Use of medicines and vaccines in a farm disease control strategy

Tackling any disease on farm can be difficult, although infectious diseases remain among the most demanding due to their diverse nature and complex relationships with host and environment. Understanding the pathophysiology of each disease is crucial to providing optimum control strategies on a farm by farm basis.

This article aims to provide a sound framework for implementing farm-based strategies, using multifaceted approaches to optimise disease control. The diseases used are chosen to illustrate a range of infectious diseases commonly seen in cattle practice (all of which have been seen during the author’s first few years as a recent graduate), using examples to highlight key control components and approaches.

Host-agent-environment factors

Infectious disease relies on the interaction between three main factors: the infectious agent (pathogen), the host and the environment. Of the three, we are most able to affect the host and the environment. Factors such as good nutrition, sound vaccination protocols and reducing stress will positively influence the host’s defence, and environmental aspects such as good hygiene and reduced stocking densities will reduce pathogen load, thus improving the host’s ability to deal with most pathogens. The ability of infectious agents to conceal themselves within the host and environment means sound diagnostic protocols are necessary to devise targeted control strategies.

An infectious disease agent will be “successful” if it is able to infect at least one other host before the current host either cures or dies. We can reduce the success of a disease by reducing transmission within the group – typically by improving the immune competency of the susceptible population (for example, stress reduction and vaccination) and by minimising infection pressure (for example, by biosecurity, improved husbandry and vaccination to reduce agent shedding).

Biosecurity

Biosecurity is often a “Cinderella” of infectious disease control, but must form the backbone to such plans. Food and water sources, bedding, contractors and machinery, co-grazing and quarantine procedures all form part of on-farm biosecurity, but this is often an issue we as veterinarians need to take more control of by helping farmers to identify their blind spots in this area.

Before embarking on any control strategy for a farm, we must have knowledge of two things: what diseases are on farm and what diseases are at risk of being introduced. With this information we can appropriately vaccinate and treat any imports to the farm during quarantine and identify areas of risk – for example, close proximity to neighbouring farms or a high buy in of cattle at different times of the year, and tailor the control plans to meet the requirements of the farm. Even within a farm, biocontainment is crucial to reduce disease spread – for example, farm workflows moving from most susceptible cattle to the least susceptible are vital for maintaining a healthy population.

Medicine choices

Before we look at treating infected cattle, we must have a working knowledge of pathogens found on the individual farm. Only then can we draw up appropriate first and second line protocols specific to the farm (for example, penicillin would have very little effect on cases of calf pneumonia on a farm with a known history of Mycoplasma outbreaks). Systematic diagnostic protocols are necessary to provide this information.

The treatment of a disease relies on the interaction between the host (cow), the drug (the active ingredient and composition) and the disease. Panel 1 illustrates some important factors to consider when identifying treatment choices.

Understanding the mechanism for the animal contracting the disease is important. For example, a group of young heifers with high lungworm burden most likely grazed on a contaminated pasture, so this aids the farmer in developing an understanding of the process that has led to a group of coughing youngstock.

Among the most important aspects when considering treating infectious diseases is the speed at which the appropriate treatment is given. The best farmers will be able to pick out subtle changes in their cattle’s behaviour long before many other clinical signs manifest, allowing crucial time to reduce the potential spread of disease.

Farmer compliance, however, has the potential to be one of the largest barriers to treatment success and is one that must be addressed each time we draw up a treatment plan. Sound communication with the farmer must be relied on to ensure both parties understand the importance of each other’s priorities to produce a shared decision on a treatment strategy.

Vaccine choices

While a detailed discussion of all vaccine options available on the UK market is outside the remit of this article, it will focus on the main aspects when considering a vaccination plan (Panel 2), with illustrations where appropriate.

Similarly to producing a treatment plan, we must understand the diseases on farm and those at risk of being introduced to the farm. We must also address the aim of vaccination – is it to reduce the incidence of disease, for example in herds with endemic leptospirosis? Or is it to prevent disease occurrence in high-risk situations?

