Maestanyglwyden Project Results: Mastitis Field Trial

Alana Jackson

Cain Farm Vets, Llansantffraid 

 

Project description:

Mastitis is a problem seen on sheep farms throughout lactation and at weaning. Mastitis affects different farms with varying severity – including seeing dead ewes (as a result of toxic mastitis) and/or premature culling out of young ewes with affected udders. This, together with mastitis treatment costs and knock-on slower lamb growth rates (due to reduced milk availability), makes ewe mastitis a costly problem and an animal welfare issue. 

 

Maestanyglwyden (MTG) farm background 

MTG is a sheep and beef farm based on the Powys/Shropshire border near Oswestry. There are nearly 1000 ewes, consisting of Texel crosses, Mules and Beltex-cross ewes. The ewes are lambed in batches, with ~100 lambing at the end of January (sold off as couples), ~300 due in February, ~500 in March and ~100 ewe lambs lambing in April. 

Mastitis has been a major cause of losses in the MTG flock over the last five years; around 10% of ewes will get mastitis each year. In real numbers, this equates to 80-90 ewes/year getting mastitis. Losses are mainly seen due to premature culling, reduced lamb vigour, and occasionally through sheep dying as a result of toxic mastitis. 

The MTG flock lambs inside, housed approximately six weeks pre-lambing. There are small individual pens that freshly lambed ewes are moved into within two hours (hrs) of lambing. After 24-48 hours in the individual pens, the ewes and lambs are moved into larger pens with multiple ewes and lambs. After a further 24-48 hours in large group pens, ewes and lambs are moved outside, weather dependant. In-lamb ewes and group pens are bedded on straw, blown in with a straw chopper. Individual pens are bedded up with straw by hand. 

 

Project aims: 

The main aim of the project is to see which factors are affecting the mastitis incidence in the Maestanyglwyden flock. As well as employing good nutrition and hygiene throughout lambing, a proportion of the flock was vaccinated using the VIMCO vaccine, to help reduce potential antibiotic usage on farm, and to boost the immunity of the ewes against one type of mastitis-causing bacteria (Staphylococcus aureus). 

Body condition score (BCS) was taken pre-lambing, and BCS was taken at point of a mastitis event/case. The ambient temperature and rainfall were also taken throughout the study to see if there was any correlation between mastitis cases and weather patterns. 

 

Field trial outline:

During March, approx. 500 twin-bearing ewes were due to lamb; these were the field trial flock. 

Half of these were vaccinated with VIMCO and half were left unvaccinated. All the ‘trial sheep’ were kept in the same shed throughout lambing, and no special treatment was given to either vaccinated nor unvaccinated ewes. The ewes were not randomly allocated to their group (vaccinated or unvaccinated), but every other ewe was allocated to the alternative group. 

The ‘trial sheep’ were BCS scored pre-lambing (during their routine handling in February). The ewe ID (ear tag number) and incisor number (age) was also recorded, and whether they had received the vaccine. The ewes were fluke drenched (Flukiver®) and routinely docked, leaving as much wool cover over the udder as possible, clipping only around the tail area. 

Pre-lambing metabolic blood samples were taken (two to three weeks pre-lambing) from 20 ewes within the flock to assess whether the feed offered pre-lambing was meeting the late gestation ewes demand. 

The VIMCO-vaccinated ewes were given two injections: one five weeks pre-lambing, and one three weeks pre-lambing. The use of the vaccine was to ascertain whether the null hypothesis – which showed that there is no significance between the incidence of mastitis in the vaccinated and unvaccinated ewes – was correct. 

Temperature loggers were placed on farm, and took the ambient temperature every 10 minutes. This data has been collated to give the minimum, maximum and average temperature, and temperature variation per day over the trial (plotted in Figure 1). 

Rainfall measures were placed on farm, to record rainfall volume once a week. This rainfall measure is plotted in Figure 2. 

Cases of mastitis were recorded throughout lactation. The ear tag and BCS of the ewes was recorded, and a milk sample was taken if possible. Milk samples were then sent off to the lab for bacterial culture.  

