Introduction
Brucella abortus, a gram-negative bacteria, causes Brucellosis, is a highly significant zoonotic cattle disease across the globe that causes large production losses. Affected male cattle’s are sterile. Affected females experience reduced milk production, increased calving intervals, birth of weak calves and abortions in the last trimester of gestation, which may lead to retention of placenta and metritis. The abortus harbors large quantities of the pathogen, which can then infect other cattle that comes into contact. Disease is transmitted via contact of contaminated fluids with the conjunctiva and nasal or oral mucosa. The organism populates in regional lymph nodes before spreading to the mammary glands, reproductive tract and the rest of the body through blood.
This study aims to discuss possible implications of Brucella control in the UK, based on Northern Ireland as a model.
BRUCELLA IN NORTHERN IRELAND
The eradication scheme was started in Northern Ireland in 1963 after it was discovered that 12% of milk sold contained Brucella. By early 1980s, the disease was seemingly cleared.
In 1991, a cluster of 8 outbreaks occurred near County Amagh. 32 herds were found to be seropositive, with 8 of them occurring very near (2.5km) County Amagh, all within 15 months of the primary outbreak. These 8 herds experienced high burden with 15.3% within herd prevalence, as compared to 5.4% in the rest of the herds. The spread is highly indicative of the ripple-like effect of this infection, whereby the ripples, or in this case the prevalence of Brucella, has highest infectivity at its core. The rest of the province however, continued to stay disease free.
The outbreak in County Amagh was controlled until a recrudescence in 1997, spreading the disease to 60 herds. The incident prompted a 3-fold increase in non-routine tests, which are essentially short-term interval risk tests. Most of the triggers for these tests happened in winter, when the animals were housed and the higher stocking density contributed to higher animal prevalence.
3 primary outbreaks occurred from September 1997-1998 resulted in 61 secondary and tertiary outbreaks. The first primary outbreak was in County Londonberry as a result of the sale of 10 infected bulls to different herds, though detecting them had been difficult as some needed to be tested multiple times before a positive result was achieved. Fortunately, as in most cases of infections in bulls, the infection was not spread within the new herds. The same area faced a second primary outbreak that led to five secondary outbreaks in 8 farms more than 30km away.
The third primary outbreak was in County Amagh. It was found that the infection had been present since the summer grazing season.
30 more herds were tested seropositive within the next 9 months, and by 2000, a total of 285 herds were tested seropositive. From a mean annual herd incidence of 0.035% and mean animal incidence of 0.01% in 1990-1996, numbers had risen to 0.48% and 0.11% respectively for periods 1997-2000. The numbers continued to rise and peaked in 2002, afterwhich it started to decline as the program was intensified to include culling of female offspring of reactors and pre-movement testing. Numbers reduced to an annual herd incidence of 0.56% by 2006.
RESULTS AND FINDINGS
It was found that cumulative incidence was 0.5% higher in dairy herds than in non-dairy herds. In non-dairy herds, incidence was higher in larger and denser herds. A year increase in age increases the odds ratio for seropositivity by 1.6, making older cattle much more susceptible to disease than younger cattle. Abortions are cardinal signs of the disease. Herds with high occurrence of abortions most likely faced higher in-herd prevalence than herds with less abortion cases. There is a strong relationship between reactor cattle and their offspring, indicating that all calves from reactors should be culled.
The most common reason for spread of the organism is locality as 40% of spread of the disease between 1990-2000 is due to contiguous infected herds. Farms are more likely to be infected if they are located close to seropositive farms. Transmission was suspected to be due to cross contamination by human traffic especially if there is shared management, shared machineries and pastures.
Another significant risk factor is cattle movement. It is suggested that up till 2006, 20% of breakdowns is due to the movement of cattle between herds through sales. 3.1% of animals sold from an infected herd were found to be positive at their destination herd. Cattle in the destination herd immediately becomes 19 times more likely to be infected compared to an unexposed cattle.
Only 1% was due to the importation of cattle from Republic of Ireland. 19% of seropositive cases were due to false positives and non-compliance with animal regulations.
POSSIBLE IMPLICATION OF CONTROL MEASURES IN THE UK
Complete eradication of Brucella would require highly dependable on-going surveillance and stringent eradication program in the event of an outbreak. It will be time consuming, resource intensive and extensive epidemiological investigations will be required.
Farmers need to improve farm management and be consistent with biosecurity measures and calving management. They need to be educated on the importance of complying with regulations and reporting promptly and accurately. Besides biosecurity, perhaps the most important step will be to enforce stricter between farm movement restrictions, including more tests and individual risk assessment of each cattle before they should be allowed to move. Farms identified as high risk, such as larger, high density or those in inner rings of outbreaks, or farms with history of breakdown, should have within herd movement restrictions imposed on them.
58% of breakdowns were discovered during a non-routine test after putative links were made, which were commonly contiguity to infected herds. Another 15% of discoveries were made during the routine biennial and annual screening test. 20% were during post-abortion tests, and 6.9% were made during restricted tests, which are essentially tests acting upon epidemiological evidence or previous laboratory results. It is thus very important that authorities continue to invest time and money into these tests, especially non-routine and post abortion tests. Prompt testing of contiguous herds and short-interval testing are recommended to be continued for up to 3 years after the disease has been cleared.
The current primary screening test, the SAT, was estimated to be only 79-91% sensitive, which may have been the reason for the repeated times when 2-3 tests were required to achieve a positive result. This is despite the fact that the cattle tested had been exposed for 3 months and had all epidemiological evidence pointed to a seropositive result. Though it is possible the discrepancy was due to the cattle’s inability to seroconvert as infective dose was low, we need to be able to identify ways and methods that can pick up on these latent infections promptly. False positives not only skew epidemiological studies of disease, making it harder to identify disease characteristics, but also encourage the spread of disease and make outbreaks almost impossible contain. To increase the accuracy of testing, other serological tests should be run concurrently or in parallel with the SAT. These tests include ELISA, RBPT, AGID or FPA. CFT should still remain as the secondary confirmatory test due to its high specificity to eliminate any false positives. PCR of grouped samples can also be considered as a routine check. Other tests include annual serological screening of mating animals, bulk-tank milk test using ELISA and serological screen of all breeding animals over 30 months of age at slaughter.
As part of the surveillance program, farmers should be compelled to report all abortions to the authority for further investigation within a stipulated time.
In the event of an outbreak, all seropositive animals are to be slaughtered in a specified slaughterhouse and herds may be depopulated. Calves from reactors should be culled. Contiguous herds, both in inner and outer rings, should face strict movement restrictions until 2 tests a few weeks apart produce seronegative results.
FURTHER DISCUSSION
A cattle that has been exposed due to a newly bought herd mate is 19 times more likely to be seropositive than an unexposed cattle. Between 1997-2003, this exposure had been responsible for 20% of the infected animals. We recommend that a cost-benefit analysis be conducted to further quantify the necessity of such a stringent culling program. It will be wise to explore the option of lowering culling rates. Evaluation of best possible alternatives should be based on, but not limited to, financial impact on farm and government, animal welfare, public health and sustainability.
Besides control, policy makers can also perhaps look into the possibility of channeling resources to research in areas with history of repeated primary outbreaks such as County Amagh, to identify and rectify the main causes of problems.
Surveillance and eradication programs practiced in Northern Ireland should be modified to tailor-fit demographics of cattle farming in the UK. They should be dynamic and adaptive to current trends and disease status as discovered in epidemiological studies of the UK and not just within Northern Ireland.