Respiratory problems caused by Pasteurella multocida in dairy buffaloes in Pakistan

THE dairy colony is a distinct type of livestock production system that is different from the mixed crop-livestock production system generally found in Pakistan (Afzal 1999). Dairy colonies are appearing around all the major cities in Pakistan, and there are many in Karachi, the largest metropolitan city in the country. Of these, the Landhi dairy colony has more than 190,000 animals at one location, and is probably the largest congregation of buffaloes in one place. High-yielding animals are kept for milk production, and the rate of turnover of animals is high (10 to 12 per cent per month). Calves are not raised in the dairy colonies. All the buffaloes are stall-fed with feed consisting of concentrate, minimal green fodder (5 to 10 kg/day) and wheat straw (5 to 6 kg/day) per animal, regardless of production level. Due to market demand for high butterfat (7 per cent) milk in Karachi, the animals are also given rapeseed oil daily or weekly (Afzal 2003). Poor hygiene practices are the hallmark of most of the dairy colonies in Karachi. The animals are overstocked and provided with only approximately 1 m width of space. There is no mechanism of waste disposal. Dried dung is sold, but most of the animal waste is washed into open channels that ultimately drain into the sea, resulting in pollution and environmental problems. The concept of disinfection of premises is alien to the dairy farmers (Afzal 2003). These conditions have always favoured a high incidence of disease. Rinderpest resulted in high mortality in the 1980s and 1990s in the dairy colonies of Karachi (Hussain 1997, Rossiter and others 1998). With the technical assistance of the Food and Agriculture Organization of the United Nations (FAO) and the concerted efforts of veterinary professionals in Pakistan, the country declared provisional freedom from the disease in January 2003. The last outbreak of rinderpest involved eight animals on one dairy farm in Karachi in 2000 (Hussain and others 2001). High mortality during 2003 brought with it the fear that rinderpest had returned. However, the clinical signs of disease were different, being more suggestive of haemorrhagic septicaemia, but without the most typical sign of swelling around the throat. Nevertheless, Pasteurella multocida was isolated from blood samples, and there was an encouraging response to treatment in animals treated for haemorrhagic septicaemia early in the disease. It was thought that respiratory viruses may also have been involved in the pathogenesis of the disease. The first clinical sign was sudden onset of anorexia. Animals had a high temperature (40·5 to 41·1°C) in the initial stage of the disease, and within 24 to 72 hours they developed dyspnoea and laboured breathing. Slight to moderate nasal discharge was seen in a few animals. The skin was normal in appearance. No ocular discharge was observed and faeces were normal, although some farmers reported constipation in a few animals. Morbidity and mortality on the 10 dairy farms (1512 animals) under investigation were 6·5 per cent and 1·5 per cent, respectively. The overall case mortality was 22·4 per cent. Most of the farmers had vaccinated the animals 3 to 5 months previously with alum-precipitated vaccine against haemorrhagic septicaemia. Paired sera were collected from 27 affected buffaloes during the acute and convalescent stages of the disease (15 days apart). These sera were analysed at the virology department of the Veterinary Laboratories Agency – Weybridge, for antibodies to bovine viral diarrhoea (BVD), infectious bovine rhinotracheitis (IBR), parainfluenza virus type 3 (PI3) and respiratory syncytial virus (RSV) by ELISA. Suggested positive titres were an optical density (OD) ratio greater than 0·2 for BVD and greater than 0·24 for IBR, PI3 and RSV. A rise in OD ratio of 0·2 or more indicated seroconversion in paired acute and convalescent sera. Nasal swabs and blood samples were also collected for isolation of bacteria. Serological analysis showed that four animals had positive titres for BVD. None of the animals seroconverted to BVD in the paired sera. In total, 20 of the 27 buffaloes had significant titres to IBR, but only four of these animals seroconverted in the paired sera. In addition, 18 buffaloes showed positive titres for PI3, but only two of these animals showed significantly increased titres in the paired sera analysis. Only eight buffaloes had positive titres for RSV and none of the animals seroconverted in the paired sera testing. P multocida was isolated from nasal swabs and blood samples collected during the acute febrile stage of the disease. Antibodies to rinderpest were not detected in any of the sera when analysed by competitive ELISA. Bacteriological and serological studies indicated that the disease was caused by infection with P multocida. Lack of seroconversion in a large number of paired sera against different respiratory viruses indicated that these viruses did not have a significant role in the aetiology of the disease. However, it was apparent that many of the animals had been exposed to IBR and PI3 viruses. The lack of typical clinical signs for haemorrhagic septicaemia could be attributed to the fact that the buffaloes were carrying respiratory viruses that altered the pathogenesis of the disease, resulting in laboured breathing but no swelling around the throat. This aspect of the disease requires further investigation. It is also interesting to note that the recommendations of vaccine manufacturers to inoculate animals every six months did not confer protection in the dairy colony production system, particularly in Karachi. This may be attributed to overcrowding of animals or to the transfer of animals from the low-humidity environment of the Punjab to the high-humidity environment of Karachi. It also raises a question on the quality of some brands of haemorrhagic septicaemia vaccine being used in Karachi, as a higher prevalence of disease was seen on farms where these vaccines were being used. These aspects need further investigation if a rational vaccination strategy against haemorrhagic septicaemia is to be propagated in the dairy colonies of Karachi.