When ewes were weaned, their udders were examined for further evidence of mastitis (abnormal intra-mammary masses). Ewes that had abnormal udders/evidence of chronic mastitis at weaning were recorded and culled. 

 

RESULTS

1.    Pre-lambing metabolic bloods

Blood samples were taken from 20 ewes (a mixture of twin-, triplet- and single-bearing ewes) for metabolic profiling (three weeks pre-lambing). These were taken to see whether the ewes’ diet (pre-lambing) was meeting their protein and energy demands. The twins’ (within the study) results are shown below. 

The ewes were body condition scored (BCS) when the blood samples were taken. In this lowland flock, the aim for BCS should be 3-3.5 out of 5.[1] 

The β-hydroxybutyrate (BOHB) analysis shows the ewe’s current energy balance. If the BOHB value is too high, it suggests the ewe is in negative energy balance, and having to breakdown her own fat reserves to meet her energy demands. 

Urea-N analysis provides information about the effective rumen degradable protein (ERDP) intake, to assess the ewe’s current protein provisions in her diet. Urea-N that is too low suggests the ewe is not receiving sufficient protein in her current diet. This can result in poorer colostrum quality and quantity, with knock-on effects on milk production. 

Albumin provides information about the ewes’ long-term protein status. Underlying diseases and inflammatory processes can affect albumin levels. Diseases such as fluke or gut worms can result in decreased albumin levels. 

Magnesium and copper status were also analysed. Magnesium is an important enzyme-activating element, especially in energy, carbohydrate, lipid and protein metabolism pathways. Magnesium levels also give some indication of calcium intakes. Copper levels are taken, firstly as an indicator of the likelihood of deficiency in late-pregnancy, which can lead to ‘swayback’ in lambs. However, copper toxicity is also becoming more commonly diagnosed within Texel ewes. Copper levels can also be elevated with underlying inflammatory disease issues. 

 

Results:

TWINS

Diet: The twins were receiving 1.5lb of 18% protein concentrates per day. They had access to molasses lick, and were fed ab lib big bale haylage (14% crude protein and 10.1ME).

Ewe ID

BCS

BOHB (mmol/l)

UreaN (mmol/l)

Alb (g/l)

Mag (mmol/l)

Cu (umol/l)

7902

3.5

0.47

2.82

26.30

1.06

27.70

2111

3.5

0.47

3.11

28.50

0.94

19.30

5115

3

0.43

2.30

29.40

1.06

21.60

7352

3.5

0.37

2.32

31.90

1.08

21.40

5096

3

0.46

2.40

30.80

1.12

22.40

743

4.5

0.45

2.34

30.00

0.95

24.00

2019

3

0.31

2.00

27.70

1.05

27.70

AVERAGE

3.43

0.42

2.47

29.23

1.04

23.44

Result discussion:

From these results in the twins, you can see that one ewe out of the seven ewes blood-sampled was over-conditioned – i.e. too fat (4.5/5) – with the rest being within the target BCS (3-3.5). 

The twin-bearing ewes’ diet show that their current energy and protein levels were being met. There are marginal albumin levels in over half of the sampled twin-bearing ewes, and elevated copper levels in all but one ewe. 

Reduced albumin levels are marginal and could be due to a fluke burden – these ewes were fluked on the day they were blood-sampled. 

The ewes have high copper levels, despite no copper supplementation. 

 

2.    Temperature and rainfall results

The graph in Figure 1 shows the minimum, maximum and average temperature (blue, red and purple lines respectively); temperature variation is plotted as the green area, against the time (in weeks) along the horizontal axis. 

From Figure 1, you can see that the temperature varied throughout the trial. The largest variation in temperature (difference between the minimum and maximum temperature recorded) occurred between week 6-9 and again between week 19-21.

Rainfall is plotted in Figure 2 as volume in a bar chart. The time (in weeks) is plotted on the horizontal axis. 
 

From Figure 2, you can see there were weeks with very little or no rainfall through the trial: between week 6-9, week 14-17, and again between week 20-22. 

 

3.    Mastitis incidence

The total number of mastitis cases were calculated (both from the vaccinated ewes and the unvaccinated ewes). 

