Calf health and immunity

Workshop for Animal Health Professionals, 09-09-1999
Teagasc [Irish Agriculture & Food Development Authority] Homepage

Dr. Bernadette Earley PhD <bearley@grange.teagasc.ie>

Introduction

Immunity against infectious diseases is a complex phenomenon, involving interaction between different cell types, each having unique function(s). The immune response is classified into two main categories: (1) humoral immune response, also known as antibody mediated immunity; and (2) cell-mediated immunity (CMI). Lymphocytes called B cells produce the antibodies, which circulate in the blood (humoral immunity). The antibodies neutralise antigens by removing them from the body's circulation, causing them to clump, or making them more susceptible to other immune cells. Lymphocytes, which mature in the thymus gland are called T cells, are active in cell-mediated immunity, i.e. they activate other cells that cause direct destruction of antigens or assisting B cells. CMI involves direct attack on foreign cells by lymphocytes (cytotoxic T lymphocytes and natural killer (NK) cells), phagocytosis and intracellular killing of foreign or infected cells by macrophages under T lymphocyte control.

Calf mortality

Calf mortality can best be subdivided into 4 main categories:

• Abortions (foetal loss at less than or equal to 270 days gestation).
• Perinatal mortality (foetal loss at greater than 270 days gestation and mortality during the first 24 hours of life).
• Neonatal mortality (calves born alive that die between 24h and 28 days).
• Older calf mortality (calves born alive that die between 29 and 84 days).

Morbidity and mortality of the young calf represent a major cause of economic concern for beef producers. Septicaemia and enteric disorders caused by strains of Escherichia coli, Rotavirus, Neospora, Coronavirus, Cryptosporidium or by Salmonella species are the main cause of neonatal mortality. Respiratory infections and salmonellosis are the main causes of death in older calves.

Disease is not a simple matter of exposure of a susceptible animal to an infectious agent such as a bacterium, virus, or fungus. Calves are exposed to infectious organisms from the moment of birth, and natural defence mechanisms usually preclude the establishment of disease. Animals develop disease because of a complex relationship between the host (animal), the infectious agent (bacterium, virus, fungus, or toxic agent), and the environment. Control of the agent is largely based on prevention of exposure, immunity, and chemotherapeutic agents (drugs). Diagnosis and correction of health problems usually involves clinical, immunological, haematological and therapeutic approaches.

Calf diarrhoea

Outbreaks of diarrhoea in calves are associated with the interaction of potentially pathogenic enteric microorganisms with the calf’s immunity, nutritional state and environment. A single infectious agent may trigger an outbreak, but most outbreaks are due to mixed infections. The most common organisms involved are Escherichia coli., Salmonella Species, Rotavirus, Coronavirus and Cryptosporidium. Dehydration, acidosis, impaired growth rate or deaths are the major consequences.

Escherichia coli infections (colibacillosis)

To be enteropathogenic, E. coli strains must be able to adhere to the small intestine. The K99 antigens allow adherence of the organisms to the mucosal wall. E. coli then produces an enterotoxin that causes excess secretion of fluid into the intestinal wall resulting in diarrhoea. Loss of body fluids in this way leads to dehydration. In animals deprived of colostrum, in particular IgM (immunoglobulin), a rapidly developing shock like syndrome (circulatory failure) may occur, which can cause sudden death.

Prevention of calf diarrhoea

Preventative measures adopted to control calf diarrhoea include:

• Vaccination against E. coli infection.
• Hyperimmunisation of the dam with the E. coli antigen provides passive immunity to the calf (via ingested colostrum) during the first 2 to 3 weeks of life, i.e. before it can produce its own antibodies.

Salmonellosis

The organisms Salmonella dublin or Salmonella typhimurium are the main causes of salmonellosis in calves. Salmonella typhimurium DT104 has been recognised as highly pathogenic to calves, resulting in a high incidence of mortality, has a wide range of antibiotic resistance and is capable of rapidly developing new resistance patterns. It is also important as a cause of human food poisoning.

In acute cases of calf salmonellosis, a septicaemia may occur, accompanied by blood stained diarrhoea. Calves affected more severely have elevated body temperature (greater than 40^oC) are debilitated, have reduced feed intake and a greater disturbance of body fluid balance. The calf may become infected as early as the second day of life with highest incidences occurring at 1 to 5 weeks of age. Outbreaks of Salmonella typhimurium are usually associated with purchased calves. Salmonellosis, superimposed on calves with pre-existing pneumonia, exacerbates the clinical signs and pulmonary damage. Thus, purchased calves should be carefully examined on arrival so that any infection may be quickly diagnosed. Prevention is again highly dependent on ensuring that the calf has adequate passive immunity.

