- Disease susceptibility is the tendency of an animal to become infected, develop clinical disease, or experience adverse health effects when exposed to a pathogen or disease-causing condition. In animal breeding, disease susceptibility is an important health trait because differences in susceptibility can influence disease incidence, production, fertility, survival, longevity, welfare, treatment costs, and overall efficiency. Understanding the genetic and environmental factors that influence susceptibility can help breeders develop populations that are healthier and more resilient.
- Disease susceptibility is a complex quantitative trait influenced by many genes and environmental factors. The observed phenotype can be simplified as P=G+EP = G + E, where PP represents the observed disease outcome, GG represents genetic effects, and EE represents environmental effects. Genetic effects may include additive genetic effects, dominance, and epistasis, while environmental influences include pathogen exposure, nutrition, housing, climate, hygiene, stocking density, vaccination, stress, age, and management.
- Disease susceptibility is closely related to disease resistance, but the two concepts describe different aspects of the host–pathogen relationship. Resistance generally refers to the ability to prevent infection or reduce pathogen burden, whereas susceptibility describes the tendency to become infected or develop disease. In practical breeding programs, these concepts may be measured through related traits such as disease incidence, pathogen load, disease severity, treatment frequency, or mortality.
- An animal may be exposed to a pathogen without becoming clinically ill. Therefore, disease susceptibility should not be interpreted simply as whether an animal has ever been exposed to a disease. Differences in pathogen exposure, immune response, management, nutrition, and environmental conditions can all influence whether disease develops. Accurate genetic evaluation requires methods that help separate genetic differences from differences in exposure and management.
- The biological basis of disease susceptibility includes several components of the animal’s physiology and immune system. These include physical barriers, innate immunity, adaptive immunity, pathogen recognition, inflammatory regulation, tissue repair, and metabolic condition. Genetic variation affecting any of these processes may contribute to differences in susceptibility. Because immune and physiological systems involve many biological pathways, susceptibility to complex diseases is usually polygenic.
- Disease susceptibility can be measured in different ways. A simple measurement may record whether an animal developed a disease or remained healthy. Other measurements may include disease severity, pathogen burden, duration of infection, number of disease episodes, treatment requirements, recovery time, or mortality. The appropriate measurement depends on the disease and production system.
- Many disease outcomes are recorded as binary traits, such as diseased versus healthy. However, an underlying biological liability to disease can vary continuously. This makes threshold models particularly useful for genetic evaluation. Under a threshold model, genetic and environmental factors contribute to an underlying liability, and disease is expressed when that liability exceeds a particular threshold.
- The heritability of disease susceptibility varies among diseases, populations, environments, and measurement methods. Many disease-susceptibility traits have low to moderate heritability because environmental exposure and management can account for a substantial proportion of phenotypic variation. Nevertheless, even when heritability is low, genetic differences may exist and can contribute to long-term genetic improvement if accurate records and appropriate selection methods are used.
- A low heritability should not be interpreted as evidence that disease susceptibility is entirely environmental. Rather, it indicates that a relatively small proportion of the observed variation under the conditions of measurement is attributable to additive genetic variation. Improving the quality and quantity of disease records, accounting for environmental differences, using information from relatives, and incorporating genomic data can improve the accuracy of genetic evaluation.
- Disease susceptibility is often disease-specific. Genetic susceptibility to one pathogen does not necessarily imply susceptibility to another. Different pathogens interact with different host receptors, immune pathways, tissues, and physiological systems. Consequently, genetic evaluation should generally focus on specific diseases or biologically related groups of diseases rather than assuming that all disease susceptibility is controlled by the same genetic factors.
- Parasite susceptibility is an important example in livestock production. Sheep and goats, for example, may differ genetically in their susceptibility to gastrointestinal parasites. Animals with higher parasite burdens may experience reduced growth, poor body condition, reduced fertility, and increased mortality. Measurements such as fecal egg counts can provide information about parasite burden and can contribute to genetic evaluation of parasite resistance or susceptibility.
- In dairy cattle, mastitis susceptibility is another important example. Mastitis can reduce milk production, alter milk composition, increase treatment costs, and increase the probability of premature culling. Traits such as somatic cell count can provide indirect information about udder health and mastitis-related susceptibility. However, environmental factors such as milking hygiene, housing, pathogen exposure, and management also have substantial effects.
- Respiratory and gastrointestinal diseases are important examples in several livestock species. Young animals may be particularly vulnerable because their immune systems are still developing. Nutritional status, housing conditions, stocking density, ventilation, pathogen exposure, and stress can all influence disease susceptibility. These environmental effects make careful experimental design and statistical adjustment important for genetic evaluation.
- Disease susceptibility is also affected by age and physiological stage. Young animals may have different susceptibility from mature animals, while pregnancy, lactation, reproduction, or periods of rapid growth may alter physiological demands and immune function. Genetic evaluations should therefore consider the age and production stage at which disease records are collected.
- Nutrition can strongly influence disease susceptibility. Energy and protein deficiencies, mineral imbalances, poor feed quality, or inadequate access to nutrients can impair immune function and increase the likelihood of disease. Conversely, excessive nutritional intake can contribute to metabolic disorders that may also increase health risks. Genetic susceptibility therefore needs to be interpreted within the nutritional environment in which animals are raised.
- The environment can create substantial genotype–environment interaction (G×E) for disease susceptibility. Genetic differences that are relatively small under low pathogen exposure may become much more important under high disease pressure. Similarly, an animal genotype that performs well in one climate or management system may not have the same health advantage elsewhere. Breeding programs should therefore consider the target environment when evaluating disease-related genetic differences.
