Tendencias de consumo relacionadas con el ser humano 1 El triunfo de los sentidos

con el consumo

4.1. Las nuevas tendencias de la publicidad en el siglo

4.1.1. Tendencias de consumo relacionadas con el ser humano 1 El triunfo de los sentidos

One of the bottlenecks identified in the operation of the EHQ was the follow-up work

associated with traces involving movements of people, animals or materials on to or off infected places (IPs) in the period leading up to diagnosis. In the Temuka FMD scare of 1981, in addition to

investigating the sources of garbage fed to the pigs, some 50 direct movements had to be traced (Ryan

et al., 1981). The Southland movement study, reported in Chapter 1, revealed a mean of 50 movements

on to or off farms over a two week period, with a range of 2 to 144. When secondary movements were also considered, this swelled the numbers of movements to an average of 68, range 41 to 132, over the same time period. In a large epidemic involving multiple IPs being identified daily, such as the UK 1967-8 epidemic where there were 80 new farms identified as infected per day at the height of the emergency, the number of traces to investigate would quickly place an overwhelming demand on manpower resources.

Investigation of traces involves first establishing whether or not there is a risk of FMD having been transferred with the particular movement. The traces are assigned a risk rating, ranging from very high to nil risk. The second step involves a team of telephonists confirming the movement, the fate of the moved items (in particular whether or not they have been transferred to another property) and whether or not there are any signs of disease. This process identifies “at-risk” properties, and potentially contaminated equipment and vehicles (see Figure 5.1).

Once this preliminary work has been completed, the Disease Investigation Group (DIG) has the responsibility to visit those properties deemed necessary, and take appropriate action. This may involve visiting the farm daily to check for signs of the disease, or in high risk situations, considering the farm as a “dangerous contact” and slaughtering the animals irrespective of the appearance of clinical signs (pre-emptive slaughter). The aim is to eliminate further opportunities for transmission of the disease due to pre-clinical excretion of FMD virus (Burrows, 1968a; Sellers et al., 1968; Sellers & Parker, 1969).

The assignment of risk ratings is based on a set of decision rules which is provided to the tracing group (under the manual system). In order to convert these rules into expert system rules, the concept of “conveyors” of the virus, and “encounters” between conveyors and other conveyors, or between conveyors and properties was developed.

Figure 5.1 Flow diagram of tracing process in Emergency Headquarters.

A conveyor is any moveable object, living or inanimate that can transport the virus. An encounter is any event that potentially allows the transmission of virus between conveyors, or between conveyors and properties. Properties can be thought of as immovable objects. Typically, but not necessarily, a movement off a farm involves three conveyors. For example, in the case of sheep leaving an infected farm bound for some destination, the group of sheep is one conveyor, the transport is a conveyor, and the driver of the vehicle is a third. Each of these three conveyors will probably have a different risk rating, and will need to be traced individually, as they have different opportunities for disease transmission (encounters).

The principal issues to consider in the assignment of risk are date of movement, whether on to or off the farm, type of conveyor and whether there was any direct contact with susceptible animals. The priority for tracing is reduced slightly if the movements being considered relate to a suspect premises (SP) or pre-emptive slaughter premises rather than an IP.

1. Date of movement

The date of movement relative to the infection date of the farm is critical. First of all, one needs to consider if the infection date of the farm is known. If it is, based on some established

transmission event, then all the movements can be evaluated according to this date (see Figure 5.2). All movements on to the farm (other than the source) can be dismissed. For movements off the farm, all movements after the date of infection are regarded as dangerous.

Figure 5.2 Chronology of events relating to infection of foot-and-mouth disease, and the evaluation

of the risks associated with movements on to and off the property when infection date is known.

