Guinea Pigs (Laboratory) - Pathology

Introduction

Guinea pigs or “cavy” originate from South America and in the wild are a type of burrowing animal. They are nocturnal herbivores and were originally domesticated by the Incas as a source of nutrition. Guinea pigs are widely used in experimentation for a number of reasons, including highly developed young at birth (due to a relatively lengthy gestation period), the ability of the young to eat solid food at age 2-3 days, being fully haired and completely mobile. These factors reduce many issues and complications regarding the use of experimentation animals. Guinea pigs are also among the easiest laboratory animals to handle and can be housed in open top cages due to their in ablility to climb or jump.

Guinea pigs have been used for a broad spectrum of scientific purposes including the production of antibodies, tumor studies, disease studies and nutritional studies. Guinea pigs have been used since the 17th century and in 1990 were even launched into space as scientific passengers by China. One of the most common experimental uses of guinea pigs in the past has been for the standardisation and assessment of vaccines.

Strains and Stocks

When compared with the continuous and long term breeding strains of mice such as the F344 and LEW, the breeding of guinea pigs is not as well developed and there are few specific strains in use. The outbred strains available are mainly the Hartley, Dunkin-Hartley and Albino. Inbred strains available are strains “2” and “13”, although these are uncommon compared to the outbred strains and are only used for very specific research. There are relatively less guinea pig breeders available of the quality required for experimental use and there are likely to be considerable variations between strains from different breeders. The total number of experimental guinea pigs in use as of 2007 was 2% of the total animals used.
Other types used within experimental research are the hairless strains that were developed in the 1980s specifically for dermatological use. These strains are often a result of a spontaneous genetic mutation from the strains described above. The recently popularised “skinny pig” is a type of guinea pig originally developed for laboratory use.

Physiology

Please find details of Guinea Pig physiology within the WikiNormals section. Physiological information available includes “General”, “Biochemical”, “Haematological” and “Breed” parameters.

Anatomy and Histology

This section has been included to allow familiarisation with the peculiarities of guinea pig anatomy to provide a context for some of the disease and pathological headings found below. Therefore only anatomical areas with specific features warranting emphasis have been included below;

Reproductive System

In males the inguinal canal remains open throughout the guinea pig’s life and this can lead to an increased risk of abdominal organ prolapse post-castration. Males also have coiled vesicular glands which extend into the abdomen approximately 10cm in a similar manner to a females uterine horns.

Urinary System

Guinea pig urine is opaque and creamy yellow with a normal pH 9. The urine may also contain crystals of ammonium phosphate and calcium carbonate.

Respiratory System

Although guinea pigs have a normal mammalian respiratory system, a healthy guinea pig will only breathe through its nose and mouth breathing is therefore a sign of respiratory distress.

Digestive System

Guinea pigs have a longer colon than other rodents, accounting for 60% of the length of its intestines. Guinea pigs also have a relatively large caecum for the digestion of cellulose and it is able to contain up to 65% of the gastrointestinal contents at any time. The stomach is non-glandular and the female spleen is significantly larger than a male spleen.

Diseases

For ease of use, the diseases of guinea pigs listed below are by body system, or where this is not appropriate in an “Other” category displayed after the body system sections. Those diseases listed below are not exhaustive but rather highlight common diseases encountered with laboratory guinea pigs.

Integument System

Perhaps the most common finding in guinea pigs are skin problems related to diet, specifically a low vitamin C diet. Diet should always be considered when evaluating the integumentary system.

Dermatitis

Mainly seen in strain 13 guinea pigs and characterized by alopecia and erythema of the ventrum.
At necropsy there will be erythema, scabbing and cracks in the epidermis with associated hair loss. Histologically, there is marked epidermal cleavage parakeratotic hyperkeratosis and minimal inflammatory response.

Dermatophytosis (Ringworm)

The causal agent of this mycotic infection is commonly either Trichophyton mentagrophytes or Microsporum gypseum. Areas of alopecia and seborrhoea can be found and hair can easily be plucked. Microscopic examination of hair plucked from areas of lesion is sufficient to confirm diagnosis.
Guinea pigs can suffer from a wide range of mycotic infections which can range in severity from mild skin changes to cystitis, pneumonia and reproductive disorders.
Macroscopic pathology will include circumscribed lesions that are erythematous, edematous, and scaly with alopecia. Pustules are usually due to secondary bacteria. On microscopic examination there is hyperkeratosis, epidermal hyperplasia and pustules in the epidermis and hair follicles. Arthrospores may be seen with H&E staining.

