Mastitis in the female dog and cat

Mastitis is an inflammatory condition of the mammary glands, generally due to bacterial infections, which typically occurs during lactation but which can also be observed in late pregnancy or at the end of a false pregnancy.

AETIOLOGY AND PATHOGENESIS

In the bitch and queen, most cases of mastitis are caused by environmental contaminant bacteria, such as coliforms, staphylococci and streptococci, usually acquired via an ascending route. In the dog, risk factors for the development of mastitis include: poor hygiene conditions, high environmental contamination, hot and humid weather conditions, pre-existing mammary lesions, a low number of suckling puppies and kittens, early or abrupt weaning, mammary gland engorgement and galactostasis, poor overall status of the gravid female and/or of the new mother, metritis, severe parasitosis or systemic diseases. The infection can occur by hematogenous spread, but more frequently via the ascending route through the milk ducts or through microtraumas caused by the nails and teeth of neonatal puppies and kittens starting from the second-third week of age.

Mastitis can be responsible for a high neonatal mortality rate, especially if the infection is caused by Staphilococcus aureus, type G Streptococcus and β-haemolytic Escherichia coli. In humans, an increase in the number of cases of mastitis due to methicillin-resistant Staphilococcus aureus has been reported, a phenomenon which is also spreading among companion animals; moreover, this type of mastitis is highly resistant to treatment.  In the dog, Staphilococcus aureus has been associated with gangrenous mastitis in patients affected by leukocytosis and thrombocytopenia. In experimental dog models, Staphilococcus intermedius has been shown to induce a type of mastitis characterised by mammary glands which appear considerably increased in volume, hot, oedematous and painful. Nevertheless, the isolation of the bacterium within canine milk does not seem to be correlated with the onset of septicaemia in suckling puppies. Of the great variety of germs isolated from canine milk, only Escherichia coli, Klebsiella pneumoniae and β-haemolytic Streptococcus were found to be possible causes of septicaemia in suckling puppies.   

SYMPTOMATOLOGY AND DIAGNOSIS

Mastitis is generally an acute condition, which can endanger the survival of the new mother, although mild, subclinical or unapparent forms are also frequent; chronic mastitis is instead rare. 

Mastitis may affect only a portion of a single mammary gland, the entire gland or more than one gland (Fig. 1) and the condition can be mild or unapparent, but characterised by the presence of stunted growth of the neonatal animals and by the morbidity of suckling puppies and kittens. In some cases, the disorder can lead to septicaemia and to neonatal mortality.

In overt clinical cases, the mammary glands appear increased in volume, hyperaemic and painful; when all mammary glands are involved, mastitis is associated with general conditions of malaise, at times also severe, principally characterised by fever, anorexia, apathy, vomiting, sepsis and shock. The females may show no interest for the newborn puppies and kittens, they may refuse to breastfeed them, or they may allow lactation but with signs of nervousness or even of aggression against the newborns, up to the point of cannibalism. In mild and protracted cases, the only sign present may be the stunted growth of the newborn puppies and kittens.

Physical examination can be helpful in the identification of mastitis. Normally, the mammary glands of lactating females are voluminous (especially if some time has elapsed from the last suckling) and hot, but never reddened or dark in colour, bruised and painful or particularly hot to the touch. The gentle daily squeeze of all the nipples, in compliance with hygiene standards, allows to assess the characteristics of the milk (presence, colour, appearance, consistency). In the bitch, normal milk is white, whereas colostrum, produced in the first 1-2 days after whelping, tends to be yellowish. In the presence of mastitis, mammary secretions are more watery, transparent and sticky, with precipitates or clots, and with a white to greenish-yellow or white to blood-brown colour.

When ignored, mastitis may evolve with the formation of abscesses and fistulae or of extensive open gangrenous lesions, loss of the affected gland and systemic involvement. In addition to the physical examination, blood tests may detect the presence of leukocytosis; in order to set up a targeted therapy, a bacteriological test of mammary secretions, avoiding any possible contamination and with the addition of an appropriate antibiogram, is also important. In the chronic form, the mammary glands may appear wrinkled, or palpation may reveal the presence of fibrous, hard, extremely small  (0.5-2 mm) nodular formations, which may be responsible for a possible relapse of acute mastitis in subsequent lactations. The acute forms of mastitis must be differentiated from galactostasis, mammary lesions and from skin diseases of the breast region, which are in fact very common during breastfeeding; the condition must also be differentieated from mammary tumours  with an inflammatory component (e.g. inflammatory mammary carcinoma)and, in the queen, from feline mammary hyperplasia. In cases of subclinical mastitis, the diagnosis depends on the bacteriological test and, possibly, on cytology; the determination of haptoglobine, a protein expressed in the acute phase of the disorder, may also be used as a diagnostic test for the subclinical form. The nodular lesions of the chronic forms, instead, must be differentiated from neoplastic mammary lesions.

