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vol.23 número2TRIAGEM SOROLÓGICA DE DOENÇAS INFECCIOSAS: II. ASSOCIAÇÃO ENTRE PREVALÊNCIA E NÍVEIS DE POSITIVIDADE AO ELISAINTOXICAÇÃO CRÔNICA POR Pteridium aquilinum (L) Kuhn (Polypodiaceae) EM BOVINOS índice de autoresíndice de assuntospesquisa de artigos
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Ciência Rural

versão impressa ISSN 0103-8478

Cienc. Rural vol.23 no.2 Santa Maria maio/ago. 1993

http://dx.doi.org/10.1590/S0103-84781993000200015 

SEROLOGICAL SCREENING FOR INFECTOUS CATTLE DISEASES.

III. CHOICE OF SENTINEL ANIMALS.

 

LEVANTAMENTO SOROLÓGICO DE DOENÇAS INFECCIOSAS DE BOVINOS.

III. ESCOLHA DE ANIMAIS SENTINELAS.

 

José Alfonso Barajas-Rojas1 Hans Riemann2 Charles Franti3

 

 

SUMMARY

A herd of Cattle (Holstein-Zebu crosses) was screened bimonthly for two years by ELISA for IgG antibodies to infectious disease agents. The herd was composed of 3 age groups: Young animals ( < 4 months of age), developing animals (4-36 months of age) and producing animals ( > 36 months of age). Ali young animals received colostrum within two hours after birth. The overall consistency of ELISA results, i. e agreement between two consecutive tests was about the same for the three age groups and was lowest for infections of intermediate prevalence (40-50%). The interpretation of changes in seroprevalence is most straight forward for young animals. Young animals positive at their first test reflect the immune status of their mothers while young animals born seronegative but seroconverting reflect the incidence rate. It is concluded that young animals when tested repeatedly are the best sentinel animals.

Key words: ELISA, seroconversion, young, sentinels, infectious diseases.

 

RESUMO

Um rebanho bovino (cruza Holandês-Zebu) foi amostrado a cada dois meses durante dois anos pela prova do ELISA para a detecção de anticorpos para doenças infecciosas. O rebanho era composto de três grupos etários: animais jovens (menos de '4 meses de idade); animais em desenvolvimento (4-36 meses de idade); e animais em produção (mais de 36 meses de idade). Todos os animais jovens receberam colostro dentro das duas primeiras horas após o nascimento.A consistência total dos resultados do teste ELISA, isto é, a concordância entre dois testes consecutivos foi praticamente a mesma para os três grupos e foi menor para infecções com prevalência intermediária (40-50%). A interpretação de alterações de soroprevalência em animais jovens é simples. Animais jovens positivos ao primeiro teste refletem o "status imunológico" de suas mães, enquanto que, animais jovens que nascem soronegativos mas soroconvertem, refletem a taxa de incidência. Conclui-se que animais jovens testados repetidamente são os melhores animais sentinelas.

Palavras-chave: ELISA, soroconversão, jovens, sentinelas, doenças infecciosas.

 

 

INTRODUCTION

Serological screening for antibodies to disease agents can be a very tool in determining the health of a herd of some other population. There are however difficulties in interpretation of test results because, among other things, the test results are influenced by pregnancy status age, season and the prevalence of the infection in the herd. (BARAJAS-ROJAS et al, 1993a, 1993b, 1993c. In the present study we evaluate the usefulness of selecting a certain age group in this case young animals, as sentinels for infections occurring in a herd.

 

MATERIAL AND METHODS

The study was conducted in a cattle herd at the Center for Research, Teaching and Extension in Tropical Livestock (Centro de Investigación y Extensión en Ganadería Trópical-ClEEGT) of the Faculty of Veterinary Medicine of the National Autonomous University of México. The Center is located in the tropics of México, in the North-Central part of the state of Veracruz. Blood samples were collected bimonthly and he sera subjected to ELISA for infectious disease agents as described earlier (BARAJAS-ROJAS, 1993a). Three age groups of animals were tested: Young animals ( < 4 months of age), developing (4-36 months of age) and producing animals ( > 36 months of age).

