DIVA strategy in foot-and-mouth disease sero-surveillance with special reference to Haryana
Anu Malik and Swati Dahiya
Department of Veterinary Microbiology, Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar-125004, Haryana
In India, foot-and-mouth disease (FMD) is one of the most important viral diseases of domestic animals affecting large ruminants mainly cattle, buffalo, pigs, sheep, goat or wildlife population. FMD is considered to be a major threat to animal health, dairy sector and livestock economy leading to financial losses to farmers as there is lesser milk production and reduction in working capacity of affected animals. Also, farmers bear with the burden of economic hardship due to increased expenditures on medication, quarantine, isolation, shelter, etc. The affected animal shows symptoms of high fever, blister like vesicles in mouth, teats and inter-digital space of foot, excessive salivation with reduced feed intake and drop in milk production. Pigs show presence of vesicles on the snout and lameness as characteristic sign of FMD infection.
FMD is caused by FMD virus (FMDV) having RNA as genetic material, classified in the Genus Aphthovirus within the Family Picornaviridae. FMD is highly contagious and transmitted by respiratory route through aerosols and close contact. The disease is characterised by high morbidity which may reach upto 100% in susceptible livestock population with lesser mortality in adults while high mortality is reported in young calves due to necrotic myocarditis. Owing to safeguard the animal health and economy of our country, Government of India has initiated the FMD Control Programme (FMD-CP) since 2003-2004 under the 10th Five Year Plan in 54 districts of India later covering the whole country. Now the National Animal Disease Control Programme (NADCP) has been initiated by the Govt. of India to control FMD with vaccination since 2019. The major component of these control programmes included mass vaccination of all cattle and buffaloes at six monthly intervals against FMDV, public awareness campaigns and sero-monitoring of disease.
In country like India where control of FMD through slaughter policy is not possible, control by vaccination in the only practically adopted measure. However, vaccination raises the critical issue of differentiation of infected from vaccinated animals (DIVA). Hence, active surveillance of the disease on random basis is required for effective control. Vaccination with inactivated vaccines (devoid of non-structural proteins, NSP of FMDV) elicits antibody response against the structural proteins of the virus similar to the infection. The sero-monitoring of disease i.e., pre- and post-vaccination antibody monitoring is being done in the country by a very sensitive technique called solid phase competitive enzyme linked immunosorbent assay (SPC-ELISA) developed by Scientists of ICAR-Directorate of FMD (ICAR-DFMD). This test detects the antibodies against the structural proteins of FMDV. Apart from development of antibodies to structural proteins of FMDV, the antibodies against NSPs of FMDV are also induced during natural infection but not after NSP free FMDV vaccination. As per the OIE guidelines sero-surveillance should be performed by an assay capable of differentiating infected from vaccinated animals (DIVA strategy) in the regions which are adopting vaccination to control FMD. Thus, the differentiation between the FMDV infected and vaccinated animals is being done in the labs using NSP based ELISA (3AB3 NSP-ELISA) developed by Scientists of ICAR-DFMD.
The best tool for sero-surveillance of virus circulation is identification of antibodies against NSPs of FMDV in the serum sample of cattle/ buffalo/ sheep/ goat/ pigs which are considered to be good indicators of FMDV infection for discrimination of infected animals from vaccinated ones. Several recombinant NSPs based ELISA for FMDV diagnosis namely 2B, 2C, 3A, 3AB, 3B, 3ABC and 3D are available for use in DIVA. Of these, 2C, 3A, 3ABC or its derivatives such as 3AB3 based NSP-ELISA tests are more promising. In India, DIVA ELISAs have been developed employing 3AB3, 3ABC, truncated 2C (2Ct) NSPs and the assays have been validated for field application. Of these, the 3AB3 NSP-ELISA is being used countrywide for sero- surveillance. The NSP-ELISA as developed and standardized using Escherichia coli expressed recombinant r3AB3 antigen by the ICAR-DFMD, Mukteswar, Uttarakhand is being used for screening of serum samples of animals. r3AB3 NSP-ELISA particularly, have emerged to be the most reliable indicator and appear to produce conclusive evidence of previous infection, whether or not the animals have been vaccinated. It has been demonstrated that the antibody response against 3AB3 could be used as a reliable serological marker for DIVA.
DIVA test is normally recommended to be conducted in young animals of age group 6-12 months as adults being multiple vaccinated with anti-FMDV vaccines may result in development of antibodies against NSPs also, resulting in false positive DIVA test. As reported by ICAR-DFMD in its Annual Report 2019, in India, 30,413 bovine serum samples collected at random from various parts of the country were tested using r3AB3 NSP-ELISA (DIVA) for assessing the prevalence of NSP-antibody (NSP-Ab) positive animals, which is an indicator of FMD virus exposure regardless of vaccination status and virus circulation. The test revealed overall seropositivity in 20.8% animals, which is slightly higher than the previous year’s average of 19.98%. The sero-surveillance against FMD is being carried out by 27 Regional and Collaborating Centres across the country and ICAR-International Centre for FMD (ICAR-ICFMD) at Bhubaneswar.
The FMDV circulation was monitored in cattle and buffaloes of Haryana by the testing the serum samples collected by field Veterinarians/ para staff of Department of Animal Husbandry & Dairying, Govt. of Haryana. The samples were tested by the Scientists of Regional Research Centre on FMD, Department of Veterinary Microbiology, LUVAS, Hisar for detecting the antibodies against 3AB3 NSPs of FMDV. A decreasing trend has been observed in the DIVA reactivity of animals of Haryana i.e., 8.1%, 7.1% and 4.4% during 2018-19, 2019-20 and 2020, respectively. This DIVA data (virus circulation) has a direct correlation with reduction in FMD outbreaks (Nil) and higher percentage of animals having protective antibody titres (SPC-ELISA), thus indicating that the FMDV activity in Haryana has decreased after FMD+HS combined vaccination since 2019. Thus, it is of utmost importance that the inactivated FMDV vaccines being used under NADCP throughout the country are certified as “NSP Free” by the manufacturers for successful adoption of NSP-ELISA based DIVA strategy for FMD sero-surveillance which in turn aids in the assessment of effectiveness of vaccination based FMD control programme.
Several nations have gained FMD-free status through stamping out policy and/ or systematic mass vaccination programme and enjoy economic benefits from international trade in animals and livestock products. As control of FMD through eradication by slaughter policy is not practicable in our country due to ethical and socio-economic reasons, therefore, the only way out to control FMD is by routine vaccination to achieve protective antibody response against FMD. Use of DIVA strategy is crucial in identifying the potential disease free zones (DFZs) with vaccination in the country. It is expected that with the successful implementation of NADCP (earlier FMD-CP) across the whole country, India will be having DFZs with vaccination in phased manner followed by FMD free country.
