Clinical variation in foot and mouth disease: pigs

Understanding the molecular epidemiology of foot-and-mouth-disease virus

The use of molecular epidemiology is an important tool in understanding and consequently controlling FMDV. In this review I will present basic information about the disease, needed to perform molecular epidemiology. I will give a short introduction to the history and impact of foot-and-mouth disease, clinical picture, infection route, subclinical and persistent infections, general aspects of the transmission of FMDV, serotype-specific epidemiological characteristics, field epidemiology of FMDV, evolution and molecular epidemiology of FMDV. This is followed by two chapters describing the molecular epidemiology of foot-and-mouth disease in global surveillance and molecular epidemiology of foot-and-mouth disease in outbreak investigation.

Serological survey for foot-and-mouth disease virus in wildlife in eastern Africa and estimation of test parameters of a nonstructural protein enzyme-linked immunosorbent assay for buffalo

In this study we estimate the seroprevalence of foot-and-mouth disease virus (FMDV) in wildlife from eastern and central Africa. Sera were sourced from between 1994 and 2002 from a rinderpest surveillance program. Our study compared a nonstructural protein enzyme-linked immunosorbent assay (Cedi test) with a virus neutralization test. The study shows that there is only a low seroprevalence of FMDV in sampled nonbuffalo species. The seroprevalence in the Cape buffalo was high for SAT2, lower for SAT1, and lowest for SAT3. As the SAT2 serotype was most prevalent, the Cedi test largely reflected the occurrence of SAT2-positive animals. The results also suggest that SAT2 became dominant around 1998, with a large increase in seroprevalence. The sensitivity and specificity of the Cedi test were estimated by comparison to the combined virus neutralization test results from all three SAT tests. A Bayesian implementation of the Hui-Walter latent class model was used to estimate the test parameters. The model permits estimation in the absence of a gold standard test. The final model, using noninformative priors and assuming conditional independence of test performance, estimated Cedi test sensitivity at 87.7% and specificity at 87.3%. These estimates are similar to those for domestic bovines; they suggest that the Cedi test is a useful tool for screening buffalo for infection with the various serotypes of FMDV.

Review of identification and traceability legislation for pigs in Australia

OBJECTIVE

To assess Australia's capability to trace pig movements in the event of an exotic disease outbreak by highlighting the commonalities and inconsistencies between the various state and territory legislations in defining how pig producers are located; their pigs are identified; and whether movement records are required post-farm gate.

PROCEDURE

A review of the identification and traceability legislation applicable to pigs in Australia was undertaken over a 6 month period. The appropriate legislative Acts and Regulations were downloaded from the Australasian Legal Information Institute internet site (http://www.austlii.edu.au/) and reviewed.

RESULTS

The Australian pig industry currently uses the branding method (tattooing) to identify pigs for sale or slaughter, with each state responsible for its own pig identification and movement control systems. Areas of concern identified included inconsistencies with the minimum weight or age of pigs that require identification; discrepancies between methods of tattoo registration and shortfalls in documentation for recording pig movements.

CONCLUSION

Our findings highlight the flaws in Australia's current state legislative Acts and Regulations for identifying pigs and tracking their movements, which compromise the ability of jurisdictions to meet the endorsed National Performance Standards. Improvements in these areas will enhance security to the pig and other livestock industries in the event of future exotic disease outbreaks.

Quantification of foot and mouth disease virus excretion and transmission within groups of lambs with and without vaccination