Mange

This mite infection is commonly caused by Trixacarus caviae, a sarcoptiform mite. Clinical signs usually occur 3-5 weeks after infection and lesions are usually visible around the head, shoulders and back. Hair is easily plucked and skin is usually seborrhoeic and pruritic. In some cases self trauma may also compound problems.
Microscopic examination of skin scrapings is usually sufficient to confirm diagnosis. Guinea pigs are also succeptible to Demodex infections with similar clinical signs and diagnosis.

Lice

Lice infestations can be caused by a number of different species with the most common being Gliricola porcelli. Gyropus ovalis and Trimenopen jenningsi are also common. The lice can often be seen in the fur and eggs can be seen as either black or white specks adhering to the hair shafts of the fur.
Microscopic examination of the hair and skin scrapings is sufficient for diagnosis.

Other Mite Infestations

Fur mites or Chirodiscoides caviae can be found in pairs within the fur but are commensal. Fur mites do not usually produce clinical signs, even during heavy infestations.

Rabbit fur mites or Psoroptes cuniculi can also be found within the fur and cause crusting and scaling dermatitis. This mite can also cause extreme pruritis

Fly Strike

Although this condition is uncommon in guinea pigs, the prescence of larvae of blowfly colonising a moist wound can lead toxic shock. Guinea pigs appear far less susceptible to fly strike than rabbits and in most cases prognosis for recovery is good.

Alopecia

Alopecia usually occurs hormonally, during pregnancy or post-natally. In most cases the hair loss will not be accompanied by pruritis. Bilateral symmetrical alopecia can be seen in older females suffering with ovarian cysts. Post-parturition hair loss is often bilaterally on the flanks and ventral abdomen.

Liver Disease

Hair loss is a common cause of hair loss once parasitic and fungal infections have been removed. During liver disease the overall fur covering is sparse and the skin is thickened. Often this general hair loss is accompanied with seborrhea.

Abscesses

Abscessess are mainly caused by fighting and various types of bacteria have been isolated as the causal agent. These include Pseudomonas aeruginosa, Pasteurella multocida, Corynebacterium pyogenes, Staphylococcus aureus, Streptococcus spp. and other Enterobacteria. There will be localised swelling and most abcessess occur in the throat area.

Reproductive System

Pseudopregnancy

Although this condition is rare in guinea pigs, its duration is 17 days. Care must be taken that sows do not develop mastitis during pseudopregnancy.

Pregnancy toxaemia

Pregnancy toxaemia is a metabolic disorder similar to twin lamb disease in sheep. Predisposing factors include obesity and stress leading to negative energy balance. The energy imbalance progresses to ketoacidosis together with aciduria, proteinuria and hyperlipaemia. There is also a fatty degeneration of the liver.
In cases of stress the sow stops eating, becomes depressed and will have a ruffled appearance. Some sows may also salivate profusely and the distinctive smell of ketones (pear drops) may be present. The condition is more prevalent in hot weather.
Necropsy will show an empty stomach and pathology including fatty liver and kidneys, multiple fetae with varying levels of haemorrhage and large fat reserves in the abdomen.

Hypocalcemia (Eclampsia)

Pregnant sows can develop an acute calcium deficiency due to the metabolic demands of parturition combined with lactation. Obesity and stress are key predisposing factors.
Pathologic findings will include fatty liver and kidneys and in general are similar to the macroscopic findings of pregnancy toxaemia but usually more severe. At the point of onset of clinical signs the prognosis is poor.

Miscarriage and Abortion

If abortion occurs early in gestation (up to 40 days) the sow will commonly not be systemically affected. Later than 40 days will result in systemic illness with severe blood loss with potentially fatal consequences.

Toxoplasmosis

A Toxoplasmosis infection is caused by a protozoan parasite called Toxoplasma gondii in which the definitive host is the cat. Guinea pig infection is caused by consumption of food contaminated with cat faeces, i.e. a faecal-oral route. Tissue cysts from the parasite can be found in muscle, heart, eye and brain. Toxoplasmosis can also cause vulval bleeding and abortion.

Mastitis

Predisposing factors for the development of this infection are poor standards of environmental cleanliness or a build up of faecal contamination. Various species of bacteria have been found to be involved including Pseudomonas aeruginosa, Pasturella multocida, Corynebacterium pyogenes, Staphylococcus aureus, Streptococcus spp and various types of Enterobacteriacae.
Affected mammary glands will be warm, hard and swollen and if the infection is heavy the gland may become ulcerated and necrotic. Mastitis in guinea pigs can also cause systemic illness.

