The diseases we treat in veterinary medicine can broadly be divided into two categories: congenital diseases which are present in animals from birth due to inherent genetic or chromosomal abnormalities and acquired diseases which develop after birth due to the many different harmful things we are regularly exposed to in our environment. We can further divide acquired diseases into infectious diseases and non-infectious diseases, which are also sometimes called communicable and non-communicable diseases, respectively.
Infectious diseases present a unique challenge to us as clinical veterinarians because any animal that is actively infected with a pathogen is at risk of spreading disease to other animals as well as the people who are in contact with them in the special case of zoonotic diseases. This is a very different situation from non-infectious diseases like trauma, nutritional deficiencies, degenerative disorders, toxicities, and neoplasia where the animal is sick, but generally not at risk of causing others around itself any harm. It’s our job to use our clinical knowledge to protect our patients, our clients, our staff, our families, and of course ourselves against these diseases.
There are 5 main categories of infectious diseases that affect animals and humans, including:
Did you know?
Of the 1,407 known human pathogen species, 816 (58%) are zoonotic. Among emerging or re-emerging pathogens, 130 of 177 (73%) are zoonotic.
Woolhouse MEJ, Gowtage-Sequeria S. Host range and emerging and re-emerging pathogens. Emerging Infectious Diseases. 2005;11(12):1842.
Despite the diversity of pathogens, they all follow the same fundamental principles that govern infection and transmission.
Fundamentally, we know that in order for a pathogen to persist in a population long term, it must get copies of itself out of its currently infected host and into at least one other susceptible host on average before (1) the current host dies naturally or as the direct result of infection or (2) the current host mounts an immune response or receives some kind of treatment that kills the pathogen.
The term we use to describe the ability of diseases to spread is the basic reproductive number or R0, which you may have heard discussed in the news recently with the COVID-19 outbreak. This measures the average number of secondary infections caused by a single infected individual interacting with an entirely susceptible population or, in other words, if you were to become sick with a disease, how many other people on average are you going to infect before you either recover or die.
The following table lists estimates of R0 for several common infectious diseases of animal populations.
| Disease system | R0 estimate | Reference |
|---|---|---|
| Bovine herpesvirus 1 in dairy cattle | 7 | Hage et al. (1997) |
| Paratuberculosis in dairy cattle | 11 | Roermund et al. (1999) |
| Bovine viral diarrhoea virus in dairy cattle | 4 | Moerman et al. (1993) |
| Staphylococcus aureus mastitis in dairy cattle | 0.5–8.0 | Lam et al. (1998) |
| Rabies virus in dogs | 2.0–2.5 | Coleman and Dye (1996) |
| Escherichia coli O157 in beef calves | 2.9–5.6 | Laegrid and Keen (2004) |
The interpretation for BVD, for example, would be that the average infected dairy cow manages to infect 4 other cows in the herd on average before recovery or death. In general, the more susceptible individuals that a single infected individual manages to infect (high R0 values), the faster disease will spread through the population and the harder it will be to control the disease. The primary exception to that rule is with fast-moving diseases in small populations where the pathogen can deplete the population of susceptible animals. Diseases with R0 values close to 1 will tend to remain at a steady state with the prevalence of infected individuals staying relatively constant over time. The closer R0 gets to zero, the faster disease will disappear from the population.
If we unpack the concept of R0 a little further, there are three primary things that influence its value:
1. Number of contacts an infected individual makes with other individuals during the infectious period
This will depend on the length of the infectious period and the behaviours of the infected individual during that time. Contacts can include direct contacts such as face-to-face interactions as well as indirect contacts such as through contamination of the environment. The primary reason we have the Level 4 lockdown for COVID-19 is to reduce the number of contacts that infected individuals may have, particularly those that are subclinically infected and may not be aware that they are infectious to others. It is important to also remember that being infected with a pathogen can also cause individuals to change their typical contact patterns. A classic example is rabid animals or mice infected with Toxoplasmosis that lose their inhibition to avoid contact with other animals. Or if you have signs of a respiratory infection, why it’s important to stay home so you don’t contact other people.