Table 1. A table showing the number of ewes vaccinated and unvaccinated, and the number of ewes with mastitis from the respective groups. 

Table 1:

Vaccinated

Unvaccinated

Totals

Average BCS

3.53

2.86

 

Total number

225

214

439

Number of mastitis cases

14

16

30

% of mastitis cases

6.2%

7.5%

6.8%

After the total number of ewes was calculated, there were 439 ewes included within the trial. Other ewes either didn’t lamb in March (lambing earlier or later), or had either more or fewer lambs at foot. To keep as many variables the same, only twin-bearing/-rearing ewes lambing in March were included. 

From Table 1, you can see that the percentage of mastitis cases in the vaccinated was 6.2%, compared to the unvaccinated at 7.5%. 

Another observation is that the average BCS of the vaccinated group was higher at 3.53, compared to the unvaccinated group, which was lower at 2.86. 

There were fewer ewes in the unvaccinated group than the vaccinated group (214 compared to 225 respectively), but more cases of mastitis in the unvaccinated compared to the vaccinated group. 

 

4.    Cases plotted onto temperature and rainfall graph

The graph in Figure 6 shows all the collected data (min + max temperature and rainfall volume), as well as the number of mastitis cases found. 

 

5.    Mastitis culture results

Ewes with a case of mastitis were cultured where possible. These milk samples were collected into a sterile pot and sent off for culture to the Quality Milk Management Services Ltd (QMMS) laboratory. 
Culture results were produced on nine mastitic milk samples. The results are shown in the Figure 4 bar chart and Figure 5 pie chart. 

Figure 4 shows that three cultures found a pure Staphylococcus aureus (S. aureus) growth, two cultures found S. aureus plus other bacterial growth. Pure Mannheimia haemolytica (M. haemolytica) was cultured from one sample, with two other samples growing M. haemolytica and other bacterial growth. Streptococcus sp. with other bacterial growth was cultured in one sample. 

Figure 5 presents the culture result in a pie-chart design. From the pie chart, you can see that over half of the cultured mastitic milk samples involved S. aureus, either as a pure growth or in combination with other bacteria. 

M. haemolytica was also cultured in over a quarter of the cultured samples (either pure or in combination with other bacteria). 

 

Statistical analysis: 

Data were entered in an Excel spreadsheet, with Ewe ID recorded (tag number), together with BCS (score from 1-5) and incisor number (2,4,6,8). Throughout the trial, a case of mastitis was entered into the data as score 1, with the date of mastitis recorded. At the end of the trial, if no case of mastitis was entered, ewes were assigned a score of 0. 

Totals were calculated; the number of ewes getting mastitis and those who did not (number diseased + and - respectively) in the vaccinated (exposure +) and unvaccinated group (exposure -). These figures were put into OpenEpi 2x2 Table Statistics. 

Open Epi 2 x 2 Table

 

Disease

Totals

 

(+)

(-)

 

Exposure

(+)

14

211

225

(-)

16

198

214

Totals

 

30

409

439

The data were analysed using a Chi square test (two-sided (tailed) test), which showed the p value being 0.6, and therefore, there was no statistical difference between the outcome (getting mastitis) between the two groups (vaccinated vs unvaccinated). 

The Fishers exact was p= 0.7. There was a small effect, but no statistical difference between the incidence of mastitis between the vaccinated and unvaccinated group. 

 

DISCUSSION 

Mastitis is still the major problem in the MTG flock. However, this year the overall mastitis rate within the flock has improved from ~10% to 6.8% within the trial flock. Reduction in mastitis rates is most likely due to a combination of factors: reduced stocking rate in the lambing shed (increasing the shed size by one-sixth), improved hygiene, and improved nutrition up to peak lactation and until six weeks after turn-out. 

The statistical analysis showed that there was no statistical difference in the mastitis incidence rate between the vaccinated and unvaccinated group of ewes in this small trial, with only a few ewes developing mastitis. The positive effect of the vaccine appears to be subtle, and having more ewes within the study may have shown that vaccination could significantly reduce the mastitis incidence rate. 

