2 American Foulbrood in the New Zealand Context

2.1 History of American Foulbrood in New Zealand

Information on the numbers of beehives infected with the disease was not recorded during the early period of beekeeping development in New Zealand. Part of the reason was that beekeepers attempted to manage the disease, rather than destroy infected hives.

Honey bee colonies with light infections were "shook swarmed". Bees were shaken from infected hives into hives that contained only foundation. While the method was often effective at eliminating the disease, painstaking effort was required, and some hives still developed heavy infections and had to be destroyed.

Early attempts at managing AFB using "shook swarming" make interesting reading:

The districts in which the Ruakura State Apiary is situated were amongst the worst in the Dominion for foulbrood. The colonies I started the State Apiary with that were already on the farm were affected. By constant attention and treatment we were able to keep the disease from spreading and when we left for the Christchurch Exhibition (1906) there were six out of over 70 slightly affected with foulbrood. When we returned in the following June we found the disease had spread through robbing to nearly every colony. Early in the following season we treated a number of the worst cases and replaced bad with clean combs. As this did not turn out as satisfactory as we hoped, I hoped to treat the whole of the colonies the next spring.⁴

In 1950, it was decided that the incidence of AFB could not be reduced further if shook swarming continued to be used. Beekeepers were therefore instructed by the Department of Agriculture to "destroy the contents of diseased hives and to sterilise thoroughly any remaining hive equipment by approved methods."⁵

2.2 Incidence of American Foulbrood Disease in New Zealand

There were no reliable AFB disease statistics collected between 1950 and 1960. By 1961, however, the incidence of AFB had reduced to 0.23% of hives. The decline in disease levels during the 1950's may have been due to the move away from shook swarming (managing AFB), and the adoption of the practice of destroying diseased hives.⁶

The percentage of beehives reported to be infected increased over the next 30 years, reaching a peak of 1.2% in 1990. The NBA instituted an American Foulbrood Control Programme in 1991. The programme included the inspection of approximately 4% of the nation's apiaries by government inspectors, voluntary inspections carried out by NBA branches (called "diseaseathons"), the counselling of beekeepers with AFB problems, a research programme elucidating the factors contributing to the spread of AFB and an extensive education programme. During the seven years the programme was in existence, the reported incidence of the disease decreased by an average of 12% per annum, reaching a low of 0.38% in 1998, the last year of the programme. (see Figure 1).

Figure 1. The percentage of hives in New Zealand reported to have AFB each year.

New Zealand is fortunate to have detailed statistics on AFB incidence. The data are based both on beekeeper reports of AFB findings in their hives (including an annual disease declaration by every beekeeper every year), and on reports by trained inspectors. The data have also been collected over a number of years. Very few other countries have comparable data. The New Zealand statistics therefore provide useful information on the spread of AFB in a beekeeping industry that does not use antibiotic drugs to control the disease.

Although New Zealand's AFB disease statistics are more comprehensive than most, the information must still be treated with caution. The figures rely heavily on information provided by beekeepers to the Ministry of Agriculture. Even though it is a statutory requirement in New Zealand for beekeepers to report diseased colonies, in the past:

1. not all beehives were inspected,
   
2. not all AFB infections were detected in those beehives that were inspected, and
   
3. not all cases of AFB were reported when found.

2.2.1 Frequency Distribution

This assumption is supported by an analysis of New Zealand AFB statistics that shows that the fewer beehives belonging to a beekeeper, the greater the percentage of those hives infected with AFB (see Figure 2). The analysis should be treated with caution, however, since beehives belonging to part-time beekeepers with small hive holdings (1-50 hives) make up less than 15% of the AFB hives found each year (see Figure 3).

Because commercial beekeepers with more than 500 hives own most of the beehives in New Zealand, these beekeepers also have over half of the AFB hives. Beekeepers with larger hive holdings tend to also have more experience finding AFB, since 70% of them find at least one of their hives infected with AFB each year, compared to about 1% of beekeepers who own less than five hives (see Figure 4).

Figure 2. Annual percentage of AFB hives found for beekeepers with varying numbers of hives.

Figure 3. Annual percentage of hives reported to have AFB that are owned by beekeepers with varying hive holdings.

Figure 4. Percentage of beekeepers with varying hive holdings who find 1 or more AFB hives each year.

2.2.2 Geographic Distribution

2.2.3 Antibiotic Feeding

Although under some circumstances antibiotics can eliminate AFB disease from honey bee colonies, it is probably not possible to eliminate AFB from New Zealand by using antibiotics. Antibiotics are commonly used to control AFB, rather than to eliminate it. It has been estimated that in areas where antibiotics are used as an AFB preventive, between 10 and 20% of beehives would break down with the disease if antibiotic feeding ceased.^7,8 To avoid devastating economic losses, beekeepers in the United States have found that they must feed antibiotics on a continuing basis.⁹

Antibiotic treatment has three important disadvantages, which are also problems typically associated with antibiotic programmes to control most animal diseases:

1. Bacterial resistance - With prolonged antibiotic use, it is possible that resistant strains of AFB could become a problem. Strains of AFB resistant to oxytetracycline, the main antibiotic used against AFB, have been reported in Argentina.
   
2. Drug residues - Information from countries where antibiotics are fed indicates that antibiotic residues will end up in honey produced for the export and local markets. This may result in honey having to be tested, which would increase product costs.
   
3. Applicator error/negligence - The most significant problem with antibiotics is that everywhere they are used, applicator error/negligence results in a continuing incidence of American foulbrood in beehives.