Preputial infections

This type of infection is usually caused when a foreign body such as saw dust becomes trapped in the prepuce causing preputial dermatitis. This can lead to a swelling of the prepuce but also cause inability to copulate. Similarly it has been found that saw dust can also block the vagina of the female also leading to an inability to achieve copulation.
To decrease the risk of preputial infections and vaginal blockage wood shavings are superior to saw dust.

Urinary System

Polydipsia

Guinea pigs should consume approximately 10ml/100g but will vary dependant on the water content of the diet. Polydipsia often accompanies chronic renal failure and diabetes. Polydipsia is also associated with abdominal swelling and hydronephrosis of both kidneys.

Polyuria

Guinea pigs should have a urinary output of approximately 20-25ml per day. Polyuria is seen as a consequence of polydipsia. Excess urine output can lead to urine scalding and dermatitis.

Haematuria

“Bloody urine” can be a common symptom found in guinea pigs, both male and female. Cystitis, neoplasia of the bladder or uterus, urolithiasis and cystic endometritis are all causative factors for haematuria.

Acute Renal Failure

This condition is uncommon but where is does occur it is invariably fatal in guinea pigs. Oxalic acid poisoning can result from the consumption of large amounts of beetroot, spinach or dock leaves once the stems of these vegetables have turned woody. Clinical symptoms prior to death include depression, salivation and muscle tremors.

Chronic Renal Failure

Chronic renal disease may initially be indicated via polydipsia together with a period of anorexia, depression, diarrhoea, collapse and then death. Staphylococcol pododermatitis, chronic intersititial nephritis, diabetes and hydronephrosis have all been shown to lead to chronic renal failure in guinea pigs.

Diabetes

Diabetes is a common occurrence in guinea pigs together with polydipsia and weight loss despite a strong appetite. Some guinea pigs will develop cataracts and glycosuria.
Exogenous insulin is not required in treatment for this disease and guinea pigs often have spontaneous remissions.

Respiratory System

Epistaxis

Bleeding from the nose is often a symptom of vitamin K deficiency caused by a lack of greenstuffs in the diet.

Nasal Discharge

Mucopurulent discharges are often a symptom of upper or lower respiratory tract infections. Respiratory tract infections are also associated with “ruttling” which is a wheezy breathing sound. Predisposing factors for respiratory infections are a change in environmental temperature, humidity or ventilation. Ammonia build up as a consequence of dirty bedding/environment is known to weaken the resistance of the respiratory tract.
Any nasal discharge should not be confused with a milky type of fluid secreted from the eyes and nose as part of the normal guinea pig grooming process.

Pneumonia

The clinical symptoms of pneumonia include dyspnoea, ruttling (see above), sneezing, nasal discharge and coughing. If left untreated, guinea pigs can become depressed anorexic and it can become a fatal infection. In guinea pigs pneumonia can be caused be various bacterial and viral agents including Bordetella bronchiseptica, Streptococcus zooepidemicus, Streptococcus pneumonia, Klebsiella pneumonia, Pseudomonas aeruginosa and Pasturella spp.
Diagnosis is via isolation of the causal agent from a culture of the nasal discharge.
Bordetella Pathology: Bordatella has an affinity for ciliated respiratory epithelium. Necropsy may reveal mucopurulent rhinitis, purulent bronchitis and tracheitis. Exudate can also be found in the tympanic bullae. There may also be pulmonary consolidation (usually anterioventral). Pleuritis or pyosalpinx may be seen.
Microscopic pathology may include acute to chronic suppurative bronchopneimonia with heterophilic infiltration. There will also be generalised obliteration of the normal architecture together with fibrinous exudate.
Streptococcus Pathology: necropsy will show fibrinopurulent bronchopneumonia, with or without fibrinous pleuritis and pericarditis. Lungs will often contain a sanguinous fluid. Histologically, an acute fibrinosuppurative bronchopneumonia is present with thrombosis of pulmonary vessels. Infiltrating cells may be elongate and form palisading patterns in affected airways.

Adenovirus

Adenoviridae do not require co-infection with bacteria in order to become pathogenic within the respiratory system. The infection may have no clinical symptoms but in susceptible animals such as those that are stressed or immune-deficient it can be fatal.
Macroscopic pathology will include consolidation of the cranial lobes of the lung, necrotizing bronchitis, bronchiolitis with desquamation of the epithelial lining and inflammatory cell infiltration. Airways may contain high levels of cell debris, WBC’s and fibrin. Nuclei often contain distinctive basophilic inclusions 7-15u in diameter.

