It’s not a topic that initially sparks excitement or wonder, or any positive feelings at all. But all living things have had to deal with infections at some point in time, and their ubiquity has earned them a special place in both science and medicine. The mechanisms by which infections survive and spread between hosts can be ingenious, if not a bit terrifying. In order to deal with infections, and protect against them, we need to understand how they work.
Simply put, a clinical infection is where someone is infected and looks sick, where a subclinical infection is where someone has an infection, but does not demonstrate any obvious outward signs. Think Typhoid Mary, who was one of the first non-symptomatic, but still infectious, typhoid cases known. She unknowingly passed this infection on to some members of the families for whom she was cooking, resulting in their deaths. The problem with having a subclinical infection is that you may not know you are infected with anything, so you could continue acting as normal, meeting up with people and transmitting the infection on to the unknowing masses, as Mary did.
In a livestock/wildlife setting, this can be a problem when farm animals come into contact with wildlife that do not look sick. Exposure of domestic animals to subclinical infections in wildlife can lead to losses in productivity of the domestic animals (as is the case when cattle contract foot-and-mouth disease) or to losses due to animal fatalities (as with cattle with bovine tuberculosis).
Wildlife vs. livestock
Domestic livestock are bred for anthropogenic purposes. We want them to grow meat, or wool, or babies, and so on. This means we are selecting certain animals to breed based on certain characteristics that we see as favourable. However, many genes don’t act as independent entities. DNA is messy. Bits work with other bits.
So, while you may think that the characteristics you have chosen to become more pronounced work alone, the genetic information responsible for that trait could also be connected to an undesirable trait that ends up showing as well. This artificial manipulation of certain characteristics does not happen in free-ranging wildlife. Instead, natural selection prevails: wild animals with weak immune systems tend to die off before reaching reproductive maturity, so their genes do not get passed on within a population. While domestic animals with artificially selected traits may be surviving in the environment in which they were bred, they may not fare so well when they are moved to a new environment, or into contact with new animals. Under novel circumstances, these animals may not be able to cope with stressors or unfamiliar attacks on their immune system. This is evident when cattle and wild bovines, like African buffalo, make contact.
My infection of choice
To better understand the risk of disease transmission between buffalo and cattle, I joined a team of veterinarians and disease ecologists working with a herd of African buffalo in Kruger National Park (KNP), South Africa. The specific focus of this project was originally Foot and Mouth Disease (FMD), but the scope of the work has expanded considerably over the course of two-and-a-half years of data collection.
From 2014, we sampled the same cohort of buffalo every three months, collecting a range of measurements (body length, body girth, pregnancy status, horn width, body condition) and samples (including blood, faeces, milk, hair), as well as testing for a number of other diseases and infections (bovine tuberculosis, brucellosis, bovine respiratory syncytial virus, to list a few).
While this project has produced valuable data to answer countless different questions about disease transmission, my research specifically focuses on two closely related species of bacteria called Anaplasma marginale and A. centrale. These two species are mainly spread between animals (also referred to as ‘hosts’) by ticks. Anaplasma marginale and A. centrale invade the red blood cells of host animals, multiplying many times within the cell until it bursts (called lyses) and infects the next cell. These infections can cause the host to become anaemic through the destruction of blood cells. To add insult to injury, the host’s immune system deals with the infection by destroying its own red blood cells to stop the bacteria spreading to all cells, which can also cause the host to become anaemic. In other words, their method of dealing with this infection can be a double-edged sword.
If A. marginale infects cattle, it can cause the disease called anaplasmosis. Symptoms include weight loss, decreased milk production and anaemia, with more severe cases resulting in temporary infertility and abortions. The most severe cases result in death, which occurs in 29-49% of infections in cattle over the age of two. Cattle under the age of two tend to be better protected against the severity of the infection, and become persistently infected, meaning the infection cycles back throughout their lifetime.
