A survey on milk somatic cell count and sub-clinical mastitis prevalence in dairy cattle on small dairy farms in Hochiminh City, Vietnam

Nguyen The Thao, Vo Lam and Ngo Van Man*
 

Department of Animal Husbandry and Veterinary Sciences,
Angiang University, Vietnam

ntthao@agu.edu.vn

^* Department of Animal Nutrition, Nong Lam University, Vietnam

Abstract

A survey on milk somatic cell count and sub-clinical mastitis in dairy cattle on smallholder dairy farms in Hochiminh City was conducted in two districts: District Number 12 and Cuchi District, where the distribution of dairy cattle was more than 52% of the total heads. A total of 360 milk samples was collected from 360 dairy cows, and somatic cells counted by using Delaval cell counter. The sub-clinical mastitis was classified by SCC0 (negative reaction, milk somatic cell counts <400,000) and SCC1 (positive reaction, milk somatic cell counts >400,000).

The results of this survey showed that the average of somatic cell counts in dairy milk was 1,301,891 (SD=902,539; n=360) and 69% of milking cows were considered to have sub-clinical mastitis. The increased sub-clinical mastitis in the dairy cows was positively correlated with increasing crossbred generation, herd size, and parity number. The F2 generation had the lowest sub-clinical prevalence (65%) and F4 the highest (72%). Fifty-three percent of the cows kept in small herds (<5 head) was positive for sub-clinical mastitis, whereas, 80 % of the cows kept in larger herds (>10 head) were positive. The cows at second parity had 59.4 % prevalence of sub- clinical mastitis and the prevalence was 75.9% for the cows which had more than four parities. Cows which were milked by family labor had a lower prevalence of  positive reactions (65.6%) than cows milked by hired labor. However, the Chi-square test did not show any significant difference.                              

The relationship between high milk somatic cell counts and daily milk yield was negative. Cows with low milk yield (<14 kg/day) had 83.3% positive for sub-clinical mastitis, while 63.6% of the cows with a high milk yield (>16 kg/day) were positive (Chi square =19.31, DF = 2, P-value = 0.001).

Key words: dairy cattle , generation, herd size, labor, milk yield number of calvings, smallholder, somatic cell count, sub-clinical mastiti,.

Introduction

As in many developing countries, in order to meet the demand of national self-sufficiency of milk production, with support from the government, dairy production development in Vietnam has made good progress in the last decades (Man, 2001). Milk production in Hochiminh City dominates the national production, and in 2005, the dairy population was 56,162 head and milk production 130,054 tons, corresponding to 54% and 66% of the figures for the whole country, respectively. Most of the production is from smallholder systems (DARDH, 2006).

Mastitis is generally regarded as the most frequent and costly disease in dairy cows worldwide. Depending on the differences in management practices and animal care, the impact of the disease varies between herds. The rate of new infections within a herd is increased by management factors that facilitate the spread of pathogens from the environment to the cows or from cow to cow (Barkema, 1998). Mastitis cause losses in the form of costs for veterinarians and treatment, discarded milk due to treatment with antibiotics, decrease in milk production, premature culling, replacement of animals, more work, poor milk quality and increased risk of infection in the future (Sara Ahlner, 2003).

Sub- clinical mastitis is the most prevalent form of mastitis, and in which there is no detectable inflammatory change in the udder and no observable abnormalities in the milk, but the concentration of milk somatic cell count (SCC) is increased (Akers, 2002). The higher count of somatic cells in milk usually precedes the clinical form and reduces milk production and milk quality (Tainturier, 1997; Gianneechini et al., 2002). Anri and Kanameda (2002) reported that if the SCC in milk is around 200,000, the milk yield will decrease by around 3%, and 7% if the milk SCC is 300,000. Consequently, the most common way to detect sub-clinical mastitis is by measuring the somatic cell count (SCC) in the milk. A high SCC can indicate that the udder is infected with mastitis as mentioned above.