Infection of the ewe’s udder, resulting in mastitis, can be broadly categorised into two factors: non-animal related (including environmental/climatological factors, housing hygiene, nutrition etc) and animal-related factors (including litter size, immune status/function, anatomic factors/teat placement etc.).[14]

Management and control of mastitis in flocks therefore needs a multi-factorial approach. However, most studies agree that teat damage is one of the biggest risk factor for a ewe developing mastitis (damage of the skin and other natural teat/mammary gland defences).

Healthy teats are the first line of defence for a ewe to prevent mastitis.[19] The skin surface of the teats has fatty acids present, which have bacteriostatic properties, limiting bacterial numbers around the teat end. Keratinised lipids line the internal teat canal; these entrap invading bacteria, and are flushed out at the next ‘milking’ event. The local teat musculature closes the teat end, with closure achieved around 20-30 minutes after milking, making entry harder for invading bacteria. In addition to these defences, there are also lymphoid nodules at the border of the teat duct and teat cistern (mammary gland), which help remove any bacteria that manage to get up the teat canal into the cistern. Any teat skin/teat damage makes ewes more susceptible to mastitis. 

 

Non-animal related factors

Nutrition:

Ewe nutrition is thought to be key in preventing teat damage, and therefore mastitis. 

Monitoring the ewes’ nutrition pre-lambing, by BCS and pre-lambing metabolic bloods, can ensure that ewes start their lactation in the best metabolic state. In the MTG flock, the pre-lambing metabolic bloods showed that the ewes’ current energy (BOHB) and protein (UreaN) demands were being met in late gestation. 

One study found ewes that were underfed protein during pregnancy had four times greater odds of suffering acute mastitis.[12] Alongside this, ewes that develop pregnancy toxaemia were also found to be predisposed to mastitis after lambing.[14] A study conducted in dairy goats found that goats with an elevated BOHB before parturition had a lower milk yield during lactation.[10] This suggests a negative energy balance pre-lambing could have prolonged knock-on effects, reducing milk production during lactation.

Reduced milk production may result in increased suckling, increasing the risk of traumatic teat lesions. Traumatic teat lesions are most likely to occur when the lambs’ demand for milk is not met, with a peak occurrence three to four weeks into lactation.[13]

Most farms were found to reduce additional feeding to ewes after one month post-lambing.[12] This is when the lambs’ demand for milk is highest. Reducing the feed availability to the ewe at this point could reduce milk production, creating hungry lambs that may suckle for longer and more vigorously, increasing the risk of teat damage and mastitis. 

The ewes’ average BCS (taken pre-lambing) was calculated for both the vaccinated and unvaccinated ewes in the trial-flock. From Table 1, you can see the unvaccinated ewes were thinner than the vaccinated. This difference in BCS could have affected the likelihood of the unvaccinated ewes getting mastitis, as low BCS is a risk factor for ewes developing mastitis.[2]

The MTG flock were fed for longer this year after turn-out, to ensure the ewes were receiving enough protein and energy for six weeks after lambing. The weather pattern this year meant that six to nine weeks into the trial was very dry (Figure 2), with reduced grass growth. The MTG flock is set stocked, meaning the grass availability is limited by rainfall/grass growth. Despite the additional feed supplied to help maintain milk production, there was still a peak in mastitis cases during this dry period, as seen in Figure 6. 

Eight cases of mastitis occurred within 23 days from 16 April (week 7) through to 9 May (week 10). This peak of cases may have coincided with peak lactation, but this is unknown, as the exact lambing date was not recorded for each ewe. Interestingly, this peak in cases did not occur during the wettest time, as we would have predicted, but peaked several weeks after the largest rainfall volume was measured. 

