The occurrence and distribution of foot and mouth disease virus types in India from 1971 to 1985

Transcription

1 Rev. sci. tech. Off. int. Epiz., 1988, 7 (2), The occurrence and distribution of foot and mouth disease virus types in India from 1971 to 1985 D. RAMARAO and B.U. RAO * Summary: Data on 8,099 foot and mouth disease virus strains identified in India between 1971 and 1985 are analysed. An index of relative frequency is defined to examine the virus type-specific activity in a particular year. Using this index, it was observed that different Indian states consistently maintained their respective ranks for each virus type. Thus it was possible to delineate the relative dominance of each type, even though all four types (O, A, C and Asia 1) had been reported throughout the country. The interval between two peak years of activity varied from 28.3 months for type O to 42.9 months for type C with values of 32 months for Asia 1 and 33 months for type A. Although these differences were not statistically significant, the interaction between type and group of States was significant. KEYWORDS: Aphthovirus - Disease statistics - Epidemiology - India - Serotypes. The occurrence and distribution of foot and mouth disease (FMD) virus typer in India has been reported by Seetharaman and Datt (4) and Datt et al. (1, 2). Presented here are the results of an analysis of reported data on 8,099 FMD virus strains identified from all over India during the period , by examining about 15,000 specimens collected from field outbreaks. MATERIALS AND METHODS Data on virus types The field samples from disease outbreaks were collected, preserved and sent to the appropriate Regional Laboratory of All India Coordinated Research Project for Epidemiological Studies on Foot and Mouth Disease by scientists and staff of the project Disease Investigation staff, field veterinarians and livestock inspectors of the Veterinary Departments of the State Governments. Depending upon their quality, specimens were processed on receipt, either by a direct complement fixation test or by inoculating cell culture or suckling mice, and the virus isolates were identified by complement fixation test. * Southern Regional Station, Indian Veterinary Research Institute, Bangalore, India.

2 358 The work was carried out in eighteen Epidemiological Units, eight Regional Laboratories, a Central Referral Laboratory under technical direction and control of the Project Coordinator, with periodical reviews and recommendations by workshops. As such, although the promptness of collection and the quality of field samples might have varied, uniformity was largely maintained, with the same standard reagents and techniques adopted in processing of the samples, both for virus isolation and type identification. Experimental method To analyse the type-specific activity of a particular FMD virus type i. in a year j. a relative frequency index (RFI) Iij was defined as the number of specimens n j, identified as type i in the jth year per 100 total specimens n.j recognised as one of the four virus types that year. The year j was considered as a peak year of activity (PYA) for virus type i, if Iij was significantly greater than l ij-1 by one tailed t-test at a 5% level. The time elapsed between two PYA for type i was taken as the virus type activity cycle (VTAC) of that type. RESULTS AND DISCUSSION In India as a whole, of the four FMD virus types O, A, C, Asia 1 known to occur, every alternate (51.2%) virus identified in the past fifteen years proved to be O (Table I), while Asia 1 was identified about once in five strains (22.3%). Whereas the virus type A was recorded once in every six (16.2%), type C occurred once in ten strains (10.3%). The quinquennial totals for the four virus types differed significantly (x 2 = df: 6; P< 0.001) indicating that the pattern of relative preponderance of the different types varied from one five year period to another, as indeed from one year to another. The virus types had an ostensible regional preference. Figure 1 (a, b, c and d) illustrates the geographic distribution of RFI for the four types during the period studied. Of the 8,099 virus strains identified, 8,009 were from 18 States, each having an Epidemiological Unit of the Project, and 90 from 10 smaller States and Union Territories. No information on virus types was available from three smaller Union Territories. The 18 States had significant rank correlations (at 1% level) when their RFI were examined for the past 5, 10 and 15 years, indicating that, over these years, the States had maintained their relative ranks (by RFI) consistently, in respect of all four serotypes. All four virus types have been recognised in each of the 18 States during the past 15 years. Even then, as can be seen from Fig. la, type O had emerged as the dominant type in the 8 States of Jammu and Kashmir, Punjab, Himachal Pradesh, Haryana, Uttar Pradesh, Bihar, Sikkim and Kerala, with an overall RFI of This was in sharp contrast to the RFI of 44.5 in the other ten States (t= 19.7; df:8007; p< 0.001). Similarly, type Asia 1 was remarkably dominant in Madhya Pradesh, Gujarat and Karnataka, with an overall RFI of At the other end of the scale only 5.3% of types were Asia 1 in Kerala and Tamil Nadu (Fig. lb). Type A (Fig. lc) had a special predilection for Andhra Pradesh (RFI: 32.8) and Karnataka (RFI: 24.2).

