of seropositivity (OR 2.25; 95% CI 1.08–4.69). Age-stratified analysis showed IgG seropositivity of 8.1% in the 20–29 yr group and 5.9% in the 30–39 yr group, with no IgG positives in participants aged ≥40 yr; the association with age was significant (P=0.045). IgM seropositivity increased with age, ranging from 9.3% in the 20–29 yr group to 25.9% in the 40–49 yr and 22.0% in the ≥50 yr groups, with a significant age association (P=0.033). District-wide variation was also marked, with seropositivity ranging from 0% in Mangan to 36.5% in Soreng, and differences across districts were statistically significant (P<0.001) ( Table II).

Globally, brucellosis risk varies considerably, with Asia accounting for nearly half of the population at risk.⁴ In India, reported human seroprevalence ranges widely, from <1% in some northern regions to over 25% in certain endemic areas.^5,6 The Brucella seroprevalence observed in the present study is comparable to reports from other parts of India. A community-based study from central Gujarat documented an overall seropositivity of 22.6% using ELISA,⁷ while a laboratory-based study reported 17.5% seropositivity by agglutination testing, with many individuals remaining asymptomatic.⁸ The absence of significant geographic clustering in the present study could suggest that Brucella exposure among veterinary personnel in Sikkim is relatively widespread rather than geographically localised. Such a distribution is consistent with the occupational nature of brucellosis, where risk is determined largely by routine animal handling, veterinary procedures, and contact with potentially infected livestock rather than place of residence alone. The absence of clear geographic clustering may also reflect frequent inter-district movement of livestock and personnel, facilitating uniform exposure across regions.⁹

Worldwide, leptospirosis is estimated to account for more than one million infections and nearly 59,000 deaths annually.¹⁰ India is recognised as a leptospirosis hotspot, although the disease remains underdiagnosed.^11,12 Studies from southern India have reported IgM ELISA positivity rates of 25–27% among clinically suspected cases,¹³ while a retrospective analysis from northern India documented an increase in seropositivity from 11.7% to 20.5% over five years.¹⁴ The seroprevalence observed in the present study is comparable to these reports, despite being derived from an occupational population rather than clinically suspected cases.

Leptospira seropositivity showed significant geographic clustering, with higher positivity in Soreng (36.5%) and Namchi (27.2%), while Gangtok, a predominantly urban district, showed low seropositivity (4.9%). This pattern is consistent with observations from other Indian settings, where leptospirosis burden is higher in rural environments.¹³ Brucella and Leptospira seropositivity was higher in technical than non-technical staff, though statistically not significant.

In summary, this study provides the first systematic evidence of exposure to Brucella and Leptospira among veterinary personnel in Sikkim. While Brucella exposure appeared relatively uniform across demographic and geographic strata, leptospirosis showed clear male predominance and significant geographic clustering. These findings highlight the relevance of integrated zoonotic surveillance among occupational groups in this ecologically distinct Himalayan setting and provide baseline data to inform future One Health–oriented surveillance efforts.