Each vaccination strategy must include age and temporal considerations as each disease will have different periods of risk, so the target must be to complete vaccination (considering the demonstrated onset of immunity – that is, when the animals will be “protected”) before the period of risk.

Infectious bovine rhinotracheitis

Infectious bovine rhinotracheitis (IBR), caused by bovine herpesvirus 1, is capable of producing dramatic effects when infecting a naïve herd. Due to the disease pathophysiology and the nature of the herpesvirus, treatment is often unrewarding and prognosis is poor.

Severely reduced host defences – for example, the breakdown of the mucociliary escalator caused by necrotising tracheitis (Figure 1) – means secondary bacterial infections are common. Appropriate antibiotics should be selected based on diagnostic screening on farm. NSAIDs should also be used alongside antibiotics to reduce pulmonary inflammatory reactions typically seen in clinical cases of bovine respiratory disease¹.

In addition to the problems arising from an acute outbreak, IBR can have serious long-term implications due to its effects on reproduction and latency associated with the herpesvirus, therefore control must be initiated early on. The aim of vaccination should be clear before initiating a regime to allow the correct vaccine and protocol to be chosen.

Cattle Health Certification Standards outlines programmes for IBR control that identify different levels of health status associated with each programme² – this allows a farm-based strategy to be considered. For example, marker vaccines are advisable for both eradication and vaccinated, monitored free programmes to allow differentiation from vaccinated animals to those exposed to wild virus. Closed herds with sound biosecurity measures and good vaccination protocols should use the inactivated vaccine (Figure 2) as it will reduce the amount of active IBR circulating by reducing any latent carrier from recrudescing.

Care must be taken with calf vaccination protocols as some commonly used multivalent respiratory vaccines include a non-marker IBR element. Regardless of the vaccination protocols taken, biosecurity must remain an important part of IBR control.

Particular care should be taken with herds seeking to produce high health status bulls or those exporting cattle, as these animals should not be vaccinated due to ongoing eradication schemes throughout Europe that prevent importation of vaccinated animals. Biosecurity is paramount in these situations as cattle for export as IBR-negative should have no contact with a virus (either wild or vaccinal).

Bovine viral diarrhoea virus

Unlike IBR, infection of bovine viral diarrhoea (BVD) can remain unrecognised within a herd for a number of years due to its transient limited clinical signs in adult cattle. However, an acute outbreak in a naïve herd can have very large impacts on farm economics.

The main consideration with BVD control is the risk of producing or buying in persistently infected (PI) cattle. PI cattle are produced following contact between the susceptible (first trimester) foetus with the BVD virus in utero, most commonly due to a non-vaccinated dam contracting an acute infection, either from another transiently infected individual or from a PI individual. The unborn foetus becomes immunotolerant to the BVD virus due to the incapacity of the calf’s immune system to overthrow the virus. PI calves, if born alive, remain as a source of infection throughout their lives.

Risk analysis forms a large portion of assessing which control measure should be taken. Knowing the BVD status of the farm is important to recognise what steps need to be taken – BVD-free farms must decide whether biosecurity measures alone are enough to maintain a disease-free herd. In the author’s experience, this is often not enough – one of her practice’s larger dairy herds with no immediate neighbours saw a spike in bulk milk antibody titre last summer and since then has had more than 14 PI heifer calves born, which, along with milk loss and other fertility issues, has cost it in excess of £25,000.

Moreover, understanding the risk for disease entry must be considered, especially on farms that have close neighbours, buy in cattle or attend shows. Farmers who buy in cattle run the risk of introducing the disease, especially by buying in-calf cattle, due to the “Trojan horse” effect of carrying a PI calf.

Undoubtedly, vaccination is a large part of BVD control. Several vaccines are available on the UK market, but the appropriate one must be considered on a farm-by-farm basis, bearing in mind the strains present on farm and the BVD and risk status. Several aspects of the vaccines available must be considered (Panel 2) – for example, a short vaccinal course and onset of immunity may be important in a naïve herd in the face of a virus incursion, where this may be less important in a stable farm controlling disease by vaccination, testing and biosecurity.

