**11. Use of fly control to manage heifer mastitis**

Historically, the major association between flies and intramammary infections has been with the development of summer mastitis, in which the biting fly, *Hydrotoea irritans*, is the proven vector (Chirico et al., 1997). Summer mastitis is an isolated seasonal problem primarily in July, August, and September in heifers and dry cows of northern Europe, and may be controlled by insecticidal sprays. In the US, fly control is used to reduce these insect pests on farm premises, and subsequently reduce animal stress, but its application as an adjunct management practice for preventing new cases of mastitis and reducing SCC has not really been considered or embraced by producers.

Surprisingly, very little research has been conducted on the relationship between mastitis in adult cows and fly control; most studies in this regard have been carried out in dairy heifers. An initial survey performed at Louisiana State University showed that prevalence of mastitis in bred heifers was significantly lower in dairy herds that used some form of fly control for their lactating cows, dry cows, and heifers compared with herds applying no fly control (Figure 9) (Nickerson et al., 1995). The greatest reductions were in numbers of *Staph. aureus* and the environmental streptococci, both major mastitis pathogens in adult cows associated with elevations in SCC.

352 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

were greater in vaccinated heifers compared with controls.

reduce the introduction of *Staph. aureus* to the milking herd.

new antigens and adjuvants are added to vaccine preparations.

**11. Use of fly control to manage heifer mastitis** 

been considered or embraced by producers.

were all quite different.

In addition, overall milk yield, production of fat and protein, and number of days in milk

The decrease in frequency of new *Staph. aureus* intramammary infections at calving (60.9%) in vaccinates using Holstein heifers is higher than the 44.7% reduction observed in a Louisiana trial using the same vaccine in Jersey heifers (Nickerson et al., 1999). In both trials, SCC at calving were reduced by approximately 50%. The 60.9% efficacy found in the present trial is also higher than the 40.2% efficacy observed by Giraudo et al. (1989), the 46% efficacy observed by Nordhaug et al. (1994), and the 52% efficacy observed by Sears et al. (1990). However, it is difficult to compare among the latter three trials as the vaccine formulations

The question becomes: Is it economically feasible to use this commercial vaccination protocol on young dairy heifers? Based on an average of 1,864 more lb milk per vaccinated heifer, which translates to 18.64 hundredweights (cwt) of milk (1,864/100), at the current (2008) price of \$25.00/cwt, an increased income of \$466.00/heifer would be realized (18.64 cwt x \$25.00/cwt = \$466.00). If each heifer was vaccinated beginning at 6 months of age until calving, this would entail vaccinations at 1) 6 months, 2) a booster 2-weeks later, and subsequently at 3) 12 months, 4)18 months, and 5) 24 months, or a total of 5 immunizations through calving. At \$1.50/dose, this cost would total \$7.50, which when subtracted from the increased income from milk production, would yield a net income of \$458.50 per heifer (\$466.00 - \$7.50). This figure does not include the costs of labor involved in the immunization process; however, it is evident that vaccination is well worth the cost of the vaccine. Not only does it reduce new infections in first calf heifers at parturition, it may also

It is obvious that use of experimental and commercially available *Staph. aureus* vaccines can be used to prevent new infections, especially when used in heifers. Efficacy has been shown to range between 44 to 61% with reductions in SCC of 50%. This prevention strategy may represent a major control mechanism for managing *Staph. aureus* in the future, especially as

Historically, the major association between flies and intramammary infections has been with the development of summer mastitis, in which the biting fly, *Hydrotoea irritans*, is the proven vector (Chirico et al., 1997). Summer mastitis is an isolated seasonal problem primarily in July, August, and September in heifers and dry cows of northern Europe, and may be controlled by insecticidal sprays. In the US, fly control is used to reduce these insect pests on farm premises, and subsequently reduce animal stress, but its application as an adjunct management practice for preventing new cases of mastitis and reducing SCC has not really

Surprisingly, very little research has been conducted on the relationship between mastitis in adult cows and fly control; most studies in this regard have been carried out in dairy heifers. Results of this survey also demonstrated that bred heifers having teats with bite lesions and scabs caused by the blood-sucking horn fly (*Haematobia irritans*), exhibited a 70% frequency of intramammary infection compared with a 40% frequency in heifers with normal teats. Such infections are always associated with elevated SCC in excess of 5 million/ml in these young animals. See Figure 10 below illustrating horn flies and lesions on heifer teats.

**Figure 9.** Prevalence of mastitis in Louisiana dairy herds with and without a fly control program.

Since that first survey, researchers have proven through DNA studies that the horn fly is not only responsible for teat lesions on heifers, but is indeed a vector in the transmission of mastitis-causing bacteria, such as *Staph. aureus*, from heifer to heifer (Owens et al., 1998). Such mastitic heifers serve as sources of IMI for transmission to the entire lactating and nonlactating herds.

Once it was established that the horn fly was a vector in the transmission of mastitis-causing bacteria, the next step was to develop management practices to reduce flies and lower the prevalence of intramammary infections. Insecticide-impregnated tags placed on the tail switch in close proximity to the udder during the spring and summer months were successful in reducing horn fly populations by 60% as well as the incidence of mastitis during the first 2 months after placement (Nickerson et al, 1997). In heifers with tail tags, mastitis incidence increased from 8.6 to 15% (1.7-fold increase), while in controls, incidence

increased from 17.1 to 52.4% (3.1-fold increase). As observed above, infections were associated with marked elevations in SCC. However, after 2 months, tags fell off and replacing them was impractical from a management standpoint.

**Figure 10.** Udder of a 10-month-old heifer illustrating horn flies and lesions on teat ends.

In a subsequent trial, the daily dietary supplementation of an insect growth regulator helped to suppress fly populations but not enough to prevent new cases of mastitis in dairy heifers (Owens et al., 2000).

Lastly, the use of an insecticidal pour-on every 2 wk for 6 wk followed by treatment with insecticidal ear tags reduced fly populations and decreased the incidence of new *Staph. aureus*  by 83% during a 6-mo trial in heifers during the warm season in Louisiana (Owens et al., 2002). Mastitis in heifers caused by *Staph. aureus* was associated with SCC in excess of 10 million/ml.

These studies demonstrate that, during the warm and humid months of the year, horn flies do serve as vectors in the transmission of heifer mastitis, which is associated with elevated SCC in these young dairy animals. Although research has not been conducted to show this same association in lactating and dry adult cows, it is assumed that fly populations play some role in the elevation of mastitis and SCC observed in the hot summer months. And, with the proposed reduction in the SCC legal limit to 400,000/ml in the USA, and in light of the fact that milk buyers are imposing their own limits, some as low as 250,000/ml, it is imperative that dairymen utilize all possible means to prevent new cases of mastitis and their associated SCC. A simple fly control program can serve as an important adjunct to the basic 5-point plan of mastitis control and assist dairymen in lowering their bulk tank SCC and earn quality premiums for their product.
