Farm Animal Medicine and Surgery. Graham R Duncanson
Читать онлайн книгу.
serovar Hardjo and L. interrogans serovar Hardjo)
These bacteria will cause abortion but more commonly cause infertility. They are associated with transitory illness in cows which show a ‘milk drop’ syndrome. The condition is readily diagnosed on serology. It can be spread from infected urine and infected ditches, and also from infected sheep which are symptom-less carriers. It is a zoonosis. There are good vaccines available.
Leptospira serovar Pomona
This organism is not a primary cattle pathogen and is found in many mammalian species. It has been isolated in rare instances from aborted cattle fetuses.
Listeria monocytogenes
This bacterium causes primarily a neurological pyrexic, zoonotic disease. Abortions are normally sporadic but can affect up to 20% of the herd at any stage of gestation. Death of the fetus occurs 2 days before the abortion so the fetus is normally decomposed. The disease is normally associated with the feeding of silage. Treatment with antibiotics is helpful. There is no vaccine available.
Mycotic abortion
Aspergillus fumigatus is the main causal organism of mycotic abortion. Infection comes from eating mouldy bedding or feeding stuffs. The organism spreads in the bloodstream to the placenta, and causes late abortions. Diagnosis is by demonstration of the fungus in the placenta, which has ‘ring-worm’-like lesions.
Neospora caninum
This protozoan was first seen in dogs and spreads from them by the faecal-oral transfer to cattle. It has only recently been associated with bovine abortion. The organism is common in the UK but it may not be as common as serology might suggest. The definitive diagnosis is from immunohistochemistry of fetal brain tissue. This will differentiate N. caninum from Toxoplasma gondii. The main method of spread is vertical transmission from infected dams to congenitally infected calves. There is no vaccine available.
Parachlamydia acanthamoeba
This bacterium was first found in Switzerland causing suppurative and necrotizing placentitis. It is now found in southern Scotland. It can be diagnosed on PCR. It may have zoonotic implications.
Rift Valley fever
This viral disease caused by a bunyavirus is found in Africa and the Middle East. The main method of spread is by Aedes mosquitoes. The abortion rate can be 100%, with 10% of adult cattle dying, and showing icterus and pyrexia. Diagnosis can be confirmed with serum and heparanized blood that show high titres and by virus isolation. There is no treatment but there is a vaccine available.
Salmonellosis
This is a rare cause of abortion and might just be the raised temperature from a systemic infection, but in some instances S. dublin or S. typhimurium are isolated from the fetus or the placenta. The spread is via the contaminated environment. There is a vaccine against S. dublin available.
Schmallenburg virus (SBV)
The Orthobunyavirus that causes this disease of cattle is named after the town of Schmallenburg in North West Germany where it was first discovered in August 2011. By the time of writing, it had spread to most of Western Europe. SBV is spread by Culicoides obsoletus and C. dewulfi and is reportable but not notifiable in the UK. Clinical signs seen in cattle include fever, reduced milk yields, inappetence, weight loss and diarrhoea. These signs are transitory, but the most important result is the formation of congenital abnormalities if the virus affects the cow when 8-16 weeks pregnant. The abnormalities seen are limb contracture, brain abnormalities and a twisted neck (see Plate 3). These may result in the need for an embryotomy or a Caesarean section. Cows after parturition do not appear to be shedding virus.
Ureaplasma diversum
This mycoplasma is found in healthy animals and so its role in causing abortion is unclear. It is certainly transmitted venereally, but it is likely that it requires another agent to cause an abortion.
Wesselsbron disease
This viral disease, found in Africa, is caused by an arthropod-borne (mainly Aedes spp. mosquitoes) Flavivirus (Wesselsbron virus) which causes abortion and a fatal disease in neonatal calves. It is zoonotic and difficult to differentiate from RVF.
Endometritis
This condition is seen commonly post calving, particularly if there has been fetal membrane retention. The normal causal organisms are Arcanobacterium pyogenes, Streptococcus spp. and E. coli. In the acute form, the cow will be ill, show pyrexia, have swollen hocks and a bloody smelly vulval discharge. Treatment would be penicillin parenterally and NSAIDs. In the chronic form, the cow will appear normal except for a purulent smelly vulval discharge. Treatment should be aimed at bringing the cow into oestrus with a prostaglandin injection. Uterine washouts were recommended historically but show poor evidence of efficacy.
Mastitis
Mastitis can be attributed to contagious or environmental pathogens or, in the case of summer mastitis, to the fly Hydrotaea irritans.
Contagious pathogens
• These are Staphylococcus aureus, Streptococcus dysgalactiae, S. agalactiae, Corynebacterium bovis, coagulase-negative Staphylococcus spp. (these often colonize the teat canal and are opportunist pathogens), Mycoplasma bovis, M. californicum and Mycoplasma spp.
• They cause inflammation and pain in the udder, as well pyrexia and painful and swollen hocks.
• A milk sample should be taken for culture and sensitivity before treatment is started and then this treatment can be modified when the results are known.
• Although short milk withhold periods are important in dairy cows, effective antibiotic treatment is more important in the long term, not only for the affected cow but also for the rest of the group -bearing in mind that these pathogens are contagious.
• It is important to use the same antibiotic administered into the mammary gland as is used parenterally.
• Mycoplasma spp. present in the udder are not controlled by antibiotics.
• NSAIDs are useful in treatment.
• Dexamethazone should be used with care, making sure that the cow is not pregnant and the organism is being controlled by the antibiotic.
• There is a vaccine available against S. aureus.
Environmental pathogens
• These are E. coli, Klebsiella pneumoniae, Streptococcus uberis, Pseudomonas aeruginosa, Bacillus cereus, Pasteurella spp., Candida spp. and Aspergillus spp.
• E. coli and K. pneumoniae are both very serious pathogens which normally infect newly calved cows. They both produce toxins which cause a peracute disease called ‘toxic mastitis’.
• Toxic mastitis is a specific disease syndrome often causing acute toxicity before mastitis is seen in the udder, and culture of the milk may often be unrewarding.
• Only 30% of affected animals are likely to survive toxic mastitis.
• Treatment