Dehydration

Dehydration is the main cause of death in most cases of diarrhoea, thus oral or intravenous re-hydration of the calf is of primary importance. The excretion of large volumes of fluid in the scouring calf results in metabolic acidosis and depletion of sodium (Na⁺), potassium (K⁺) and chloride (Cl^-).

Field investigation studies indicate that neonatal diarrhoea and diseases of the digestive system are the leading causes of dairy calf mortality. Prevention of infectious disease begins with the effective transfer of immunity to the newborn calf through intake of adequate colostrum. At birth the calf has no antibodies in its system. The antibodies (immunoglobulins) obtained from colostrum must be consumed within the first few hours of life in order to provide the calf with a disease defence mechanism until it builds up its own resistance through its own active immune system. The interactive effects of serum immunglobulin (Ig) deficiency and disease on blood and immune characteristics of calves were evaluated at Teagasc, Grange. Serum immunoglobulins (IgG) were measured quantitatively by single radial immunodiffusion (sRID) and calculated via an internal Ig-standard.

Research has shown that mart purchased dairy calves with low immunoglobulins are more susceptible to neonatal infections than similar calves with high serum IgG levels. Of 444 dairy calves examined in a recent study at Grange (Table 1), 171 remained healthy, 18 received treatment for enteric disease, 133 received treatment for respiratory disease while 122 calves received treatment for both enteric and respiratory disease and 35 calves died (mortality 7.9% mean zinc sulphate turbidity (ZST) units 9.11 ± 0.91 vs. surviving calves 12.07 ± 0.26 P < 0.003).

Table 1: Relationship between the serum concentrations of Ig, ZST Units and haptoglobin and the requirement to treat calves for enteric disease and respiratory disease. Values are expressed as mean Ig concentration (mg/ml) ± s.e.m., mean ZST (Units) ± s.e.m. and mean haptoglobin levels (g Hb/l) ± s.e.m..

Figures in brackets represent the number of healthy calves and calves treated for disease.

* P < 0.01 versus healthy calves. Correlation ZST with Total Ig levels; Healthy calves 0.585 P < 0.0001; Respiratory disease only; 0.629 P < 0.0001; Enteric disease only 0.820 P < 0.0001; Respiratory disease and enteric disease 0.854 P < 0.0001.

Serum haptoglobin (acute phase protein) levels were significantly increased in the calves treated for respiratory disease alone and for both respiratory disease and enteric disease (P < 0.01). The incidence of respiratory disease and enteric disease and requirement for frequent antibiotic treatments was lower in dairy calves with a higher Ig concentration and ZST Units (P < 0.05). In addition, serum IgG₁ levels and zinc sulphate turbidity (ZST) units were significantly higher in the Charolais X, The Limousin X and the Simmental X in spring-born suckled calves compared with the dairy calves. There was no outbreak of respiratory disease in suckled calves and indicators of infection, namely the white blood cell count was normal at 28 days postpartum.

Calves from Charolais cows had lower IgG₁ (P = 0.02), IgG₂ (P = 0.001), total Ig serum concentrations (P = 0.02) and ZST Units (P = 0.005) than calves from Limousin cows at 48 hours postpartum. The IgG₁ serum levels and ZST units were significantly higher in the Charolais X, Limousin X and the Simmental X suckled calves at 28 of age when compared with the dairy calves at the same time intervals (Table 2). There was no significant difference in IgG₂, IgA and IgM serum levels between calves from the suckler herd and those from the dairy herd at 28 days of age with the exception of higher IgG₂ serum levels in Charolais X purchased calves. Serum Ig concentrations for IgG_1, IgG_2, IgA, IgM and total Ig were significantly lower (P £ 0.01) in calves from Limousin X cows at 28 days when compared with values obtained at 48 hours postpartum. In calves from Charolais cows IgA and IgM serum concentrations were significantly decreased at 28 days postpartum (P £ 0.03) when compared with values obtained 48 hours postpartum.

Table 2: Serum immunoglobulin levels and ZST units in suckled calves (28 days of age) and mart purchased dairy calves (28 days of age). The values are expressed as mean Ig concentration (mg/ml) + s.e.m and mean ZST (Units) + s.e.m.