Sheep are well known to be susceptible for foot and mouth disease virus (FMDV), but it is unknown whether the infection can spread and persist in a sheep population. We therefore quantified virus transmission by performing experiments with FMD virus strain O/NET/2001 in groups of lambs. We used six groups of four lambs each, in which half of each group was inoculated and the other half was contact-exposed. To quantify the effectiveness of a single vaccination we also included six groups of four lambs each, vaccinated with O Manisa vaccine, 14 days prior to inoculation. Oropharyngeal fluid was obtained with a swab (OPF-swab), and blood samples were collected daily to determine virus excretion and serological response. We calculated the transmission rate beta (the number of new infections per day per infectious animal), and the reproduction ratio R (the number of secondary infections caused by one infectious individual). The mean daily virus excretion and the number of days the lambs excreted virus in the OPF differed significantly between vaccinated and non-vaccinated lambs. The transmission rate beta in the unvaccinated groups was 0.105 (95% confidence limit 0.044; 0.253) per day. The duration of the infectious period (T) was 21.11 (95% confidence limit 10.6; 42.1) days. With the final size of infection we estimated the reproduction ratio R in the non-vaccinated groups to be 1.14 (0.3; 3.3), and in vaccinated groups 0.22 (0.01; 1.78). Virus transmission as quantified by the final size did not differ statistically between the vaccinated and the non-vaccinated groups of lambs. In conclusion, FMDV seems able to persist in a sheep population, although the reproduction ratio was only slightly larger than one. As a consequence, vaccination might only have a small effect on transmission, in spite of the fact that virus excretion and duration of virus excretion is significantly reduced after vaccination.

The effect of vaccination on foot and mouth disease virus transmission among dairy cows

The aim of this study was to quantify the effect of a single vaccination of dairy cows on foot and mouth disease virus (FMDV) transmission. To estimate if vaccination could significantly reduce virus transmission, we performed two replicates of a transmission experiment with one group of vaccinated and one group of non-vaccinated dairy cows (ten animals per group). Half of both groups were intranasally inoculated, with FMDV field isolate O/NET2001, and housed with the other half of the group (contact-exposed cows) from the next day onwards. Virus transmission was quantified by estimating the reproduction ratio R, which is the average number of secondary cases caused by one infectious animal. In the non-vaccinated groups all cows became infected and Rnv was significantly above 1. In the vaccinated groups infection was demonstrated in three inoculated cows, and no transmission was observed (Rv was 0, not significantly below 1). Transmission was significantly reduced in the groups of vaccinated cows when compared to the groups of unvaccinated cows. Our findings indicate that after a single vaccination cows are protected against infection of FMD and that most likely no virus transmission will occur within a vaccinated herd.

Predictions for the timing and use of culling or vaccination during a foot-and-mouth disease epidemic

First-fortnight incidence (FFI) is a modelling parameter that can be used to predict both the prevalence and duration of a foot-and-mouth disease (FMD) epidemic at regional and national levels. With an indication of how long an epidemic may last by the end of week two, it becomes possible to estimate whether vaccination would be economically viable from the start of an epidemic. Where FFI indicates that an epidemic is unlikely to last for as long as an export ban on agricultural produce, it may be inappropriate to implement a policy of 'vaccination to live'. Alternatively where FFI indicates that an epidemic will equal or exceed the ban length, then the benefits of vaccination should be considered at an early stage, during or after the first fortnight. Since blanket vaccination of the national or regional herds and flocks would be both costly and heighten the risk of producing carrier animals, targetting vaccination through risk assessment becomes useful.

Foot-and-Mouth Disease: Host Range and Pathogenesis

In this chapter the host range of foot-and-mouth disease (FMD) under natural and experimental conditions is reviewed. The routes and sites of infection, incubation periods and clinical and pathological findings are described and highlighted in relation to progress in understanding the pathogenesis of FMD.

Determinants of early foot-and-mouth disease virus dynamics in pigs

This paper provides a quantitative description of the early infectious process in pigs experimentally infected with foot-and-mouth disease virus (FMDV), obtained by dose-dependent, time course studies of viral load in serum. Pigs were inoculated by the intravenous or intradermal/subcutaneous route with FMDV and housed together in groups or individually. The effects of dose, inoculation route and exposure intensity on the replication of FMDV in vivo and the development of disease were studied. It was shown that the higher the dose, the shorter was the time to the start of active viraemia and to the onset of clinical signs. Exposure intensity and housing conditions influenced the viral dynamics of FMDV. Increasing the exposure intensity, by increasing the number of infected pigs housed together, had the effect of synchronizing the infection and reducing the variance in the start of active viraemia. Increasing the number of pigs housed together also increased the interaction between the pigs and the activity of individual pigs, which had the effect of shortening the time to the onset of clinical signs such as vesicle formation. Intradermal inoculation was more effective than intravenous inoculation for transmitting FMDV to pigs, resulting in shorter times to the start of active viraemia and in higher clinical scores.

