Streptococcus zooepidemicus (see modern nomenclature below) is a Lancefield (the conventional serological classification scheme for these bacteria) group C streptococcus, commonly found in animals and which can cause zoonotic (acquired from animals) disease in humans. Other members of this group are S. equi (which causes the contagious disease called 'strangles' in horses, but does not infect humans), S. dysgalacticae and S. equisimilis. S. zooepidemicus is an opportunistic pathogen of animals and causes a variety of suppurative lesions and mild inflammation of the upper respiratory tract. There are 15 different serotypes, of which Type 2 is one of the most common and infects a number of species, including humans. The organism is found widely in veterinary practice. It rarely appears to cause human infection, but when it does, human illness tends to be severe.
Modern nomenclature of group C streptococci
S. zooepidemicus = Streptococcus equi subspecies zooepidemicus
S. equisimilis = S. dysgalactiae subspecies equisimilis
S. dysgalactiae= S. dysgalactiae subspecies dysgalactiae
S. equi = S. equi subspecies equi
S. zooepidemicus is commonly found in a number of animals, particularly horses, and is an occasional cause of mastitis in cows. It is a commensal of the skin, mucosa of the upper respiratory tract and in the tonsils of many animal species, and is regarded as an opportunist pathogen. In horses it is a common agent of wound infections, and causes secondary disease after a viral infection in the upper respiratory tract of young animals. S. zooepidemicus may sometimes be isolated from open strangles lesions where it is presumed to be a secondary invader; it is not isolated from unopened lesions of typical strangles cases. In cattle, S. zooepidemicus can cause acute mastitis when it enters a wound in the teat. Neither disease nor suspicion of disease in animals is reportable or notifiable.
S. zooepidemicus infection in humans usually occurs in the respiratory tract, both as a cause of acute pharyngitis and as a commensal organism. The infection may be complicated by spread into the lungs or via the bloodstream, to cause meningitis, endocarditis and septic arthritis. Cases of post-streptococcal nephritis/glomerulonephritis (which is more usually associated with Group A streptococci) have also occurred albeit infrequently. The more severe human infections have been associated with drinking unpasteurised milk, often when the cows had mastitis. Most human cases occur sporadically, though outbreaks have also occurred. A recent outbreak in Finland was associated with the consumption of fresh goats cheese. Streptococcus zooepidemicus in humans is not notifiable under general public health legislation in the UK.
The incubation period may be short, 1-3 days, though for nephritis this is usually 7-14 days after the preceding respiratory tract infection. Presenting clinical features are generally those of septicaemia, bacterial endocarditis, meningitis, deep vein thrombophlebitis or mycotic aneurysm. Nephritis may present with malaise, headache, vague loin discomfort, dark urine, and oedema around the eyes and on the backs of the hands and feet. Most fatalities occur in infants or the elderly. Young people affected may only experience a short 'flu-like illness which may not come to the attention of general practitioners.
Streptococcus zooepidemicus infection is usually treated with penicillin, but is also susceptible to co-trimoxazole, erythromycin and cephalosporins. Prevention of human infection transmitted through milk is by pasteurisation. People infected with S. zooepidemicus should not undertake milking or handling milk or other dairy products.
People in close contact with animals, especially horses and dairy cattle are potentially at risk, though it should be remembered that infection appears to occur very infrequently. In agricultural areas where the terrain is generally unsuitable for beef and cereal production, small producer-retailers may produce farm-bottled unpasteurised milk, consumers of which may be unaware of the lack of pasteurisation. Current interest in 'healthy foods' may encourage the public to consider that unpasteurised cows' milk may be "healthier". Pasteurisation has been the most important factor in the reduction of streptococcal outbreaks resulting from milk.
Very few cases of human infection with S. zooepidemicus have been reported in the UK in recent years. Between 1990 and 2006, there were eleven laboratory confirmed reports of infection with S. zooepidemicus (less than one per year). However, between 1983 and 1984 there were eight deaths during 32 outbreaks associated with milk and milk products contaminated by S. zooepidemicus.
A serious outbreak was reported in 1984, when S. zooepidemicus was reported from 11 in-patients admitted between 31 March and 4 June 1984, to two hospitals in a district of West Yorkshire. Seven patients died, although the organism was not completely responsible in all of them. Nine of the patients were aged 70 years or older, one was 52 years of age and another was a 1-day old premature infant whose mother was thought to be infected. A further fatal case was reported from a neighbouring district (total 12 cases with 8 deaths). 10 of the patients lived within a 1.5 mile radius and 11 of the cases drank unpasteurised green-top bottled milk supplied by two retailers who both obtained their supplies from one dairy farm with a herd of 17 cows. Milk from the cows was pooled, filtered and bottled on the farm after cooling. Initially, samples of milk, specimens from milk filters and swabs from farm environment and workers was negative until about 6 weeks later S. zooepidemicus was isolated from a bottle of raw milk offered for sale and originating from this farm. One patient also drank unpasteurised milk but apparently from a different supplier. This second source occasionally supplied milk to the first farm, but pasteurised and bottled. It appeared that this one case may have inadvertently consumed unpasteurised, green top milk originating from the first farm.
Whilst the 17 cows on the first farm appeared healthy, S. zooepidemicus was isolated from the udder of one cow; subsequently two other apparently healthy cows were found to be excreting the organism intermittently. The animals were slaughtered. The source of infection in the cows was unclear but an indistinguishable isolate of S. zooepidemicus was recovered from the vaginal swab of a mare, the only horse on the farm, not known to have grazed in the same field as the milk herd. The farmer subsequently ceased milk production.
The first cases coincided with the usual febrile illnesses of late winter and early spring so that others arising in younger adults may not have come to the attention of general practitioners. Since the milk from the herd was pooled contamination of bottles may have been widespread.The severity of illness in this outbreak did not appear to correlate with the quantity of milk consumed. Most sporadic cases of infection appear to be associated with close domestic animal contact, particularly horses, or with the consumption of unpasteurised cows' milk.
Two cases of human infection with S. zooepidemicus were identified in West Yorkshire in 1988. The index case regularly drank green-top milk and developed a severe febrile illness associated with septicaemia. He had several domestic pets and his daughter, who did not live at the family home, kept a horse. Milk and filter samples were negative as were throat swabs from families of cases, farm workers, and other members of the wider population were negative for S. zooepidemicus. Mouth and rectal swabs from the horse belonging to the daughter of the first case were positive for S. zooepidemicus, and the strains appeared similar to the patients' organism. Mouth and rectal swabs from other pets were negative. There was no microbiological or epidemiological evidence to show that milk was involved in his illness. The second case occurred in a patient admitted to the same hospital at the same time but with no epidemiological risk factors. It was considered that this second case could have resulted from cross-contamination of specimens within the hospital.
In 1994, infection was identified in a patient with meningitis who consumed his own unpasteurised milk; S. zooepidemicus was isolated from the bulk milk tank and from a cow. Subsequent cases have also had meningitis and endocarditis. The last fatality from this infection occurred in 1996, in a patient with lobar pneumonia and meningitis.