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Calves are young and vulnerable animals. In a short period, calves have to develop from a monogastric into a ruminant animal. This transition requires them to grow in both size and mass, while dealing with transportation, man handling and changing environments. All these challenges together cause a lot of stress that may lead to a lower immune system.

What is stress
What exactly is stress? In some way, we have probably used the term stress ourselves to indicate a state of negative wellbeing regarding our physical or psychological condition. The same definition seems to be true for animals and the types of stress that are caused by different stressors. Some sorts of stress are part of a natural response – like for instance the stress caused by fear, which is meant to avoid dangerous situations, such as predators. Acute (short lasting) stress can therefore be an acceptable and even positive event. Part of the development of the (growing) organism in the end is also the ability of dealing with such stresses as a net result of the complex interaction between genetics and experience.
As a general principle, however, it is desirable to strive for a low stress environment in order to reduce the amount of unpredictable stressors to a minimum. Despite the possibility of stress (reactions) being a natural response, they are not on the same level as the homeostatic systems ‘more common’ to the functioning of the body of the animal like managing blood oxygen levels, blood pH, body temperature, etc. These so called allostatic systems and especially the longer they last or when chronic, are more profound and influence a wider range of functions and systems. This is also their function: to activate (physiological) changes to cope with a certain situation. Further illustrating this, another strong example of such a reactive system is the immune system. The immune system, being a system that also works on a wide range of body functions, is itself even affected by both minor and major stressful events. Individuals subjected to different kind of levels of stressors are at a greater risk from illnesses, although some sources also indicate that short acute stress in animals with a well-functioning immune system (a high ‘immunocompetence’) may even contribute to an improved immune reaction. In other words, the impact of allostatic systems is quite influential. Stress and immune systems causes and effects are closely related.
The impact of stress on the immune system
A bit more on the relationship between stress and the immune system. On one hand, physiological stressors can have a direct impact on the immune system, if they can trigger an inflammatory response. On the other hand, a stressor of a more psychological nature may increase the risk of disease. During its life, a calf undergoes many moments and phases in which it is most vulnerable and susceptible to diseases.
Due to the development of the calf’s immune system in combination with the many changes the animal goes through and the situations it is put in in the early days and weeks of life, the risk of being afflicted by disease is drastically increased. These changes and situations are most of the times well known to us: (simply already) being born, being transported and being mixed with other animals, being handled, being weaned, transport, different feeding regimes, housing, and much more. They all pose a risk to the calf’s health, but the good thing about these ‘predictable’ stressors is that they can be mitigated to a certain extent. This extent is determined by the capability to manage them – the efforts made by the producer mostly. They may however, present themselves in varying levels of severity also dependent on the capability of the animals to cope with them. In case they cannot be mitigated, there is the possibility to intervene. How to properly mitigate or intervene is highly dependent on the root cause of the stressor or the (cause of the) problem the animal is undergoing.
Causes of calf distress
One of the most common causes for calf distress (septicemia, diarrhoea) and calf mortality are enteric diseases and it is strongly influenced by ongoing other stressors. One out of three calves experiences enteric diseases caused by pathogens or issues caused by nutrition in the critical phase after 2-3 weeks of age. The economic impact of having sick calves is significant.
Understanding the impact of specific stressors on these sick calves is better done when looking a bit closer to what is happening in these animals, especially in the gastro-intestinal tract.
The gastro-intestinal tract, which has the function to digest food and absorb nutrients that the growing calf needs can be disrupted by wrong nutritional regimes and/or applications and enteric diseases. An important contribution to the normal functioning of the gut is made by the numerous small folds on the gut wall called villi. The gut villi are responsible for increasing the surface area of the gut, allowing for better access to nutrients entering the gut and improve digestion by enzymes. Asides from this role, the gut wall itself is also a mucus covered barrier that provides protection of the internal environment from the external environment. Which means, that a healthy and functional gut and gut wall is very important to avoid bacteria, toxins or other undesirable substances from entering the bloodstream of the animal. Some E. coli strains, rotavirus, coronavirus and cryptosporidium can all cause villus atrophy, reduce their size (height) and functionality and thereby further exacerbating the problems in the gut and gut wall. Keep in mind though that there are also strains, of for example E. coli that do not cause any problems. On top of all this, 35% of the cases is also a mixed infection. In case the gut function is disrupted, the animal may suffer from diarrhoea. This diarrhoea is a consequence of the gut (wall) being unable to perform its function properly, an attempt to correct an imbalance or due to the influence of bacterial toxins. It may also be caused by a combination of any of these and is therefore the result of any combination of increased secretion of water, malabsorption of nutrients and an increase in gut movement.
Ways to solve the problem
There are several ways to reduce morbidity and mortality of calves caused by diarrhoea. These are all focused on removing, preventing or reducing some sort of stressor - in its broadest sense, support the immune system, or both.