A review of foot-and-mouth disease with special consideration for the clinical and epidemiological factors relevant to predictive modelling of the disease

Modelling the epidemiology of foot-and-mouth disease (FMD) has been undertaken since the early 1970s. We review here clinical factors and modelling procedures that have been used in the past, differentiating between those that have proved to be more relevant in controlling FMD epidemics, and those that have showed less significance. During the 2001 UK FMD epidemic, many previously developed FMD models were available for consideration and use. Accurate epidemiological models can become useful tools for determining relevant control policies for different scenarios and, conversely, inaccurate models may become an abuse for disease control. Inaccuracy presents two opposing difficulties. Firstly, too much control (in terms of animal slaughter for 2001) would negatively impact the farming community for many subsequent years, whilst too little control would permit an epidemic to persist. Accuracy however, presents the optimal permutation of control measures that could be implemented for a given set of conditions, and is a prerequisite to boosting public confidence in the use of epidemiological models for future epidemics.

Comparison of two 3ABC enzyme-linked immunosorbent assays for diagnosis of multiple-serotype foot-and-mouth disease in a cattle population in an area of endemicity

The development of a serological test for foot-and-mouth disease virus (FMDV) which is quick and easy to use, which can identify all seven serotypes, and which can differentiate vaccinated from convalescing or potential virus carriers would be a major advance in the epidemiological toolkit for FMDV. The nonstructural polyprotein 3ABC has recently been proposed as such an antigen, and a number of diagnostic tests are being developed. This paper evaluates the performance of two FMDV tests for antibodies to nonstructural proteins in an unvaccinated cattle population from a region of Cameroon with endemic multiple-serotype FMD. The CHEKIT-FMD-3ABC bo-ov (CHEKIT) enzyme-linked immunosorbent assay (ELISA) (Bommeli Diagnostics/Intervet) is a commercially available test that was compared with a competitive 3ABC ELISA (C-ELISA) developed in Denmark. The tests were compared with the virus neutralization test as the "gold standard." Diagnostic sensitivity and specificity were examined over a range of test cutoffs by using receiver operating characteristic curves, which allowed comparison of the overall performance of each test. The results indicated that the CHEKIT ELISA kit was 23% sensitive and 98% specific and the Danish C-ELISA was 71% sensitive and 90% specific at the recommended cutoff. These results have important implications if the tests are to be used to screen herds or individual cattle in surveillance programs, at border crossings for import-export clearance, or following emergency vaccination in an outbreak situation.

Foot-and-mouth disease

Foot-and-mouth disease (FMD) is a highly contagious disease of cloven-hoofed animals. The disease was initially described in the 16th century and was the first animal pathogen identified as a virus. Recent FMD outbreaks in developed countries and their significant economic impact have increased the concern of governments worldwide. This review describes the reemergence of FMD in developed countries that had been disease free for many years and the effect that this has had on disease control strategies. The etiologic agent, FMD virus (FMDV), a member of the Picornaviridae family, is examined in detail at the genetic, structural, and biochemical levels and in terms of its antigenic diversity. The virus replication cycle, including virus-receptor interactions as well as unique aspects of virus translation and shutoff of host macromolecular synthesis, is discussed. This information has been the basis for the development of improved protocols to rapidly identify disease outbreaks, to differentiate vaccinated from infected animals, and to begin to identify and test novel vaccine candidates. Furthermore, this knowledge, coupled with the ability to manipulate FMDV genomes at the molecular level, has provided the framework for examination of disease pathogenesis and the development of a more complete understanding of the virus and host factors involved.

The pathogenesis and diagnosis of foot-and-mouth disease

The pathogenesis of foot-and-mouth disease (FMD) is reviewed, taking account of knowledge gained from field and experimental studies and embracing investigations at the level of the virus, the cell, the organ, the whole animal and the herd or flock. The review also addresses the immune response and the carrier state in FMD. Progress made in understanding the pathogenesis of the disease is highlighted in relation to developments in diagnosis and methods of control.