Leptospirosis

Similarly to BVD, leptospirosis has the potential to go unnoticed for some time, although its zoonotic potential³ means its presence on farm should be taken seriously. Leptospirosis in UK cattle is caused by two main genomospecies of the serovar hardjo (Leptospira interrogans serovar hardjo and Leptospira borgpetersenii serovar hardjo).

Clinical signs tend to be non-specific, although naïve herds may experience abortions, changes to milk and a drop in milk yield⁴. While streptomycin (in preparations with penicillin) has been used historically for treatment of acute infections, no licensed product exists for the treatment of leptospirosis under UK drug licensing, so rules of the cascade, including appropriate withdrawals of seven days for milk and 28 days for meat, must be adhered to.

Understanding the herd status is important for implementing disease control. Calves in endemic herds should be vaccinated as soon as possible (adhering to vaccine licensing) as they can become exposed to the disease in the birth canal. Both cattle and sheep can act as carriers for the disease, which is transmitted when naïve animals come into contact with infected urine, water or products of abortion, therefore strict biosecurity is also necessary for control of this disease. Naïve animals entering an endemic herd must be vaccinated while in quarantine to ensure the effects of acute infection are not seen.

Lungworm

Although cases of lungworm (husk/parasitic pneumonia) submitted to the Animal and Plant Health Agency have fallen over the past few years⁵, it still remains an important infectious disease risk in some areas of the UK. Preventing lungworm infection relies on an understanding of the disease epidemiology and life cycle (Figure 3) and identifying local risk areas alongside control measures such as vaccination, anthelmintic use and biosecurity.

In areas with high lungworm risk, cattle should be vaccinated prior to their first grazing season. The vaccine involves two doses of irradiated L3 larvae, thereby allowing the animal to gain resistance before encountering the parasite.

Anthelmintics are widely effective in the treatment of infected cattle and longer duration products offer some periods of control, although challenge/clinical disease is unpredictable and severity is not proportional to challenge (although cattle with high worm burdens will have poorer prognosis of full recovery). Cattle most at risk from the disease are youngstock during their first grazing season, as immunity develops on exposure to the parasite. Control plans should also include removing lungworm from carrier animals such as older cows, bulls or bought-in replacements.

Clostridia

Although clostridial disease is often forgotten, it should be considered within any herd health plan as its effects can be spontaneous and potentially very damaging to a susceptible population. It is caused by a variety of Clostridium species – Gram-positive, anaerobic bacilli that cause rapidly fatal diseases. Clinical signs associated with clostridial disease in cattle can be widespread; however, the most common clinical sign is death or rapidly dying cattle.

Farmers must understand eradication of clostridial disease is not possible as the organisms are widespread/ubiquitous in endemic areas. Entry into a host may be gained through various different methods, including ingestion, absorption from the gut due to abnormal commensal overgrowth, or entry through the skin via wounds⁴. Cattle grazing at pasture are at greatest risk from the disease and often there appears to be a stress “trigger” initiating the disease process.

The main focus on disease control must be vaccination as treating individually affected animals, unless identified extremely quickly, is often unrewarding. Monovalent and multivalent vaccines are available for cattle, and should target known problem strains on farm. It is important risk periods are covered, often giving annual boosters prior to turnout.

Summary

Infectious diseases are challenging to control due to their variations and relationships with the host and environment. We should develop comprehensive, farm-based approaches that target the current and future threats to cattle health using up-to-date knowledge of the farm’s disease and pathogen status, risk status and disease control priorities. Biosecurity remains a key component of infectious disease control and is often poorly implemented or overlooked.

Above all, we must have farmer compliance and commitment to tackling such diseases, which can only be achieved if we listen to clients, understand their priorities and frustrations, and devise both practical and farm-based options for them. Only then will we tip the balance of infectious disease in favour of the cattle.