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Also known as

Chronic mastitis


This is a severe form of mastitis in cattle caused by colorless algae of the genus Prototheca. This particular algae was first linked to bovine mastitis in 1952, but was not considered a significant pathogen until recently. The species most often associated with bovine mastitis are P. zopfii and P. wickehamii.

Prototheca mastitis has now become what is considered an emerging disease. It was once thought to be a problem only for dairies in tropical regions, but incidence of this pathogen is now increasing steadily in many countries. For instance, a 2019 study published in the Journal of Dairy Science indicated that Prototheca zopfii (genotype 2) is the third most common mastitis pathogen in southeast Poland.

A 2018 study of a large confinement operation in Germany involving 248 Prototheca-infected cows showed that 74% of them had clinical signs of mastitis. A whole-herd survey identified 80 cows that were infected without a history of clinical mastitis, and 27% of those cows were infected in more than one quarter.

Prototheca is widespread in the environment of dairy cows, particularly in wet or humid areas. Infections occur when the teat end is exposed to large numbers of the organism; the infectious dose is thought to be higher than for other pathogens. In other words, it takes more of them to cause a true infection.

Although Prototheca has primarily been considered an environmental pathogen, some recent studies have found that one particular strain of Prototheca zopfii is found in the majority of mastitis cases. This, along with the fact that there is no effective treatment for Prototheca and infected cows typically develop chronic infections, suggests that this algae should be considered as a contagious pathogen.

In many cases, cows infected with Prototheca go undetected until the affected quarter is nearly dry. Unlike other mastitis pathogens, initial immune reaction in the udder to Prototheca is not very strong. Most cases are subclinical, with a few episodes of mild clinical mastitis where the milk may be only slightly abnormal.

However, these infections so not clear up; they remain chronic with progressive decrease in milk production as the pathogen damages more of the gland. The somatic cell count (SCC) of affected cows may be only slightly elevated or may be greater than 1,000,000 cells per milliliter. In cases where the SCC is only slightly elevated it may be due to the dilution effect of the healthy quarters.

Some studies have found increased rates of Prototheca in early lactation and in herds with compromised immunity.

Prototheca may be diagnosed by bulk tank or individual cow culture, but Prototheca may grow very slowly on traditional culture media. This may result in some false negative culture results. However, since Prototheca is becoming more common, diagnostic laboratories may use selective culture media or other techniques to improve detection of Prototheca in a milk sample.

Since Prototheca is prevalent in the environment and could conceivably contaminate bulk tank milk, a positive bulk tank culture may not necessary mean that infected cows are present in the herd, but due to the economic significance of Prototheca infection, a positive bulk tank culture does warrant further investigation. It may be wise to culture cows with elevated SCC. Strain-typing in the lab is a way to determine if Prototheca found in bulk tank milk is likely to be the result of environmental contamination or mastitis infection.

Prototheca can exist in a herd as a subclinical infection with no signs, or maybe just slightly abnormal milk or as a clinical infection showing watery milk. Sometimes large clots are found in watery milk, along with reduced milk yield.

Chronic infections will decrease milk production as the organism continues to damage the udder by infiltrating various types of white blood cells, and will increase the cow’s somatic cell count.

Most mammary infections with prototheca show abnormal milk but without severe systemic signs such as going off feed, depression or high fever. Milk from cows with prototheca mastitis often have elevated somatic cell counts to the point that the bulk tank milk SSC may also be elevated. Due to the detrimental impact of prototheca infections on milk quality and lack of response to treatment, culling is advised for infected cows.


  • Slightly abnormal milk
  • Watery milk with occasional clots
  • Reduced milk yield
  • Increased somatic cell count in milk


Prototheca are colorless one-celled algae found in cow manure, soil and water, bedding, feed, barns and barnyards. Infected cows can also serve as reservoirs for the infection for other cows and usually become chronic cases, so this organism is considered to be both an environmental and contagious pathogen. As algae, Prototheca have different cell walls than Gram-negative or Gram-positive bacteria, and don’t respond to antibiotic treatment.

Transmission occurs through the teat end. Cow-to-cow transmission is always a concern. Risk factors include wet areas on the farm, periods of warm temperatures and heavy rainfall, recycled sand bedding washed with effluent from a dairy lagoon, poor hygiene practices, contaminated intra-mammary infusion equipment, and cow-to-cow transmission in the milking parlor or in a hospital pen. As with other pathogens that are resistant to antibiotic therapy, poor hygiene is a major factor.

Prototheca can be found in wet areas containing decaying manure and plant material.   They can also be found in flowing water, standing water, water tanks, water runoff from silage, well water, milking parlor wash water, manure, teat dip containers, milking machine liners, teat end swabs and feed troughs. These algae have also been found in feces of rats trapped on dairies.

Prototheca have been found in sites on dairies that have cows infected with prototheca and dairies that do not have mastitis caused by prototheca. This indicates that prototheca are widely dispersed in the environment of dairy cows on dairies with and without prototheca problems.

Infections are thought to occur when cows’ teats are exposed to high populations of algae during intervals between milkings, but new infections can occur in situations where a high percentage of cows are infected with prototheca and milking techniques are poor.

The reported level of infection within dairy herds is anywhere from 4 to 40% of milking cows. In most herds few, if any, cows are infected. Herd outbreaks with greater than 10% of cows infected are rare, yet this pathogen can often be detected in bulk tanks.

The fact that it may take a high dose of these pathogen to create infection may account for the sporadic nature of new infections caused by prototheca. The udder is in constant contact with water sources on the dairy. All stages of lactation appear to be equally susceptible to new infections, including dry cows.

Most prototheca infections quickly become entrenched in the udder and develop into long-term chronic infections. These infections may persists across dry periods and last for several lactations.

Prototheca mastitis should be suspected when non-responding clinical cases of mastitis occur. Milk culture of cows with prototheca mastitis will show typical algae using standard laboratory procedures as recommended by the National Mastitis Council.

When numerous cows in the herd are infected, the bulk tank somatic cell count may become elevated above 400,000 cells per milliliter.

Prototheca are widely dispersed within the dairy environment, and finding prototheca in the environment does not mean the source of infection has been located.


Due to the economic impact a positive Prototheca or false negative result could have on a dairy farm, diagnostic laboratories now offer more advanced diagnostic tools, like PCR, that can provide better results to veterinarians and producers--enabling more rapid segregation of infected cows from the herd, and culling animals as needed.

Cows should be kept out of obvious wet areas, particularly those with decaying manure or plant matter. In some cases, there is no obvious risky site. A few outbreaks have occurred on arid Western dairies. These outbreaks occur in most cases during periods of warm weather with high rainfall when teat-end exposure is extreme.

Use of sprinkler pens, flush alleys and cooling misters do not appear to increase the risk of new prototheca infections.

Cows identified on culture as being infected with prototheca should be identified and milked last in the milking order until they can be culled.

All cows with elevated cell counts should be identified and cultured for prototheca if prototheca has been identified as a mastitis-causing organism in a herd. Prototheca-positive cows should be culled. Early detection and culling of infected cows will protect bulk tank milk quality, decrease the need for milk discard and reduce frustration due to treatment failures.

There is no easy solution to dealing with prototheca infections, so culling is the best policy to prevent spread to other cows.


There are no effective or approved treatments available for Prototheca mastitis. Prototheca is unresponsive to antibiotic treatment. Therefore, no treatment for prototheca should be attempted with antibiotics.

The use of SCC data to identify potentially infected cows for culture is recommended.
Affected cows should be clearly marked, and milked last in the milking order until they can be culled.

About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.