One of the key technologies in our armoury is molecular diagnostics because this group of technologies is able to determine infection earlier than any other technique by detecting the pathogen’s own genetic material - even before the body has started to mount an immune response. This is crucial for cutting the time a human or animal is infectious for before it can be isolated. This reduces the number of others it can infect, the so-called replication rate, which is the key factor changing an isolated disease problem into an epidemic.
There is currently a significant outbreak of equine flu among race horses causing major race meetings to be abandoned and making a significant impact on training programs and animal welfare (although the disease is seldom fatal). This community is particularly vulnerable to epidemic disease because thoroughbred racehorses travel extensively across the world to race and are drawn from a narrow gene pool meaning there is little natural immunity in the population once a pathogen develops an effective method of attack. Good data exits on the dynamics of equine flu outbreaks [1] and using that data it’s possible to construct a simple model of infection spread and test the effect of reducing the time an animal spends in the infectious state before being quarantined.
Using estimates of spreading dynamics and time in the infectious state (determined by PCR tests taken during an equine flu outbreak in 2003) we modelled the expected evolution of an epidemic. We simulated a population of 20 000 horses (approximately the number of race horses in the UK) for the case where the average horse is infectious for 10 days, 7 days (the estimate for the 2003 outbreak which was managed using the best available testing at the time) vs. 5 days. Five days is a time which could easily be achieved by regular testing with a low cost, hand held, PCR system and the extent of epidemic suppression is dramatic. Still more dramatic is the price of leaving an infected animal in the population even longer (in this case 10 days) which is plausible when relying on symptom recognition by busy stable staff.
Simulation showing how the number of infected horses evolves during an epidemic depending how quickly they are quarantined
The data presented is a gross simplification because the industry changes its animal husbandry practices substantially once an epidemic is obvious, but these changes are both expensive and disruptive. A regular testing program, or testing that starts much earlier in an epidemic’s lifecycle, would reduce severity dramatically. Such practices require the availability of low cost diagnostics which are easy to use making it acceptable to start testing even when evidence of an outbreak is too scant to justify major disruption. Recent developments in PCR make this feasible and we look forward to new techniques in both human and animal care to make epidemic disease outbreaks ever rarer, even as our intrinsic vulnerability to them becomes greater.
References
01. Joseph Hughes et. al. PLOS Pathogens Vol. 8, Issue 12, Dec 2012