As calves are not born with a completely functional immune system, colostrum is the single most important factor to deal with infectious causes for neonatal diarrhoea originating from the cow, the environment and other calves. Four litres of colostrum at the right temperature, of good quality (> 50g IgG/L and hygienic (disease free)) should be provided within 1 hour after birth and up to 6 litres within 12 hours. Calves that do not receive sufficient good quality colostrum have a so-called ‘failure of passive transfer’ with a much higher risk of getting ill and possibly dying. Monitoring the quality of the colostrum allows for effective intervention if needed by providing better quality colostrum from other cows, if this is kept frozen in stock. Otherwise, supplementing with colostrum replacers is another possibility, although farm-specific colostrum is preferred especially if cows are vaccinated against common pathogens on that farm. The situation of the calf after birth is further improved, if there is a clean calving area available with fresh bedding and the calf is separated from the cow as soon as possible. This should also apply to (male) calves going on transport at 2 weeks of age or that are handled intensively. Not only are those animals an additional temporary threat to the health status of the farm if they don’t receive colostrum, they obviously also benefit a lot from having a properly functioning immune system whilst being faced with changing environments, food and water deprivation during transport (sometimes up to 3 days) and mixing with other calves. When animals (finally do) start drinking milk or milk replacer, ensure this follows mostly the same rules as providing colostrum to minimize digestive disturbances: good quality milk or milk replacer provided at regular intervals at the right temperature and when applicable at the indicated quantity and concentration.
As stated earlier, the cow, other calves and the environment are possible causes for diarrhoea and the challenge is to prevent the spreading of disease and to reduce the infection pressure. Work with the predictable stressors! Sick animals can be separated from healthy animals and should be fed last to avoid carry-over contamination by the person feeding. Additional care can be provided to sick animals by first double-checking if feeding practices are correct. Next, supportive additives can be provided or some colostrum added in the milk to have some antibodies on gut level. Ensure that the overall hygiene of the pens is good. This means that calves are best housed individually in the first weeks on dry, clean and disinfected pens. Preferably, younger calves are fed first as older animals may shed increasing amounts of pathogens in their feces. Lastly, avoid overcrowding; evaluate temperature and ventilation in the barn to keep stress at acceptable levels both for the calf as well as for the cow.
Where rehydration comes in
Then, once diarrhoea does occur, identifying the cause is a crucial first step. The sooner treatment starts, the better the recovery and the lower the risk of calves dying. Antibiotics treatment only to be applied if the cause of the infection is clearly evaluated, the targeted pathogen is sensitive for that antibiotic and it is not used to target something unspecific, rendering it useless or even detrimental. Therefore, treatment protocol is mainly defined by providing rehydration, as the calf is losing large amounts of water and electrolytes: minerals that maintain water balance, through the feces. This water is extracted from the calf’s body fluids. The resulting imbalance in water and minerals in the body causes a series of physiochemical occurrences that acidifies the blood and leads to metabolic acidosis. If the dehydration is left untreated, the ongoing metabolic acidosis will halt major body functions of the calf and in severe cases can result in death of the animal.
Providing rehydration products that contain electrolytes to correct the balances, buffering agents to counter the ongoing metabolic acidosis and some energy thus prove to be an effective treatment. In case of malabsorptive diarrhea caused by rotavirus, coronavirus or cryptosporidium, it is also beneficial to provide smaller milk meals and ensure a low tonicity rehydration product. As long as the dehydration of the animal is below 8% of body water lost, even though it looks depressed but is typically still able to stand, oral hydration solutions can be provided voluntarily. Something to be cautious of though is that calves can be extremely dehydrated and still be able to drink by themselves properly. However, when dehydration is more than 8% of the body water lost an intravenous solution should be given as standard because it may otherwise not work fast enough to resuscitate the animal. Similarly important is that calves can be just slightly dehydrated and still have severe metabolic acidosis ongoing. These calves may then refuse to drink. At that point, drenching would be a possibility to provide a rehydration solution, but limited to once a day with maximum two liters.
Rehydration products are potent solutions for animals that truly have an ongoing problem: they are in a vicious circle that has to be corrected with specific components in very specific ratios. A rehydration product that is not tailored to the situation and the type of stressor the animal is in or is undergoing will be either less functional, completely dysfunctional, or even have a reverse effect. Therefore, healthy animals that could use some hydration support, like animals after transport as an example, generally do not benefit from rehydration or electrolyte products designed for diarrheic calves and vice versa.
Final recommendation
Make sure to investigate all options and consult your veterinarian or feed advisor about the best options on your farm and for your calves. Use solutions from companies that invest in research and provide support on digestive disorders and in particular on the topic of hydration. Effective treatment of diarrheic calves and providing healthy calves a good start by minimizing their stress deserves all attention. If we do that right, we can start saving some calves!

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