Comparison of Culture of Goat Milk with Indirect Measurement of Somatic Cell Count Using a Commercial Kit, and Antimicrobial Susceptibility of Bacterial Isolates

Introduction

Mastitis is one of the most serious economic and health problems of dairy goats worldwide. The inflammation of the mammary gland can cause gross variations in milk composition, reflected by physical, chemical, and bacteriological changes. There may be a decrease of important components like lactose, casein, fats, and minerals such as calcium, phosphorous, and potassium, and increases in other unimportant components such as serum proteins and chlorides (Oliszewski et al., 2004). While the clinical form is characterized by symptoms, including abnormalities in milk, the subclinical form is subtle, and the milk often appears to be normal. The subclinical form constitutes a reservoir of microorganisms that can prompt infection of other animals within the herd, and may precede the clinical form. According to Philpot (1984), it is utterly futile to think that mastitis can be controlled by the treatment of clinical mastitis. Somatic cell count (SCC) is the indicator most commonly used for mastitis detection in the bovine. The SCC values, and the type of bacteria involved in clinical and subclinical mastitis in goats, can vary considerably between different countries and between regions in the same country. The relative and absolute importance of different pathogens may also vary from one geographic area to another. It is important to generate baseline information for accurate diagnosis and preventive/control measures. While determination of actual SCC values for goats requires a direct microscopic method using the DNA-specific stain pyronin Y-methyl green (Petersson et al., 2011); commercial kits such as “PortaCheck” have recently appeared on the market. “PortaSCC® goat milk test” (Porta-test) is based on a chemical reaction between a dye on the test strip and an enzyme found on the surface of somatic cells in the milk. This test comes with claims to detect a lower level of somatic cells than the standard California Mastitis Test (CMT), which uses a concentrate that will gel in relation to somatic cell count and was developed to determine bovine mastitis. Therefore, the Porta-test should have a greater sensitivity for identifying subclinical mastitis in the caprine. The sensitivity and specificity of the Porta-test for the bovine is reported to be 76% and 94%, respectively when compared against the Bently 300 Somatacount machine as the gold standard (Leslie et al., 2006). The California Mastitis Test is reported to have a range of sensitivity and specificity from 73-89% and 75-85% respectively, however the gold standard used to confirm these findings is bacterial culture (Ruegg and Rienemann, 2002). It has been noted that factors such as caprine arthritis-encephalitis virus infection and stress can contribute to an increase in SCC in the caprine, making the sole use of SCC for predicting mammary infection in the goat difficult (Lerondelle et al., 1992).  Many farms currently rely on the Porta-test to monitor udder health.  It is more costly than the California Mastitis Test; however, a more accurate interpretation of on-farm somatic cell counts can be a useful screening tool. This study was primarily intended as a comparative study of culture results with Porta-test results, considering the fact that there has been no published report on such comparison. The study also determined the types and numbers of potential intramammary bacterial pathogens from lactating goats in Grenada, and their antimicrobial susceptibility.

Materials and Methods

All goats selected for this study were from the island of Grenada, and were assigned a number and identified with regard to age, stage of lactation if known, herd location and any illness or treatment history. The animals in this study were not kept in a traditional farm setting, and were kept in very rugged terrain, which is typical of this island. A total of 150 animals, representing all known lactating animals that met the selection criteria on the entire island during the time of study, were included. Any does with signs of clinical mastitis, visible signs of damage to the udder, or showing outward signs of illness were excluded from the study. To determine subjects and udder half, the simple random sample method was used. Seventy four udder halves were randomly chosen for the study.   Individual udder half samples were identified as Right (R) or Left (L), and samples were collected as outlined by Quinn et al., (1994). The teat was thoroughly wiped with 70% ethyl alcohol, paying particular attention to the teat orifice.  One minute was allowed to elapse before sample collection.  A small amount of milk was first stripped from the udder, and then a capped sterile wide mouth tube was used to collect the sample. The samples were then labeled and stored at 4°C for not more than 2 hrs before testing and culture.

Indirect somatic cell count tests were performed in the laboratory on samples brought to room temperature, using the Porta-test protocol outlined by the manufacturer. For Porta-test, SCC (cells/ml) of <500,000 is interpreted as normal, 500,000-1,000,000 as weak udder infection, 1,000,000-1,500,000 as indicative of possible udder health problems, and over 1,500,000 as indicative of udder infection.

For culture, a 0.01 ml sample of milk was plated on Columbia blood agar and MacConkey agar using a calibrated loop. The plates were incubated at 37°C for up to 72 hours, examined for the number and type of colonies and the information recorded. A plate was considered positive if there was greater than one colony cultured. The bacteria were identified based on colony morphology, hemolytic patterns, Gram’s stain; and tests as required for catalase, coagulase, oxidase, and finally using API bacterial identification strips (“ID32 STAPH”, rapid ID32 STREP, api Coryne, api 20E, and api 20NE; bioMerieux) (Hariharan et al 2004). Additional tests including esculin hydrolysis using Edward’s agar, and CAMP tests (Quinn et al., 1994) were utilized for certain isolates.

The number of samples positive on culture from each of the 4 Porta-test SCC categories (Table 1) were compared for statistically significant differences using the chi-square test.