Foot and Mouth Disease

Tuesday, 29 March 2011

Foot and Mouth Disease [FMD] is a highly contagious and usually acute virus infection of cloven hoofed animals; eg cattle, sheep, goats, pigs and camels. It does not affect humans. There are 7 distinct types in the world with 3 naturally occurring in Africa south of the Sahara.(SAT-1, SAT-2, SAT-3). In Africa buffalo act as the natural reservoir host for these types, seldom showing any clinical signs but carrying and shedding the virus intermittently over long periods of time. FMD is usually a transient, mild disease  with clinical signs that vary from almost none to difficulty in eating and walking because of severe blistering and ulceration of the mouth and tongue, as well as between the hooves, drop in milk production and rarely death. However due to its high rate of transmission it is a  controlled disease worldwide and its importance is a result of the devastating economic effects of trade restrictions rather than any direct health impact on susceptible animals. SAT types of FMD typically spread much slower and present as relatively less pathogenic than the O & Asia types that have caused such devastation in recent times in the UK, Europe and Asia.

Can we control the disease by vaccination?
As a result of high variability during replication, a characteristic that is typical of FMD, there are several groups (topotypes) within each FMD type so that close matching of vaccines with active field strains causing outbreaks are essential to achieve effective control.Vaccine-related antibodies also interfere with the easy detection of outbreaks Outbreaks are detected based on typical lesions and /or the detection of antibodies in the blood of animals collected during routine surveillance activities. The latter is what happened in KwaZulu Natal [KZN] during February 2011.

What are antibodies?
Antibodies are formed by a healthy animal/person in response to exposure to an infectious agent (this could be through infection or by using a vaccine) and consist of proteins programmed to assist in the destruction of that specific agent. We can therefore use them to indicate whether an animal had been in contact with the FMD virus some time in the past. The concentration of antibodies can also be measured as an indication of how recent the exposure was, or even as a prediction of how effective a particular vaccine will be.  Different serological tests vary in their accuracy (specificity) as well as their sensitivity so a surveillance programme typically consist of  screening tests with more specific tests on “positive” cases, e.g to distinguish between antibodies formed as a result of exposure to vaccine or field infection.

How can we detect the virus itself?
FMD viruses can be isolated in specific cell culture systems, but this is a very time consuming, expensive exercise and due to the contagious nature of the virus must be done in extremely biosecure laboratories (level 4). Only a few laboratories in the world can manage this, with Pirbright as the world reference centre for FMD viruses.
Molecular techniques including Polymerase Chain Reaction (PCR) are standardised into kit form and work only with inactivated virus or even portions of it so is a much safer option. Unfortunately the exquisite sensitivity of PCR tests will pick up tiny remnants of viral RNA for several weeks after the FMD virus infection has passed through the animal. Cross contamination between samples is another constant challenge for lab personnel working with these extremely sensitive techniques.  A positive PCR result thus indicates viral replication but cannot replace conventional virusisolation entirely.

Can other diseases cause similar clinical signs and/or lab results and therefore confuse matters?
From the above it should be clear that any unusual or unexpected lab result must be examined very carefully. The general guideline here is to interpret all FMD results from a population perspective and not the individual animal/sample, using several different tests to guide deductions. Experienced field personnel as well as further diagnostics should be involved in any outbreak investigation to distinguish between oral lesions caused by mechanical injury (thorns etc), and viral infections such as: Bovine Viral Diarrhoea Virus, Infectious Bovine Rhinotracheïtis virus, Vesicular Stomatitis Virus, Lumpy Skin Disease Virus, Bluetongue Virus and Malignant Catarrhal Fever as these are often indistinguishable on visual examination only.  

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