Milk has been an important food source for humans since the domestication of dairy animals began. It is a common component of the animal-derived food products that comprise many diets. Methods to ensure the safety and quality of goat milk and milk products are necessary with the expansion of goat dairy industry and use of goat milk in various sectors of food processing. Therefore, it is important to make sure that marketed goat milk is unadulterated and safe for human consumption.
Drugs and medications that are labeled for food animals are required to list the meat and milk withdrawal times on their label or packaging materials. This is because certain drugs stay in the meat and/or milk of food animals for extended periods of time, and the residual and potential cumulative effects of drugs and medications on humans consuming these food animals are of ongoing concern.1 The U.S. Food and Drug Administration (FDA) governs the approval and disposition of medications and drugs that go to market, and
Many, but of course not all of the medications addressed by such notifications are antibiotics. Government agencies are duly concerned that drug residues in meat or milk destined for human consumption can result in human populations developing resistance to these medications, as well as the pathogens they were designed to fight. Antimicrobial resistance (AMR) is the term applied to this phenomenon. Since new antibiotics are not being developed as rapidly as in previous decades, there is legitimate concern that "super bugs" may develop resistance to all known forms of antibiotics.1 Multiple Drug Resistant (MMR) tuberculosis is one of the more noteworthy diseases that have developed as a result of AMR. Thus, the producers of food animals should carefully evaluate the appropriateness of every drug regimen.
In the case of goats, veterinarians tend to recommend drugs with shorter withdrawal times. Unfortunately, many drugs with shorter withdrawal times tend to be less effective than those with longer withdrawal times. For example, the dewormer fenbendazole has a very short withdrawal time. Even though its efficacy is reported to be less than that of ivermectin, many veterinarians will often recommend its use over the latter when prescribing for goats and other food animals.1 This is understandable; since veterinarians and other medical professionals are held to a high standard, the veterinarian may reason that the risk in recommending off label or extra label usage justifies the use of less efficacious drugs when prescribing for food animals.
In such cases however, a more prudent approach would be for the veterinarian to be informed regarding what a client's goats are being raised for. If they are breeding stock animals or pets, then withdrawal time is far less critical. If, however, the goat is being raised to produce milk or meat for human consumption, then withdrawal times are far more significant. Administration of products which are not labeled for use in goats (off label or extra label) is not illegal; it just means that the products’ manufacturers have not spent the time or money to complete and submit detailed research studies to obtain government approval to label them for use with goats. Although the exact regulations vary from country to country and state to state (or province), regulatory authorities expect that a veterinarian–client relationship exists and that animals are under the clinical control of the veterinarian before drugs are prescribed.
That said, there are medications which are illegal for use in certain species of food animals. Enrofloxacin is one of these. Enrofloxacin is legal for use in cattle but illegal to administer to goats or other food animals.1 The banning of such drugs comes from concerns over humans developing resistance to such antibiotics as a result of drug residue in meat and milk. These medications are available by prescription only, and should be used in very limited fashion.
All but a few of the medications used for goats have been developed for other species of animals. This is because research, development, testing, and obtaining government approval for drugs are expensive and time-consuming. Thus, manufacturers must see sufficient demand for new drugs so they can earn back their costs and make a profit. As a result, producers often learn which remedies are effective from other goat raisers, from veterinarians who knowledgeable about goats, or by trial and error in their own herds. Often, the dosage amounts of drugs for larger livestock are simply adjusted for the goat's faster metabolism.
In some cases, manufacturers should be consulted about suitable drug doses for goats. Drug metabolism shows species variation, with the result that dose rates and excretion times for goats should not be extrapolated from sheep and cattle data. The elimination half-life of some drugs that are eliminated mainly by hepaticmetabolism is, in goats, about half that for sheep.2
Draxxin®is the branded form of tulathromycin, a macrolide antibiotic that is labeled for the treatment of BRD associated with Mannheimia haemolytica, Pasteurella multocida, Histophilus somni (Haemophilus somnus), and Mycoplasma bovis in beef and non-lactating dairy cattle. Draxxin® is also used for the control of respiratory disease in cattle at high risk of developing BRD.3 It is also FDA-approved for the treatment of bovine foot rot (interdigital necrobacillosis) associated with Fusobacterium necrophorum and Porphyromonas levii, as well as for the treatment of infectious bovine keratoconjunctivitis (IBK) associated with Moraxella bovis.
Recommended dosages for Draxxin® in goats are as follows (Adapted from Plumb’s Veterinary Drugs, extra-label): 2.5 mg/kg SC; repeat in 7 days if necessary. Based on pharmacokinetic studies in meat and dairy goats, and FDA guidelines: a 34-day meat withdrawal interval and a 45 day milk withdrawal time are recommended.3
While Draxxin® is a macrolide antibiotic, it is structurally unique in that it has three amine groups (tribasic), while erythromycin and azithromycin have one (monobasic) and two (dibasic) groups, respectively. It is believed that tulathromycin’s tribasic structure allows it to better penetrate gram-negative pathogenic bacteria and its low affinity for bacterial efflux pumps may allow the drug to remain and accumulate within the bacteria.
The mechanism of action of tulathromycin is similar to other macrolides in that it inhibits protein synthesis by penetrating the cell wall and binding to the 50S ribosomal subunits in susceptible bacteria.3 It is considered a bacteriostatic antibiotic although there is the potential for a time-dependent bactericidal action, particularly with high concentrations. The drug possesses bactericidal activity particularly for Mannheimia haemolytica and Pasteurella multocida.1,3
Draxxin®’s efficacy is enhanced by its ability to accumulate and persist in pulmonary epithelial lining fluid, macrophages and neutrophils. The drug shows notable accumulation and long persistence in lung tissue. Neither time-dependent nor concentration-dependent models may accurately predict or describe the drug’s efficacy.