Protothecosis and other algal diseases

H F VISMER AND J A PICARD

Protothecosis

Introduction

Protothecosis is a rare algal infection caused by chlorophyllfree (alchloric) Prototheca spp. that affect both healthy and immunocompromised humans^{6, 7, 51, 74} and animals.^{56, 67} These organisms are the cause of chronic, non-treatable mastitis in cows,^{1, 18, 21, 30, 33, 63} cutaneous, ocular, enteric or systemic disease in dogs,^{31, 32, 40, 42, 64, 67} and cutaneous disease in domestic cats.^{12, 22, 24} Rare infections have been recorded in a sheep,⁶⁹ deer,²⁵ fruit bat (Pteropus lylei),⁴⁴ a platypus (Ornithorhynchus anatinus),⁴⁸ Atlantic salmon (Salmo salar),²⁶ and a corn snake (Elaphe guttata guttata).¹⁹

Protothecosis is most probably found throughout the world, although most reports of disease in animals have originated from Europe, the USA, New Zealand and Australia. 1, 18, 30, 63 Two cases of protothecosis in domestic animals in South Africa have been described: one in a dog with a disseminated infection,32 and the other in a cow with mastitis caused by a mixed infection of a Prototheca sp., Staphylococcus aureus and Streptococcus agalactiae.²⁸

Aetiology

The genus Prototheca is closely related to the genus Chlorella and contains five species,^{2, 52, 57} but protothecosis in animals is only caused by Prototheca zopfii Krüger and P. wickerhamii Tubaki and Soneda, with P. moriformis, P. stagnora and P. ulmea not being incriminated as pathological agents.^{2, 13, 57} Most outbreaks of protothecal mastitis in cattle are caused by P. zopfii, although P. wickerhamii has occasionally been isolated.^{16, 23} In dogs and cats P. zopfii has been incriminated in disseminated disease and P. wickerhamii in cutaneous disease.^{22, 27} Most cases in humans have been attributed to P. wickerhamii.^{6, 74} The species typically produce thick-walled cells (sporangia) that, at maturity, divide by irregular cleavage, forming 2 to 15 aplanospores (endospores) that enlarge on rupture of the sporangia.^{10, 33} One to 3 per cent of the sporangia will cleave to form thick-walled resting cells (hypnospores).¹⁶

Generally, the sporangia of P. zopfii are round and 14 to 15 μm in diameter, and those of P. wickerhamii are oval to cylindrical and 7 to 13 μm in diameter, but this characteristic can be influenced by environmental conditions.⁷²

The organisms are cultured using Sabouraud’s dextrose agar or potato dextrose agar plus chloramphenicol (0,1 g/l ) and incubated aerobically at room temperature for up to seven days. The optimum incubation temperature is 30 °C and growth is inhibited by the presence of cycloheximide,⁷² tetracycline, ⁴⁵ and aminoglycosides.61 Colonies of the organisms are pale, friable, and cream-coloured and must be distinguished from those of yeasts.30 On blood agar the colonies are grey.¹⁶ Cultured organisms appear as ovoid to spherical nonbudding cells 4 to 20 μm in diameter, containing variable numbers of endospores.³⁰ They often have a mulberry-like appearance. Species identification is done using the API 20C⁴⁹ and API ZYM (bioMérieux, France) sugar assimilation tests.⁸ Both P. zopfii and P. wickerhamii assimilate glucose and galactose, and, additionally, P wickerhamii assimilates trehalose.⁷² A selective medium known as Prototheca enrichment medium (PEM) allows the isolation of these algae from the environment or milk samples^.55 Dull, creamish-white colonies (1 to 2mmin diameter) develop after 48 to 72 hours of incubation.¹

Epidemiology

Prototheca spp. are common in aqueous habitats and have frequently been isolated from the soil, plants and the slime flux of trees.^{1, 32} Other sites from which these algae have been isolated include domestic or municipal sewage, fresh and marine water, animal faeces, human fingernails, sputum and skin scrapings, potato skins, clams, crabs, beef and pork products.³⁸ Epidemiological studies following outbreaks of protothecal mastitis in dairy herds indicate that P. zopfii and P. wickerhamii are associated with wetness and the presence of organic material.³² In the dairy herd environment these algae are found in organically rich soil, demanuring and feeding aisles, drinking troughs, slurry basins and sewage.^{1, 17, 30, 32}

Humansand monogastric animals such as rats and pigs are thought to play a role in spreading protothecal organisms by ingesting them in their feed, and then passing them intact in their faeces for them to continue their saprophytic cycle.^{1, 54} Prototheca zopfii is also transmitted in the milk and faeces of affected animals and in this manner contaminates the environment. ^{1, 17, 55} The organisms are fairly resistant to milk pasteurization. ⁴³ They can be harboured in a non-lactating bovine udder and shed during the subsequent lactation.^{17, 23, 30}

Transmission is thought to be from a contaminated environment and there is no evidence that there is direct transmission from animal to animal.⁴⁷ Although there is no evidence of animal to human transmission, some fear that the drinking of raw milk could create faecal shedding or disease in humans.⁴³ Improper chlorination of, and the presence of organic debris in milking equipment may result in them playing a role in the transmission of these algae.¹ Wet, hot weather conditions and the geographic location of dairies may also influence the prevalence of mastitis.^{18, 53} In one study in Brazil it was found that the occurrence of Prototheca spp. mastitis was higher in large dairy herds in cows kept in free stalls...