Controlling streptococcal infections on a sow farm, a field experience

Introduction

Currently, streptococcal infections and colibacillosis are the two most recurrent pathologies in both the lactation and nursery phases and are responsible for significant losses in the swine industry. In this new production scenario, prophylactic vaccines and hygiene are some of the best measures that can be adopted to control both of these pathological processes.

Clinical case

This clinical case aims to explain how to approach streptococcal infection control within the current context of reduced antibiotic use becoming more and more present.

This commercial sow farm produces 6kg piglets and has an inventory of 2,000 sows. It is located in a highly pig-dense area in southeastern Spain. Initially, the farm received replacement animals from different sources, but in the last few years future breeding gilts have been brought in from the same source, the company's own multiplier.

Clinical symptoms of streptococci infection were frequently present in piglets in this source's farrowing unit. The main clinical expressions were arthritis throughout the lactation period, both in the forelimbs and hind limbs, and meningitis mainly at the end of the lactation period (from 23 days onwards). The percentage of losses and animals that did not recover in the farrowing unit ranged from 1.5 to 2%.

Video: 1. Arthritis in piglets in the farrowing pen.

In the nursery phase, it was also common to find meningitis, arthritis, and/or pneumonia, appearing as single cases or outbreaks, which could also be expressed as septic shock affecting animals with higher body condition in this case.

Video 2. Streptococcal septicemia in the farrowing pen.

Approach and control

The change in the current scenario based on the limitation and categorization of antibiotics and the lack of available tools for controlling streptococcal infections has forced us to look for alternatives for their control.

Faced with the problem of streptococcal infections on this farm, the control implemented focused on different aspects: management of the facilities and use of autogenous vaccines. Reducing stress in nursery piglets is also essential. To do this, we increased the space allowance per piglet and improved the minimum temperature and ventilation conditions, since poorly ventilated barns increase the risk of streptococcal infection. Autogenous vaccines are a useful alternative for dealing with this pathology although they have certain limitations.

Video 3. Onset of meningitis in piglets.

The primary problem with Streptococcus suis is that there are about 35 different serotypes, and there is no scientific evidence of cross-immunity between them. Several can coexist on the same farm, although there is a clear epidemiological relationship between the serotypes and the source of the farm's replacement animals. We find that in many farms several serotypes- both pathogenic and non-pathogenic- can coexist. This makes it difficult to know which serotype is responsible for the disease and, therefore, to choose the appropriate serotype to manufacture the autogenous vaccine.

Making an autogenous vaccine. The first attempt...

Once it became clear that one of the ways to control streptococcal infection was to use autogenous vaccines, we got started.

We started to send samples to the laboratory to isolate Streptococcus suis, and this is when a problem appeared; many of these samples were negative although it was evident that the animals presented clinical symptoms compatible with streptococcal infection. This experience reminded us that it is vital to know how to sample properly, choosing the appropriate animals that are clinically affected to achieve representation of the serotypes responsible for the pathology on the farm. Choosing animals that have not been previously treated with antibiotics is also crucial. Collecting samples from animals in the middle of a disease outbreak usually guarantees success and sampling should be as broad as possible, covering cases both in the farrowing unit and in the nursery. The main samples of choice are joints, heads and blood with anticoagulant.

Thus, we were able to isolate several Streptococcus suis serotypes. In this first attempt, we determined the virulence factors to determine which of them were pathogenic. It is important to know which serotypes are pathogenic since, in the case of this farm which began by bringing in gilts from various origins, the number of Streptococcus serotypes present on the farm was very high.

To type the strain, the main virulence factors determined were:

• Gen mrp. It encodes the muramidase-released protein but is not clearly related to the virulence of the strain in question.
• Gen epf. It encodes extracellular factor, although the absence of the protein it encodes does not determine whether or not the strain in question is virulent.
• Gen sly. It encodes suilysin, a hemolysin with cytotoxic characteristics.

Once the serotypes determined to be pathogenic were chosen, we made the autogenous vaccine using aluminum hydroxide as an adjuvant. This adjuvant was used since it was the only one available in the laboratory.

We blanket vaccinated the entire breeding herd on the farm and revaccinated them a month later. Blanket vaccination was decided upon with the objective of reducing the infection pressure on the herd.

However, the results were not as expected since clinical manifestations were not reduced in the farrowing unit or nursery.

Video 4. Streptococcal infection in farrowing pens.

Making an autogenous vaccine. The second attempt...

Convinced that autogenous vaccines should have a positive effect on streptococcal infection control since we use them for other pathologies and they are very helpful, we decided to try again, tweaking some parts of the manufacturing process.

Samples were sent for pathogen isolation in the same way as in the previous case.

Once Streptococcus suis was isolated, we determined the sequence types of each serotype in addition to the virulence factors.

It is important to determine the virulence factors but they are not the only ones to be identified since there are strains that do not contain some of the virulence factors and are extremely pathogenic.

Table 1. Streptococcus suis strains isolated from the farrowing unit and nursery to manufacture the autogenous vaccine. (Souce: Own preparation)

The capsule polysaccharide antigens were determined after typing some virulence factors to determine whether the strain was pathogenic or not. This determination is performed with molecular technology by performing a qPCR on the different genes encoding the main virulence factors.

Molecular identification of the strain by performing multilocus sequence typing (MLST) allows us to type the bacterium and compare it with strains identified as pathogenic in cases where we identify several strains in our sampling. This laboratory technique is highly standardized and can provide very precise information about the pathogenicity of previously isolated strains.

As previously mentioned, we must evaluate both the virulence factors and the sequence type (ST) of each of our strains and choose those with the most pathogenic character.

After choosing the strains, it is important not to exceed the number of strains to be included in the autogenous vaccine to avoid instability.

In addition to choosing the strains of greatest pathogenic interest, the choice of adjuvant is key since it is essential to generate an adequate immune response in the animal. Several studies support that using aluminum hydroxide as an adjuvant is not a good option since no humoral response is evidenced in the animal, with the oil adjuvants being more immunogenic (M. Gottschalk et al., 2022).

Once the autogenous vaccine was obtained with this new strain selection protocol and the new adjuvant, we blanket vaccinated the farm's breeding herd and revaccinated after 3-4 weeks. In addition to vaccinating the breeding sows, a vaccination protocol was established for gilts in the acclimation period on the farm, administering 2-3 doses during the gilt development phase.

With this second attempt, we managed to reduce practically all clinical expressions of streptococcus infection in lactation. Its appearance is also slowed slightly in the nursery. Although cases appear, the number of antibiotic therapies against streptococcus infections is reduced and we manage to control the clinical manifestations more effectively.

Currently, the farm continues to use autogenous vaccination by blanket vaccinating the entire breeding herd in line with the farm's clinical picture. It continues vaccinating gilts three times during the acclimation period. It is important to check the cases that appear to ensure the serotypes do not vary.

Conclusions

For an autogenous vaccine to work:

• Sampling should be as broad as possible in order to have the largest possible number of strains responsible for the disease on the farm.
• Establish screening by serotyping and detecting virulence factors.
• Perform type sequences in cases where there are several strains with the objective of differentiating between pathogenic and opportunistic strains on the farm.
• There is a very significant epidemiological relationship between the multiplier of origin and the recipient sow farm.
• Be clear about the adjuvant used in the vaccine to ensure adequate immune response.
• Establish an appropriate vaccination protocol for the farm in which the gilts have to be a fundamental factor since they are usually the ones that pass on the infection to their offspring.