Mycoplasma mycoides mycoides large colony (MmmLC) in dairy goats

by Dr Scott McDougall, Animal Health Centre, PO Box 21, Morrinsville

Introduction

The finding of MmmLC in a dairy goat herd in July 2001 led to a disease investigation undertaken by MAF, because MmmLC was previously considered exotic to New Zealand. The conclusions of the investigation were that MmmLC has probably been present in New Zealand for some time, and given the movement of dairy goats between herds, that many herds may be infected.

What happened during the outbreak?

Over a short period of time approximately half of a group of 1-3 week-old kids on a dairy goat property developed swelling of the joints and lameness in one or more legs (termed “polyarthritis”), fever and reduced feed intake. Approximately 10% of the affected kids died or were euthanased. A veterinarian took appropriate samples and the laboratories detected MmmLC. Subsequent examination of bulk milk samples found that MmmLC was present in the bulk tank milk on two occasions in early spring, 2001. The bulk tank milk from this farm was on-sold to a calf rearer in the Waikato at this time. On the calf rearing property approximately 10% of the calves fed this milk developed polyarthritis, conjunctivitis and/or pneumonia, and MmmLC was isolated.

One doe on the original property had intractable clinical mastitis that had not responded to antibiotics, and this doe seemed the likely source of MmmLC . Colostrum from this and other does had been pooled for feeding the kids resulting in a large number of kids being exposed to MmmLC.

What is MmmLC?

Mycoplasmas are a group of simple microbial organisms that lack a true cell wall and hence survive for less than 2 weeks away from an animal host. MmmLC can cause a range of clinical conditions in goats including mastitis, pneumonia and multiple joint infections (polyarthritis). The initial infection of a herd may be manifest as an explosive outbreak. More commonly, infection results in a small number of goats showing minor or no clinical signs. MmmLC is present in most dairy goat populations internationally. With increasing intensification of production systems the disease may be seen more commonly and in more severe forms. In countries where the disease is present, intermittent ‘flare-ups’ occur in individual herds, with intervening periods where there is little serious disease.

How is it spread?

Carrier animals that may have no signs of disease spread MmmLC. Infected does may excrete large numbers of MmmLC in milk following kidding, and hence infect their kids. Where pooled colostrum is fed, many kids may be infected by one doe. MmmLC is also present in discharge from infected joints, vaginal secretions, the external ear canal and in ear mites. Infection may spread from doe to doe during milking by spread via the liners in the milking machines and milkers’ hands, and by close association of infected animals.

What are the clinical signs?

Where MmmLC has not been previously present in a herd and when conditions lead to widespread exposure, severe outbreaks of disease can occur. In such cases up to 40% of does and 90% of kids may show clinical signs. Kids are more susceptible than older animals and death may occur in up to 50% of infected kids.

In does, an initial fever is followed by reduced milk production (or total cessation of milk production) and mastitis. The mammary gland becomes hard and may contain abscesses. One or, more commonly, many joints are infected resulting in lameness. The respiratory tract may also be involved with signs of pneumonia (seen as coughing and difficulty in breathing) and conjunctivitis. Abortion may also occur.

In kids, lameness and pneumonia are the main signs but conjunctivitis and nervous system signs can also occur. Often a high proportion (>50%) of a group of kids can be infected and show clinical signs.

Where the disease has been present for a long period of time (i.e. it is ‘endemic’), then only a small number of animals may exhibit mild clinical signs. Does may be carriers with no clinical signs at all. The concentration of MmmLC in the milk increases to very high levels around kidding, and this is probably the major source of infection to other animals.

MmmLC should be suspected where a high proportion of kids in a group show signs of lameness and pneumonia and/or a number of does have mastitis or pneumonia which is not responsive to antibiotic treatment.

What do you do if you suspect the disease?

Confirmation of MmmLC in an individual animal requires growth (‘culture’) of the organism from joint fluid, milk, ear swabs or lungs.

The MmmLC status of a herd may be assessed by aseptically collecting milk samples from a proportion of does (see Table 1). As does are more likely to shed MmmLC around kidding, milk sampling should occur at this time. At present blood antibody tests for MmmLC are not particularly accurate and they have not proven very useful in determining whether MmmLC is present in either an individual animal or a herd.