3 359 TABLE I FMD virus types identified in India in and their Relative Frequency Indexes (RFI) Year/Period O Asia 1 A C. Total No. RFI No. RFI No. RFI No. RFI * Apr.-Dec * , * * * * , * * , , * * 1, * * * * * , , , , , ,099 * Peak year of activity. Source: Compiled from the Annual Reports of the Project Coordinator, All India Coordinated Research Project on Foot and Mouth Disease, Indian Council of Agricultural Research for the years 1971 to Virus type C was characterised by a low profile even though it was responsible for several outbreaks in 1981 and Tamil Nadu (RFI: 24.2), Kerala (RFI: 17.9) and Maharastra (RFI: 17.2) were the regions in which it was most prevalent during the past 15 years (Fig. Id). Furthermore, occurrence was sporadic in Himachal Pradesh, Punjab, Uttar Pradesh, Rajasthan, Madhya Pradesh and Bihar. In these regions type C was recognised only 78 times (RFI: 2.9) in a total of 2,705 virus strains identified. To examine the VTAC for different types, the peak year of activity for all of India (Table I) might be misleading in view of the wide differences between States noted above. To form homogeneous regions, contiguous states were examined for significant or high correlations between Iy for at least two virus types. Accordingly, the States of Himachal Pradesh, Haryana, Punjab and Rajasthan were grouped

4 360 FIG. 1 Geographic distribution of Relative Frequency Indexes for the four FMD virus types identified in India,

5 361

6 362 together as North-Western Region; Orissa, West Bengal and Assam, the Eastern Region; Gujarat and Maharastra, the Western Region and Karnataka, Kerala, Tamil Nadu and Andhra Pradesh, the Southern Region. Uttar Pradesh alone represented the Northern Region. The dominance of type O, highest in North-West (Fig. 2) dropped slightly in the North, and declined further towards the South, while at the same time, the prevalence of type C increased. FIG. 2 FMD virus type differentials in five Regions, In all five Regions, 55 cycles of virus type activity (VTAC) were noted. The average VTAC length for type O was calculated to be 28.3 months based on 18 VTAC and for Asia 1 it was 31.9 months (15 VTAC). Virus type A had an average of 33.0 months (14 VTAC); while type C, the longest average length of 42.9 months based on 8 VTAC. This conformed with the general experience of type O being most frequent and type C the least. However, when these differences were examined by analysis of variance, it was noted that the type x region interaction was significant at 1% level. When this was taken into account, the types failed to differ significantly in VTAC length. ACKNOWLEDGEMENTS The authors are grateful to the successive Project Coordinators, and all the scientists of the All India Coordinated Research Project for Epidemiological Studies

7 363 on Foot and Mouth Disease, and the Indian Council of Agricultural Research, for the secondary data used in this paper. * * * FRÉQUENCE D'APPARITION ET RÉPARTITION DES TYPES DU VIRUS APHTEUX EN INDE DE 1971 A D. Ramarao et B.U. Rao. Résumé : Les auteurs analysent les données relatives à souches du virus aphteux identifiées en Inde de 1971 à Ils définissent un indice de fréquence relative pour étudier, année par année, l'activité spécifique des types du virus. L'utilisation de cet indice permet de constater que les différents Etats de l'inde ont régulièrement conservé leurs rangs respectifs concernant chaque type de virus. Ainsi, la dominance relative de chaque type a pu être définie, bien que les quatre types O, A, C et Asia 1 aient été signalés dans tout le pays. L'intervalle entre deux années d'activité maximale a varié de 28,3 mois pour le type O à 42,9 mois pour le type C, avec un intervalle de 32 mois pour le type Asia 1 et de 33 mois pour le type A. Ces différences ne sont pas statistiquement significatives ; par contre, on observe une interaction significative entre types de virus et groupes d'etats. MOTS-CLÉS : Aphthovirus - Epidémiologie - Inde - Sérotypes - Statistiques de maladie. OCURRENCIA Y DISTRIBUCIÓN DE LOS TIPOS DE VIRUS AFTOSO EN LA INDIA DE 1971 A D. Ramarao y B.U. Rao. Resumen: Los autores analizan los datos referentes a cepas del virus aftoso, identificadas en la India entre 1971 y 1985, y definen un índice de frecuencia relativa para estudiar, año por año, la actividad específica de los tipos del virus. La aplicación de dicho índice permite comprobar que los diferentes Estados de la India han conservado regularmente sus respectivas posiciones en la clasificación respecto a cada'tipo de virus. De este modo, ha podido definirse la relativa dominancia de cada uno, a pesar de haberse señalado los cuatro tipos (O, A, C y Asia 1) en todo el país. El intervalo entre dos años de actividad máxima ha oscilado entre 28,3 meses para el tipo O y 42,9 meses para el tipo C, con un intervalo de 32 meses para el tipo Asia 1 y 33 meses para el tipo A. Estas diferencias no son significativas estadísticamente, pero, en cambio, se observa una interacción que sí es significativa entre tipos de virus y grupos de Estados. PALABRAS CLAVE: Aftovirus - Epidemiología - Estadísticas de enfermedad - India - Serotipos. * * *

8 364 REFERENCES 1. DATT N.S., RAO B.U. & SHARMA G.L. (1968). - The incidence and distribution of the different types of foot and mouth disease virus in India. Bull. Off. int. Epiz., 69 (1-2), DATT N.S., NATARAJAN C, RAO B.U., NEGI B.S. & GOEL A.C. (1969). - Continuation of studies on the incidence and distribution of different types of foot and mouth disease virus in India. Bull. Off. int. Epiz., 71 (9-10), Project Coordinator. All India Coordinated Research Project for Epidemiological Studies on Foot and Mouth Disease: Indian Council of Agriculture Research: Annual Reports for the years 1971 to SEETHARAMAN C. & DATT N.S. (1951). - Frequency of occurrence of different strains of foot and mouth disease virus in India. Indian J. vet. Sci., 21,