Digestive System

Cytomegalovirus (Salivary Gland Virus)

CMV often causes subclinical infections and associated immunosupression. CMV is transmitted via the placenta.
Microscopic pathology will include the appearence of eosinophilic intranuclear inclusion bodies which are often accompanied by basophilic cytoplasmic inclusions in the ductal epithelial cells of the salivary glands. In some cases there is also a a lymphoid cell infiltrate.

Infectious Causes of Diarrhoea

Salmonella
Salmonella typhimurium and Salmonella enteriditis are the common causative agents and are often associated with contaminated food and bedding. Diarrhoea may be haemorrhagic, cause septicaemia and can also cause sudden death.
Macroscopic pathologic findings include enlarged liver, spleen, intestine and lymph nodes containing small white necrotic foci in chronically infected animals. Pregnant animals may have a purulent metritis. There may also be hyperplasia in the Peyer's patch and splenomegally. Microscopically there may be granulomatous hepatitis, splenitis and lymphadenitis with the areas of necrosis are surrounded by mononuclear cells and neutrophils.
There are often no findings in animals with acute infections.
Diagnosis is via isolation and culture of the Salmonella spp via faecal samples.
Chronic cases and those that recover from a Salmonella infection are likely to become carriers and should be destroyed to prevent further outbreaks.
Yersinia (pseudotuberculosis)
Acute forms of this disease can cause septicaemia and death within 48hrs. Chronic cases have more progressive clinical symptoms but will usually culminate in death after 3-4 weeks. During this 3-4 week period any young may become congenitally or neonatally affected. Diagnosis is via culture of Yersinia pseudotuberculosis from blood, lymph nodes or faecal samples.
Yersinia infections are transmitted via wild birds and other rodents contaminating their food, particularly green foods. Once the disease is endemic in a breeding population some infections can be passed vertically in the milk. Macroscopic pathologic lesions include enlarged or abscessed lymph nodes and necrosis of liver and spleen.
Clostridia
Normally this species are present in the large intestines as commensals but during certain circumstances such as antibiotic administration they are able to proliferate and cause fatal enterotoxaemia. Clostridium species can be cultured from the lumen of the infected intestine.
Pathologic examination will show necrotizing ileitis and typhlitis. Hepatic lesions may be present and will be characterized by focal coagulative necrosis in periportal regions.
Antibiotic-Induced Enterotoxemia
Bacterial overgrowth is associated with enterocolitis, septicemia and death. Antibiotics implicated include penicillin, erythromycin, lincomycin, chlortetracycline, oxytetracycline, bacitracin, dihydrostreptomycin, cefazolin, cephalexin, cephalothin, ampicillin, clindamycin & gentamycin. All of the above species have been implicated as potential causative agents.
Macroscopic pathology during necropsy will include a distended cecum containing blood and hemorrhagic serosal and mucosal surfaces. Microscopic pathology during necropsy will include extensive vascular congestion, submucosal hemorrhage and oedema, and sloughing of the absorptive epithelium throughout the GI tract.

Parasitic Causes of Diarrhoea

Nematodes
The only nematode that affects guinea pigs is Paraspidodera uncinata and is usually restricted to outdoor housed animals. Heavy infestation can cause enteritis and the Paraspidodera resides in the caecum and can be 11-28mm long.
Protozoa
A wide range of commensal protozoa are carried and are not pathogenic except for Coccidia and Cryptosporidium species. Protozoan infections are generally the result of poor husbandry. Contact with rabbits can allow other rare protozoan infections in guinea pigs.
Eimeria caviae is the coccidial species contracted by guinea pigs eating contaminated food, however coccidiosis in guinea pigs is rare. Droppings will be slimy and contain blood. Oocysts will be present in the faeces and can be seen on microscopic examination.
Cryptosporidium species that colonise the intestine will cause weight loss and diarrhoea.
Microscopic pathologic findings will include flattened and irregular villi in the ileum together with a granulomatous infiltrate into the lamina propria and absorptive areas of the intestinal epithelium. In some cases the parasite itself may be visualised via parasitophorous vacuoles within the apical portion of the infected enterocyte.
Ascaridae
Ascaridae are not a natural parasite of guinea pigs but if housed outdoors in areas which has been contaminated with dog or cat faeces guinea pigs will suffer from weight loss, unthriftiness and diarrhoea.

Musculoskeletal System

Pododermatitis

The origin of the swelling and ulceration of the foot pad is usually bacterial, most commonly Staphylococcus aureus. Treatment of advanced infections is rarely successful.