Figure 5.3 Chronology of infection of foot-and-mouth disease, and the evaluation of risks

date based on the maximum incubation period of FMD, and a most likely date based on typical interfarm incubation periods (see Table 5.1). The veterinarian on the farm estimates the age of the oldest clinical lesions. The earliest date of infection is then the date at which clinical signs would have appeared, minus 14 days. The most likely incubation period is computed according to the species of animal(s) involved. All movements on to or off the farm are then evaluated according to these dates (see Figure 5.3). All movements on to the property at or around these dates are held under suspicion as possible sources of infection, given that the source is unknown at this juncture. For movements off the farm, the more recent movements constitute greater risks of transmission, given that the weight of virus exposure to the items has become greater as the disease has spread within the farm.

Table 5.1 FMD Incubation Periods (Garland & Donaldson, 1990).

Range Usual Airborne

Cattle 2-14 days 2-3 days 4-14 days

Sheep 2-11 days 3-5 days

Pigs 2-13 days 3-5 days

2. On to or off

This has been discussed in part (a) above in relation to date of infection of farm. In general, movements on to the IP are considered as potential sources of infection, and movements off are considered as possible transmission events.

3. Type of conveyor

Conveyors are classified and coded according to the following groups: • SAN - susceptible animals - cattle, pigs, sheep, goats, deer, other ungulates; • NSA - non-susceptible animals - dogs, cats, horses, poultry;

• PER - persons - these can include frequent animal-handlers such as farmers and farm- labourers, stock and station agents, veterinarians, farm-advisors, artificial insemination operators etc., as well as persons who are unlikely to have contact with the animals such as young children, visitors from town etc.;

• ANP - animal products - meat, wool, hides, milk, velvet etc.; • NAP - non-animal products - hay, grain etc.;

• VEH - all vehicles, as well as fomites - farm equipment such as tractors or gumboots that could mechanically transfer the virus;

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In general, susceptible animals constitute the greatest risk, given that they can actually become infected, and thereby act as multipliers of the virus, as well as remaining infectious for relatively long periods. It is possible to estimate the probability that at least one animal in the movement group is infected, using probability theory. The calculation is analogous to testing for the presence of disease by sampling a population of a certain size where we have an expected prevalence of disease (Cannon & Roe, 1982). The population size is the number of susceptible animals on the IP, the sample size is the number of animals in the movement group, and the number of diseased animals in the population is taken from the simulated number of infected animals on the farm as at the movement date, as generated by the On-Farm simulation model of FMD.

Let

N be the population size,

d be the number of infected animals in the population, n be the number of animals in the movement group

and ß be the probability that at least one animal in the movement group is infected,

(where ! = factorial.)

This formula is based on the hypergeometric distribution, and involves sampling without replacement. When the population size on the farm is large (500 or more animals), then the binomial distribution, based on sampling with replacement, is a sufficient approximation. The formula then becomes:

Where livestock holdings have received a group of susceptible animals off an IP, the estimated probability value could be used to ascertain the priority for dangerous contact slaughter (pre-emptive slaughter).

People, and presumably non-susceptible animals, can carry and exhale FMDV for up to 48 hours after exposure to the virus (Sellers et al., 1970; Sellers et al., 1971b). They therefore need to be traced for this period.

Dairy farms on a dairy tanker route are investigated by the dairy liaison officer at the EHQ, who has access to details of the various routes directly from the milk processing sites. The assignment of risk to dairy tanker movements involves consideration of whether IA restrictions have been

Figure 5.4 The risks associated with dairy tanker pickups when infected area restrictions are

imposed after the date at which FMD virus would be found in milk.

excreted into the milk.

It is known that FMDV can be secreted in milk up to 4 days prior to the onset of clinical signs (Burrows, 1968a). The On-Farm simulation model of FMD (discussed in chapter 4) estimates the date at which the milk collected from a dairy farm would be expected to contain FMDV. These outputs are used by the expert system in the assignment of risk.

