For full Northumberland Report 1968 see http://www.warmwell.com/northum.html
(b) Characteristics of Foot-and-Mouth Disease Virus
224. Strains of foot-and-mouth disease virus from the 1967/1968 epidemic were studied in susceptible animals under experimental conditions. In brief, the experiments indicated that:
(a) the virus could be excreted by cattle for five days, by sheep for five days and by pigs for ten days before clinical signs of the disease appeared;
(b) the virus was excreted in milk at an earlier date in a higher concen-centration than had previously been thought; it was also more difficult to destroy than laboratory experiments had shown;
(c) the strain of the virus isolated during the 1967/1968 epidemic was particularly stable under certain laboratory conditions.
225. Speculation arose as to whether these characteristics contributed to the serious nature of the 1967/1968 epidemic, Since then, however, other strains of foot-and-mouth disease virus have been similarly examined and it is clear that the early excretion shown by the 1967/1968 strain is common to a number of strains isolated both in this country and abroad. It was found that strains can nevertheless show considerable variation in such properties as stability under laboratory conditions and it may be that such characteristics will provide useful markers in future in identifying the origin of strains responsible for outbreaks. 93
226. It has been known for some time that foot-and-mouth disease virus is excreted from infected animals before the appearance of clinical signs and the slaughter policy as practised by the Ministry of Agriculture over the years has been based on this knowledge. The precise periods of excretion were, however, not known and neither was it realised that they could be of such long duration.
227. Experiments have now provided precise information on the amount of virus excreted as an aerosol by cattle, sheep and pigs infected with foot-and-mouth disease virus.
Pigs were found to excrete a much larger amount of the virus than either cattle or sheep.
Further the maximum recovery of virus from cattle and pigs occurred before the vesicles had ruptured and in sheep even before the appearance of lesions. The lesions in sheep when they did develop were often not obvious and were thus difficult to detect; this could be a very significant factor in the epidemiological pattern.
In summarising the epidemiological position the Animal Virus Research Institute state that in general “sheep act as maintenance hosts, pigs as amplifiers and cattle as indicators”.
(c) Windborne Transmission of Foot-and Mouth Disease Virus
228. Naturally it has not been possible to carry out experiments in the field to investigate the transmission of foot-and-mouth disease virus by wind because of the obvious risks involved; however experiments in the laboratory have shown that the virus in the aerosol form is very fragile when humidity is low and persists for long periods when the humidity is high.
There was evidence from the 1967/1968 epidemic to indicate that rain-bearing winds played an important part in spreading the disease. Furthermore retrospective studies have been made of previous outbreaks of the disease in relation to the meteorological conditions existing at the time and these support the important part played by wind and humidity in the spread of infection.
For example in one of the studies made of an outbreak which occurred in East Keswick (between Wetherby and Leeds) in October, 1960, meteorological experts, without any knowledge of the pattern of spread, on the basis of the wind tracks and humidity as well as other weather conditions prevailing at the time, indicated where the outbreaks should have occurred. Their indications were correct but of equal importance, they were also able to indicate correctly those areas which remained free of the disease.
It is considered that the virus can be carried by wind, if the conditions are ideal, over distances of more than sixty miles.
This approach will be valuable in dealing with future outbreaks particularly in indicating where to look for secondary outbreaks associated with wind spread once a primary outbreak is reported.
229. In the 1967/1968 epidemic, very few outbreaks of foot-and-mouth disease occurred to the south and west of the initial outbreak, that is into the wind, compared with outbreaks to the north and east of Oswestry. We commented in Part I of our Report that this could not be accounted for, at least over a ten mile range, either by a difference in concentration of holdings or of concentration in livestock. Since then further analysis of livestock densities in sectors emanating from the initial outbreak over a wider range has been undertaken and the results are recorded in Map IV. This, read in conjunction with Map V, provides further support for the theory that wind was a significant factor in spread during the first month of the epidemic. 94
230. Recent work has shown that disposal of slurry from animal houses by spreading it on farm land can give rise to aerosols which, under suitable conditions of wind and moisture, would distribute micro-organisms over long distances. Although the investigation was not done with slurry containing foot-and-mouth diseases virus, it is nevertheless clear that the virus, if present, could become airborne and travel over considerable distances.