My post earlier this week about culling considerations for control of disease outbreaks in animals generated some good discussion and ideas (and a surprisingly small number of “you’re an idiot” emails). Although that post was focused primarily on culling in response to influenza outbreaks, we can also think about it little more broadly and how it would apply to other disease outbreaks. Australia has a really nice related guidance document that I hadn’t seen before, and it was great to receive it from a couple of people: Risk based assessment of disease control options for rare and valuable animals. It’s a good read for anyone interested in the topic.
So, I’ve expanded the assessment table I created with a few more rows, with some more explanation below:

Likelihood animals/group have been exposed
If most of the group has been exposed, that’s bad. If it’s possible or likely that there’s only been limited exposure, containment is more feasible.
For example, if a farm brought in a new animal and kept it isolated after arrival (a good but underused routine infection control measure), or it was kept with a small, separate group, then the rest of the farm/group might not have any realistic risk of exposure if that new animal turns out to be a disease carrier. It takes work to sort out what the risk is depending on how long the new animal has been there, the quality of the infection control practices, management issues and physical proximity to the other animals, but it can be done (and may well be worth the work in some cases).
Rapid and effective vaccination
Vaccination is often “too little, too late” for a facility-level outbreak. However, some vaccines (usually modified live vaccines) can produce very rapid and effective protection. Vaccination of this kind could also be considered for adjacent facilities to help avoid the need for culling (in some situations culling may be applied to all susceptible animals within a certain radius of an affected property, not just on the one property). If layered onto a situation where there hasn’t likely been widespread exposure of other animals on the farm or adjacent farms, rapid and effective vaccination of these other animals makes managing the outbreak without a call a more reasonable and feasible option.
Effective post-exposure prophylaxis
Although not very common, there are some situations where we might have an effective post-exposure treatment that can prevent infection in exposed animals. If we can do that (without causing more risk or damage), then it decreases the likelihood that culling is necessary.
International trade and regulatory restrictions
As I said before, I hate to see us acting based on regulations that might not be up-to-date, logical or fit the situation. However, I’m practical enough to know that doesn’t always matter. While we want to make sure regulations are practical and reasonable (and updating regulations is typically a very slow process), it doesn’t change the fact that they can be important. Trade issues can be even more important if there’s a potential loss of a multi-million or multi-billion dollar export market because of the presence of a disease.
Immunity after infection
This factor is perhaps a bit more obscure, but it’s worth considering. It comes down to how likely it is that the pathogen can be effectively, rapidly and safely contained and eliminated. If infected animals have solid post-infection immunity, the disease will typically burn through a group and die out as the number of susceptible animals decreases. If animals don’t have good post-infection immunity, they can get infected again (and again, and again), which can lead to ongoing cycling of the infection in a group. That makes it much harder to contain.
Testing options
Accurate, rapid and accessible testing for the pathogen in question is important. Without it, we’re flying blind: we have less confidence in our understanding of the disease status of individuals, which makes it harder to implement control measures and determine when an animal or facility is no longer a risk.
Is the species known or reasonable suspected to be able to transmit the pathogen?
This overlaps with some of the rows in the first version of the culling assessment table (i.e. transmission risk to humans/domestic animals/wildlife). However, it probably deserves its own row since since it’s a key factor and “transmission risk” covers a broader range of considerations. Being infected is not always the same as being infectious. It’s possible for animals to be infected but to pose little or no risk of transmitting the pathogen. If they are infected but are unlikely to be shedding the pathogen, the risk they pose to others is very low, as is the value of culling such animals.
Risk to people involved in culling
Culling may mean a lot of people having a lot of contact with infected animals, which is a significant risk, especially if it’s a zoonotic disease. Culling can be done with lots of infection control measures to mitigate the risk, but there’s always still some degree of risk to consider. With some animals there may also be physical risks/dangers to people, depending on the mechanism used for culling, which also ties into the last factor on this list.
Humane aspects of culling
Culling needs to be done effectively and humanely. That’s always a controversial topic, but it needs to be part of the equation. Human health risks will always win out over animal health (and usually welfare) concerns, but we need to make sure both sides are fairly assessed. The flip side of this is also humane aspects of not culling, i.e. what is the likelihood of animal suffering with highly infectious and very severe / fatal diseases (like highly pathogenic avian influenza in poultry) if the animals are not culled.
There are probably other things we could add to the list so the table might get even bigger. That makes it messier, but it’s important to consider all the different (and sometimes competing) factors to make sure we make the best decisions.