Where MmmLC is suspected or where the MmmLC status of a herd needs to be established, contact your veterinarian to arrange collection of appropriate samples for testing.

Treatment

MmmLC is sensitive to a number of antibiotics in the laboratory, but response under farm conditions is variable. This may be due to inappropriate choice of antibiotics, under dosing, too short a duration of treatment and the fact that MmmLC may survive in joints where antibiotics may not be particularly effective. During an outbreak antibiotics may limit the severity of disease, the number of animals showing clinical signs and reduce the number of deaths. However, some treated animals may apparently recover but remain carriers of the infection. Hence antibiotic treatment should be used to limit the effect of an outbreak of MmmLC, while recognising that this is unlikely to lead to eradication of the disease from a herd even if every animal is treated. Contact your veterinarian to get a diagnosis and advice about appropriate antibiotics before treating animals.

Internationally a number of vaccines have been tested but none are available in New Zealand at present. Although vaccination reduces the severity of clinical signs, individual animals may still become carriers.

What can be done to control MmmLC?

The major route of infection of MmmLC is directly from animal to animal. Does excreting MmmLC in the milk are probably the major source of infection for kids via the feeding of pooled colostrum or milk, and to other does via milking. Minimising opportunities for spread between animals offers the best option for control.

Specific control measures include:

  • Ensuring adequate functioning of the milking machine (i.e. have the milking machine tested by registered milk machine tester annually; replace the liners as suggested by the manufacturer)
  • Teat spraying following every milking for the entire season
  • Milking ‘clean’ (uninfected and/or young) animals first
  • Identifying and culling suspect animals (e.g. repeated clinical cases of mastitis; chronically lame animals)
  • Isolating and treating any sick does and kids
  • Minimising spread from dams to kids by “snatch kidding” (i.e. prevent kids from suckling does and provide colostrum from known uninfected does) and/or feeding of pasteurised, pooled colostrum (heat milk to 56 oC for 20 to 60 minutes)
  • Raising kids in small rather than large groups
  • Ensuring that kid rearing facilities are kept clean, because MmmLC is readily killed by most disinfectants.
What can be done to minimise risk of introducing MmmLC?

Certification of individual animals or herds as being MmmLC ‘free’ is not presently possible given the current limitations of blood tests and the time and cost of culturing the organism. Minimising the risk of introduction and/or spread of disease should be attempted by use of management practices that prevent spread between and within herds.

Introduction of infected stock following purchase or loan is a major potential source of introduction. Thus, between-herd movements of animals should be minimised: herd owners should aim to have ‘closed-herds’ wherever possible. Where stock are to be purchased or loaned, the management practices of the source herd should be closely examined, and stock sourced only from herds that do not pool colostrum, have policies to isolate and treat sick animals and that practice good milking hygiene as listed above.

Sharing of milking equipment at shows risks spread of MmmLC. Following the milking of an infected goat, the liners and residual milk within the liner and claw may transfer MmmLC to any goat subsequently milked using this equipment. Thorough disinfection of communal milking equipment at shows will minimise this risk. Any goat with mastitis, pneumonia, lameness or a fever should not be shown. MmmLC may also spread by transfer from goat to goat on milkers' hands, and so a high level of personal hygiene should be used when milking.

Milk collected from does should not be fed to goats from other herds or to cattle or sheep unless pasteurised.

There is no risk to humans either from drinking goat’s milk or from eating meat from infected animals or animals that may have been fed milk containing MmmLC.

Conclusions

MmmLC may have been present in New Zealand dairy goat herds for some time, and has probably caused periodic minor disease. The number of herds and goats infected is not presently known. The likely economic loss due to MmmLC is probably relatively small for the dairy goat industry, although disease outbreaks may result in significant impacts for individual owners.

For the individual herd owner, continuing care with introduction of goats, sourcing animals from herds that practice good management techniques, and continuing basic sound management to minimise disease spread are the best means of minimising the risk of introducing or spreading the disease.

Monitoring and control of MmmLC is the responsibility of individual herd owners. Where MmmLC is suspected or where programmes to minimise risk of MmmLC introduction or spread are required, contact your veterinarian for further advice. 

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