- Disease susceptibility is closely associated with immune response. Genetic variation can affect the speed, magnitude, and regulation of immune responses following pathogen exposure. However, disease susceptibility cannot be reduced to immune response alone. Disease outcomes also depend on pathogen characteristics, exposure level, tissue susceptibility, physiological condition, and environmental factors.
- The distinction between disease susceptibility, disease resistance, and disease tolerance is important. Resistance generally concerns the ability to prevent or reduce infection. Susceptibility describes the likelihood of infection or disease development. Tolerance describes the ability to maintain health and performance despite infection or pathogen burden. An animal may therefore be relatively susceptible to infection but show high tolerance, while another animal may resist infection effectively. These different mechanisms can have different implications for breeding.
- Disease susceptibility can affect fertility and reproductive performance. Infection and disease can interfere with reproductive physiology, conception, pregnancy maintenance, semen quality, offspring survival, and reproductive efficiency. Animals with repeated health problems may therefore have reduced lifetime reproductive performance. Genetic relationships between susceptibility and fertility should be considered when constructing breeding objectives.
- Disease susceptibility also affects survival and longevity. Severe disease can increase mortality and involuntary culling and can shorten productive life. Repeated disease episodes may reduce performance and increase the probability that an animal is removed from the breeding or production population. Selection to reduce susceptibility to economically important diseases can therefore contribute indirectly to improved survival and productive longevity.
- There are also important relationships between disease susceptibility and production traits. Disease can reduce milk yield, growth, meat production, egg production, wool production, and feed efficiency. Genetic associations between production and health can sometimes create trade-offs, although the strength and direction of these relationships vary among populations and production systems. Balanced breeding objectives are therefore essential.
- Genetic correlations help breeders understand how selection for disease susceptibility or resistance may influence other traits. Favorable genetic correlations can allow simultaneous improvement of health and production, while unfavorable correlations may require compromises. Genetic correlations with fertility, survival, growth, feed efficiency, body composition, and production should therefore be considered when developing selection programs.
- Genetic evaluation of disease susceptibility can use estimated breeding values (EBVs). An EBV predicts an animal’s genetic merit for a particular trait using information from the animal itself, relatives, progeny, contemporary groups, and other available records. Statistical approaches such as BLUP (Best Linear Unbiased Prediction) help separate genetic differences from systematic environmental effects and can improve selection accuracy.
- Genomic selection provides additional opportunities for reducing disease susceptibility. Genomic estimated breeding values (GEBVs) use genetic-marker information across the genome to predict genetic merit. This is especially useful for diseases that are difficult or expensive to measure directly, occur late in life, or require substantial pathogen exposure before reliable records can be obtained. Genomic information can allow selection decisions to be made earlier and may increase the accuracy of genetic evaluation.
- Genetic research can also identify genomic regions associated with disease susceptibility using QTL mapping, genome-wide association studies (GWAS), and other genomic approaches. Some variants may increase or decrease susceptibility to particular pathogens. However, complex disease traits usually involve many genetic variants, and genetic effects may depend on pathogen strain, population, environment, and management. Findings from one population should therefore be validated before being applied broadly.
- Accurate disease recording is essential for genetic improvement. Useful information can include confirmed diagnoses, disease incidence, disease severity, pathogen identification, treatment records, number of disease episodes, mortality, culling reasons, laboratory measurements, and repeated health observations. Consistent definitions are particularly important because differences in disease-recording practices can reduce the reliability of genetic evaluations.
- It is also important to distinguish true genetic susceptibility from apparent susceptibility. An animal may develop disease because it experienced greater pathogen exposure, poorer nutrition, inadequate housing, or other environmental challenges rather than because it has inherently poor genetic resistance. Conversely, an apparently healthy animal may simply have experienced little exposure. Genetic evaluation methods attempt to account for these differences through contemporary-group information, relatives, environmental records, and appropriate statistical models.
- Reducing disease susceptibility can provide significant economic benefits. Fewer disease cases can reduce veterinary expenses, medication costs, mortality, production losses, reproductive problems, and premature culling. Healthier animals may also require fewer interventions and remain productive for longer periods. Genetic improvement in disease-related traits can therefore contribute to sustainable animal production.
- Disease susceptibility is also directly relevant to animal welfare. High susceptibility can increase the risk of pain, illness, impaired function, and premature death. Reducing genetic susceptibility to important diseases can therefore support welfare when combined with effective management, vaccination, biosecurity, nutrition, and veterinary care. Genetic selection should complement rather than replace appropriate disease-prevention practices.
- Long-term breeding programs must also consider genetic diversity. Strong selection against susceptibility may be beneficial, but excessive emphasis on a narrow set of genetic variants can increase inbreeding or reduce genetic diversity. Maintaining genetic variation is important because populations may encounter new pathogens and changing environmental conditions in the future.
- A balanced breeding objective should therefore combine disease-related traits with production, fertility, survival, welfare, and other economically important characteristics. Selection index methods can combine information from multiple traits and assign appropriate importance to each. This allows breeders to reduce disease susceptibility while maintaining desirable levels of production, reproductive performance, longevity, and adaptability.
- Overall, disease susceptibility is an important health-related trait that reflects the interaction between an animal’s genetic background and its environment. Although pathogen exposure, nutrition, management, climate, and other environmental factors strongly influence disease outcomes, genetic variation can contribute to differences in susceptibility within populations. The use of heritability, genetic correlations, breeding values, genomic selection, accurate disease records, and balanced selection objectives can help reduce susceptibility to economically and biologically important diseases. The most effective long-term approach combines genetic improvement with vaccination, biosecurity, nutrition, hygiene, parasite control, appropriate housing, and veterinary management to produce healthier, more resilient, productive, and welfare-compatible animals.
