What do we know about Streptococcus uberis mastitis and why is it so frustrating?
Streptococcus uberis is recognised in many countries around the world as the problem mastitis bug that can cause difficult to cure cases that often recur. This is epitomised by the common phone call conversation farmers have with their veterinarians saying “I treated the cow and all seemed to be going well until just before I was going to return her milk to the bulk tank when the mastitis came back again”. Hardly surprising then that it gets called the frustrating mastitis bug. Successful treatment is often elusive in these situations and so preventing Streptococcus uberis mastitis cases is much more rewarding than attempting to treat your way out of a problem.
What is Streptococcus uberis and where can you find it?
Streptococcus uberis is a bacterium which commonly causes both clinical and sub-clinical mastitis in dairy cattle. It can colonise both animals and the environment and is predominantly found in manure and organic matter, including bedding materials. Streptococcus uberis has been found in the alimentary tract (lips, tonsils, rumen, rectum and faeces), respiratory tract, urogenital tract, infected wounds and abscesses and coat of cattle as well as the teat orifice, teat canal, teat skin and infected mammary glands of cattle.
How does Streptococcus uberis spread?
Streptococcus uberis spread is mainly via the alimentary tract from cows licking and grooming with around 50% of dairy cow skin samples being positive for Streptococcus uberis to faecal contamination of the cow environs with around 25% of cow faeces being positive for Streptococcus uberis and grazed pasture having higher levels of Streptococcus uberis than ungrazed pasture. Faecal contamination of bedding in loose yards or cubicle (free stall) beds in housed cattle will also result in high levels of Streptococcus uberis with particularly organic cow bedding such as straw or sawdust facilitating further multiplication. The presence of Streptococcus uberis in the environment results in contamination of teats and teat ends and subsequent intramammary infection with high bacterial counts resulting in increased infection rates.
When do intramammary infections with Streptococcus uberis infection occur?
Dairy cows are most commonly infected from environmental sources in the dry period often resulting in clinical cases in the subsequent lactation although infection can also originate in lactating cows with the first 75 days of lactation being a greater risk period than the rest of the lactation. Up to 60% of clinical cases of mastitis can have their origins in the dry period with highest infection rates see just after drying off and around parturition. Strain typing has shown that some Streptococcus uberis infections can become persistent and then have the potential to behave in a contagious (cow-cow transmission) manner. In this situation often there are only a few different strains found on a single farm as opposed to the multiple heterogeneous strains found in herds where environmental sources are important.
How common is Streptococcus uberis mastitis in Canada?
Streptococcus uberis mastitis is found throughout the world and is most common in countries with a developed dairy industry. It is designated as an environment associated major pathogen and often accounts for approximately one third of clinical cases (Fig 1).
Figure 1. Relative percentage of mastitis pathogens in clinical mastitis samples with positive bacteriologic growth on Canadian dairy farms.
Adapted from R. Olde Riekerink et al., JDS vol. 91, no. 4, pp. 1366-1377, 2008.
How long do Streptococcus uberis intramammary infections last?
The duration of Streptococcus uberis infections can vary considerably with around 60 % of infections lasting less than 30 days; however, approximately 18 % can become chronic and last > 100 days. There are reports of Streptococcus infections persisting through dry periods and lasting up to 20 months. These sub-clinical infections are increasingly recognised as problem. Chronically infected cows can act as an important source of new infections in herds where udder adapted strains are present. For the most part, environmental sources generally remain the major concern.
What does Streptococcus uberis mastitis look like?
As with any intramammary infection, the changes in the appearance of the milk produced from quarters infected with Streptococcus uberis can vary from no visible changes in subclinical cases to the usual milk clots, swollen udders and even through to watery cases with sick cows in clinical cases. Streptococcus uberis clinical mastitis cases are most commonly Grade 1 (milk affected only) or Grade 2 (milk and udder affected), with only a few cases progressing to Grade 3 (milk, udder and cow affected).
How can Streptococcus be controlled?
5 point plan and Milking routine
Significant improvements in udder health (clinical mastitis rates and bulk tank somatic cell counts) have been made over recent decades in the dairy industries around the world by application of measures based around controlling contagious mastitis pathogens. Three of the five points in the control plan include treatment and culling to remove existing infections. These measures have little effect on udder health impacts from environmental mastitis pathogens – although they would be beneficial where persistent udder-adapted strains were prevalent.
Minimising teat end exposure to Streptococcus uberis and other environmental mastitis pathogens such as E.coli by optimising cow environment hygiene management including udder preparation prior to milking will significantly reduce new intramammary infection rates.
Although removal of existing infections by successful treatment of environmental pathogens, such as Streptococcus uberis, will improve udder health in the cow under treatment, the benefit to other cows in the herd is minimal compared to contagious pathogens. This is because new Streptococcus uberis infections are more likely to come from environmental sources than the less common udder adapted existing intramammary infections.
Treatment of clinical mastitis in lactation is a welfare requirement; however, cure rates are significantly less than with treatment at drying off.
Dry cow treatment
New infection rates during the dry period are high, so prevention with hygienic management and use of internal teat seal where permitted is as important as treatment with antibiotic at drying off to remove existing infections and prevent new infections.
What about the future?
It is often said that prevention is better than cure, and mastitis control is no exception. Improvements in milk quality and udder health following decades of application of the 5 point plan has perhaps stalled a little following a shift of predominant pathogens from contagious towards environmental origin. Fig 2.
There is no denying the improvements seen in the control of environmental mastitis to date by improving cow environment hygiene management and use of vaccination against E. coli and coliforms; however, there is always a place for any new tools to help reduce mastitis new infection rates caused by Streptococcus uberis.
Treatment of Streptococcus uberis mastitis is known to be difficult in terms of both success and recurrence and persistent infections can lead to significant use of antibiotics at both a cow and herd level to try and contain this frustrating pathogen. In the current climate of “One Health” (human and animal health and medicine use being considered together) and the aim for both responsible and reduced antibiotic use in the dairy sector, relying on treatment for the control of mastitis and in particular Streptococcus uberis mastitis will not be sustainable.
It is clear that all mastitis control plans need to include treatment of clinical mastitis for the welfare of dairy cows. It is also clear, particularly with Streptococcus uberis mastitis cases which can be difficult to treat successfully, often recur and can spread from cow to cow, mastitis control plans reliant on treatment will be doomed to fail if they don’t contain significant control measures to reduce mastitis new infection rates from all sources.
To control this frustrating mastitis pathogen, we need more tools in the toolbox to reduce new Streptococcus uberis infections from both the environment and infected cows. Consequently, a successful vaccine against Streptococcus uberis mastitis has long been an aspiration of many veterinarians and farmers the world around.