The dairy industry procedures that come into operation in New Zealand following the

declaration of an infected area require dairy tankers to be fitted with special virus filters over tanker air vents, and drivers to wear waterproof clothing and carry garden-type spray equipment with disinfectant (NASS, 1990). The driver is required to disinfect himself, all hose couplings and any spilt milk. Precautions are taken both as the tanker enters a farm, and as it leaves. It is believed these procedures reduce the risk associated with tanker pickups to virtually nil (Westergaard, 1982).

Figure 5.4 shows the changing risk assessments when IA restrictions are imposed after the date FMDV would be found in the milk. Figure 5.5 shows the situation when IA precautions are brought into operation after the farm is infected, but before virus would be secreted into the milk. Figure 5.6 shows what happens when IA precautions are already in operation before the farm becomes infected.

Figure 5.5 Risk assessment associated with dairy tanker pickups when infected area restrictions

are imposed after the date of infection of a farm, but before FMD virus is excreted in the milk.

Figure 5.6 The risks associated with dairy tanker pickups when infected area restrictions are

It is important to know if there has been any direct contact with animals, especially infected animals. The risk rating of most of the conveyors is raised if contact has occurred.

Based on the above classifications, risk ratings of very high, high, medium, low, very low or nil are given to each conveyor according to the following rules:

Rule 1

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (expected infection.date+2) and conveyor.type = susceptible animal

Then possible_source and risk very high

and trace back until disease found Rule 2

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (expected infection.date+2) and conveyor.type = person with animal contact

Then possible_source and risk high

and trace back 2 days Rule 3

If infection.date and farm.source not known and conveyor on to farm

and encounter in (earliest clin.s.-14) to earliest clin.s. and conveyor.type = animal product with animal contact

Then possible_source and risk high

and trace back to origin Rule 4

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (expected infection.date+2) and conveyor.type = vehicle with animal contact

Then possible_source and risk high

and trace back to last C&D Rule 5

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (earliest clin.s.-14) or (expected infection.date+2) to earliest clin.s.

and conveyor.type = susceptible animal

Then possible_source and risk high

and trace back till disease found Rule 6

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (earliest clin.s.-14) or (expected infection.date+2) to earliest clin.s.

and conveyor.type = person with animal contact

Then possible_source and risk medium and trace back 2 days Rule 7

If infection.date and farm.source not known and conveyor on to farm

and encounter in (earliest clin.s.-14) to earliest clin.s.

and conveyor.type = (animal product or non-animal product) with no animal contact

Then possible_source and risk very low and trace back to origin

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (earliest clin.s.-14) or (expected infection.date+2) to earliest clin.s.

and conveyor.type = vehicle with animal contact

Then possible_source and risk medium

and trace back till last C&D Rule 9

If encounter before (earliest clin.s.-14)

then risk nil Rule 10

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (expected infection.date+2) and conveyor.type = non-susceptible animal or (person with no animal contact)

Then possible_source and risk low

and trace back 2 days Rule 11

If infection.date and farm.source not known and conveyor on to farm

and encounter in (earliest clin.s.-14) to earliest clin.s. and conveyor.type = hay with animal contact

Then possible_source and risk medium and trace back to origin Rule 12

If infection.date and farm.source not known and conveyor on to farm

and encounter in (earliest clin.s.-14) to earliest clin.s.

Then possible_source and risk low

and trace back to origin Rule 13

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (expected infection.date+2) and conveyor.type = vehicle with no animal contact

Then possible_source and risk low

and trace back to last C&D Rule 14

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (earliest clin.s.-14) or (expected infection.date+2) to earliest clin.s.

and conveyor.type = non-susceptible animal or (person with no animal contact)

Then possible_source and risk very low and trace back 2 days Rule 15

If infection.date and farm.source not known and conveyor on to farm

and encounter in (expected infection.date-2) to (earliest clin.s.-14) or (expected infection.date+2) to earliest clin.s.

and conveyor.type = vehicle with no animal contact

Then possible_source and risk very low Rule 16