IgG₁ concentration * P < 0.01 versus mart purchased calves; Total Ig ** P < 0.01 versus mart purchased calves;
ZST *** P < 0.01 versus mart purchased calves. · P < 0.01 versus Charolais X Beef Breed mart purchased.

Colostrum feeding

Colostrum feeding should continue as long as possible after birth. While immunoglobulins are not absorbed after 24 hours they continue to provide local protection in the intestinal tract. Maternal colostrum provides the main source of immunoglobulins for the newborn calf. Considerable variation between cows with respect to immunoglobulin concentration in the colostrum is reported in the literature. In a study carried out at Grange, there was no significant between breed difference in suckler cows (Charolais X, Limousin X, Simmental X and Hereford X beef breeds) in colostrum IgG₁ concentrations in either the front or back quarter of the udder (Table 3). Colostrum obtained from Holstein X Friesian cows had significantly lower (50%) IgG levels than that obtained from the beef x breed types.

Table 3: Colostrum IgG₁ levels in secretion of the front and back quarters of the udder. The values are expressed as mean levels (mg/ml) ± s.e.m.

* P < 0.01 versus beef x cows.

Heifers and high-yielding cows (Holstein) produce more dilute colostrum resulting in reduced IgG concentrations. It is of vital importance that all calves are fed 2 litres of colostrum within one hour of birth and receive a second 2 litre feed 4 to 6 hours later. Thus, Friesian X Holstein calves should be assisted to suckle or hand fed 4 litres of colostrum in two feeds within 6 hours of birth. The problem of low serum antibody levels with dairy calves is increased due to the use of high-yielding cows.

Factors influencing Ig in calf serum

The major factors influencing Ig concentration in calf serum (antibody production) are:

• Volume of ingested colostrum
• Timing of feeding
• Antibody concentration in colostrum - first milking colostrum has

twice the antibody concentration of second milking colostrum.

Calf pneumonia

The underlying cause of bovine respiratory disease (BRD) is extremely complex with the involvement of viruses, bacteria and Mycoplasma. The incidence of infection (morbidity) is usually high, but the mortality rate is variable. Viruses that have been predominantly isolated from outbreaks of calf pneumonia are infective bovine rhinotracheitis (IBR), respiratory syncytial virus (RSV), parainfluenza-3 virus (PI-3 virus), and bovine virus diarrhoea/mucosal disease (BVD/MD virus). Predisposing factors affecting immunocompetence (ability to fight infection) are stress, overcrowding, inadequate ventilation, draughts, fluctuating temperatures, poor nutrition and/or concurrent disease. In most cases it would appear that the primary infective agent is viral, producing respiratory tract damage that is subsequently extended by Mycoplasmas and secondary bacterial infections. Viruses are unaffected by antibiotics, however, antibiotic treatment is usually administered to kill off secondary bacterial infections and offer the calf the opportunity to fight the disease. Mycoplasma species are resistant to antibiotics, which act on the cell wall, and an antibiotic specific to the cell nucleus is required to inactivate it. In order to direct the appropriate treatment strategy, nasal swabs should be submitted to the Regional Veterinary Laboratory for accurate identification of the pathogen(s) involved. In addition, Mycoplasmas are known to suppress the calf’s immunity to disease. Of the mycoplasmas, M. bovis is the most pathogenic and can act in unison with Pasteurella species to produce a very severe form of pneumonia. Following suppression of immunity the animal’s ability to withstand an attack from Pasteurella and other organisms is reduced. Pasteurella haemolytica is an important secondary agent in respiratory disease. Pathological changes occur in the lung tissue leading to consolidation and respiratory distress. Negative consequences for the welfare of animals that survive a respiratory disease attack are that they may end up as "respiratory cripples" with permanent lung damage.

Colostral antibodies usually adequately protect calves against disease unless environmental stress is excessive. Even low-grade infections can compromise respiratory immune mechanisms, enabling bacteria to invade the lung and cause pneumonia. Excessive stress causes production of cortisol by the adrenal glands, which in turn further suppresses the immune system, resulting in bacterial or viral pneumonia. The haematological profiles of suckled calves and mart purchased dairy calves at 28 days of age were examined at Grange. Friesian calves had lower blood Cu levels, compared with suckled calves (P < 0.01) while alkaline phosphatase (ALP) activity was lower in mart purchased Charolais X, Friesian and Limousin X calves (P <0.01). Total antioxidant status (TAS – a measure of oxidative stress) was higher in Charolais X and Friesian purchased calves (P < 0.01) compared with Limousin X Friesian suckled calves and Limousin X Friesian purchased calves. Glutathione peroxidase (GPx – selenium requiring enzyme) activity was higher in suckled calves than purchased calves (P < 0.01).

