In order to generate accurate estimates of the economic and social impacts of an infectious disease, there are certain pieces of information that must be collected about both the pathogen and the host population. Some of this information can be obtained from work conducted in other countries, while other pieces require New Zealand-specific data due to differences in how dairy, beef, and sheep production systems are structured. The following provides a brief summary of key areas that are important to capture for priority infectious diseases.
The purpose is to create a standardised framework for capturing information on the epidemiology, economics, and social impacts of infectious diseases to assist decision-makers in determining how best to prioritise the limited resources available for disease control.
Having a good understanding of how farm systems work is important for understanding the potential ways that infectious disease can impact farms as well as for evaluating the logistics and feasibility of integrating different biosecurity recommendations into farm management calendars. This is also called the demographic structure of the livestock industries. At the national level, this includes having accurate information on the number, size, and location of each farm in New Zealand that contain livestock species of interest and then tracking how the populations change dynamically over time through the births, movements, and deaths of animals. At the farm level, this includes having accurate information on how the herd is structured into management groups, how animals progress through those different groups from birth to slaughter or death, dates of routine husbandry and management events, and key performance indicators that are used to track herd performance. There are various methods that can be used to capture and display this information in a standardised form
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.
In order to estimate the burdens of an infectious disease, we need to have accurate information on how many farms and animals are currently affected within a livestock industry.
Prevalence describes the percentage of individuals in the population that are currently infected with the pathogen while incidence describes the rate at which currently susceptible individuals become infected with the pathogen over time. This is important information to be able to assess the current disease burdens for infected farms and the risk of susceptible farms experiencing new disease outbreaks. The term seroprevalence is used to describe the percentage of animals that have antibodies against a disease. This usually overestimates the true prevalence of animals that are infected with the disease because it may include animals that have recovered from infections as well as those in chronic carrier states depending on the disease pathogenesis. The two primary means of obtaining this data are through prospective cross-sectional surveys of farms or abattoirs or through analysis of national laboratory test accession data. The latter often overestimates disease prevalence since many diagnostic tests are only performed when there is a high index of suspicion for disease.
We are also usually interested in knowing the prevalence and incidence of infected animals within each herd to estimate the herd-level impacts of disease. This data is ideally collected by running an initial cross-sectional survey to determine the infection status of individual herds and then conducting longitudinal studies where individual animals and/or herds are sampled at multiple occasions over time to estimate the rates of new infections and recovery from existing infections.
Estimates for prevalence and incidence are difficult to extrapolate from published reports from other countries as these are highly influenced by country-specific factors including the demographic structure of the livestock industries, availability disease control interventions, weather and climate variables, quality of the data available from national animal health recording systems, and implementation of any national or voluntary disease control programmes.
For pathogens to persist in a host population, infected animals must infect at least one other susceptible host before they either recover from the disease or are removed from the population through death or culling. Given the number of endemic pathogens in New Zealand, there are clearly ample opportunities for disease to spread within and between livestock herds particularly if farms are not implementing appropriate biosecurity measures to mitigate the risk of diseases spreading through these contacts.
Livestock industries operate as metapopulations with four different scales for how diseases spread:
Border biosecurity encompasses the activities that regulatory authorities take at the border to prevent foreign animal diseases from entering New Zealand as well as the activities taken to meet sanitary requirements for exporting New Zealand livestock and livestock products to other countries. This has often been the traditional definition of biosecurity. On-farm biosecurity encompasses the actions taken by farmers to prevent the spread of diseases across their farm boundary as well as the actions taken to prevent disease from spreading within the farm environment.
Horizontal transmission refers to the spread of infectious diseases from living animal to living animal. This can be either through direct contact where the animals are in close physical proximity to each other (i.e. directly mixing in the same group or in direct nose-to-nose contact over shared boundaries) or through indirect contact where the pathogen spreads through environmental contamination, airborne or waterborne transmission, fomites (contaminated objects), or vectors (insects, wildlife species, or domestic animals) that physically move the pathogen between hosts). Vertical transmission refers to infectious diseases that pass from dams to the fetus across the placenta during pregnancy resulting in a calf or lamb that is already born infected with the pathogen.
There are a limited number of routes that pathogens can use to gain entry to and exit from a mammalian host with the most common being through the respiratory tract, digestive tract, and reproductive tract (Figure 4). This determines which bodily fluids and types of contact pose the greatest risk of a successful transmission event. Other host factors such as immune status, nutritional status, genetic susceptibility, and behavioural traits can also influence the risk of transmission.
Most farms in New Zealand will manage animals in separate groups based on their age, sex, physiological status, and health status. There will always be a natural flow of disease between management groups as animals progress through the different stock classes from birth through slaughter (i.e. calf to R1 replacement heifer to R2 replacement heifer to mixed-age cow). However, the prevalence and incidence of disease will also be influenced by the degree of direct and indirect contact between those management groups at any given time.
Important pathways for the spread of disease between groups on farm:
Any time an animal or an object that has been in direct or indirect contact with another animal crosses the farm boundary, there is a risk of introducing infectious diseases. These contacts can include:
The contact patterns between livestock farms are commonly described using network analysis where each node represents a farm and each connection between nodes represents a potential transmission pathway between farms (Figure 6). The contacts can be weighted by the frequency of occurrence or volume of product moved between farms to estimate risk of disease transmission occurring through the contact.