There were 17 cases of mastitis detected at weaning; no correlation between weather can be made about this cluster of cases, as these were chronic cases that had been missed during lactation. It does highlight that over half of the mastitis incidents were only identified at weaning. These ewes would not have received any treatments and their lambs would not have received any additional supplementary feeding. One study suggested that if lambs (from affected ewes) are not supplementary fed, they would ‘cross-suckle’ other ewes in the gang, potentially transmitting bacteria to other ewes’ teats within the same flock.[5] 

 

Bacterial Involvement: 

The bacteria involved in ewe mastitis in ‘suckler’ ewes is thought to be primarily caused by Staphylococcus aureus and Mannheimia haemolytica.[14] This was also the picture found from the culture results from the MTG trial flock (Figure 5). S. aureus is a commensal found on the skin surface, as well as being isolated from the nares and tonsils of lambs. [15] M. haemolytica has also been isolated from the nares and tonsils of nursing lambs.[16] The skin of the teat can become contaminated with bacteria, and organisms enter the teat following lambs suckling,[17] prior to teat-end closure. 

S. aureus infection in the udder can result in abscess formation (chronic infection), which can subsequently rupture, releasing the bacteria. The bacteria can be shed into the environment,[18] or form more abscesses within the udder. S. aureus is able to ‘hide’ in the ewe’s udder, forming biofilms under which the bacteria can survive, with resistance to host defences (e.g. white blood cells/immune cells) and antibiotics.[20] This demonstrates why culling ewes with any case of mastitis is important: to prevent ewes shedding bacteria and returning the following year with another case of mastitis. 

Weather and other environmental conditions also affect how well these bacteria can survive. Studies have shown that M. haemolytica survive longer in cold and wet conditions (being isolated from pasture occupied by sheep with pneumonia and from bedding used by ewes suffering with mastitis).[17] In cattle, it has been shown that S. aureus colonisation of the teat skin is also greater in colder conditions. 

 

Lambing environment:

One study looking into risk factors of ewe mastitis concluded that indoor management before, during and after lambing was associated with an increased risk of mastitis.[3] Increased concentration of bacteria within the sheep shed can be the result of high stocking densities, insufficient bedding and limited ventilation[5] – all of which can lead to an increase in intramammary infections. This year, MTG increased the space for lambing ewes by one-sixth, meaning that the lying space per ewe increased from 1.5m2/ewe to 1.9m 2/ ewe. This is higher than the recommended 1.2-1.4m 2 lying space for unshorn pregnant 60-90Kg ewes.[4] More individual pens were also created this year, so freshly lambed ewes were moved into individual pens more promptly. Reducing the stocking density (more space per ewe) will improve the hygiene and pathogen load within the shed. 

 

Outside conditions (rainfall and temperature):

Throughout the trial, there was a large variation in temperature. The prediction of the trial was that we would see an increased number of mastitis cases during the coldest and wettest time. This cold and wet correlation was not shown in this field-trial. The peak of cases (as mentioned above) between week 7 and week 10 did, however, occur when the lowest minimum temperatures were recorded, but did not coincide with the highest rainfall. 

Further work into ambient temperature effect is needed to see any correlation. One study found that during the colder weather, lambs suckled more frequently, with teats becoming chapped. This teat damage then resulted in increased colonisation of the teat duct from bacteria, leading to mastitis cases,[11] which could be similar to findings from the MTG trial flock. 

 

EWE RISK FACTORS:

Litter size 

Increased litter size (ewes suckling two or more lambs)[5] is a known risk factor for mastitis[3] – thought to be due to increased suckling time (from more lambs), with increased milk demand. The higher the number of suckling lambs per ewe is usually accompanied by more frequent suckling events, with a longer total sucking time. This will increase the risk of teat lesions and also the time the teat canal is open. Both of these could increase the likelihood of bacteria entering the mammary gland. 

Ewes bearing multiple lambs during pregnancy have increased energy demands, and are therefore predisposed to pregnancy toxaemia (and mastitis) after lambing, as mentioned above.[14] 

 

Immunosuppression/immunocompromise:

Ewes with a worm (gastro-intestinal worms) or fluke (trematode) burden were found to be more at risk to mastitis.[23] Again, this is thought to be due to a protein drain/deficiency in the ewe, which negatively impairs her immune function. 