In order to get more information in response to the demands of dairy producers to improve milk production in this area, this study aimed to: 1) evaluate the prevalence of sub- clinical mastitis and 2) identify the factors that affect sub-clinical mastitis in dairy cows under smallholder conditions in Hochiminh City.
 

Materials and methods

 Study area

District number 12 and Cu Chi District in Hochiminh City were selected as the study areas, are located about 20-40 km from the center of Hochiminh City, and are classified as peri-urban areas. Hochiminh City is characterized by tropical conditions with the dry season starting in October/November and ending in April/May. Short rains occur in April and May and heavy rainfalls in August to September. The city is 5-15 meters above sea level. The mean maximum and minimum air temperatures are 33.3⁰C and 25.9⁰C, respectively, and the mean maximum and minimum air relative humidities are 81 and 68% respectively, and average rainfall is around 1769 mm (Southern regional hydro-meteorological center, 2006).

Selection of studies farms

Totally 120 smallholder farms (approximately 6% of the total number of small-scale dairy farms based on census records of the two districts) were chosen randomly from the districts (60 farms per site). Sampling was done randomly from the list of farmers provided by the offices of agriculture in the districts.

Data collection 

Milk samples were taken from individual cows randomly, which corresponded to at least 20 percent of cows per herd depending on the herd size at the studied farms. Milk samples were analyzed and somatic cells counted by a fluorescence method, using Delaval cell counter DCC (Delaval, Tumba, Sweden).

A total of 360 healthy cows without any mastitis inflammation and more than 7 days after caving were chosen for collection of milk samples. Individual information of the cows, such as herd size, breed, age, number of calvings, kinds of milking, and milk yield were was collected directly by interviewing the respondants.

According to Schalm et al. (1971), SCC1 refers to cows with SCC>400,000 cells/ml, which has a positive relation with sub-clinical mastitis, and SCC0 to cows with SCC<400,000 cells/ml (negative relation).

Cows for sampling milk were also selected to determine the breathing rate and rectal temperature twice per day, and micro-environments such as air temperature, and relative humidity were recorded at the same time, at 08.00h and 14.00h.  

Data management and statistical analysis

Collected data from the questionnaires were checked and transferred into the same unit of measurement. The quality variables were coded into categorical values. Collected data were then entered into worksheets of Microsoft Excel 2003 (Microsoft Corporation, copyright © 1985-2003). The statistical analyses were performed using SPSS for Windows version 14.02 (SPSS Inc., copyright © 1989-2005). Descriptive statistics with mean, median, frequencies, max, min, and range were used. The independent-samples T-test procedure was used to compare the means of the quantitative variables. Pearson’s correlation was used for chi-squared statistics evaluation for categorical variables.

 

Results

Heat stress

The preliminary analysis results of the heat stress situation show that mean Temperature and Humidity Index (THI) was 81 in morning and 85 in the afternoon, and breathing rates were 54 and 70, and body temperature 38.8 and 39.3 in the morning and afternoon, respectively (Table 1).

SCC scores

The 360 milk samples from the 120 farms in Hochiminh City were counted for SCC. The average milk SCC was 1,301,891 (SD=902,539; n=360), of which, 249 cows (69%) were found to have a total  SCC of more than 400,000 cells, and were considered to be infected with mastitis. There were 111 (30.8%) cows with total milk SSC of less than 400,000 cells (Table 2).

Effect of generation on SCC

The generation is defined by the level of Holstein-Friesian blood in the cow.  The results in Table 3 indicate that the prevalence of sub-clinical mastitis was lowest in F2 cows (65.1%) and increased up to the F4 generation and above (72.1%). The proportion in the F1 generation was 68.3%. However, the Chi-square test did not show any significant differences between generations.

Effect of herd size on SCC

As can been seen from Table 4, the number of cows with mastitis infection increased  with increasing herd size. The highest percentage infected was 80.8% in the cows on farms with a herd size of more then ten cows. The proportion in cows in a herd size of 6-10 was 68.6%, and in herds of 1-5 cows was lowest (53.3%). However the differences were not significant.