Respiratory immune defence mechanisms

There are two main divisions of the respiratory tract, namely, the upper and lower regions each having their own unique immune defence mechanism. The presence of mucus and ciliated epithelium bind and clear infectious agents from the upper respiratory tract, thus preventing bacterial pathogens from establishing infection. Cold, foggy climatic conditions retard the activity of the ciliated epithelium in removing infectious agents. In addition, virus infected cells produce a substance called interferon, which inhibits the spread of infection to neighbouring cells. The alveolar macrophage of the lower respiratory tract serves a unique function in providing the first line of defence against an infectious attack. Thus, when the innate defence mechanisms of both the upper (mucus, ciliated epithelium) and the lower (alveolar macrophage) respiratory tract becomes impaired by viruses, respiratory secretions increase in amount and viscosity thereby resulting in reduced mucociliary clearance. As a consequence, opportunistic bacterial pathogens invade the lower respiratory tract and increase the extent and severity of lung damage.

Numerous vaccines provide a range of combinations of live and/or killed antigens. These include IBR, RSV and PI-3 Pasteurella spp., and Haemophilus somnus. Intramuscular modified-live virus vaccines quickly induce long-lasting immunity. Intranasal modified-live virus vaccines induce immunity at the mucosal surface. However, experience at Grange has shown that response to these vaccines were only evident in calves over two months of age when their own immune system was active.

It is difficult to successfully vaccinate young calves against these diseases because protective colostral antibodies block the vaccine, resulting in maternal antibody interference of vaccination. The Pasteurella organisms and other bacteria that cause pneumonia are notoriously poor in provoking an antibody response (immunisation). Development of effective vaccines has been difficult. It is difficult to get newborn calves to effectively respond to these vaccines.

A haematological profile of a respiratory disease outbreak in purchased Friesian calves, in which 55/71 were treated for respiratory disease was evaluated at Grange. The total white cell counts were higher in the calves with respiratory disease when blood sampled at 14 and 28 days after arrival. Serum haptoglobin levels were significantly increased in the calves treated for respiratory disease alone and for both respiratory disease and enteric disease (Table 1). The incidence of respiratory disease and enteric disease and requirement for frequent treatments was lower in dairy calves with a higher Ig concentration and ZST Units (P < 0.05). Suppression in cell mediated immune response as measured by lymphocyte blastogenesis was significantly greater in dairy calves treated for respiratory disease indicating that lymphocytes from calves with respiratory disease manifest an impaired capability to blast in vitro. Friesian calves with respiratory disease had reduced concentrate intake, elevated rectal temperature and increased respiratory rates. Infectious bovine rhinotracheitis virus (IBR), bovine respiratory syncytial virus (RSV) and parainfluenza type 3 virus (PI-3) were detected in nasal smears.

Microparticles as non-live viral vectors for respiratory tract vaccination

Non-live subunit vaccines encapsulated in biodegradable microparticles is a novel approach to achieving a protective immune response against specific pathogens in the hosts’ respiratory tract. The potential of microparticles composed of poly(lactide-co-glycolide) containing a model antigen, ovalbumin (OVA), was investigated in calves at Grange. The Mean size of the microparticles was determined by Scanning Electron Microscopy to be 2-3um. Three groups of calves were each administered 0.5, 1.0 or 5.0mg of encapsulated OVA. All animals received the appropriate booster dose of encapsulated OVA intranasally 5 weeks after the primary inoculation. Sera samples and nasal mucosal washings were collected weekly for 10 weeks following primary immunisation. The presence of OVA-specific IgA and IgG antibody content in nasal washings and sera was detected using an ELISA procedure. The immune response in the upper respiratory tract of young calves was determined. In all inoculated calves significant levels of ovalbumin-specific secretory IgA was detected as early as one week post-administration and levels persisted throughout the duration of the study. Peak levels of IgA were detected in all calves at week 3. Microparticles incorporating antigens show potential in the quest for generating complete protection in the young bovine against respiratory tract pathogens. Due to the ability to control the release of antigen from the particles by altering the polymers’ properties this system may in the future alleviate the need for repeated "booster" administrations.

Immunological research has led to an appreciation of the role and relative importance of individual proteins/cells in the immune response to infection and possible mechanisms of protection.

The key to successful calf rearing is disease prevention, not treatment. Once disease occurs, producers can only seek to minimise losses. Economically, returns are greatest when prevention is stressed.

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