However, it should be noted that every single contact represents a potential transmission risk and all it takes is one biosecurity break for a farm to experience a disease outbreak.
A common feature amongst all livestock contact networks is that there are a small number of farms that have a disproportionately large number of contacts and are considered a high risk of both acquiring and spreading disease to other farms in the industry. Long distance trades of animals can also result in wider dispersal of pathogens across the livestock industries.
The direct production losses associated with infectious disease outbreaks on farms will depend on the impacts that the pathogen can have on an individual animal and then how that scales up to the herd level in terms of the overall prevalence and incidence of animals that become infected.
The direct effects of animal diseases on production are related to decreased feed and water intake, increased energy expenditure, damage to tissues and organs, and potential mortality. Depending on the animal’s age and physiological status, this may manifest as:
For each of these factors, it is important to know the likelihood of animals experiencing this effect, the magnitude of the effect, the duration of the effect, and whether animals can return to normal production levels following resolution of the disease. Thinking about animals as a series of cashflows over their lifespan, the most economically devastating time to experience mortality is before female animals complete their first lactation (dairy) or wean their first calf (beef) since they will have incurred significant rearing costs without recovering the expenses. The replacement cost of a cow before its first calving has been estimated at NZ$1,445. Any disease episodes that the animal experiences will reduce the total amount of revenue that the animal would have been capable of generating for the farm over the course of its lifespan.
The farm level impacts of an infectious disease outbreak will depend on a number of factors including how the disease was introduced (i.e. purchase of an infected animal versus contact with a fomite), when the disease was introduced relative to critical physiological periods (i.e. during the mating period versus during the dry period), how the animals within the herd are segregated into management groups (i.e. managed as a single mob versus split into multiple isolated mobs), general health status of the animals (i.e. ideally free from other diseases with strong immune systems), and other biosecurity measures that are implemented on farm to prevent disease from spreading between animals (i.e. maintaining a sick pen, cleaning equipment and clothing between animals, and practicing good waste management).
The effects are usually reported as a change in key performance indicators from the baseline values for the herd or flock. Depending on the farm type, these may include:
For dairy in particular, it is often difficult to extrapolate values from the published literature on economic impacts to the New Zealand farming systems since many of the key performance indicators used in intensive herds in North America and Europe are designed for year-round calving herds and the production levels in intensive herds are significantly higher than pastoral herds due to the combination of management, diet, and genetics.
For notifiable diseases or diseases that are under national control programmes, outbreaks of disease on a farm may result in control measures such as movement restrictions, strategic culling, and/or total depopulation. This can result in significant costs as well as potential welfare issues if farms are unable to move cattle on and off for grazing, if the animals fetch lower market prices than expected, if farmers are completely unable to sell into certain markets, and if farmers must replace animals that were culled. For New Zealand, the loss international trade market access would have devastating impacts on the livestock industries which are largely dependent on exports to generate revenue.
Many infectious diseases are zoonotic meaning that they can infect humans as well as animals. The public health costs associated with these diseases are often expressed in economic values as the direct treatment costs as well as the costs of lost work productivity. Another common measure is the disability-adjusted life year (DALY) which is a time-based measure that combines years of life lost due to premature mortality (YLLs) and years of life lost due to time lived in states of less than full health or years of healthy life lost due to disability (YLDs).
Any disease that reduces production efficiency has the potential to increase the environmental impacts of the livestock industries by requiring more animals and/or more inputs to produce the same amount product. For example, lambs that are placed on less effective anthelminthic regimens take significantly longer to reach target slaughter weight and produce an extra 10% emission of CO2 per kg of weight gain. Foot lesions in dairy cattle in dairy cattle can also increase emissions by 4 (0.4%) kg CO2e/ per ton of fat-and-protein-corrected milk (t-FPCM per case of digital dermatitis, by 39 (4.3%) kg CO2e/t FPCM per case of white line disease, and by 33 (3.6%) kg CO2e/t FPCM per case of sole ulcers . It is interesting to note that greenhouse gas emissions are starting to be integrated with traditional cost-benefit analyses to aid in decisions around prioritising which endemic diseases of livestock to control. However, biosecurity measures that alter farming demographics to reduce contact rates may not always result in lower greenhouse gas emissions. One study in New Zealand examined the impacts of dairy herds switching from sending animals offsite for grazing to maintaining self-contained operations as a biosecurity measure to prevent disease outbreaks. The results showed that an average herd would experience a 15% decrease in farm profitability, leach 3% to 7% more nitrogen into the environment, and produce 7% to 10% more greenhouse gas emissions per hectare. Other strategies that target different areas of farm management may be more effective.
Farming is already considered a stressful occupation due to long hours, large amounts of paperwork, and relative isolation, but infectious diseases can also add to the psychological burden from factors like culling animals at the wrong time, stigma for farmers that rely on breeding and pedigree animals, stress from loss of income and subsequent impact on meeting family obligations, increased time spent treating affected animals, and distress from observing illness and death in affected animals.