Effect of calving number on SCC   

A positive correlation was found between the calving number and milk SCC (Table 5). The prevalence of sub-clinical mastitis increased with the calving number. The prevalence was lowest (59%) in cows at second calving, and highest in cows at fourth calving or above (76%).

Effect of test-day milk yield on SCC

The SCC indicated that there was highly negative correlation between milk yield on the test day and milk SCC. In total 83% cows with milk yields of under 13 kg/day were infected with sub-clinical mastitis, followed by 67% and 64% for cows with yields of 13-16, and more than 16 kg/day, respectively. The proportion of infected cows with milk yields under 13 kg per day was significantly higher (p<0.001) than the cows that had yields of more than 16 kg per day and (p<0.05) compared with the cows with milk yields of 13-16 kg per day.

Effect of milking labor on SCC

The milking labor affected the SCC, and 76% of cows milked by hired labor were positive for sub-clinical mastitis, which was higher than the proportion of positive cows milked by family labor (66%). 

Discussion

Values for THI indicated that dairy cattle in Hochiminh City were in the “danger” category (USDC-ESSA, 1970), as the breathing rate and body temperature were significantly higher than reported by Fuquay (1981) and Stowell (2000). The cows thus are kept in an unfavorable environment, and their health would be affected negatively. This situation can partly explain the high percentage of dairy cows that were positive for sub-clinical mastitis in the survey area. Smith et al. (1985) studied rates of infection with environmental pathogens, and suggested that the stress of high temperatures and humidity could have increased the susceptibility to infection as well as increased the number of pathogens to which cows were exposed. A study in Florida (Elvinger, et al. 1991, cited by Harmon, 1994) showed a significant increase in SCC of milk from cows subjected to either heat stress or housed in a thermo- regulated environment, and were 145,000 and 105,000 cells/ ml, respectively.

In total 69% of the samples in Hochiminh City were found with milk SCC > 400,000, which can be compared with Pitkälä et al. (2004), who found that the proportion of cows with milk SCC >300000 in Finland was 38% in 1995 and 31% in 2001. In similar conditions to the present study, Xuan (2005) reported that 61% of the dairy cows in DongNai province, bordering with Hochiminh City, were positive with sub-clinical mastitis, tested by the California Mastitis Test. The proportion in Uruguay, presented by Gianneechini et al. (2002), was 52.4% and Sara Ahlner (2003) reported an average 42.2% per farm (range 10.5% to 92.3%). Wilson et al., (1997) found that the prevalence of sub-clinical mastitis was 50% in New York and Pennsylvania. In general, during the survey we found that almost the farmers interviewed have a limited understanding of sub-clinical mastitis. None of the farmers used any preventive methods to prevent infection, except that water was used to clean the udders before and after milking. Meanwhile, Norman et al (2000) asserted that mastitis control programs also have important effects. 

The trend of increased percentage of the prevalence of sub-clinical mastitis in higher generations was found in current study, and is in agreement with the report by Xuan (2005), with the lowest value in F2 (30%), and highest with F5 (56%). Also Phat (1999), using the California Mastitis Test, showed that F2 (12%) was lowest and F3 highest (14%). However, the prevalence was higher in F1 generation in the present study, and after consultation with the farmers could be explained by the possibility that the F1 milking cows might not have adapted to the milking conditions, thus leading to problems during milking, causing injuries to the cows. An additional reason could be heredity reactions in the milk let-down mechanism, that could have resulted in them keeping milk in the udder, which would create a good medium for infection.  

Harmon (1994) found that herd size did not affect SCC. However, Normal et al (2000) in a survey in the USA, found that the larger herds generally had lower SCC than smaller herds and supported the premise that mastitis management is better in larger herds than in smaller herds. In contrast Xuan, (2005) reported that at household level, in smaller herds it may be easier to take good care of the animals, and maintain a good hygiene and ventilation. Thus the cows are more healthy, and sub-clinical mastitis can be prevented. In his study the percentage of dairy cows positive with mastitis were 13%, 29%, 50% in herd sizes of 1-5, 6-10, and more than 10 head, respectively. In our study the increasing SCC level with herd size followed  a similar trend.

