Elizabeth Plunkett, Graduate Research Associate, Department of Animal Sciences, The Ohio State University
Disease presents a significant threat to animal productivity, leading to billions of dollars in annual losses. As a producer, you do everything in your power to mitigate disease risks in your operation, but unfortunately, disease is opportunistic and will strike when least expected. While disease can be challenging to mitigate, it is often easy to spot by observing cow behavior and productivity. The signs that we initially observe and associate with disease are the consequences of physiological adjustments that are a part of the host’s response to an abnormality. At first glance, these responses might seem counterproductive. For example, why would a cow go off feed at a time when her metabolic requirements increase exponentially? However, the physiological adjustments present at the onset of disease are a set of highly organized and calculated survival mechanisms that have been evolutionarily favored for centuries. Understanding the biological rationale behind these responses reveals how they collectively contribute to the cow’s initial defense against pathogens.
Reduced Feed Intake:
Interleukin-1 (IL-1) is a cytokine that plays a crucial role in regulating the immune response. When IL-1 is released, it triggers a decrease in appetite. Although it might appear unfavorable when sick cows stop eating, it is actually a protective mechanism. Listed below are several plausible reasons why this is a beneficial response during sickness, and for more information into this perplexing phenomenon, I encourage you to read Brown and Bradford (2021), which reviews these mechanisms in greater detail.
- When cows consume less feed they divert less energy towards digestion, which is an energetically costly process. Creating less energy usage allows for more energy to go towards fighting the infection and fueling vital organs.
- The pathogen may be present in the feed so limiting intake helps reduce overall pathogen load. Additionally, if the pathogen is in the feed, reducing intake would allow the gastrointestinal tract time to heal and prepare for future intake.
- Cows may simply lack the motivation to eat. If feeding requires them to walk a great distance or fight for their food, they may not feel up to the challenge when they are sick. It may be more energetically costly for them to approach a feed bunk rather than staying put to conserve energy.
- Looking at this from an evolutionary perspective, limiting feed intake during sickness is a prey instinct. Historically, cows are grazing animals; therefore, during times of sickness, it is unwise to put themselves in such a vulnerable position when they can’t promptly respond.
- Eating less helps reduce iron intake which is an important mechanism to ‘starve’ the invading bacteria. Some bacteria require iron for proliferation and growth; therefore, limiting intake helps reduce nutrient availability for the invading pathogen.
Decrease in Plasma Iron Concentration:
Iron is an essential mineral for some bacteria as they need it to grow and proliferate. During bacterial infections in cattle, we see a reduction in iron concentrations. This reduction is not due to increased excretion but rather the redistribution of iron throughout the body. For example, during mastitis, transferrin concentrations in milk will increase as transferrin will work to bind free iron to keep bacteria from acquiring it and utilizing it to multiply. The release of IL-1 also causes iron concentrations to decrease. Additionally, the reduction in iron concentration acts synergistically with fever to hinder bacterial growth in the early stages of disease. While they both work to suppress bacterial growth, one is not dependent upon the other and one can occur while the other does not.
Fever:
Fever is a temporary increase in the body temperature set point, not to be confused with hyperthermia which is an increase in body temperature without an adjustment made to set point. Fever is controlled by IL-1, and it helps make the body less welcoming to infections as some bacteria and viruses can’t survive at higher temperatures. Immune cells also have enhanced performance during fever, primarily due to the actions of IL-1, but also due to the heat fever creates. To reach and hold this new higher temperature, the body must make several changes. It does this by reducing heat loss and/or generating more heat. For example, sick animals often curl up in an attempt to reduce their surface area and limit heat loss. Additionally, blood flow will shift to internal organs, leaving the extremities to feel cold to the touch (e.g., cows’ ears feed cold). Another way the body reduces heat loss is called piloerection, which is when hair follicles stand up. While the body does everything it can to retain heat, sometimes this is not enough. If it can’t hold the heat, the body will increase heat production by speeding up metabolism, which is energetically costly. Fever can increase overall metabolism by 30-50%, creating more stress on the cow if this response is prolonged.
Depression and Lethargy:
Just like when we are sick and all we want to do is stay in bed, cows behave in similar patterns. There are multiple benefits of depression and lethargy during disease. Cows may curl up and move less as a strategy to conserve body heat and minimize the use of nutrients for non-essential functions. IL-1 also plays a role in inducing sleepiness, as it is involved in promoting rest and reducing activity in response to illness. This is why an activity monitoring system can help us detect illness in cows because they will be moving less to conserve energy. This depressive state helps the cow conserve energy for essential functions, allowing her to focus on launching her defense.
These physiological adjustments that occur during disease work with one another to provide an initial defense mechanism against invading pathogens. These responses work together to protect the host, but they neither the same purpose nor do they depend upon one another. Even cows subjected to the same pathogen will have variations in these responses and may present with different clinical signs. Making the approach to resolve the issue is difficult and case-dependent. While these mechanisms are biologically normal, their prolonged presence can create just as much damage as the invading pathogen. Leading furthermore to the question, we continue to ask ourselves: When do we treat and when do we let these processes run their course?
Stay tuned for the next article which will answer this question and many more!
Disclaimer: This article is intended to explain and inform readers about some common physiological responses observed in ill lactating cows. It does not constitute veterinary advice or recommendations for treating animals. For specific concerns and treatment plans, please consult a licensed veterinarian.
Sources:
- Brown, W.E., and B.J. Bradford. 2021. Invited review: Mechanisms of hypophagia during disease. Journal of Dairy Science 104:9418–9436. doi:10.3168/jds.2021-20217.
- Hart, B.L. 1988. Biological basis of the behavior of sick animals. Neuroscience and Biobehavioral Reviews 12:123–137. doi:10.1016/S0149-7634(88)80004-6.