2. Probability that at least one of those contacts is with a susceptible individual who can become infected
This will depend on the overall level of immunity in the population as well as other underlying health issues they may have. Common factors that can influence the level of immunity are recovery from previous infections, immunization, maternal antibodies. This is why elderly and immunocompromised people in particular have been advised to stay home as much as possible during the COVID-19 outbreak.
3. Probability that transmission will successfully occur as the result of the contact
This will depend on the total pathogen load the susceptible individual is exposed to as well as the level of biosecurity and hygiene measures that are in place to prevent disease from occurring through the contact. This is why people have been advised to wear face masks, wash their hands, and avoid touching their face if they are out in public places during the COVID-19 outbreak.
Given the number of infectious diseases that are currently present in animal populations, they have clearly found very efficient ways of spreading from host to host to keep that R0 value greater than 1. We often divide the main transmission pathways into three main groups:
Direct Contact: This is probably the most efficient form of transmission and as the name suggests, occurs when a susceptible individual comes into direct physical contact with an infected individual. This can be further subdivided into:
Indirect Contact: Many of the diseases we treat can also survive outside the host for prolonged periods of time and are at risk of being spread indirectly through:
Vectorborne: These are diseases that rely on insect carriers including mosquitoes, fleas, lice, and ticks to be transmitted from host to host. A good example in New Zealand is Theileria orientalis (ikeda) which is a parasite that spreads between cattle through tick bites – a tick will latch onto an infected cow, pick up the parasite from ingesting blood, migrate over to another cow, and inject the parasite when they go to take their next blood meal.
Some diseases can infect multiple species of animals beside the primary host and these reservoir species can serve as a constant source of re-exposure. A great example of this is bovine tuberculosis (Mycobacterium bovis) in New Zealand cattle. One of the many reasons why it is a difficult pathogen to eradicate is because it is also present in possum populations which can get infected by coming in direct contact with infected cattle grazing out on pasture and then spreading to other cattle through direct contact. There is no way to treat either possums or cattle for bovine tuberculosis and, if you have been following the 1080 debate over the past few years, very difficult from both a practical and political perspective to control possum populations.
Infectious diseases can gain entry into the body through pretty much any surface or opening although the ones preferred by each disease tend to be associated with the body system they infect. For example, COVID-19 which is a respiratory pathogen primarily infects individuals through either inhalation directly into the lungs or through direct contact with the conjunctiva or mucous membranes of the nasal passages. That’s why it’s really important not to cough or sneeze all over the place when you are sick and why it’s also important to minimise touching your face with your hands.
It’s worth noting that some transmission pathways like direct contact or indirect spread through fomites are common across many different infectious diseases and so taking simple measures to prevent one disease from spreading will most likely have knock-on effects for multiple other pathogens as well. However, you still need to have a good understanding of the specific transmission risk factors for each pathogen so you can make targeted and prioritised recommendations to prevent or control disease outbreaks.
The disease pathogenesis describes the different infection, immunological, and clinical states that a susceptible animal may progress through from the time it is initially exposed to the pathogen until it either recovers from the infection or dies. This information is important to know because it tells us the timeline for when we can expect animals to have the pathogen, antibodies, and clinical signs to assist with diagnosis of the disease and also provides information about the potential impacts on animal health, welfare, and performance.
A major concern is infectious diseases where animals pass through a state of being subclinical, infected, and infectious because they are spreading disease with no obvious outward indication that they are sick. Some infectious diseases also have chronic carrier states where the animal remains infected and potentially infectious for extended periods of time because the immune system is unable to fully clear the pathogen. The latter is a common characteristic of many infectious diseases that are currently endemic in New Zealand. Diseases like bovine tuberculosis where the pathogen is maintained in wildlife populations or theileriosis where the pathogen is carried in tick populations also present unique challenges because of the logistical and social issues associated with implementing control measures in non-domestic species.
2. Diagnostics and Surveillance
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