Milk SCC has been found to increase with parity (Valde et al., 2004; Detilleux et al., 1997). In the present study, the trend of increased milk SCC with increasing number of parities was also found, except in the first parity. This also agrees with the results reported by Phat (1999), with 5.97% at the first calving and the highest proportion at the parities of more than four (13.2%), and Xuan, 2005 showed an increase from 23.2% at the first parity to 62.2% at the parities of more than seven.  Tainturier (1997) and Valde (2004) showed that the prevalence of sub-clinical mastitis in dairy cattle increased with increasing number of parities and age. This can be explained by the fact that the risk of infection increases with age, probably because the immune system in an older cow is not efficient. The “abnormal” result of prevalence of sub-clinical mastitis in the first parity may be explained by the first parity cows not having adapted to the milking conditions, which may have created favourable conditions for infection.

The daily milk yield is associated with milk SCC. In our study, SCC had a negative correlation with milk yield (P<0.001). Similar results were found in the studies of Wilson et al. (1971), Xuan (2005) and Phat (1999). Emanuelson and Funke (1991) and Miller et al. (1993) found a ”dilution effect” due to an inverse relationship between milk yield and milk SCC. In these studies it was assumed that a dilution effect caused the regression of milk yield on milk SCC. Miller et al. (1993) suggested that the observed negative relationship between milk yield and SCC may partly reflect both the true biological effects of udder inflammation and a dilution effect.

Interestingly, the survey showed that 45% of the dairy farmers employed laborers for milking. To the question of whether there was a relationship between the milk SCC in the dairy and source of labour for milking, it was found that there was higher milk SCC prevalence in the cows that were milked by hired labour compared with family labour. Although our results followed the trend of many other studies, the Chi- square test did not show any statistically significant differences. The possible explanation for this was summarized by Valde et al. (2004) and other authors mentioned above. The milk SCC also varied according to cows factors, and older cows were at higher risk of sub-clinical mastitis than younger cows. Higher exotic blood levels also resulted in a higher ratio than lower levels.  In a cow population with a high culling/replacement rate the cows will be younger and less susceptible to sub-clinical mastitis, and this may in turn lead to a lower incidence rate. In this study, recorded milk SSC was randomized at household level, and the prevalence of sub-clinical mastitis could have been related with several associated factors. For example the F1 and F2 generations in the studied farms were older and had more parities than the F3 and F4 generations, while the F1 and F2 cows were mainly kept in the larger farms while the F3 and F4 cows were kept in small farms. In addition, conditions on the smallholder farms were different and many of the cows produced much lower yields than the mean milk yield. Hence, it was impossible to determine which were the main factors affecting the prevalence of cows having high milk SCC.

In addition, additional factors, such as hygiene and management routines, that were not studied could have been associated with high milk SCC. Another possible explanation for these results could be the limited number of the samples, because of the lack of DDC cassettes used to collect milk and for counting somatic cells.
 

Conclusion and recommendation

Base on this study, it can be concluded that:

• The sub-clinical mastitis prevalence in dairy cattle is high, and could affect milk production and milk quality in smallholder farms in Hochiminh City.

• The milk somatic cell counts were related to herd size, parity number, proportion of HF blood and source of labour for milking, although these relationships were not significant. However there was a highly significant negative correlation between milk somatic cell count and milk yield.

• There is a requirement for further research, with a larger number of samples and  factors studied, in order to identify the main causes of mastitis,  so that mastitis control programs can be set up, to increase milk yield, milk quality and benefits for the farmers. 

Acknowledgments

The authors are grateful to the Swedish International Development Agency-Department for Research Cooperation with Developing countries (Sida-SAREC) for supporting this study. Thanks are also given to the farmers, Department of Agriculture and Rural Development in Hochiminh City, who supported and made it possible to conduct this study. Further acknowledgment goes to the Delaval Company for kindly providing  the DCC machine for efficient analysis of SCC.

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