 

Providing protection: 

The MTG flock field-trial looked into whether ewes vaccinated against S. aureus bacteria were more protected (measured as a lower incidence rate of mastitis), compared to unvaccinated ewes. The licence of the vaccine used in the MTG ewes only states, however, that it will ‘reduce the incidence of sub-clinical mastitis’.[21] Therefore, the vaccine cannot be expected to reduce clinical cases; however, boosting the ewes’ immunity to S.aureus will have been beneficial to some extent. 

Other authors have looked into vaccination to provide protection to ewes against mastitis. One study looked into using a vaccine against M.haemolytica, which is a major cause of mastitis seen in ‘meat’ sheep flocks.[22] The study used a vaccine systemically (under the skin of the udder) and found this provided no protection against M.haemolytica. However, when given intramammary (into the teat canal), the vaccine protected the udder against mastitis (due to M.haemolytica) for seven days, but not 14 days. This shows that the local protection, provided by vaccination into the udder, could prevent an infection for seven days. However, in practical terms, most sheep are not handled after the first few days, so ‘re-boostering’ ewes (every seven days) is not practically applicable. 

 

Genetics /teat angles:

There is a hereditary component of ewes to susceptibility to mastitis,[25] which may play a role in some flocks. Often ‘high-value’ animals are retained despite having mastitis, which could potentially propagate infection pressure on farm. 

Poor udder and teat confirmation are associated with higher levels of intramammary infection, detected with increased somatic cell counts (SCC).[26] Interestingly, the lambs’ growth/weight in one study was lower in the ewes that had high SCC, indicating lower milk production from the ewes – possibly due to damage to the mammary gland from bacterial infection. 

Other studies have shown that teats positioned further forward on the udder were at a higher risk of infection (teat angle),[25,16] as these teats are thought to have less protection from the weather when lambs are suckling.   
 

CONCLUSION: 

Preventing mastitis in ewes needs to be a multi-factorial approach. As mentioned above, nutrition of the ewe (before lambing and into first six weeks of lactation) is critical, to ensure the ewe can produce sufficient milk for her growing lambs. Monitoring BCS of ewes will help identify potential dietary issues (lower BCS needing more supplementary feeding etc.). 

Minimising the risk of teat damage is also really important to reduce the chance of bacterial colonisation of the teat/udder and subsequent mastitis cases. Ewes that have any case of mastitis should be treated as soon as possible, but marked for culling. Ewes with mastitis should be isolated with their lambs, to reduce the risk of cross-suckling; the affected ewes’ lambs will be hungry, due to reduced milk availability, and so cross-suckling other ewes in the field could transmit mastitis, causing bacteria to other ewes. 

The findings in this field-trial show that the vaccine did have a subtle benefit in reducing mastitis rates; however, this was not a statistically significant effect. Hygiene, stocking density and improved nutrition (into lactation) will have played a large part in reducing the mastitis cases seen in this flock this year. 

Further work into ambient temperature and rainfall would be interesting, to see if after a particular pattern in weather would highlight ‘risk periods’. This would allow farmers to increase vigilance for mastitis, or supply supplementary feed during higher risk times. 


Acknowlegdements: 

Thank you to Farming Connect for the funding that allowed the field-trial to take place. Thank you to Lisa Roberts for enabling the trial to become a reality. 

Thank you to Mr. R Morris, Maestanyglwyden, for participating within the field trial, and continued data collection throughout the trial. 

Thank you to the directors at Cain Farm Vets, Robert Edwards and Simon Wilson, for allowing the author time and access to resources to gather the data. Thank you to colleague Lucy Tubbs for helping collect the temperature data.

Thank you to Joseph Angell for help on the statistical analysis and write-up. 

 
References: 

1.    AHDB (2019) ‘Managing ewes for Better Returns’, Better Returns Programme. Available at: https://projectblue.blob.core.windows.net/media/Default/Beef%20&%20Lamb/BRP_Managing_Ewes_BR_190215_WEB.pdf. (Accessed: 2 September 2021). 

2.    Better Returns Program - Plus (2016) ‘Understanding mastitis in sheep’, Better Returns Programme. Available at: https://projectblue.blob.core.windows.net/media/Default/Imported%20Publication%20Docs/BRP-plus-Understanding-mastitis-in-sheep-180716.pdf. (Accessed: 2 September 2021). 