This phenomenon can be considered a natural vaccination of sorts, as it results in them being protected against the severity of another A. marginale infection. In cattle, A. centrale appears to only cause a mild infection, if any signs or symptoms of the disease are present at all. Interestingly, it is can be used as a live vaccine against A. marginale infection in countries like Israel, Australia and South Africa. This practice does not prevent any A. marginale infection from occurring, but appears to limit the severity of that infection in vaccinated subjects to some extent.
African buffalo, however, appear to be only subclinically infected with Anaplasma spp. A recent study showed that in animals with an Anaplasma spp. infection, those with better body condition were more likely to be infected with A. centrale than A. marginale. This indicates that there are some differences in host health depending on which infectious agent is found within the host, but how much of an impact, we currently do not know.
This is where my research fits in, as I am looking at this subclinical infection in a wildlife host. More specifically, I am evaluating the health and fitness effects of A. marginale and A. centrale infection in African buffalo. I am doing this by using quantitative polymerase chain reaction, or qPCR, which entails repeatedly heating and cooling DNA mixtures.
The devil is in the DNA details
All methods of PCR are used to indicate the presence or absence of your DNA of interest. In my case, I looked at the DNA code already available for A. marginale or of A. centrale, and chose a certain fragment for each species that was different from all others. That way, if I observed the fragment in my DNA mixture, I could be sure that the associated species of bacteria was present in the given sample. I also put a fluorescent dye in the mix that attaches only to my chosen fragments of DNA. If energy is sent into the mixture, the dye will fluoresce only if it is attached to that chosen fragment of DNA. Otherwise, it will not fluoresce and no DNA presence will be recorded.
What differentiates qPCR from other methods of PCR is that it measures the level of fluorescence after each cycle of heating and cooling of the PCR reaction. To simplify things, think of DNA as a ladder, where one side and half of each rung is complementary to the other side. If you broke the ladder down through the middle of all the rungs and threw away one side, you would be able to work out what that half was, as it would just be the complement of the side you have. When you heat the DNA, it becomes ‘denatured’, which means that the two sides of your DNA fragment split apart from each other. When you put the samples into a machine to run PCRs, you also put a bunch of other ingredients in that are the building blocks to make a new half of the DNA fragment. During the cooling period of the cycle, this new half is built again, for both sides of the original DNA fragment. In theory, each cycle causes a breaking and rebuilding of each half of the DNA fragment, meaning that each cycle causes a doubling of that DNA fragment in the sample. This is what is used to work out how much of the DNA is present in the sample. You also put in a known concentration of DNA against which to compare your unknown samples to work out how much of that DNA fragment is present, which lets you know how many copies of your species of interest is present in the sample.
While qPCRs can be completed relatively quickly, getting the heating, cooling and timing conditions can be (and has been) time consuming, as was just the sheer number of samples to run. However, the large sample size does mean that the conclusions made at the end will be more robust. And that’s where I am at currently. I am conducting statistical analyses to determine how A. marginale and A. centrale affects the host, and if we can work out if any of these factors (either host characteristics like body condition, age, other infections present, or environmental factors, like tick burden or season) can be used to predict the likelihood of becoming infected.
These analyses may have applications in a management setting, whether at the interface of wildlife and domestic stock (like around KNP), or even for management within a game reserve holding buffalo. Wildlife like African buffalo have been given a bad (and arguably unfair) reputation for spreading disease amongst their domesticated counterparts without showing the signs of illness themselves, a bit like a bovine Typhoid Mary.
Improving our understanding and management of diseases in wildlife like buffalo may redeem their reputation, allowing us to admire them for the magnificent animals they are without despairing over any damage they may or may not cause through spreading infections to domesticated livestock. This research is also a fantastic opportunity to look at how a subclinical infection affects a wildlife host over a number of seasons and in different conditions, and should be able to provide us with a bit of background in this understudied field.
So stay tuned to see what we find out and how we might apply those findings in a management setting. The topic of infections may still not fill you with excitement or wonder, but perhaps this glimpse into the inner workings of disease ecology sparks just a bit of curiosity around the subject… it certainly has for me!