Osteoporosis

This condition will occur if guinea pigs are over-supplemented with vitamin D via forced such as cod-liver oil. The excess vitamin D leads to excess calcium resorption and subsequent bone weakness. Clinical signs are usually paralysis and most commonly seen in the hind legs first.
The diet should contain a maximum of 1600iu of vitamin D per kg.

Metastatic Calcification

This condition is caused by an imbalance between calcium, phosphrous and vitamin D. Metastatic calcification is mainly seen in males in which they exhibit joint stiffness, poor weight gain and eventually death.
Macroscopic pathology will show mineralisation/calcification of the heart, blood vessels, stomach, colon, kidneys and lungs.
The diet should have a calcium:phosphorus ratio of 1:5 and the same vitamin D level as above.

Scurvy

This condition is due to a deficiency of vitamin C where an adult guinea pig requires 10-30mg per day.
Macroscopic pathology will include haemorrhages around joints and on all serosal surfaces, particularly around fascial and articular cartilages. Petechial and ecchymotic bleeding may be seen on the periosteum, muscle and gingiva.

Osteoarthritis

In guinea pigs the most commonly affected joint is the stifle and clinical symptoms can range from lameness to weight loss in guinea pigs of 9 months and older.

Osteosarcoma

Although relatively uncommon in guinea pigs they can be seen as painful bony swellings in guinea pigs over 1 ear of age.

Other

Micropthalmia

The eye will either appear small or will be non-existant with resultant partial or complete blindness. Micropthalmia is commonly associated with all-white coated guinea pigs.

Conjunctivitis

The conjunctiva will become reddened with epiphora or an ocular discharge. Often conjunctivitis is either a symptom of upper respiratory disease, irritation and allergic response. If the conjunctivitis is unilateral then trauma is likely to be the cause.

Middle Ear Disease

The affected guinea pig will hold its head over to the affected side and may also suffer balance problems. The condition is very similar in terms of symptoms to “wry neck”, see below. Pus may be seen in the ear canal and the infection may progress to the inner ear and to the meninges. Ear infections in guinea pigs are also known to accompany respiratory infections such as Salmonella and Pasturella spp.

Cleft Palate

This is a congenital abnormality resulting in the guinea pig potentially being unable to suckle or chew food correctly. In guinea pigs the cleft usually involves the hard palate only. There is no treatment and do to its inherited nature the guinea pig should be destroyed.

Cervical Lymphadenitis

Steptococcus zooepidemicus is normally present in the conjunctiva as a commensal organism. If this organism gains access to the cervical lymph nodes they can become pathogenic and in some cases death can occur due to septicaemia. Infection will cause the lymph nodes to swell. Stress has been shown to increase susceptibility.
Necropsy will show macroscopic pathology ranging from enlarged ventral and cervical lymph nodes to encapsulated abscesses filled with a thick yellow/white purulent exudate. Associated pathology found in other organ systems involved may include bronchopneumonia, otitis media, pleuritis, peritonitis, and pericarditis. Infection may vary from an acute fatal septicemia to a chronic suppurative process in the lymph nodes, thoracic and abdominal viscera, uterus, and ears.
Microscopic evidence of pneumonia, pleuritis, myocarditis, pericarditis, and peritonitis, otitis media, nephritis, arthritis, and cellulitis will be seen characterized by necrotizing suppurative inflammation or fibrinosuppurative inflammation.

Wry Neck

This condition affects only newborn guinea pigs in which they have a degree of torticollis and may be found lying on their backs to suckle due to their inability to maintain a suitable head posture. This condition is hereditary.

Epilepsy

Guinea pig epileptic fits usually last for 3 to 4 mins where there will be heavy salivation and twitching of limbs. Polyphagia and aggression can be seen post-fit. Commonly the frequency of fits will increase with age.
Guinea pigs also have fits that are not true epileptiform convulsions and are often secondary to another disease. These can include liver and kidney failure, enterotoxaemia, ketosis and septicaemia.

Cerebellar Disease

Classical symptoms of this disease are circling movements, dystocia and nystagmus, although nystagmus is rare in guinea pigs. In most cases cerebellar disease is secondary to the above mentioned middle ear infection progressing to the inner ear. Most guinea pigs do not recover from this even with treatment.

Heatstroke

Although this can occur in outdoor guinea pigs exposed to direct sunlight, heavily pregnant females are the most susceptible especially if they have been deprived of water. This can be a problem during transportation or during problems with laboratory infrastructures such as air conditioning.