The psychological impacts of infectious livestock diseases on farmers was first studied extensively in response to the 2001 foot-and-mouth disease (FMD) epidemic in the United Kingdom. In order to bring the outbreak under control as quickly as possible, the government issued immediate movement restrictions on all livestock farms across Great Britain mass depopulation of all infected herds as well as their close neighbours. The 2001 FMD outbreak also had wider impacts beyond the farming community due to additional disruptions in the tourism industry. During the first week of the outbreak, the government closed public footpaths and waterways to prevent tourists from spreading disease between farms and many tourist attractions including zoos and safari parks also closed to prevent their own animals from getting infected. This further impacted businesses around the tourist destinations and exclusion zones including shops, restaurants, and hotels. Simulation model results estimated that total tourism revenue in the United Kingdom in 2001 fell by £7.7 billion with 30% attributed to losses from overnight tourism, 36% to reductions in day-trips, and 34% to reductions in international tourism for overall losses of £179 million per week.
In a thematic analysis of social impacts of bovine tuberculosis control on farming communities in Great Britain, key sources of stress were the additional workload that movement restrictions bring, the feeling of helplessness against the disease, distress at seeing good cattle slaughtered on farm, knock-on stress on other family members besides the primary farmer, low staff morale, and lack of understanding, empathy, and support from the wider public. Auctioneers were primarily affected by reduced revenue from culled animals that were underestimated in value, lack of trust in the pre-movement testing system with blame for selling animals from positive farms shifted to the market rather than the farm or origin, and changes in the patterns of sales with farmers selling smaller batches of animals more frequently. For veterinarians, their primary role was in providing cattle testing, which on the one hand provided a key source of revenue to employ veterinarians at the practice, but on the other hand limited the time available for vets to engage in other herd health activities. Insurances companies were affected by the significant loss in revenue associated with compensating famers for bTB breakdowns.
The psychological impacts of infectious diseases are not just restricted to exotic disease outbreaks or diseases under national control. A study from Sweden on the associations between farm worker health and animal health reported that mastitis cases caused increased stress and frustration due to increased workloads from having to clean, separate, and treat mastitis cows, increased pressure to develop new management plans to improve herd-level mastitis rates, and mental distress from empathy towards the sick cows. However, it is difficult to quantify the magnitude of these impacts.
Another significant economic impact of infectious diseases is the costs associated with managing and preventing disease outbreaks. Biosecurity interventions can broadly be divided into disease-specific interventions which usually include treatments, diagnostic tests and vaccinations that typically only work against a single specific pathogen and disease-agnostic interventions which usually include management strategies aimed at physically reducing the number of direct and indirect contacts between infected animals and, in situations where contacts are unavoidable, general hygiene measures that will be effective across a broad range of pathogens.
There is wide variation in the availability of treatments for each infectious disease and it may not always be necessary to provide treatment depending on the severity of clinical signs displayed by the animal as well as the likelihood of the infection resolving on its own without intervention. Many bacterial and parasitic infections of livestock can be treated with antibiotics and anthelminthics, respectively, although is increasing pressure to use these medications as sparingly as possible to delay the development of resistance and to prevent the additional costs associated with milk and meat withholding periods. Antiviral and antifungal drugs are not routinely used in livestock. Otherwise, treatment is primarily geared towards general supportive care to manage the clinical signs and effects if infections (i.e. giving fluids to correct dehydration or NSAIDS to reduce fever, correcting electrolyte imbalances, and providing general nursing care). This can incur significant costs for drugs and other consumables as well as farmer and veterinary labour costs to provide treatment.
Most infectious disease control programmes hinge on being able to accurately identify infected animals and herds. This is particularly important for biosecurity measures that aim to the risk of disease spread through animal movements by either testing individual animals for disease prior to movement or, for diseases where the diagnostic tests for individual animals are unreliable, developing systems for assigning herds into categories based on their disease status and then attempting to limit the movements of animals from high-risk into low-risk herds.
There are three primary screening tests for identifying animals that may be sick from an infectious disease:
The performance of diagnostic tests is reported as the sensitivity (percentage of individual animals with the disease that are expected to test positive) and specificity (percentage of individual animals without the disease that are expected to test negative). These performance measures can also be applied at the herd-level where the objective is to determine if the pathogen is present on farm. Test performance can be influenced by a number of factors included:
Vaccines work by introducing the immune system to something that looks like the pathogen from a natural infection so that it can produce a stronger and faster response if the individual gets exposed to the pathogen in the future. There are several different types of vaccines based on how they trigger an immune response:
Most vaccines will not completely prevent individuals from getting infected with the pathogen but will enable them to produce a stronger and faster immune response to reduce the severity of clinical signs and/or minimise shedding of the pathogen. Vaccine efficacy measures the percentage of individuals who are protected against the disease after receiving the label recommended protocol. The most common reasons for apparent vaccine failure in the field are incorrect timing of administration, improper storage and handling of the vaccine, and high pathogen loads in the environment.
There are many different potential management strategies that can be used to reduce disease transmission and help animals achieve a faster recovery from disease.
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