Adult female beef ox, (Bos taurus)Gross lesions were identified during the postmortem inspection of the carcass at slaughter. Lymph node samples were submitted for laboratory evaluation through the United States Department of Agriculture (USDA), Bovine Tuberculosis Eradication Program. Due to the gross lesions, the carcass was condemned (not used for human consumption).

Gross Description:  

The carcass was in normal body condition. The left parotid lymph node, and to a lesser degree the adjacent lymph nodes of the head, contained irregularly shaped areas which were discolored green. On cut surface, patchy areas of green extended into both the cortex and medulla. The facial musculature adjacent to the parotid lymph node, the tracheobronchial lymph nodes, and mediastinal lymph nodes were also multifocally green.

Histopathologic Description:

Lymph node: Large coalescing aggregates of macrophages and epitheloid macrophages with abundant eosinophilic cytoplasm expand the cortical and medullary sinuses. Numerous algal cells with low numbers of eosinophils, multinucleate giant cells, lymphocytes and plasma cells are intermixed with the macrophages. The algal cells are round, 7 to 15 microns in diameter, have a thin refractile cell wall, single basophilic round nucleus and amphophilic to eosinophilic granular cytoplasm. Low numbers of larger algal cells (sporangia) are filled with 2 to 6 daughter cells (sporangiospores or endospores). Intact algal cells and empty cell walls with no internal structures (degenerate algal cells) are both free in the sinuses and within macrophages. Within the cortex, there are few lymphoid follicles.

Morphologic Diagnosis:  

lymph node: Lymphadenitis, granulomatous, severe, diffuse, chronic, with large numbers of algal organisms consistent with green algae.


Chlorella sp.

Contributor Comment:  

Algal infections are uncommon opportunistic pathogens of domestic and wild mammals and humans.(1) Culture is frequently required to make a definitive etiologic diagnosis, but even with culture, the precise genus and species of green algal infections have not always been clearly identified.(1) Chlorella spp. are commonly cited as the etiology of green algal infections, but it is important to note that not all green algal infections are due to Chlorella spp.(3,6)

Green algal infections have been described most commonly in sheep and in cattle, with individual case reports in a dog, man and several species of wild mammals.(1,4,5) The algae are found commonly throughout the world and infections have been associated with stagnant water or pasture irrigated with raw sewage.(5) Disease varies from subclinical with a localized lymphadenitis to severe clinical disease and systemic lesions.(1,2,5) Underlying immunosuppression or an overwhelming infectious dose are possible predisposing factors to causing disease by this ubiquitous organism.(5) Our case is consistent with one of the largest reports (a group of 8 cattle cases) in which green algae induced lymphadenitis was identified through postmortem slaughter inspection.(6) Green algal infections are also identified several times a year in cattle by the USDA Food Safety Inspection Service pathology laboratory, which receives samples from federally inspected slaughter facilities (S Hafner, personal communication). Based on the lesion and organism morphology on HE stained slides, the primary differential diagnoses include various types of green algae and Prototheca spp. Protothecosis occurs more commonly than disease by green algae and Prototheca spp. are achlorous, which is a key criteria for differentiating green algae from Prototheca spp.(1,5) Chandler et. al.(1) uses three criteria for differentiating Chlorella (a type of green algae) from Prototheca spp. and for making a presumptive diagnosis. In chlorellosis there are: 1) grossly visible green discoloration of the tissues due to the presence of chlorophyll in the algae, 2) spherical algal cells, with an average diameter of 9 microns that exhibit endosporulation, and 3) algal cells that have abundant large cytoplasmic granules that are strongly positive with PAS, GF, and GMS stains.(1) Other methods used to differentiate the two include impression smears of fresh tissue and transmission electron microscopy, both of which are based on the presence of numerous chlorophyll/chloroplasts.(1,5) It is important to remember that the green color of the Chlorella algae is lost during tissue fixation and processing.(1)

JPC Diagnosis:  

Lymph node: Lymphadenitis, granulomatous, diffuse, moderate to marked with numerous intrahistiocytic and extracellular endosporulating algae and moderate plasmacytosis.

Conference Comment:  

The contributor provided an excellent discussion of Chlorella spp. and the associated pathology. Conference participants discussed the differences between Chlorella and Prototheca, and the difficulties in differentiating the two entities by standard histologic methods.

In addition to Chlorella and Prototheca, other pathogens that reproduce asexually by endosporulation include Rhinosporidium seeberi and Coccidioides immitis.


1. Chandler FW, Kaplan W, Ajello L: Protothecosis and infections caused by morphologically similar green algae. In: Color Atlas and Text of the Histopathology of Mycotic Diseases, pp. 96-100. Year Book Medical Publishers, Chicago, IL, 1980.
2. Cordy DR: Chlorellosis in a lamb. Vet Pathol 10:171-176, 1973.
3. Haenichen T, Facher E, Wanner G, Hermanns W: Cutaneous chlorellosis in a gazelle (Gazella dorcas). Vet Pathol 39:386-389, 2002.
4. Quigley RR, Knowles KE, Johnson GC: Disseminated chlorellosis in a dog. Vet Pathol 46:439-443, 2009.
5. Ramirez-Romero R, Rodriguez-Tovar LE, Nevarez-Garza AM, Lopez A: Chlorella infection in a sheep in Mexico and minireview of published reports from humans and domestic animals. Mycopathologia 169:461-466, 2010.
6. Rogers RJ, Connole MD, Norton J, Thomas A, Ladds PW, Dickson J: Lymphadenitis of cattle due to infection with green algae. J Comp Path 90:1-9, 1980.

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4-1. Lymph node

4-2 Lymph node

4-3. Lymph node

4-4. Lymph node

4-5 Lymph node

4-6 Lymph node

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