TREATMENT

Although the bacteriological test and the antibiogram do allow to set up a targeted therapy, the antibiotic treatment should be initiated immediately after the diagnosis while waiting for more precise indications; this initial treatment involves the use of broad-spectrum antibiotics, such as β-lactam antibiotics (cephalexin, 10-20 mg/kg every 8-12 hours, amoxycillin and clavulanic acid, 14 mg/kg every 12 hours). On the other hand, it is important to remember that also in veterinary medicine cases of antibiotic-resistance are increasing, and the use of β-lactams has led to the development of coagulase-positive methicillin-resistant strains of Staphilococcus aureus.

Although detailed studies on the use of non-steroidal anti-inflammatory drugs for treating mastitis in the dog are lacking, these drugs can be used to relieve pain and to limit inflammatory reactions. In mild forms of the disorder, the antibiotic can be administered orally, as the oral route allows a sufficient penetration of the drug inside the gland together with adequate concentrations in the milk. In addition to a sensitivity test, the choice of the right drug must also consider other aspects, such as the solubility in lipids, the frequent acidity of mastitic milk and the pH of the drug, parameters that can affect both the distribution and efficacy of the drug used; another aspect that must be considered is the blood-mammary tissue barrier and the possibility that puppies may keep suckling their mother’s milk, which is possible when overall conditions are good and the milk appears normal from a macroscopic point of view. Breastfeeding is useful in the treatment of mild forms of mastitis, because continuous mammary gland drainage prevents mammary engorgement and assures a washout effect. In cases in which suckling is continued, cephalexin, the association of amoxicillin and clavulanic acid or macrolides, also in view of their low toxicity in newborn puppies and kittens, can be used while awaiting the outcome of the culture and of the antibiogram. On the contrary, tetracyclines and fluoroquinolones must be avoided, because of their known undesired side effects in newborn animals. Treatment must be continued for at least 2 weeks, together with the application of warm packs and with periodic milking, if the suckling puppies and kittens have been removed from the mother or show scarce interest for the mastitic glands.

In the more severe cases, characterised by the presence of fever (also >40°C), lack of appetite, vomiting, lethargy, sepsis and shock, antibiotics must be given intravenously, in combination with a support therapy. In these cases, a broad-spectrum antibiotic therapy is required, while waiting for the culture and antibiotics sensitivity test results; the antibiotics used should be active against Gram-negative germs (enrofloxacine, cefotaxime and aminoglycosides), in association with antibiotics active against Gram-positive germs (ampicillin, oxacillin, ticarcillin, cefazoline, clindamycin, etc.) and against anaerobic bacteria (ampicillin, metronidazole, clindamycin). Mammary glands presenting abscesses must undergo milk drainage and a possible deferred surgical excision.  Anti-prolactin drugs are indicated to interrupt lactation and to reduce galactostasis. For this reason, the following drugs may be taken into consideration: cabergoline at a dosage of 5 µg/kg/day for 5-7 days, metergoline at 0.1 mg/kg every12 hours for 4-8 days or bromocriptine at 30 µg/kg/day for 5-7 days. In the severe forms of mastitis, newborn puppies and kittens must be removed from maternal breastfeeding. The therapy for chronic mastitis is complex, due to the difficulty in reaching effective drug concentrations within the fibrotic foci; the first-line drugs are erythromycine, clindamycin and lincomycin, whose pharmacokinetic properties adapt well to the characteristics of the mammary gland.

PROGNOSIS

In the mild forms of mastitis the prognosis, in terms of survival, is favourable, while in severe cases it is guarded and the outcome can also be fatal; in terms of function, in the bitch the prognosis is always guarded, in view of the possibility of relapses during subsequent lactations, and in severe or chronic forms it is guarde/fatal; in the queen, instead, in the mild forms the prognosis is favourable while it is guarded/negative in the severe or chronic forms.

PREVENTION

Hygienic control of whelping/delivery boxes and nurseries is essential to restrain the environmental microbial load, while nail trimming in newborn puppies and kittens may be useful to avoid microtraumas. The daily examination of mammary glands and of secretions allows to readily identify suspicious conditions and to perform additional diagnostic investigations, limiting any negative impact on the lactating female and on the suckling puppies and kittens. Since mastitis may recur during subsequent lactations, in the dog, females should be excluded from breeding, or should be strictly monitored during the following lactations. This predisposition to recurrence has not in fact been demonstrated in the queen. The preventive use of antibiotics is ineffective and promotes the development of antibiotic resistance.

Suggested readings

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