Causative agents of environmental mastitis

Bacteria, mastitis pathogen

Although, due to more recent research with strain typing,  the division between  "contagious" and "environmental" mastitis has become less black and white, the below organisms are still classified as more environmental than contagious.

Streptococcus uberis

Gram-positive cocci that can be found in bedding material.

Transmission occurs both during milking and when the animals are lying down. Thirty percent of new infections occur in the last third of the dry period. Poor udder preparation is a key factor for these infections to develop, especially if the teats are moist and have not been disinfected. Some strains of S.uberis can become chronic for some weeks or months, causing persistent contagious-like infections than are transmitted during milking.

Some cases of S. uberis respond very well to treatment (bacteriological cure rate of 80-85 %), while others have a very poor response to treatment (lower than 40 % cure rate).

Streptococcus dysgalactiae

Gram-positive cocci found in the environment, on the teat skin and in infected udders. They almost exclusively infect teats with thelitis (inflammation at the tip). They cause non-acute clinical mastitis with deterioration of the milk quality.

Mastitis caused by S. dysgalactiae affects isolated individuals, without spreading to the whole herd. When outbreaks appear, it is usually due to defective pulsation of the milking machine that causes injuries to the teats.

Response to treatment is good, although the problem will recur if the cause of the infection (thelitis) is not eliminated.

Escherichia coli

Gram-negative cocci that are found in bedding materials. They cause acute clinical mastitis that often affects the animal's overall health status (fever and toxaemia) and can even lead to gangrenous mastitis. A typical, watery ("beer-like") secretion is excreted through the udder. There is a great variability among the different strains, and a variety of clinical signs may appear.

Certain environmental conditions favour the presentation of outbreaks of mastitis caused by E.coli (humid and warm climates). Transmission mainly occurs out of the milking parlour, while the cow is resting. Very often infection occurs during the dry period. Although it is difficult for the bacteria to multiply in the dry udder, they can remain latent and wait for the immune status of the cow and udder to decrease. While multiplying, these microorganisms release a large amount of toxin (lipopolysaccharide), which affects the overall health status of the animal. Some cows may go into shock and die.

Serratia spp.

Gram-negative bacilli. They are found in the environment; transmission occurs while the cows are lying down. They have been associated with contaminated disinfectants for post-milking dips.

They cause acute clinical mastitis that does not persist for long, but affects milk quality and forms clots, without causing the typical clinical signs of E.coli infections.

Other Enterobacteriaceae

Other species that belong to the family Enterobacteriaceae are included in this group. Cases of mastitis due to Klebsiella are often associated with sawdust or wood by-product bedding. The transmission route and clinical signs are similar to those of infections by E. coli, and they are only differentiated by means of laboratory diagnosis.

Mastitis caused by enterobacteria does not respond well to treatments.

Pseudomonas spp.

Gram-negative bacilli that are found in the environment. As occurs with Serratia spp., transmission takes places when the animals are lying down.

Mastitis due to Pseudomonas becomes chronic in more than 90 % of cases (with an increased cell count), although some cases may also be hyperacute with toxaemia and deterioration of the animal's health status. They are generally observed in individual animals. When an outbreak with several affected cows appears, contamination of the drinking water should be suspected.

These bacteria are very resistant to many types of antibiotics.


The species of interest are Enterococcus faecalisE. faecium, E. durans and E. ovinun. They are found in faeces and bedding materials. They often cause clinical mastitis with deterioration of the milk quality, without affecting the overall health of the animal. They possess protective capsules that lead to some infections becoming chronic relatively easily.

Mastitis caused by enterococci can resolve spontaneously or have a very variable resistance to antibiotics, which complicates their treatment.

Arcanobacterium pyogenes

Gram-positive bacilli that cause clinical mastitis in non-lactating animals.

The main transmitters of these bacteria are flies, although they can also be found on the skin and in bedding materials. The disease appears in dry cows and in animals with difficulties at milking (crushed and injured teats).

These cases of mastitis are accompanied by severe inflammation with purulent and foul-smelling secretions, caused by the association of the pathogen with other anaerobes.

The pathogen's sensitivity to antibiotics is good, but the lesions it causes are so severe that treatment only solves 5 % of cases.

Prototheca spp.

Achlorophyllous algae have a growing prevalence. They are associated with a lack of hygiene when administering intramammary treatments.

They are found in contaminated water, feed and bedding materials. They cause chronic mastitis, which can lead to deterioration of the milk quality, but always without affecting the animal's overall health.


Yeasts are introduced by means of intramammary treatments. The risk is high with prolonged or non-conventional treatments, poor hygiene or with treatments at non-standard doses. They cause severe inflammation with oedema and deterioration of the milk quality. The use of antibiotics may worsen the state of the udder. There is no real satisfactory treatment.

Coagulase-negative staphylococci (CNS)

This group includes a large number of species, some more pathogenic than others. The most representative species are: S. hyicusS. epidermidisS. saprophyticus, S. simulansS. cromogenesS. xilosus and S. sciuri. They are found on the skin and in the environment.

They have traditionally been considered as pathogens secondary to mastitis. Their meaning in mastitis development or protection is not entirely clear. It has been suggested that they can cause acute clinical mastitis, but also  important increases in somatic cell counts. Some species behave as contagious pathogens while others behave as environmental pathogens.

In general, both pre-dips and post-dips are good methods for the control of this heterogeneous group.

Bacillus cereus

Gram-positive bacilli that are found in the environment. Cases of hyperacute mastitis caused by B. cereus are seometimes identifiable by the dark bloody colour of the milk, and many of them evolve to gangrenous mastitis.

They are usually present as outbreaks in cows that rest in outdoor pens or when the conditions of humidity and temperature are favourable to the proliferation of the microorganism.

Previous Article Next Article