3.    Cooper, S., Huntley, S.J., Crump, R., Lovatt, F., Green, L.E. (2016) ‘A cross-sectional study of 329 farms in England to identify risk factors for ovine clinical mastitis’, Preventative Veterinary Medicine, Vol 125, pp. 89-98. doi: 10.1016/j.preventmed.2016.01.012. 

4.    Better Returns Program () ‘Reducing lamb losses for Better Returns’, Better Returns Programme. Available at: https://farmantibiotics.org/wp-content/uploads/2018/01/BRP-Reducing-lamb-losses-for-better-returns-manual-14-231115.pdf. (Accessed: 2 September 2021). 

5.    Gelasakis, A.I., Mavrogianni, V.S., Petridis, I.G., Vasileiou, N.G.C., Fthenakis, G.C. (2015) ‘Mastitis in sheep – the last 10 years and the future of research’, Veterinary Microbiology,  Vol 181 (1-2), pp. 136-146. doi: 10.1016/j.vetmic.2015.07.009. 

6.    Grant, C., Smith, E.M., Green, L.E. (2016) ‘A longitudinal study of factors associated with acute and chronic mastitis and their impact on lamb growth rate in 10 suckler sheep flocks in Great Britain’, Preventative Veterinary Medicine, Vol 127, pp. 27-36. doi: 10.1016/j.preventmed.2016.03.002. 

7.    Ratanapob, N., VanLeeuwen, J., McKenna, S., Wichtel, M., Rodriguez-Lecompte, J.C., Menzies, P., Wichtel, J. (2018) ‘The associated of serum β-hydroxybutyrate concentration with fetal number and health indicators in late-gestation ewes in commercial meat flocks in Price Edward Island’, Preventative Veterinary Medicine, Volume 154, pp. 18-22. doi: 10.1016/j.prevetmed.2018.03.009.

8.    Kalyesubula, M., Rosov, A., Alon, T., Moallem, U., Dvir, H. (2019) ‘Intravenous infusions of glycerol versis propylene glycol for the regulation of negative energy balance in sheep: a randomized trial’, Animals (Basel), Volume 9(10). doi: 10.3390/ani9100731.

9.    Sarginson, N.D. (2007) ‘Pregnancy toxaemia’, Diseases of sheep, Volume 7, pp. 359-362.

10.    Karagiannis, I., Panousis, N., Kiossis, E., Tsakmakidis, I., Lafi, S., Arsenos, G., Boscos, C., Brozos, Ch. (2014) ‘Associations of pre-lambing body condition score and serum β-hydroxybutyric acid and non-esterified fatty acids concentrations with periparturient health of Chios dairy ewes’, Small Ruminant Research, Vol 120 (1), pp. 164-173. doi: 10.1016/j.smallrumres.2014.05.001. 

11.    Fragkou, I.A., Papaioannou, N., Cripps, P.J., Boscos, C.M., Fthenakis, G.C. (2007) ‘Teat lesions predispose to invasion of the ovine mammary gland by Mannheimia haemolytica’, Journal of comparative pathology, Vol 137(4), pp. 239-244. doi: 10.1016/j.jcpa.2007.08.002. 

12.    Grant, C., Smith, E.M., Green, L.E. (2016) ‘A longitudinal study of factors associated with acure and chronic mastitis and their impact on lamb growth rate in 10 suckler sheep flocks in Great Britain’, Preventative Veterinary Medicine, Vol 127, pp. 27-36. doi: 10.1016/j.preventmed.2016.03.002. 

13.    Cooper, S., Huntley, SJ., Green, L.E. (2012) ‘A longitudinal study of risk factors for teat lesions in 67 suckler ewes in a single flock in England’, Preventative Veterinary Medicine, Vol 110, pp. 232-241

14.    Vasileiou, N.G.C., Mavroginni, V.S., Petinaki, E., Fthenakis, G.C. (2019) ‘Predisposing factors for bacterial mastitis in ewes’, Reproduction in domestic animals, Vol 54(10), pp. 1424-1431. doi: 10.1111/rda.13541. 

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