 

RESULTS

The consistency of serological results was evaluated by comparing consecutive tests (2 months apart). The tests may agree or a positive test may turn negative and vice versa. Positivity and negativity was defined by the cut off point for the ELISA values. The number of test reversals expressed as percent of total number of tests performed are shown in table I and the absolute number of reversals from negative to positive and from positive to negative are shown in table II. The ratio of reversals positive to negative and negative to positive over the two year testing period is a measure of trends in prevalence and agrees fairly well with earlier estimates (BARAJAS-ROJAS et al, 1993a).

 

 

 

 

The percent of reversals in both directions are plotted for all animals against percent prevalence in figure 1 which suggests that the frequency of reversals is a function of prevalence of positive tests. In other words the highest agreement between neighbor tests is found when the prevalence is either high or low. Since most of prevalences for young animals fall below 35% and there are rather few in the mid range (Figure 2) there are indications that this group of animals yield the most consistent results especially when compared to producing (adult) cattle (Figure 3).

 

 

 

 

 

 

Figures 4-7 show how well incidence respectively prevalence at birth, in young animals predict prevalences among developing animals (Figures 4 and 5) and among producing animals (Figures 6 and 7).

 

 

 

 

 

 

 

 

Table III shows number of young animals (not number of tests) that are seropositive shortly after birth or seroconvert within a period of 4 months. Table IV is based on the same data but allows for more easy comparisons. Prevalence rate at birth represents the immune status of the mothers and is thus a measure of period prevalence of infection among the mothers with the length of the period undefined. Four months incidence durings the beginning of fife represents incidence. The relative incidence among young suggest different degrees of activity of agents during the time period involved.

 

 

 

 

 

DISCUSSION

It is not surprising that young animals turn out to be the best sentinels of herd infection; they exhibit more consistent test results and better discrimination between reactors and non reactors as shown previously (BARAJAS-ROJAS et al, 1993c). The value of young animals as sentinels of course depends on husbandry practices. Under the conditions of the present study there were close contacts between the young animals and the producing animals. A complete separation of calves from the adult population and feeding of pasteurized colostrum or milk replacer might change the situation drastically.

To maximize the usefulness of young animals as sentinels they should be bled and tested monthly or bimonthly until about half a year of age.

 

ACKNOWLEDGEMENT

We apreciate the assistance in sample and data collections of Dr. Eusebio Garcia Neria and Dr. Rebeca Acosta and personnel of the Centro de Investigación Enseñanza y Extensión en Ganadería Tropical-CIEEGT of the Faculty of Veterinary Medicine. National Autonomous University of Mexico City.

 

REFERENCES

BAAJAS-ROJAS, J.A., RIEMANN, H.P.. FRANTI, F.E. Application of enzyme linked immunosorbent assay (ELISA) for epidemiological studies of diseases of livestock in the tropics of Mexico. Rev Sci Tech Off Int Epiz, v. 12 (1), 1993a. In Press.         [ Links ]

BARAJAS-ROJAS, J.A., RIEMANN, H.P., FRANTI, C.E. Serological screening for infectious cattle diseases I. Influence of reproductive status. Ciência Rural v. 23, n. 1, p. 69-72, 1993b         [ Links ]

BARAJAS-ROJAS, J.A, RIEMANN, H.P., FRANTI, C.E. Serological screening for infectious Cattle diseases II. Association between prevelence and level of ELISA response. Ciência Rural, v. 23, n. 2, p. 193-196, 1993c.         [ Links ]

 

 

1Doctor in Veterinary Medicine. MSc, PhD. Prof Titular Dept of Virology and Immunology, Faculty of Veterinary Medicine, National Autonomous University of México, México City 04510, México.

2Doctor in Veterinary Medicine, PhD, Professor Dept of Epidemiology and Preventive Medicine, University of California, Davis, CA 95616, USA.

3Statistician PhD, Professor Dept of Epidemiology and Preventive Medicine, University of California.

Recebido para publicação em 21.10.92. Aprovado para publicação em 10.03.92.

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